diff --git a/.github/workflows/build.yaml b/.github/workflows/build.yaml
index 3406704..54ba8bf 100644
--- a/.github/workflows/build.yaml
+++ b/.github/workflows/build.yaml
@@ -24,7 +24,8 @@ jobs:
     - name: Set up JDK
       uses: actions/setup-java@v1
       with:
-        java-version: 1.8
+        # See comment on moditect-maven-plugin in pom.xml
+        java-version: 12
     - name: Compile
       run: ./mvnw clean test-compile -B
     - name: Test
diff --git a/.gitignore b/.gitignore
index 7adafbb..8227300 100644
--- a/.gitignore
+++ b/.gitignore
@@ -2,6 +2,13 @@
 *.iml
 .idea/
 
+### VS Code ###
+.vscode
+.project
+.settings
+.classpath
+.factorypath
+
 ### Maven ###
 target/
 /.mvn/wrapper/*.jar
diff --git a/README.md b/README.md
index c2c2418..c53047d 100644
--- a/README.md
+++ b/README.md
@@ -28,6 +28,7 @@ It's decoupled from any JSON library, but contains a separate module for Jackson
 - Java 8
 - Any build tool using Maven Central, or direct download
 - Jackson (optional)
+- Gson (optional)
 
 ## Installation
 
@@ -44,6 +45,24 @@ Add the following dependency to your project:
     <artifactId>jackson-datatype-problem</artifactId>
     <version>${problem.version}</version>
 </dependency>
+<dependency>
+    <groupId>org.zalando</groupId>
+    <artifactId>problem-gson</artifactId>
+    <version>${problem.version}</version>
+</dependency>
+```
+
+### Java Modules
+
+Even though the minimum requirement is still Java 8, all modules are Java 9 compatible:
+
+```java
+module org.example {
+    requires org.zalando.problem;
+    // pick needed dependencies
+    requires org.zalando.problem.jackson;
+    requires org.zalando.problem.gson;
+}
 ```
 
 ## Configuration
diff --git a/jackson-datatype-problem/pom.xml b/jackson-datatype-problem/pom.xml
index c75b87b..4b5e3c7 100644
--- a/jackson-datatype-problem/pom.xml
+++ b/jackson-datatype-problem/pom.xml
@@ -11,6 +11,10 @@
     <artifactId>jackson-datatype-problem</artifactId>
     <properties>
         <jackson.version>2.10.1</jackson.version>
+        <module.name>org.zalando.problem.jackson</module.name>
+        <module.exports>
+            org.zalando.problem.jackson;
+        </module.exports>
     </properties>
     <dependencies>
         <dependency>
diff --git a/jackson-datatype-problem/src/main/java/org/zalando/problem/AbstractThrowableProblemMixIn.java b/jackson-datatype-problem/src/main/java/org/zalando/problem/jackson/AbstractThrowableProblemMixIn.java
similarity index 80%
rename from jackson-datatype-problem/src/main/java/org/zalando/problem/AbstractThrowableProblemMixIn.java
rename to jackson-datatype-problem/src/main/java/org/zalando/problem/jackson/AbstractThrowableProblemMixIn.java
index 996ac30..bb7bc01 100644
--- a/jackson-datatype-problem/src/main/java/org/zalando/problem/AbstractThrowableProblemMixIn.java
+++ b/jackson-datatype-problem/src/main/java/org/zalando/problem/jackson/AbstractThrowableProblemMixIn.java
@@ -1,10 +1,14 @@
-package org.zalando.problem;
+package org.zalando.problem.jackson;
 
 import com.fasterxml.jackson.annotation.JsonAnySetter;
 import com.fasterxml.jackson.annotation.JsonCreator;
 import com.fasterxml.jackson.annotation.JsonProperty;
 
-import javax.annotation.Nullable;
+import org.checkerframework.checker.nullness.qual.Nullable;
+import org.zalando.problem.AbstractThrowableProblem;
+import org.zalando.problem.StatusType;
+import org.zalando.problem.ThrowableProblem;
+
 import java.net.URI;
 
 abstract class AbstractThrowableProblemMixIn {
diff --git a/jackson-datatype-problem/src/main/java/org/zalando/problem/ExceptionalMixin.java b/jackson-datatype-problem/src/main/java/org/zalando/problem/jackson/ExceptionalMixin.java
similarity index 91%
rename from jackson-datatype-problem/src/main/java/org/zalando/problem/ExceptionalMixin.java
rename to jackson-datatype-problem/src/main/java/org/zalando/problem/jackson/ExceptionalMixin.java
index f8fc29d..e6da561 100644
--- a/jackson-datatype-problem/src/main/java/org/zalando/problem/ExceptionalMixin.java
+++ b/jackson-datatype-problem/src/main/java/org/zalando/problem/jackson/ExceptionalMixin.java
@@ -1,4 +1,4 @@
-package org.zalando.problem;
+package org.zalando.problem.jackson;
 
 import com.fasterxml.jackson.annotation.JsonIgnore;
 import com.fasterxml.jackson.annotation.JsonIgnoreProperties;
@@ -7,6 +7,8 @@
 import com.fasterxml.jackson.databind.annotation.JsonSerialize;
 import com.fasterxml.jackson.databind.ser.std.ToStringSerializer;
 
+import org.zalando.problem.ThrowableProblem;
+
 import static com.fasterxml.jackson.annotation.JsonInclude.Include.NON_NULL;
 
 @JsonIgnoreProperties(ignoreUnknown = true)
diff --git a/jackson-datatype-problem/src/main/java/org/zalando/problem/ExceptionalWithoutStacktraceMixin.java b/jackson-datatype-problem/src/main/java/org/zalando/problem/jackson/ExceptionalWithoutStacktraceMixin.java
similarity index 89%
rename from jackson-datatype-problem/src/main/java/org/zalando/problem/ExceptionalWithoutStacktraceMixin.java
rename to jackson-datatype-problem/src/main/java/org/zalando/problem/jackson/ExceptionalWithoutStacktraceMixin.java
index 45a0dc5..2f47eb3 100644
--- a/jackson-datatype-problem/src/main/java/org/zalando/problem/ExceptionalWithoutStacktraceMixin.java
+++ b/jackson-datatype-problem/src/main/java/org/zalando/problem/jackson/ExceptionalWithoutStacktraceMixin.java
@@ -1,4 +1,4 @@
-package org.zalando.problem;
+package org.zalando.problem.jackson;
 
 import com.fasterxml.jackson.annotation.JsonIgnore;
 import com.fasterxml.jackson.annotation.JsonIgnoreProperties;
diff --git a/jackson-datatype-problem/src/main/java/org/zalando/problem/ProblemMixIn.java b/jackson-datatype-problem/src/main/java/org/zalando/problem/jackson/ProblemMixIn.java
similarity index 87%
rename from jackson-datatype-problem/src/main/java/org/zalando/problem/ProblemMixIn.java
rename to jackson-datatype-problem/src/main/java/org/zalando/problem/jackson/ProblemMixIn.java
index 798c082..fccbd52 100644
--- a/jackson-datatype-problem/src/main/java/org/zalando/problem/ProblemMixIn.java
+++ b/jackson-datatype-problem/src/main/java/org/zalando/problem/jackson/ProblemMixIn.java
@@ -1,4 +1,4 @@
-package org.zalando.problem;
+package org.zalando.problem.jackson;
 
 import com.fasterxml.jackson.annotation.JsonAnyGetter;
 import com.fasterxml.jackson.annotation.JsonInclude;
@@ -6,6 +6,10 @@
 import com.fasterxml.jackson.annotation.JsonTypeInfo;
 import com.fasterxml.jackson.databind.annotation.JsonSerialize;
 
+import org.zalando.problem.DefaultProblem;
+import org.zalando.problem.Problem;
+import org.zalando.problem.StatusType;
+
 import java.net.URI;
 import java.util.Map;
 
diff --git a/jackson-datatype-problem/src/main/java/org/zalando/problem/ProblemModule.java b/jackson-datatype-problem/src/main/java/org/zalando/problem/jackson/ProblemModule.java
similarity index 87%
rename from jackson-datatype-problem/src/main/java/org/zalando/problem/ProblemModule.java
rename to jackson-datatype-problem/src/main/java/org/zalando/problem/jackson/ProblemModule.java
index a647012..31ed534 100644
--- a/jackson-datatype-problem/src/main/java/org/zalando/problem/ProblemModule.java
+++ b/jackson-datatype-problem/src/main/java/org/zalando/problem/jackson/ProblemModule.java
@@ -1,4 +1,4 @@
-package org.zalando.problem;
+package org.zalando.problem.jackson;
 
 import com.fasterxml.jackson.core.Version;
 import com.fasterxml.jackson.core.util.VersionUtil;
@@ -6,6 +6,12 @@
 import com.fasterxml.jackson.databind.module.SimpleModule;
 import org.apiguardian.api.API;
 
+import org.zalando.problem.DefaultProblem;
+import org.zalando.problem.Exceptional;
+import org.zalando.problem.Problem;
+import org.zalando.problem.Status;
+import org.zalando.problem.StatusType;
+
 import java.util.Collections;
 import java.util.HashMap;
 import java.util.Map;
@@ -34,7 +40,7 @@ public ProblemModule() {
      * @throws IllegalArgumentException if there are duplicate status codes across all status types
      */
     @SafeVarargs
-    public <E extends Enum & StatusType> ProblemModule(final Class<? extends E>... types)
+    public <E extends Enum<?> & StatusType> ProblemModule(final Class<? extends E>... types)
             throws IllegalArgumentException {
 
         this(false, buildIndex(types));
@@ -76,7 +82,7 @@ public void setupModule(final SetupContext context) {
     }
 
     @SafeVarargs
-    private static <E extends Enum & StatusType> Map<Integer, StatusType> buildIndex(
+    private static <E extends Enum<?> & StatusType> Map<Integer, StatusType> buildIndex(
             final Class<? extends E>... types) {
         final Map<Integer, StatusType> index = new HashMap<>();
 
diff --git a/jackson-datatype-problem/src/main/java/org/zalando/problem/ProblemTypeConverter.java b/jackson-datatype-problem/src/main/java/org/zalando/problem/jackson/ProblemTypeConverter.java
similarity index 79%
rename from jackson-datatype-problem/src/main/java/org/zalando/problem/ProblemTypeConverter.java
rename to jackson-datatype-problem/src/main/java/org/zalando/problem/jackson/ProblemTypeConverter.java
index 38c93f1..28c65d3 100644
--- a/jackson-datatype-problem/src/main/java/org/zalando/problem/ProblemTypeConverter.java
+++ b/jackson-datatype-problem/src/main/java/org/zalando/problem/jackson/ProblemTypeConverter.java
@@ -1,7 +1,9 @@
-package org.zalando.problem;
+package org.zalando.problem.jackson;
 
 import com.fasterxml.jackson.databind.util.StdConverter;
 
+import org.zalando.problem.Problem;
+
 import java.net.URI;
 
 final class ProblemTypeConverter extends StdConverter<URI, URI> {
diff --git a/jackson-datatype-problem/src/main/java/org/zalando/problem/StatusTypeDeserializer.java b/jackson-datatype-problem/src/main/java/org/zalando/problem/jackson/StatusTypeDeserializer.java
similarity index 84%
rename from jackson-datatype-problem/src/main/java/org/zalando/problem/StatusTypeDeserializer.java
rename to jackson-datatype-problem/src/main/java/org/zalando/problem/jackson/StatusTypeDeserializer.java
index 213917c..3f89d5f 100644
--- a/jackson-datatype-problem/src/main/java/org/zalando/problem/StatusTypeDeserializer.java
+++ b/jackson-datatype-problem/src/main/java/org/zalando/problem/jackson/StatusTypeDeserializer.java
@@ -1,10 +1,12 @@
-package org.zalando.problem;
+package org.zalando.problem.jackson;
 
 import com.fasterxml.jackson.core.JsonParser;
 import com.fasterxml.jackson.databind.DeserializationContext;
 import com.fasterxml.jackson.databind.JsonDeserializer;
 
-import javax.annotation.Nullable;
+import org.checkerframework.checker.nullness.qual.Nullable;
+import org.zalando.problem.StatusType;
+
 import java.io.IOException;
 import java.util.Map;
 
diff --git a/jackson-datatype-problem/src/main/java/org/zalando/problem/StatusTypeSerializer.java b/jackson-datatype-problem/src/main/java/org/zalando/problem/jackson/StatusTypeSerializer.java
similarity index 85%
rename from jackson-datatype-problem/src/main/java/org/zalando/problem/StatusTypeSerializer.java
rename to jackson-datatype-problem/src/main/java/org/zalando/problem/jackson/StatusTypeSerializer.java
index 0ba27e5..7ee6a80 100644
--- a/jackson-datatype-problem/src/main/java/org/zalando/problem/StatusTypeSerializer.java
+++ b/jackson-datatype-problem/src/main/java/org/zalando/problem/jackson/StatusTypeSerializer.java
@@ -1,9 +1,11 @@
-package org.zalando.problem;
+package org.zalando.problem.jackson;
 
 import com.fasterxml.jackson.core.JsonGenerator;
 import com.fasterxml.jackson.databind.JsonSerializer;
 import com.fasterxml.jackson.databind.SerializerProvider;
 
+import org.zalando.problem.StatusType;
+
 import java.io.IOException;
 
 final class StatusTypeSerializer extends JsonSerializer<StatusType> {
diff --git a/problem/src/main/java/org/zalando/problem/UnknownStatus.java b/jackson-datatype-problem/src/main/java/org/zalando/problem/jackson/UnknownStatus.java
similarity index 79%
rename from problem/src/main/java/org/zalando/problem/UnknownStatus.java
rename to jackson-datatype-problem/src/main/java/org/zalando/problem/jackson/UnknownStatus.java
index 243cfd5..f6180df 100644
--- a/problem/src/main/java/org/zalando/problem/UnknownStatus.java
+++ b/jackson-datatype-problem/src/main/java/org/zalando/problem/jackson/UnknownStatus.java
@@ -1,8 +1,7 @@
-package org.zalando.problem;
+package org.zalando.problem.jackson;
 
-import javax.annotation.concurrent.Immutable;
+import org.zalando.problem.StatusType;
 
-@Immutable
 final class UnknownStatus implements StatusType {
 
     private final int statusCode;
diff --git a/jackson-datatype-problem/src/main/resources/META-INF/services/com.fasterxml.jackson.databind.Module b/jackson-datatype-problem/src/main/resources/META-INF/services/com.fasterxml.jackson.databind.Module
index e5db920..1df0daa 100644
--- a/jackson-datatype-problem/src/main/resources/META-INF/services/com.fasterxml.jackson.databind.Module
+++ b/jackson-datatype-problem/src/main/resources/META-INF/services/com.fasterxml.jackson.databind.Module
@@ -1 +1 @@
-org.zalando.problem.ProblemModule
\ No newline at end of file
+org.zalando.problem.jackson.ProblemModule
\ No newline at end of file
diff --git a/jackson-datatype-problem/src/test/java/org/zalando/problem/BusinessException.java b/jackson-datatype-problem/src/test/java/org/zalando/problem/jackson/BusinessException.java
similarity index 78%
rename from jackson-datatype-problem/src/test/java/org/zalando/problem/BusinessException.java
rename to jackson-datatype-problem/src/test/java/org/zalando/problem/jackson/BusinessException.java
index ddbcb14..2f5de2d 100644
--- a/jackson-datatype-problem/src/test/java/org/zalando/problem/BusinessException.java
+++ b/jackson-datatype-problem/src/test/java/org/zalando/problem/jackson/BusinessException.java
@@ -1,4 +1,4 @@
-package org.zalando.problem;
+package org.zalando.problem.jackson;
 
 abstract class BusinessException extends Exception {
 
diff --git a/problem-gson/src/test/java/org/zalando/problem/CustomStatus.java b/jackson-datatype-problem/src/test/java/org/zalando/problem/jackson/CustomStatus.java
similarity index 86%
rename from problem-gson/src/test/java/org/zalando/problem/CustomStatus.java
rename to jackson-datatype-problem/src/test/java/org/zalando/problem/jackson/CustomStatus.java
index cb5f669..102f4da 100644
--- a/problem-gson/src/test/java/org/zalando/problem/CustomStatus.java
+++ b/jackson-datatype-problem/src/test/java/org/zalando/problem/jackson/CustomStatus.java
@@ -1,4 +1,6 @@
-package org.zalando.problem;
+package org.zalando.problem.jackson;
+
+import org.zalando.problem.StatusType;
 
 enum CustomStatus implements StatusType {
 
diff --git a/jackson-datatype-problem/src/test/java/org/zalando/problem/EnforceCoverageTest.java b/jackson-datatype-problem/src/test/java/org/zalando/problem/jackson/EnforceCoverageTest.java
similarity index 81%
rename from jackson-datatype-problem/src/test/java/org/zalando/problem/EnforceCoverageTest.java
rename to jackson-datatype-problem/src/test/java/org/zalando/problem/jackson/EnforceCoverageTest.java
index cd3b579..69b8c58 100644
--- a/jackson-datatype-problem/src/test/java/org/zalando/problem/EnforceCoverageTest.java
+++ b/jackson-datatype-problem/src/test/java/org/zalando/problem/jackson/EnforceCoverageTest.java
@@ -1,16 +1,13 @@
-package org.zalando.problem;
+package org.zalando.problem.jackson;
 
-import com.google.gag.annotation.remark.Hack;
-import com.google.gag.annotation.remark.OhNoYouDidnt;
 import org.junit.jupiter.api.Test;
+import org.zalando.problem.AbstractThrowableProblem;
 
 import java.net.URI;
 
 import static org.junit.jupiter.api.Assertions.assertThrows;
 import static org.zalando.problem.Status.BAD_REQUEST;
 
-@Hack
-@OhNoYouDidnt
 final class EnforceCoverageTest {
 
     @Test
diff --git a/jackson-datatype-problem/src/test/java/org/zalando/problem/IOProblem.java b/jackson-datatype-problem/src/test/java/org/zalando/problem/jackson/IOProblem.java
similarity index 89%
rename from jackson-datatype-problem/src/test/java/org/zalando/problem/IOProblem.java
rename to jackson-datatype-problem/src/test/java/org/zalando/problem/jackson/IOProblem.java
index bafaff4..8c7052f 100644
--- a/jackson-datatype-problem/src/test/java/org/zalando/problem/IOProblem.java
+++ b/jackson-datatype-problem/src/test/java/org/zalando/problem/jackson/IOProblem.java
@@ -1,9 +1,13 @@
-package org.zalando.problem;
+package org.zalando.problem.jackson;
 
 import com.fasterxml.jackson.annotation.JsonCreator;
 import com.fasterxml.jackson.annotation.JsonProperty;
 import com.fasterxml.jackson.annotation.JsonTypeInfo;
 
+import org.zalando.problem.Exceptional;
+import org.zalando.problem.StatusType;
+import org.zalando.problem.ThrowableProblem;
+
 import java.io.IOException;
 import java.net.URI;
 
diff --git a/jackson-datatype-problem/src/test/java/org/zalando/problem/InsufficientFundsProblem.java b/jackson-datatype-problem/src/test/java/org/zalando/problem/jackson/InsufficientFundsProblem.java
similarity index 91%
rename from jackson-datatype-problem/src/test/java/org/zalando/problem/InsufficientFundsProblem.java
rename to jackson-datatype-problem/src/test/java/org/zalando/problem/jackson/InsufficientFundsProblem.java
index 3c9f9cb..8697d3d 100644
--- a/jackson-datatype-problem/src/test/java/org/zalando/problem/InsufficientFundsProblem.java
+++ b/jackson-datatype-problem/src/test/java/org/zalando/problem/jackson/InsufficientFundsProblem.java
@@ -1,9 +1,11 @@
-package org.zalando.problem;
+package org.zalando.problem.jackson;
 
 import com.fasterxml.jackson.annotation.JsonCreator;
 import com.fasterxml.jackson.annotation.JsonProperty;
 import com.fasterxml.jackson.annotation.JsonTypeName;
 
+import org.zalando.problem.AbstractThrowableProblem;
+
 import java.net.URI;
 
 import static org.zalando.problem.Status.BAD_REQUEST;
diff --git a/jackson-datatype-problem/src/test/java/org/zalando/problem/JacksonStackTraceProcessor.java b/jackson-datatype-problem/src/test/java/org/zalando/problem/jackson/JacksonStackTraceProcessor.java
similarity index 80%
rename from jackson-datatype-problem/src/test/java/org/zalando/problem/JacksonStackTraceProcessor.java
rename to jackson-datatype-problem/src/test/java/org/zalando/problem/jackson/JacksonStackTraceProcessor.java
index fe0546a..5392b2c 100644
--- a/jackson-datatype-problem/src/test/java/org/zalando/problem/JacksonStackTraceProcessor.java
+++ b/jackson-datatype-problem/src/test/java/org/zalando/problem/jackson/JacksonStackTraceProcessor.java
@@ -1,4 +1,4 @@
-package org.zalando.problem;
+package org.zalando.problem.jackson;
 
 import org.zalando.problem.spi.StackTraceProcessor;
 
@@ -16,7 +16,11 @@ public Collection<StackTraceElement> process(final Collection<StackTraceElement>
         final ArrayList<StackTraceElement> elements = new ArrayList<>(collection);
 
         return elements.stream()
-                .filter(startsWith("sun.reflect", "java.lang.reflect", "com.fasterxml.jackson").negate())
+                .filter(startsWith(
+                        "sun.reflect", 
+                        "java.lang.reflect", 
+                        "jdk.internal.reflect",
+                        "com.fasterxml.jackson").negate())
                 .findFirst()
                 .map(elements::indexOf)
                 .map(subList(elements))
diff --git a/jackson-datatype-problem/src/test/java/org/zalando/problem/OutOfStockException.java b/jackson-datatype-problem/src/test/java/org/zalando/problem/jackson/OutOfStockException.java
similarity index 86%
rename from jackson-datatype-problem/src/test/java/org/zalando/problem/OutOfStockException.java
rename to jackson-datatype-problem/src/test/java/org/zalando/problem/jackson/OutOfStockException.java
index e669a32..c493468 100644
--- a/jackson-datatype-problem/src/test/java/org/zalando/problem/OutOfStockException.java
+++ b/jackson-datatype-problem/src/test/java/org/zalando/problem/jackson/OutOfStockException.java
@@ -1,9 +1,13 @@
-package org.zalando.problem;
+package org.zalando.problem.jackson;
 
 import com.fasterxml.jackson.annotation.JsonCreator;
 import com.fasterxml.jackson.annotation.JsonProperty;
 import com.fasterxml.jackson.annotation.JsonTypeName;
 
+import org.zalando.problem.Exceptional;
+import org.zalando.problem.StatusType;
+import org.zalando.problem.ThrowableProblem;
+
 import java.net.URI;
 
 import static org.zalando.problem.Status.BAD_REQUEST;
diff --git a/jackson-datatype-problem/src/test/java/org/zalando/problem/ProblemMixInTest.java b/jackson-datatype-problem/src/test/java/org/zalando/problem/jackson/ProblemMixInTest.java
similarity index 96%
rename from jackson-datatype-problem/src/test/java/org/zalando/problem/ProblemMixInTest.java
rename to jackson-datatype-problem/src/test/java/org/zalando/problem/jackson/ProblemMixInTest.java
index 532080b..1bbd165 100644
--- a/jackson-datatype-problem/src/test/java/org/zalando/problem/ProblemMixInTest.java
+++ b/jackson-datatype-problem/src/test/java/org/zalando/problem/jackson/ProblemMixInTest.java
@@ -1,8 +1,14 @@
-package org.zalando.problem;
+package org.zalando.problem.jackson;
 
 import com.fasterxml.jackson.core.JsonProcessingException;
 import com.fasterxml.jackson.databind.ObjectMapper;
 import org.junit.jupiter.api.Test;
+import org.zalando.problem.DefaultProblem;
+import org.zalando.problem.Exceptional;
+import org.zalando.problem.Problem;
+import org.zalando.problem.Status;
+import org.zalando.problem.StatusType;
+import org.zalando.problem.ThrowableProblem;
 
 import java.io.IOException;
 import java.io.PrintWriter;
@@ -245,7 +251,7 @@ void shouldDeserializeWithProcessedStackTrace() throws IOException {
         final String stackTrace = getStackTrace(problem);
         final String[] stackTraceElements = stackTrace.split("\n");
 
-        assertThat(stackTraceElements[1], startsWith("\tat org.zalando.problem.ProblemMixInTest"));
+        assertThat(stackTraceElements[1], startsWith("\tat " + ProblemMixInTest.class.getName()));
     }
 
     private String getStackTrace(final Throwable throwable) {
diff --git a/jackson-datatype-problem/src/test/java/org/zalando/problem/ProblemModuleTest.java b/jackson-datatype-problem/src/test/java/org/zalando/problem/jackson/ProblemModuleTest.java
similarity index 85%
rename from jackson-datatype-problem/src/test/java/org/zalando/problem/ProblemModuleTest.java
rename to jackson-datatype-problem/src/test/java/org/zalando/problem/jackson/ProblemModuleTest.java
index 753027b..d6da2cc 100644
--- a/jackson-datatype-problem/src/test/java/org/zalando/problem/ProblemModuleTest.java
+++ b/jackson-datatype-problem/src/test/java/org/zalando/problem/jackson/ProblemModuleTest.java
@@ -1,6 +1,7 @@
-package org.zalando.problem;
+package org.zalando.problem.jackson;
 
 import org.junit.jupiter.api.Test;
+import org.zalando.problem.Status;
 
 import static org.junit.jupiter.api.Assertions.assertThrows;
 
diff --git a/jackson-datatype-problem/src/test/java/org/zalando/problem/jackson/UnknownStatusTest.java b/jackson-datatype-problem/src/test/java/org/zalando/problem/jackson/UnknownStatusTest.java
new file mode 100644
index 0000000..10cb501
--- /dev/null
+++ b/jackson-datatype-problem/src/test/java/org/zalando/problem/jackson/UnknownStatusTest.java
@@ -0,0 +1,17 @@
+package org.zalando.problem.jackson;
+
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+class UnknownStatusTest {
+
+    @Test
+    void shouldReturnCodeAndPhrase() {
+        final int code = 8080;
+        final UnknownStatus status = new UnknownStatus(code);
+        assertEquals(8080, status.getStatusCode());
+        assertEquals("Unknown", status.getReasonPhrase());
+    }
+
+}
\ No newline at end of file
diff --git a/jackson-datatype-problem/src/test/resources/META-INF/services/org.zalando.problem.spi.StackTraceProcessor b/jackson-datatype-problem/src/test/resources/META-INF/services/org.zalando.problem.spi.StackTraceProcessor
index 65d7271..853b16c 100644
--- a/jackson-datatype-problem/src/test/resources/META-INF/services/org.zalando.problem.spi.StackTraceProcessor
+++ b/jackson-datatype-problem/src/test/resources/META-INF/services/org.zalando.problem.spi.StackTraceProcessor
@@ -1 +1 @@
-org.zalando.problem.JacksonStackTraceProcessor
\ No newline at end of file
+org.zalando.problem.jackson.JacksonStackTraceProcessor
\ No newline at end of file
diff --git a/pom.xml b/pom.xml
index 71111c1..8dc81c7 100644
--- a/pom.xml
+++ b/pom.xml
@@ -61,15 +61,9 @@
                 <scope>provided</scope>
             </dependency>
             <dependency>
-                <groupId>com.google.code.findbugs</groupId>
-                <artifactId>jsr305</artifactId>
-                <version>3.0.2</version>
-                <scope>provided</scope>
-            </dependency>
-            <dependency>
-                <groupId>com.google.gag</groupId>
-                <artifactId>gag</artifactId>
-                <version>1.0.1</version>
+                <groupId>org.checkerframework</groupId>
+                <artifactId>checker-qual</artifactId>
+                <version>3.6.0</version>
                 <scope>provided</scope>
             </dependency>
             <dependency>
@@ -78,6 +72,16 @@
                 <version>1.18.12</version>
                 <scope>provided</scope>
             </dependency>
+            <!-- 
+                Only needed because lombok has a weird dependency to it.
+                https://medium.com/@Leejjon_net/youll-have-this-problems-when-you-add-lombok-to-a-modular-java-11-micro-service-832f55911bc5
+            -->
+            <dependency>
+                <groupId>org.mapstruct</groupId>
+                <artifactId>mapstruct-processor</artifactId>
+                <version>1.4.0.CR1</version>
+                <scope>provided</scope>
+            </dependency>
 
             <!-- Modules -->
             <dependency>
@@ -154,17 +158,17 @@
             <artifactId>apiguardian-api</artifactId>
         </dependency>
         <dependency>
-            <groupId>com.google.code.findbugs</groupId>
-            <artifactId>jsr305</artifactId>
-        </dependency>
-        <dependency>
-            <groupId>com.google.gag</groupId>
-            <artifactId>gag</artifactId>
+            <groupId>org.checkerframework</groupId>
+            <artifactId>checker-qual</artifactId>
         </dependency>
         <dependency>
             <groupId>org.projectlombok</groupId>
             <artifactId>lombok</artifactId>
         </dependency>
+        <dependency>
+            <groupId>org.mapstruct</groupId>
+            <artifactId>mapstruct-processor</artifactId>
+        </dependency>
 
         <!-- Testing -->
         <dependency>
@@ -258,10 +262,69 @@
                     <artifactId>maven-resources-plugin</artifactId>
                     <version>3.2.0</version>
                 </plugin>
+                <plugin>
+                    <groupId>org.moditect</groupId>
+                    <artifactId>moditect-maven-plugin</artifactId>
+                    <version>1.0.0.RC1</version>
+                    <!-- 
+                        This plugin provides most of the heavy lifting to support Java 9+,
+                        until we move away from Java 8 (in favor of 11 or a newer LTS).
+
+                        The plugin does:
+                          - generate module descriptor sources (module-info)
+                          - resolve module dependencies via Maven
+                          - generate Java 9 compatible module-info.class (without a 2- or 3-pass compile step)
+                          - put module-info.class within META-INF/versions/9 (Multi-Release JAR),
+                            to maximize backwards compatibility with Java 8
+
+                        The plugin doesn't:
+                          - detect split package situations
+                          - warn about typos in package names
+
+                        Once we move to Java 9 (or higher) we should:
+                          - get rid of this plugin
+                          - write module-info files by hand
+                          - carefully craft and vet module dependencies
+                    -->
+                    <executions>
+                        <execution>
+                            <id>add-module-infos</id>
+                            <phase>package</phase>
+                            <goals>
+                                <goal>add-module-info</goal>
+                            </goals>
+                            <configuration>
+                                <jvmVersion>9</jvmVersion>
+                                <module>
+                                    <moduleInfo>
+                                        <name>${module.name}</name>
+                                        <exports>
+                                            ${module.exports}
+                                        </exports>
+                                        <requires>
+                                            static org.apiguardian.api;
+                                        </requires>
+                                    </moduleInfo>
+                                </module>
+                                <jdepsExtraArgs>
+                                    <!-- 
+                                        JDeps prior to Java 12 doesn't support mixed multi-release and
+                                        non-multi-release jars. Hence why we require JDK 12 to
+                                        build the project.
+
+                                        See https://bugs.openjdk.java.net/browse/JDK-8210502
+                                    -->
+                                    <arg>--multi-release=9</arg>
+                                </jdepsExtraArgs> 
+                            </configuration>
+                        </execution>
+                    </executions>
+                </plugin>
                 <plugin>
                     <groupId>org.apache.maven.plugins</groupId>
                     <artifactId>maven-compiler-plugin</artifactId>
                     <version>3.8.1</version>
+                    <!-- TODO compile once with Java 9 to check modules -->
                     <configuration>
                         <compilerArgs>
                             <compilerArg>-Xlint:unchecked,deprecation</compilerArg>
@@ -338,6 +401,11 @@
                         <generateBackupPoms>false</generateBackupPoms>
                     </configuration>
                 </plugin>
+                <plugin>
+                    <groupId>org.apache.maven.plugins</groupId>
+                    <artifactId>maven-jar-plugin</artifactId>
+                    <version>3.2.0</version>
+                </plugin>
             </plugins>
         </pluginManagement>
         <plugins>
@@ -353,6 +421,14 @@
                 <groupId>org.basepom.maven</groupId>
                 <artifactId>duplicate-finder-maven-plugin</artifactId>
             </plugin>
+            <plugin>
+                <groupId>org.moditect</groupId>
+                <artifactId>moditect-maven-plugin</artifactId>
+            </plugin>
+            <plugin>
+                <groupId>org.apache.maven.plugins</groupId>
+                <artifactId>maven-compiler-plugin</artifactId>
+            </plugin>
             <plugin>
                 <groupId>org.jacoco</groupId>
                 <artifactId>jacoco-maven-plugin</artifactId>
diff --git a/problem-gson/pom.xml b/problem-gson/pom.xml
index c26e663..426d7bf 100644
--- a/problem-gson/pom.xml
+++ b/problem-gson/pom.xml
@@ -11,6 +11,10 @@
     <artifactId>problem-gson</artifactId>
     <properties>
         <gson.version>2.8.6</gson.version>
+        <module.name>org.zalando.problem.gson</module.name>
+        <module.exports>
+            org.zalando.problem.gson;
+        </module.exports>
     </properties>
     <dependencies>
         <dependency>
diff --git a/problem-gson/src/main/java/org/zalando/problem/CustomProblemAdapter.java b/problem-gson/src/main/java/org/zalando/problem/gson/CustomProblemAdapter.java
similarity index 96%
rename from problem-gson/src/main/java/org/zalando/problem/CustomProblemAdapter.java
rename to problem-gson/src/main/java/org/zalando/problem/gson/CustomProblemAdapter.java
index 81eb74a..8b4b7c7 100644
--- a/problem-gson/src/main/java/org/zalando/problem/CustomProblemAdapter.java
+++ b/problem-gson/src/main/java/org/zalando/problem/gson/CustomProblemAdapter.java
@@ -1,4 +1,4 @@
-package org.zalando.problem;
+package org.zalando.problem.gson;
 
 import com.google.gson.Gson;
 import com.google.gson.JsonElement;
@@ -9,6 +9,8 @@
 import com.google.gson.stream.JsonWriter;
 import lombok.AllArgsConstructor;
 
+import org.zalando.problem.AbstractThrowableProblem;
+
 import java.io.IOException;
 import java.net.URI;
 import java.util.Optional;
diff --git a/problem-gson/src/main/java/org/zalando/problem/DefaultProblemAdapter.java b/problem-gson/src/main/java/org/zalando/problem/gson/DefaultProblemAdapter.java
similarity index 93%
rename from problem-gson/src/main/java/org/zalando/problem/DefaultProblemAdapter.java
rename to problem-gson/src/main/java/org/zalando/problem/gson/DefaultProblemAdapter.java
index ac49b6a..59cc510 100644
--- a/problem-gson/src/main/java/org/zalando/problem/DefaultProblemAdapter.java
+++ b/problem-gson/src/main/java/org/zalando/problem/gson/DefaultProblemAdapter.java
@@ -1,4 +1,4 @@
-package org.zalando.problem;
+package org.zalando.problem.gson;
 
 import com.google.gson.Gson;
 import com.google.gson.JsonElement;
@@ -9,6 +9,11 @@
 import com.google.gson.stream.JsonWriter;
 import lombok.AllArgsConstructor;
 
+import org.zalando.problem.Problem;
+import org.zalando.problem.ProblemBuilder;
+import org.zalando.problem.StatusType;
+import org.zalando.problem.ThrowableProblem;
+
 import java.io.IOException;
 import java.util.Map;
 
@@ -65,7 +70,7 @@ public void write(final JsonWriter out, final ThrowableProblem problem) throws I
 
     @Override
     public ThrowableProblem read(final JsonReader in) throws IOException {
-        final ProblemBuilder builder = new ProblemBuilder();
+        final ProblemBuilder builder = Problem.builder();
 
         in.beginObject();
         while (in.hasNext()) {
diff --git a/problem-gson/src/main/java/org/zalando/problem/ProblemAdapterFactory.java b/problem-gson/src/main/java/org/zalando/problem/gson/ProblemAdapterFactory.java
similarity index 90%
rename from problem-gson/src/main/java/org/zalando/problem/ProblemAdapterFactory.java
rename to problem-gson/src/main/java/org/zalando/problem/gson/ProblemAdapterFactory.java
index 3762bae..ea371e9 100644
--- a/problem-gson/src/main/java/org/zalando/problem/ProblemAdapterFactory.java
+++ b/problem-gson/src/main/java/org/zalando/problem/gson/ProblemAdapterFactory.java
@@ -1,4 +1,4 @@
-package org.zalando.problem;
+package org.zalando.problem.gson;
 
 import com.google.gson.Gson;
 import com.google.gson.JsonElement;
@@ -12,8 +12,14 @@
 import lombok.AllArgsConstructor;
 import org.apiguardian.api.API;
 
-import javax.annotation.CheckReturnValue;
-import javax.annotation.Nullable;
+import org.checkerframework.checker.nullness.qual.Nullable;
+import org.zalando.problem.AbstractThrowableProblem;
+import org.zalando.problem.DefaultProblem;
+import org.zalando.problem.Problem;
+import org.zalando.problem.Status;
+import org.zalando.problem.StatusType;
+import org.zalando.problem.ThrowableProblem;
+
 import java.io.IOException;
 import java.net.URI;
 import java.util.Collections;
@@ -24,7 +30,7 @@
 import static java.util.Objects.requireNonNull;
 import static lombok.AccessLevel.PRIVATE;
 import static org.apiguardian.api.API.Status.EXPERIMENTAL;
-import static org.zalando.problem.URITypeAdapter.TYPE;
+import static org.zalando.problem.gson.URITypeAdapter.TYPE;
 
 /**
  * Problem {@link TypeAdapterFactory}.
@@ -41,7 +47,7 @@ public ProblemAdapterFactory() {
     }
 
     @SafeVarargs
-    public <E extends Enum & StatusType> ProblemAdapterFactory(
+    public <E extends Enum<?> & StatusType> ProblemAdapterFactory(
             final Class<? extends E>... statusTypes) {
         this(false,
                 new StatusTypeAdapter(buildIndex(statusTypes)),
@@ -58,7 +64,7 @@ private ProblemAdapterFactory(
     }
 
     @SafeVarargs
-    private static <E extends Enum & StatusType> Map<Integer, StatusType> buildIndex(
+    private static <E extends Enum<?> & StatusType> Map<Integer, StatusType> buildIndex(
             final Class<? extends E>... types) {
 
         final Map<Integer, StatusType> index = new HashMap<>();
@@ -84,14 +90,14 @@ public ProblemAdapterFactory withStackTraces(final boolean stackTraces) {
         return new ProblemAdapterFactory(stackTraces, statusAdapter, subtypes);
     }
 
-    @CheckReturnValue
+    // TODO @CheckReturnValue
     public ProblemAdapterFactory registerSubtype(
             final URI uri, final Class<? extends Problem> type) {
 
         return registerSubType(uri, TypeToken.get(type));
     }
 
-    @CheckReturnValue
+    // TODO @CheckReturnValue
     public ProblemAdapterFactory registerSubType(
             final URI uri, final TypeToken<? extends Problem> type) {
 
diff --git a/problem-gson/src/main/java/org/zalando/problem/StatusTypeAdapter.java b/problem-gson/src/main/java/org/zalando/problem/gson/StatusTypeAdapter.java
similarity index 89%
rename from problem-gson/src/main/java/org/zalando/problem/StatusTypeAdapter.java
rename to problem-gson/src/main/java/org/zalando/problem/gson/StatusTypeAdapter.java
index 533509e..8c2f651 100644
--- a/problem-gson/src/main/java/org/zalando/problem/StatusTypeAdapter.java
+++ b/problem-gson/src/main/java/org/zalando/problem/gson/StatusTypeAdapter.java
@@ -1,4 +1,4 @@
-package org.zalando.problem;
+package org.zalando.problem.gson;
 
 import com.google.gson.TypeAdapter;
 import com.google.gson.stream.JsonReader;
@@ -6,7 +6,9 @@
 import com.google.gson.stream.JsonWriter;
 import lombok.AllArgsConstructor;
 
-import javax.annotation.Nullable;
+import org.checkerframework.checker.nullness.qual.Nullable;
+import org.zalando.problem.StatusType;
+
 import java.io.IOException;
 import java.util.Map;
 import java.util.Objects;
diff --git a/problem-gson/src/main/java/org/zalando/problem/URITypeAdapter.java b/problem-gson/src/main/java/org/zalando/problem/gson/URITypeAdapter.java
similarity index 87%
rename from problem-gson/src/main/java/org/zalando/problem/URITypeAdapter.java
rename to problem-gson/src/main/java/org/zalando/problem/gson/URITypeAdapter.java
index 3d0ef62..b87a75d 100644
--- a/problem-gson/src/main/java/org/zalando/problem/URITypeAdapter.java
+++ b/problem-gson/src/main/java/org/zalando/problem/gson/URITypeAdapter.java
@@ -1,11 +1,13 @@
-package org.zalando.problem;
+package org.zalando.problem.gson;
 
 import com.google.gson.TypeAdapter;
 import com.google.gson.stream.JsonReader;
 import com.google.gson.stream.JsonToken;
 import com.google.gson.stream.JsonWriter;
 
-import javax.annotation.Nullable;
+import org.checkerframework.checker.nullness.qual.Nullable;
+import org.zalando.problem.Problem;
+
 import java.io.IOException;
 import java.net.URI;
 
diff --git a/problem-gson/src/main/java/org/zalando/problem/gson/UnknownStatus.java b/problem-gson/src/main/java/org/zalando/problem/gson/UnknownStatus.java
new file mode 100644
index 0000000..427e327
--- /dev/null
+++ b/problem-gson/src/main/java/org/zalando/problem/gson/UnknownStatus.java
@@ -0,0 +1,23 @@
+package org.zalando.problem.gson;
+
+import org.zalando.problem.StatusType;
+
+final class UnknownStatus implements StatusType {
+
+    private final int statusCode;
+
+    UnknownStatus(final int statusCode) {
+        this.statusCode = statusCode;
+    }
+
+    @Override
+    public int getStatusCode() {
+        return statusCode;
+    }
+
+    @Override
+    public String getReasonPhrase() {
+        return "Unknown";
+    }
+
+}
diff --git a/jackson-datatype-problem/src/test/java/org/zalando/problem/CustomStatus.java b/problem-gson/src/test/java/org/zalando/problem/gson/CustomStatus.java
similarity index 87%
rename from jackson-datatype-problem/src/test/java/org/zalando/problem/CustomStatus.java
rename to problem-gson/src/test/java/org/zalando/problem/gson/CustomStatus.java
index cb5f669..2c7cd99 100644
--- a/jackson-datatype-problem/src/test/java/org/zalando/problem/CustomStatus.java
+++ b/problem-gson/src/test/java/org/zalando/problem/gson/CustomStatus.java
@@ -1,4 +1,6 @@
-package org.zalando.problem;
+package org.zalando.problem.gson;
+
+import org.zalando.problem.StatusType;
 
 enum CustomStatus implements StatusType {
 
diff --git a/problem-gson/src/test/java/org/zalando/problem/GsonProblemTest.java b/problem-gson/src/test/java/org/zalando/problem/gson/GsonProblemTest.java
similarity index 97%
rename from problem-gson/src/test/java/org/zalando/problem/GsonProblemTest.java
rename to problem-gson/src/test/java/org/zalando/problem/gson/GsonProblemTest.java
index 922c35f..a902f8b 100644
--- a/problem-gson/src/test/java/org/zalando/problem/GsonProblemTest.java
+++ b/problem-gson/src/test/java/org/zalando/problem/gson/GsonProblemTest.java
@@ -1,9 +1,16 @@
-package org.zalando.problem;
+package org.zalando.problem.gson;
 
 import com.google.gson.Gson;
 import com.google.gson.GsonBuilder;
 import com.google.gson.stream.JsonReader;
 import org.junit.jupiter.api.Test;
+import org.zalando.problem.DefaultProblem;
+import org.zalando.problem.Exceptional;
+import org.zalando.problem.Problem;
+import org.zalando.problem.ProblemBuilder;
+import org.zalando.problem.Status;
+import org.zalando.problem.StatusType;
+import org.zalando.problem.ThrowableProblem;
 
 import java.io.File;
 import java.io.FileNotFoundException;
@@ -13,6 +20,7 @@
 import java.io.StringWriter;
 import java.net.URI;
 import java.net.URL;
+import java.util.Collections;
 import java.util.List;
 import java.util.Objects;
 
@@ -82,8 +90,8 @@ void shouldSerializeCustomProperties() {
     void shouldSerializeCustomProblem() {
         final int balance = 10;
         final int debit = 100;
-        final InsufficientFundsProblem problem = new InsufficientFundsProblem(balance, debit);
-        problem.set("foo", "bar");
+        final InsufficientFundsProblem problem = new InsufficientFundsProblem(
+                balance, debit, Collections.singletonMap("foo", "bar"));
 
         final String json = gson.toJson(problem);
         assertThat("json", Objects.nonNull(json));
diff --git a/problem-gson/src/test/java/org/zalando/problem/IOProblem.java b/problem-gson/src/test/java/org/zalando/problem/gson/IOProblem.java
similarity index 87%
rename from problem-gson/src/test/java/org/zalando/problem/IOProblem.java
rename to problem-gson/src/test/java/org/zalando/problem/gson/IOProblem.java
index 0f0f179..aad11c1 100644
--- a/problem-gson/src/test/java/org/zalando/problem/IOProblem.java
+++ b/problem-gson/src/test/java/org/zalando/problem/gson/IOProblem.java
@@ -1,8 +1,12 @@
-package org.zalando.problem;
+package org.zalando.problem.gson;
 
 import java.io.IOException;
 import java.net.URI;
 
+import org.zalando.problem.Exceptional;
+import org.zalando.problem.StatusType;
+import org.zalando.problem.ThrowableProblem;
+
 final class IOProblem extends IOException implements Exceptional {
 
     private final URI type;
diff --git a/problem-gson/src/test/java/org/zalando/problem/InsufficientFundsProblem.java b/problem-gson/src/test/java/org/zalando/problem/gson/InsufficientFundsProblem.java
similarity index 59%
rename from problem-gson/src/test/java/org/zalando/problem/InsufficientFundsProblem.java
rename to problem-gson/src/test/java/org/zalando/problem/gson/InsufficientFundsProblem.java
index a6bb74b..3cf7d3b 100644
--- a/problem-gson/src/test/java/org/zalando/problem/InsufficientFundsProblem.java
+++ b/problem-gson/src/test/java/org/zalando/problem/gson/InsufficientFundsProblem.java
@@ -1,6 +1,10 @@
-package org.zalando.problem;
+package org.zalando.problem.gson;
 
 import java.net.URI;
+import java.util.Collections;
+import java.util.Map;
+
+import org.zalando.problem.AbstractThrowableProblem;
 
 import static org.zalando.problem.Status.BAD_REQUEST;
 
@@ -15,7 +19,14 @@ final class InsufficientFundsProblem extends AbstractThrowableProblem {
     InsufficientFundsProblem(
             final int balance,
             final int debit) {
-        super(TYPE, "Insufficient Funds", BAD_REQUEST);
+        this(balance, debit, Collections.emptyMap());
+    }
+
+    InsufficientFundsProblem(
+            final int balance,
+            final int debit,
+            final Map<String, Object> parameters) {
+        super(TYPE, "Insufficient Funds", BAD_REQUEST, null, null, null, parameters);
         this.balance = balance;
         this.debit = debit;
     }
diff --git a/problem-gson/src/test/java/org/zalando/problem/MyProblem.java b/problem-gson/src/test/java/org/zalando/problem/gson/MyProblem.java
similarity index 92%
rename from problem-gson/src/test/java/org/zalando/problem/MyProblem.java
rename to problem-gson/src/test/java/org/zalando/problem/gson/MyProblem.java
index 4715a31..3f9bac9 100644
--- a/problem-gson/src/test/java/org/zalando/problem/MyProblem.java
+++ b/problem-gson/src/test/java/org/zalando/problem/gson/MyProblem.java
@@ -1,8 +1,10 @@
-package org.zalando.problem;
+package org.zalando.problem.gson;
 
 import com.google.gson.annotations.SerializedName;
+import org.zalando.problem.Problem;
+import org.zalando.problem.StatusType;
 
-import javax.annotation.Nullable;
+import org.checkerframework.checker.nullness.qual.Nullable;
 import java.net.URI;
 
 class MyProblem implements Problem {
diff --git a/problem-gson/src/test/java/org/zalando/problem/OutOfStockException.java b/problem-gson/src/test/java/org/zalando/problem/gson/OutOfStockException.java
similarity index 85%
rename from problem-gson/src/test/java/org/zalando/problem/OutOfStockException.java
rename to problem-gson/src/test/java/org/zalando/problem/gson/OutOfStockException.java
index d775cd4..951499f 100644
--- a/problem-gson/src/test/java/org/zalando/problem/OutOfStockException.java
+++ b/problem-gson/src/test/java/org/zalando/problem/gson/OutOfStockException.java
@@ -1,7 +1,10 @@
-package org.zalando.problem;
+package org.zalando.problem.gson;
 
 import java.net.URI;
 
+import org.zalando.problem.StatusType;
+import org.zalando.problem.ThrowableProblem;
+
 import static org.zalando.problem.Status.BAD_REQUEST;
 
 class OutOfStockException extends ThrowableProblem {
diff --git a/problem-gson/src/test/java/org/zalando/problem/ProblemAdapterFactoryTest.java b/problem-gson/src/test/java/org/zalando/problem/gson/ProblemAdapterFactoryTest.java
similarity index 95%
rename from problem-gson/src/test/java/org/zalando/problem/ProblemAdapterFactoryTest.java
rename to problem-gson/src/test/java/org/zalando/problem/gson/ProblemAdapterFactoryTest.java
index 0755604..7146e88 100644
--- a/problem-gson/src/test/java/org/zalando/problem/ProblemAdapterFactoryTest.java
+++ b/problem-gson/src/test/java/org/zalando/problem/gson/ProblemAdapterFactoryTest.java
@@ -1,7 +1,8 @@
-package org.zalando.problem;
+package org.zalando.problem.gson;
 
 import org.junit.jupiter.api.Test;
 import org.junit.jupiter.api.function.ThrowingSupplier;
+import org.zalando.problem.Status;
 
 import static org.junit.jupiter.api.Assertions.assertDoesNotThrow;
 import static org.junit.jupiter.api.Assertions.assertThrows;
diff --git a/problem-gson/src/test/java/org/zalando/problem/StatusTypeAdapterTest.java b/problem-gson/src/test/java/org/zalando/problem/gson/StatusTypeAdapterTest.java
similarity index 91%
rename from problem-gson/src/test/java/org/zalando/problem/StatusTypeAdapterTest.java
rename to problem-gson/src/test/java/org/zalando/problem/gson/StatusTypeAdapterTest.java
index 0851d8e..cb39b7f 100644
--- a/problem-gson/src/test/java/org/zalando/problem/StatusTypeAdapterTest.java
+++ b/problem-gson/src/test/java/org/zalando/problem/gson/StatusTypeAdapterTest.java
@@ -1,8 +1,9 @@
-package org.zalando.problem;
+package org.zalando.problem.gson;
 
 import com.google.gson.Gson;
 import com.google.gson.GsonBuilder;
 import org.junit.jupiter.api.Test;
+import org.zalando.problem.StatusType;
 
 import java.util.Collections;
 
diff --git a/problem-gson/src/test/java/org/zalando/problem/URITypeAdapterTest.java b/problem-gson/src/test/java/org/zalando/problem/gson/URITypeAdapterTest.java
similarity index 95%
rename from problem-gson/src/test/java/org/zalando/problem/URITypeAdapterTest.java
rename to problem-gson/src/test/java/org/zalando/problem/gson/URITypeAdapterTest.java
index 5ecd172..eabb92a 100644
--- a/problem-gson/src/test/java/org/zalando/problem/URITypeAdapterTest.java
+++ b/problem-gson/src/test/java/org/zalando/problem/gson/URITypeAdapterTest.java
@@ -1,10 +1,11 @@
-package org.zalando.problem;
+package org.zalando.problem.gson;
 
 import com.google.gson.Gson;
 import com.google.gson.GsonBuilder;
 import com.google.gson.JsonIOException;
 import com.google.gson.JsonPrimitive;
 import org.junit.jupiter.api.Test;
+import org.zalando.problem.Problem;
 
 import java.net.URI;
 
diff --git a/problem/src/test/java/org/zalando/problem/UnknownStatusTest.java b/problem-gson/src/test/java/org/zalando/problem/gson/UnknownStatusTest.java
similarity index 91%
rename from problem/src/test/java/org/zalando/problem/UnknownStatusTest.java
rename to problem-gson/src/test/java/org/zalando/problem/gson/UnknownStatusTest.java
index f816fdb..3327227 100644
--- a/problem/src/test/java/org/zalando/problem/UnknownStatusTest.java
+++ b/problem-gson/src/test/java/org/zalando/problem/gson/UnknownStatusTest.java
@@ -1,4 +1,4 @@
-package org.zalando.problem;
+package org.zalando.problem.gson;
 
 import org.junit.jupiter.api.Test;
 
diff --git a/problem/pom.xml b/problem/pom.xml
index 8427b5c..f7c8ca7 100644
--- a/problem/pom.xml
+++ b/problem/pom.xml
@@ -10,4 +10,11 @@
     </parent>
     <artifactId>problem</artifactId>
     <description>An implementation of the application/problem+json draft.</description>
+    <properties>
+        <module.name>org.zalando.problem</module.name>
+        <module.exports>
+            org.zalando.problem;
+            org.zalando.problem.spi;
+        </module.exports>
+    </properties>
 </project>
diff --git a/problem/src/main/java/org/zalando/problem/AbstractThrowableProblem.java b/problem/src/main/java/org/zalando/problem/AbstractThrowableProblem.java
index 3ad294e..38aae8b 100644
--- a/problem/src/main/java/org/zalando/problem/AbstractThrowableProblem.java
+++ b/problem/src/main/java/org/zalando/problem/AbstractThrowableProblem.java
@@ -1,11 +1,8 @@
 package org.zalando.problem;
 
-import com.google.gag.annotation.remark.Hack;
-import com.google.gag.annotation.remark.OhNoYouDidnt;
 import org.apiguardian.api.API;
 
-import javax.annotation.Nullable;
-import javax.annotation.concurrent.Immutable;
+import org.checkerframework.checker.nullness.qual.Nullable;
 import java.net.URI;
 import java.util.Collections;
 import java.util.LinkedHashMap;
@@ -15,8 +12,6 @@
 import static org.apiguardian.api.API.Status.INTERNAL;
 import static org.apiguardian.api.API.Status.STABLE;
 
-@API(status = STABLE)
-@Immutable // TODO kind of a lie until we remove set(String, Object)
 public abstract class AbstractThrowableProblem extends ThrowableProblem {
 
     private final URI type;
@@ -30,29 +25,34 @@ protected AbstractThrowableProblem() {
         this(null);
     }
 
-    protected AbstractThrowableProblem(@Nullable final URI type) {
+    protected AbstractThrowableProblem(
+            @Nullable final URI type) {
         this(type, null);
     }
 
-    protected AbstractThrowableProblem(@Nullable final URI type,
+    protected AbstractThrowableProblem(
+            @Nullable final URI type,
             @Nullable final String title) {
         this(type, title, null);
     }
 
-    protected AbstractThrowableProblem(@Nullable final URI type,
+    protected AbstractThrowableProblem(
+            @Nullable final URI type,
             @Nullable final String title,
             @Nullable final StatusType status) {
         this(type, title, status, null);
     }
 
-    protected AbstractThrowableProblem(@Nullable final URI type,
+    protected AbstractThrowableProblem(
+            @Nullable final URI type,
             @Nullable final String title,
             @Nullable final StatusType status,
             @Nullable final String detail) {
         this(type, title, status, detail, null);
     }
 
-    protected AbstractThrowableProblem(@Nullable final URI type,
+    protected AbstractThrowableProblem(
+            @Nullable final URI type,
             @Nullable final String title,
             @Nullable final StatusType status,
             @Nullable final String detail,
@@ -60,7 +60,8 @@ protected AbstractThrowableProblem(@Nullable final URI type,
         this(type, title, status, detail, instance, null);
     }
 
-    protected AbstractThrowableProblem(@Nullable final URI type,
+    protected AbstractThrowableProblem(
+            @Nullable final URI type,
             @Nullable final String title,
             @Nullable final StatusType status,
             @Nullable final String detail,
@@ -69,7 +70,8 @@ protected AbstractThrowableProblem(@Nullable final URI type,
         this(type, title, status, detail, instance, cause, null);
     }
 
-    protected AbstractThrowableProblem(@Nullable final URI type,
+    protected AbstractThrowableProblem(
+            @Nullable final URI type,
             @Nullable final String title,
             @Nullable final StatusType status,
             @Nullable final String detail,
@@ -124,8 +126,6 @@ public Map<String, Object> getParameters() {
      * @see <a href="https://github.com/FasterXML/jackson-databind/issues/562">Jackson Issue 562</a>
      */
     @API(status = INTERNAL)
-    @Hack
-    @OhNoYouDidnt
     void set(final String key, final Object value) {
         parameters.put(key, value);
     }
diff --git a/problem/src/main/java/org/zalando/problem/DefaultProblem.java b/problem/src/main/java/org/zalando/problem/DefaultProblem.java
index c504878..6204998 100644
--- a/problem/src/main/java/org/zalando/problem/DefaultProblem.java
+++ b/problem/src/main/java/org/zalando/problem/DefaultProblem.java
@@ -1,8 +1,7 @@
 package org.zalando.problem;
 
 import org.apiguardian.api.API;
-
-import javax.annotation.Nullable;
+import org.checkerframework.checker.nullness.qual.Nullable;
 import java.net.URI;
 import java.util.Map;
 
diff --git a/problem/src/main/java/org/zalando/problem/Problem.java b/problem/src/main/java/org/zalando/problem/Problem.java
index 99e7e8d..fee821c 100644
--- a/problem/src/main/java/org/zalando/problem/Problem.java
+++ b/problem/src/main/java/org/zalando/problem/Problem.java
@@ -2,8 +2,7 @@
 
 import org.apiguardian.api.API;
 
-import javax.annotation.Nullable;
-import javax.annotation.concurrent.Immutable;
+import org.checkerframework.checker.nullness.qual.Nullable;
 import java.net.URI;
 import java.util.Collections;
 import java.util.Map;
@@ -15,9 +14,10 @@
 
 /**
  * @see <a href="https://tools.ietf.org/html/rfc7807">RFC 7807: Problem Details for HTTP APIs</a>
+ * 
+ * {@link Problem} instances are required to be immutable.
  */
 @API(status = STABLE)
-@Immutable
 public interface Problem {
 
     URI DEFAULT_TYPE = URI.create("about:blank");
diff --git a/problem/src/main/java/org/zalando/problem/ProblemBuilder.java b/problem/src/main/java/org/zalando/problem/ProblemBuilder.java
index c2721c6..576a75e 100644
--- a/problem/src/main/java/org/zalando/problem/ProblemBuilder.java
+++ b/problem/src/main/java/org/zalando/problem/ProblemBuilder.java
@@ -2,7 +2,7 @@
 
 import org.apiguardian.api.API;
 
-import javax.annotation.Nullable;
+import org.checkerframework.checker.nullness.qual.Nullable;
 import java.net.URI;
 import java.util.Arrays;
 import java.util.HashSet;
diff --git a/problem/src/main/java/org/zalando/problem/Status.java b/problem/src/main/java/org/zalando/problem/Status.java
index 3efbf02..a8b5377 100644
--- a/problem/src/main/java/org/zalando/problem/Status.java
+++ b/problem/src/main/java/org/zalando/problem/Status.java
@@ -1,9 +1,8 @@
 package org.zalando.problem;
 
 import org.apiguardian.api.API;
-
-import javax.annotation.Nonnull;
-import javax.annotation.Nullable;
+import org.checkerframework.checker.nullness.qual.NonNull;
+import org.checkerframework.checker.nullness.qual.Nullable;
 import java.util.Arrays;
 import java.util.Collections;
 import java.util.Map;
@@ -14,20 +13,22 @@
 import static org.apiguardian.api.API.Status.MAINTAINED;
 
 /**
- * Commonly used status codes defined by HTTP, see
- * <a href="http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10">HTTP/1.1 documentation</a>
- * for the complete list. Additional status codes can be added by applications
- * by creating an implementation of StatusType.
+ * Commonly used status codes defined by HTTP, see <a href=
+ * "http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10">HTTP/1.1
+ * documentation</a> for the complete list. Additional status codes can be added
+ * by applications by creating an implementation of StatusType.
  */
 @API(status = MAINTAINED)
 public enum Status implements StatusType {
 
     /**
-     * @see <a href="http://tools.ietf.org/html/rfc7231#section-6.2.1">HTTP/1.1: Semantics and Content, section 6.2.1</a>
+     * @see <a href="http://tools.ietf.org/html/rfc7231#section-6.2.1">HTTP/1.1:
+     *      Semantics and Content, section 6.2.1</a>
      */
     CONTINUE(100, "Continue"),
     /**
-     * @see <a href="http://tools.ietf.org/html/rfc7231#section-6.2.2">HTTP/1.1: Semantics and Content, section 6.2.2</a>
+     * @see <a href="http://tools.ietf.org/html/rfc7231#section-6.2.2">HTTP/1.1:
+     *      Semantics and Content, section 6.2.2</a>
      */
     SWITCHING_PROTOCOLS(101, "Switching Protocols"),
     /**
@@ -35,36 +36,50 @@ public enum Status implements StatusType {
      */
     PROCESSING(102, "Processing"),
     /**
-     * @see <a href="http://code.google.com/p/gears/wiki/ResumableHttpRequestsProposal">A proposal for supporting
-     * resumable POST/PUT HTTP requests in HTTP/1.0</a>
+     * @see <a href=
+     *      "http://code.google.com/p/gears/wiki/ResumableHttpRequestsProposal">A
+     *      proposal for supporting resumable POST/PUT HTTP requests in HTTP/1.0</a>
      */
     CHECKPOINT(103, "Checkpoint"),
     /**
-     * 200 OK, see <a href="http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.2.1">HTTP/1.1 documentation</a>.
+     * 200 OK, see <a href=
+     * "http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.2.1">HTTP/1.1
+     * documentation</a>.
      */
     OK(200, "OK"),
     /**
-     * 201 Created, see <a href="http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.2.2">HTTP/1.1 documentation</a>.
+     * 201 Created, see <a href=
+     * "http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.2.2">HTTP/1.1
+     * documentation</a>.
      */
     CREATED(201, "Created"),
     /**
-     * 202 Accepted, see <a href="http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.2.3">HTTP/1.1 documentation</a>.
+     * 202 Accepted, see <a href=
+     * "http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.2.3">HTTP/1.1
+     * documentation</a>.
      */
     ACCEPTED(202, "Accepted"),
     /**
-     * @see <a href="http://tools.ietf.org/html/rfc7231#section-6.3.4">HTTP/1.1: Semantics and Content, section 6.3.4</a>
+     * @see <a href="http://tools.ietf.org/html/rfc7231#section-6.3.4">HTTP/1.1:
+     *      Semantics and Content, section 6.3.4</a>
      */
     NON_AUTHORITATIVE_INFORMATION(203, "Non-Authoritative Information"),
     /**
-     * 204 No Content, see <a href="http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.2.5">HTTP/1.1 documentation</a>.
+     * 204 No Content, see <a href=
+     * "http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.2.5">HTTP/1.1
+     * documentation</a>.
      */
     NO_CONTENT(204, "No Content"),
     /**
-     * 205 Reset Content, see <a href="http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.2.6">HTTP/1.1 documentation</a>.
+     * 205 Reset Content, see <a href=
+     * "http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.2.6">HTTP/1.1
+     * documentation</a>.
      */
     RESET_CONTENT(205, "Reset Content"),
     /**
-     * 206 Reset Content, see <a href="http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.2.7">HTTP/1.1 documentation</a>.
+     * 206 Reset Content, see <a href=
+     * "http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.2.7">HTTP/1.1
+     * documentation</a>.
      */
     PARTIAL_CONTENT(206, "Partial Content"),
     /**
@@ -72,39 +87,54 @@ public enum Status implements StatusType {
      */
     MULTI_STATUS(207, "Multi-Status"),
     /**
-     * @see <a href="http://tools.ietf.org/html/rfc5842#section-7.1">WebDAV Binding Extensions</a>
+     * @see <a href="http://tools.ietf.org/html/rfc5842#section-7.1">WebDAV Binding
+     *      Extensions</a>
      */
     ALREADY_REPORTED(208, "Already Reported"),
     /**
-     * @see <a href="http://tools.ietf.org/html/rfc3229#section-10.4.1">Delta encoding in HTTP</a>
+     * @see <a href="http://tools.ietf.org/html/rfc3229#section-10.4.1">Delta
+     *      encoding in HTTP</a>
      */
     IM_USED(226, "IM Used"),
     /**
-     * @see <a href="http://tools.ietf.org/html/rfc7231#section-6.4.1">HTTP/1.1: Semantics and Content, section 6.4.1</a>
+     * @see <a href="http://tools.ietf.org/html/rfc7231#section-6.4.1">HTTP/1.1:
+     *      Semantics and Content, section 6.4.1</a>
      */
     MULTIPLE_CHOICES(300, "Multiple Choices"),
     /**
-     * 301 Moved Permanently, see <a href="http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.3.2">HTTP/1.1 documentation</a>.
+     * 301 Moved Permanently, see <a href=
+     * "http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.3.2">HTTP/1.1
+     * documentation</a>.
      */
     MOVED_PERMANENTLY(301, "Moved Permanently"),
     /**
-     * 302 Found, see <a href="http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.3.3">HTTP/1.1 documentation</a>.
+     * 302 Found, see <a href=
+     * "http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.3.3">HTTP/1.1
+     * documentation</a>.
      */
     FOUND(302, "Found"),
     /**
-     * 303 See Other, see <a href="http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.3.4">HTTP/1.1 documentation</a>.
+     * 303 See Other, see <a href=
+     * "http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.3.4">HTTP/1.1
+     * documentation</a>.
      */
     SEE_OTHER(303, "See Other"),
     /**
-     * 304 Not Modified, see <a href="http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.3.5">HTTP/1.1 documentation</a>.
+     * 304 Not Modified, see <a href=
+     * "http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.3.5">HTTP/1.1
+     * documentation</a>.
      */
     NOT_MODIFIED(304, "Not Modified"),
     /**
-     * 305 Use Proxy, see <a href="http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.3.6">HTTP/1.1 documentation</a>.
+     * 305 Use Proxy, see <a href=
+     * "http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.3.6">HTTP/1.1
+     * documentation</a>.
      */
     USE_PROXY(305, "Use Proxy"),
     /**
-     * 307 Temporary Redirect, see <a href="http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.3.8">HTTP/1.1 documentation</a>.
+     * 307 Temporary Redirect, see <a href=
+     * "http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.3.8">HTTP/1.1
+     * documentation</a>.
      */
     TEMPORARY_REDIRECT(307, "Temporary Redirect"),
     /**
@@ -112,79 +142,116 @@ public enum Status implements StatusType {
      */
     PERMANENT_REDIRECT(308, "Permanent Redirect"),
     /**
-     * 400 Bad Request, see <a href="http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.4.1">HTTP/1.1 documentation</a>.
+     * 400 Bad Request, see <a href=
+     * "http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.4.1">HTTP/1.1
+     * documentation</a>.
      */
     BAD_REQUEST(400, "Bad Request"),
     /**
-     * 401 Unauthorized, see <a href="http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.4.2">HTTP/1.1 documentation</a>.
+     * 401 Unauthorized, see <a href=
+     * "http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.4.2">HTTP/1.1
+     * documentation</a>.
      */
     UNAUTHORIZED(401, "Unauthorized"),
     /**
-     * 402 Payment Required, see <a href="http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.4.3">HTTP/1.1 documentation</a>.
+     * 402 Payment Required, see <a href=
+     * "http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.4.3">HTTP/1.1
+     * documentation</a>.
      */
     PAYMENT_REQUIRED(402, "Payment Required"),
     /**
-     * 403 Forbidden, see <a href="http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.4.4">HTTP/1.1 documentation</a>.
+     * 403 Forbidden, see <a href=
+     * "http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.4.4">HTTP/1.1
+     * documentation</a>.
      */
     FORBIDDEN(403, "Forbidden"),
     /**
-     * 404 Not Found, see <a href="http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.4.5">HTTP/1.1 documentation</a>.
+     * 404 Not Found, see <a href=
+     * "http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.4.5">HTTP/1.1
+     * documentation</a>.
      */
     NOT_FOUND(404, "Not Found"),
     /**
-     * 405 Method Not Allowed, see <a href="http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.4.6">HTTP/1.1 documentation</a>.
+     * 405 Method Not Allowed, see <a href=
+     * "http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.4.6">HTTP/1.1
+     * documentation</a>.
      */
     METHOD_NOT_ALLOWED(405, "Method Not Allowed"),
     /**
-     * 406 Not Acceptable, see <a href="http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.4.7">HTTP/1.1 documentation</a>.
+     * 406 Not Acceptable, see <a href=
+     * "http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.4.7">HTTP/1.1
+     * documentation</a>.
      */
     NOT_ACCEPTABLE(406, "Not Acceptable"),
     /**
-     * 407 Proxy Authentication Required, see <a href="http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.4.8">HTTP/1.1 documentation</a>.
+     * 407 Proxy Authentication Required, see <a href=
+     * "http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.4.8">HTTP/1.1
+     * documentation</a>.
      */
     PROXY_AUTHENTICATION_REQUIRED(407, "Proxy Authentication Required"),
     /**
-     * 408 Request Timeout, see <a href="http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.4.9">HTTP/1.1 documentation</a>.
+     * 408 Request Timeout, see <a href=
+     * "http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.4.9">HTTP/1.1
+     * documentation</a>.
      */
     REQUEST_TIMEOUT(408, "Request Timeout"),
     /**
-     * 409 Conflict, see <a href="http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.4.10">HTTP/1.1 documentation</a>.
+     * 409 Conflict, see <a href=
+     * "http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.4.10">HTTP/1.1
+     * documentation</a>.
      */
     CONFLICT(409, "Conflict"),
     /**
-     * 410 Gone, see <a href="http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.4.11">HTTP/1.1 documentation</a>.
+     * 410 Gone, see <a href=
+     * "http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.4.11">HTTP/1.1
+     * documentation</a>.
      */
     GONE(410, "Gone"),
     /**
-     * 411 Length Required, see <a href="http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.4.12">HTTP/1.1 documentation</a>.
+     * 411 Length Required, see <a href=
+     * "http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.4.12">HTTP/1.1
+     * documentation</a>.
      */
     LENGTH_REQUIRED(411, "Length Required"),
     /**
-     * 412 Precondition Failed, see <a href="http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.4.13">HTTP/1.1 documentation</a>.
+     * 412 Precondition Failed, see <a href=
+     * "http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.4.13">HTTP/1.1
+     * documentation</a>.
      */
     PRECONDITION_FAILED(412, "Precondition Failed"),
     /**
-     * 413 Request Entity Too Large, see <a href="http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.4.14">HTTP/1.1 documentation</a>.
+     * 413 Request Entity Too Large, see <a href=
+     * "http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.4.14">HTTP/1.1
+     * documentation</a>.
      */
     REQUEST_ENTITY_TOO_LARGE(413, "Request Entity Too Large"),
     /**
-     * 414 Request-URI Too Long, see <a href="http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.4.15">HTTP/1.1 documentation</a>.
+     * 414 Request-URI Too Long, see <a href=
+     * "http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.4.15">HTTP/1.1
+     * documentation</a>.
      */
     REQUEST_URI_TOO_LONG(414, "Request-URI Too Long"),
     /**
-     * 415 Unsupported Media Type, see <a href="http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.4.16">HTTP/1.1 documentation</a>.
+     * 415 Unsupported Media Type, see <a href=
+     * "http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.4.16">HTTP/1.1
+     * documentation</a>.
      */
     UNSUPPORTED_MEDIA_TYPE(415, "Unsupported Media Type"),
     /**
-     * 416 Requested Range Not Satisfiable, see <a href="http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.4.17">HTTP/1.1 documentation</a>.
+     * 416 Requested Range Not Satisfiable, see <a href=
+     * "http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.4.17">HTTP/1.1
+     * documentation</a>.
      */
     REQUESTED_RANGE_NOT_SATISFIABLE(416, "Requested Range Not Satisfiable"),
     /**
-     * 417 Expectation Failed, see <a href="http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.4.18">HTTP/1.1 documentation</a>.
+     * 417 Expectation Failed, see <a href=
+     * "http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.4.18">HTTP/1.1
+     * documentation</a>.
      */
     EXPECTATION_FAILED(417, "Expectation Failed"),
     /**
-     * @see <a href="http://tools.ietf.org/html/rfc2324#section-2.3.2">HTCPCP/1.0</a>
+     * @see <a href=
+     *      "http://tools.ietf.org/html/rfc2324#section-2.3.2">HTCPCP/1.0</a>
      */
     I_AM_A_TEAPOT(418, "I'm a teapot"),
     /**
@@ -200,51 +267,69 @@ public enum Status implements StatusType {
      */
     FAILED_DEPENDENCY(424, "Failed Dependency"),
     /**
-     * @see <a href="http://tools.ietf.org/html/rfc2817#section-6">Upgrading to TLS Within HTTP/1.1</a>
+     * @see <a href="http://tools.ietf.org/html/rfc2817#section-6">Upgrading to TLS
+     *      Within HTTP/1.1</a>
      */
     UPGRADE_REQUIRED(426, "Upgrade Required"),
     /**
-     * @see <a href="http://tools.ietf.org/html/rfc6585#section-3">Additional HTTP Status Codes</a>
+     * @see <a href="http://tools.ietf.org/html/rfc6585#section-3">Additional HTTP
+     *      Status Codes</a>
      */
     PRECONDITION_REQUIRED(428, "Precondition Required"),
     /**
-     * @see <a href="http://tools.ietf.org/html/rfc6585#section-4">Additional HTTP Status Codes</a>
+     * @see <a href="http://tools.ietf.org/html/rfc6585#section-4">Additional HTTP
+     *      Status Codes</a>
      */
     TOO_MANY_REQUESTS(429, "Too Many Requests"),
     /**
-     * @see <a href="http://tools.ietf.org/html/rfc6585#section-5">Additional HTTP Status Codes</a>
+     * @see <a href="http://tools.ietf.org/html/rfc6585#section-5">Additional HTTP
+     *      Status Codes</a>
      */
     REQUEST_HEADER_FIELDS_TOO_LARGE(431, "Request Header Fields Too Large"),
     /**
-     * @see <a href="https://tools.ietf.org/html/rfc7725#section-3">Unavailable For Legal Reasons</a>
+     * @see <a href="https://tools.ietf.org/html/rfc7725#section-3">Unavailable For
+     *      Legal Reasons</a>
      */
     UNAVAILABLE_FOR_LEGAL_REASONS(451, "Unavailable For Legal Reasons"),
     /**
-     * 500 Internal Server Error, see <a href="http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.5.1">HTTP/1.1 documentation</a>.
+     * 500 Internal Server Error, see <a href=
+     * "http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.5.1">HTTP/1.1
+     * documentation</a>.
      */
     INTERNAL_SERVER_ERROR(500, "Internal Server Error"),
     /**
-     * 501 Not Implemented, see <a href="http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.5.2">HTTP/1.1 documentation</a>.
+     * 501 Not Implemented, see <a href=
+     * "http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.5.2">HTTP/1.1
+     * documentation</a>.
      */
     NOT_IMPLEMENTED(501, "Not Implemented"),
     /**
-     * 502 Bad Gateway, see <a href="http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.5.3">HTTP/1.1 documentation</a>.
+     * 502 Bad Gateway, see <a href=
+     * "http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.5.3">HTTP/1.1
+     * documentation</a>.
      */
     BAD_GATEWAY(502, "Bad Gateway"),
     /**
-     * 503 Service Unavailable, see <a href="http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.5.4">HTTP/1.1 documentation</a>.
+     * 503 Service Unavailable, see <a href=
+     * "http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.5.4">HTTP/1.1
+     * documentation</a>.
      */
     SERVICE_UNAVAILABLE(503, "Service Unavailable"),
     /**
-     * 504 Gateway Timeout, see <a href="http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.5.5">HTTP/1.1 documentation</a>.
+     * 504 Gateway Timeout, see <a href=
+     * "http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.5.5">HTTP/1.1
+     * documentation</a>.
      */
     GATEWAY_TIMEOUT(504, "Gateway Timeout"),
     /**
-     * 505 HTTP Version Not Supported, see <a href="http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.5.6">HTTP/1.1 documentation</a>.
+     * 505 HTTP Version Not Supported, see <a href=
+     * "http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.5.6">HTTP/1.1
+     * documentation</a>.
      */
     HTTP_VERSION_NOT_SUPPORTED(505, "HTTP Version Not Supported"),
     /**
-     * @see <a href="http://tools.ietf.org/html/rfc2295#section-8.1">Transparent Content Negotiation</a>
+     * @see <a href="http://tools.ietf.org/html/rfc2295#section-8.1">Transparent
+     *      Content Negotiation</a>
      */
     VARIANT_ALSO_NEGOTIATES(506, "Variant Also Negotiates"),
     /**
@@ -252,7 +337,8 @@ public enum Status implements StatusType {
      */
     INSUFFICIENT_STORAGE(507, "Insufficient Storage"),
     /**
-     * @see <a href="http://tools.ietf.org/html/rfc5842#section-7.2">WebDAV Binding Extensions</a>
+     * @see <a href="http://tools.ietf.org/html/rfc5842#section-7.2">WebDAV Binding
+     *      Extensions</a>
      */
     LOOP_DETECTED(508, "Loop Detected"),
     /**
@@ -260,18 +346,18 @@ public enum Status implements StatusType {
      */
     BANDWIDTH_LIMIT_EXCEEDED(509, "Bandwidth Limit Exceeded"),
     /**
-     * @see <a href="http://tools.ietf.org/html/rfc2774#section-7">HTTP Extension Framework</a>
+     * @see <a href="http://tools.ietf.org/html/rfc2774#section-7">HTTP Extension
+     *      Framework</a>
      */
     NOT_EXTENDED(510, "Not Extended"),
     /**
-     * @see <a href="http://tools.ietf.org/html/rfc6585#section-6">Additional HTTP Status Codes</a>
+     * @see <a href="http://tools.ietf.org/html/rfc6585#section-6">Additional HTTP
+     *      Status Codes</a>
      */
     NETWORK_AUTHENTICATION_REQUIRED(511, "Network Authentication Required");
 
     private static final Map<Integer, Status> STATUSES = Arrays.stream(values())
-            .collect(collectingAndThen(
-                    toMap(Status::getStatusCode, identity()),
-                    Collections::unmodifiableMap));
+            .collect(collectingAndThen(toMap(Status::getStatusCode, identity()), Collections::unmodifiableMap));
 
     private final int code;
     private final String reason;
@@ -297,7 +383,7 @@ public int getStatusCode() {
      * @return the reason phrase.
      */
     @Override
-    @Nonnull
+    @NonNull
     public String getReasonPhrase() {
         return reason;
     }
diff --git a/problem/src/main/java/org/zalando/problem/ThrowableProblem.java b/problem/src/main/java/org/zalando/problem/ThrowableProblem.java
index 0883b47..8fdc4d1 100644
--- a/problem/src/main/java/org/zalando/problem/ThrowableProblem.java
+++ b/problem/src/main/java/org/zalando/problem/ThrowableProblem.java
@@ -2,8 +2,7 @@
 
 import org.apiguardian.api.API;
 
-import javax.annotation.Nullable;
-import javax.annotation.concurrent.Immutable;
+import org.checkerframework.checker.nullness.qual.Nullable;
 import java.util.Collection;
 import java.util.Objects;
 import java.util.stream.Stream;
@@ -13,8 +12,11 @@
 import static org.apiguardian.api.API.Status.STABLE;
 import static org.zalando.problem.spi.StackTraceProcessor.COMPOUND;
 
+/**
+ * 
+ * {@link Problem} instances are required to be immutable.
+ */
 @API(status = STABLE)
-@Immutable
 public abstract class ThrowableProblem extends RuntimeException implements Problem, Exceptional {
 
     protected ThrowableProblem() {
diff --git a/problem/src/main/java/org/zalando/problem/package-info.java b/problem/src/main/java/org/zalando/problem/package-info.java
index 7d55a70..5352e50 100644
--- a/problem/src/main/java/org/zalando/problem/package-info.java
+++ b/problem/src/main/java/org/zalando/problem/package-info.java
@@ -1,3 +1,6 @@
-@ParametersAreNonnullByDefault package org.zalando.problem;
+@DefaultQualifier(value = NonNull.class, locations = TypeUseLocation.PARAMETER)
+package org.zalando.problem;
 
-import javax.annotation.ParametersAreNonnullByDefault;
\ No newline at end of file
+import org.checkerframework.checker.nullness.qual.NonNull;
+import org.checkerframework.framework.qual.DefaultQualifier;
+import org.checkerframework.framework.qual.TypeUseLocation;
diff --git a/problem/src/test/java/org/example/MyProblem.java b/problem/src/test/java/org/example/MyProblem.java
index 0e56dd1..1f7275d 100644
--- a/problem/src/test/java/org/example/MyProblem.java
+++ b/problem/src/test/java/org/example/MyProblem.java
@@ -4,7 +4,7 @@
 import org.zalando.problem.StatusType;
 import org.zalando.problem.ThrowableProblem;
 
-import javax.annotation.Nullable;
+import org.checkerframework.checker.nullness.qual.Nullable;
 import java.net.URI;
 import java.util.Map;
 
diff --git a/problem/src/test/java/org/zalando/problem/EnforceCoverageTest.java b/problem/src/test/java/org/zalando/problem/EnforceCoverageTest.java
index d119dea..ffb4afb 100644
--- a/problem/src/test/java/org/zalando/problem/EnforceCoverageTest.java
+++ b/problem/src/test/java/org/zalando/problem/EnforceCoverageTest.java
@@ -1,7 +1,5 @@
 package org.zalando.problem;
 
-import com.google.gag.annotation.remark.Hack;
-import com.google.gag.annotation.remark.OhNoYouDidnt;
 import org.junit.jupiter.api.Test;
 
 import java.net.URI;
@@ -9,8 +7,6 @@
 import static org.junit.jupiter.api.Assertions.assertThrows;
 import static org.zalando.problem.Status.BAD_REQUEST;
 
-@Hack
-@OhNoYouDidnt
 class EnforceCoverageTest {
 
     @Test