Add custom Gson type adapter factories during runtime

http/media/gson/src/main/java/io/helidon/http/media/gson/GsonSupport.java

@@ -17,6 +17,7 @@
 
 import java.util.Map;
 import java.util.Objects;
+import java.util.ServiceLoader;
 import java.util.function.Consumer;
 
 import io.helidon.builder.api.Prototype;
@@ -34,6 +35,7 @@
 
 import com.google.gson.Gson;
 import com.google.gson.GsonBuilder;
+import com.google.gson.TypeAdapterFactory;
 
 import static io.helidon.http.HeaderValues.CONTENT_TYPE_JSON;
 
@@ -82,6 +84,12 @@ public static MediaSupport create(Config config, String name) {
         Objects.requireNonNull(config, "Config must not be null");
         Objects.requireNonNull(name, "Name must not be null");
 
+        GsonBuilder gsonBuilder = new GsonBuilder();
+        // Enable the registering of custom type adapters by using service providers for TypeAdapterFactory.
+        for (var factory : ServiceLoader.load(TypeAdapterFactory.class)) {
+            gsonBuilder.registerTypeAdapterFactory(factory);
+        }
+        Gson gson = gsonBuilder.create();
         return builder()
                 .name(name)
                 .config(config)

http/media/gson/src/test/java/io/helidon/http/media/gson/GsonSupportTest.java

@@ -0,0 +1,61 @@
+/*
+ * Copyright (c) 2025 Oracle and/or its affiliates.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package io.helidon.http.media.gson;
+
+import io.helidon.common.GenericType;
+import io.helidon.common.config.Config;
+import io.helidon.http.WritableHeaders;
+import org.junit.jupiter.api.Test;
+
+import java.io.ByteArrayInputStream;
+import java.io.ByteArrayOutputStream;
+
+import static org.hamcrest.CoreMatchers.is;
+import static org.hamcrest.MatcherAssert.assertThat;
+
+class GsonSupportTest {
+
+    record Book(String title, int pages) {
+    }
+
+    @Test
+    void test() {
+        var support = GsonSupport.create(Config.empty(), "gson");
+        var headers = WritableHeaders.create();
+        var type = GenericType.create(Book.class);
+        var outputStream = new ByteArrayOutputStream();
+        var instance = new Book("some-title", 123);
+
+        support.writer(type, headers)
+                .supplier()
+                .get()
+                .write(type, instance, outputStream, headers);
+
+        assertThat(GsonSupportTestBookTypeAdapterFactory.writeCount.get(), is(1));
+
+        Book sanity = support.reader(type, headers)
+                .supplier()
+                .get()
+                .read(type, new ByteArrayInputStream(outputStream.toByteArray()), headers);
+
+        assertThat(GsonSupportTestBookTypeAdapterFactory.readCount.get(), is(1));
+
+        assertThat(sanity.title(), is("some-title"));
+        assertThat(sanity.pages(), is(123));
+        assertThat(sanity, is(instance));
+    }
+}

http/media/gson/src/test/java/io/helidon/http/media/gson/GsonSupportTestBookTypeAdapterFactory.java

@@ -0,0 +1,66 @@
+/*
+ * Copyright (c) 2025 Oracle and/or its affiliates.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package io.helidon.http.media.gson;
+
+import com.google.gson.Gson;
+import com.google.gson.TypeAdapter;
+import com.google.gson.TypeAdapterFactory;
+import com.google.gson.reflect.TypeToken;
+import com.google.gson.stream.JsonReader;
+import com.google.gson.stream.JsonWriter;
+
+import java.io.IOException;
+import java.util.concurrent.atomic.AtomicInteger;
+
+public class GsonSupportTestBookTypeAdapterFactory implements TypeAdapterFactory {
+
+    static final AtomicInteger readCount = new AtomicInteger(0);
+    static final AtomicInteger writeCount = new AtomicInteger(0);
+
+    private static final TypeAdapter instance = new TypeAdapter<GsonSupportTest.Book>() {
+        @Override
+        public void write(JsonWriter writer, GsonSupportTest.Book book) throws IOException {
+            writer.beginObject();
+            writer.name("title");
+            writer.value(book.title());
+            writer.name("pages");
+            writer.value(book.pages());
+            writer.endObject();
+            writeCount.incrementAndGet();
+        }
+
+        @Override
+        public GsonSupportTest.Book read(JsonReader reader) throws IOException {
+            reader.beginObject();
+            reader.nextName();
+            var title = reader.nextString();
+            reader.nextName();
+            var pages = reader.nextInt();
+            reader.endObject();
+            readCount.incrementAndGet();
+            return new GsonSupportTest.Book(title, pages);
+        }
+    };
+
+    @Override
+    public <T> TypeAdapter<T> create(Gson gson, TypeToken<T> typeToken) {
+        if (typeToken.getRawType().isAssignableFrom(GsonSupportTest.Book.class)) {
+            return instance;
+        }
+        return null;
+    }
+}

http/media/gson/src/test/java/my/pkg/Book.java

@@ -0,0 +1,4 @@
+package my.pkg;
+
+public record Book(String title, int pages) {
+}

http/media/gson/src/test/java/my/pkg/BookTypeAdapterFactory.java

@@ -0,0 +1,49 @@
+package my.pkg;
+
+import com.google.gson.Gson;
+import com.google.gson.TypeAdapter;
+import com.google.gson.TypeAdapterFactory;
+import com.google.gson.reflect.TypeToken;
+import com.google.gson.stream.JsonReader;
+import com.google.gson.stream.JsonWriter;
+
+import java.io.IOException;
+
+public class BookTypeAdapterFactory implements TypeAdapterFactory {
+
+    private static final TypeAdapter<Book> instance = new TypeAdapter<>() {
+        @Override
+        public void write(JsonWriter writer, Book book) throws IOException {
+            writer.beginObject();
+            writer.name("title");
+            writer.value(book.title());
+            writer.name("pages");
+            writer.value(book.pages());
+            writer.endObject();
+        }
+
+        @Override
+        public Book read(JsonReader reader) throws IOException {
+            reader.beginObject();
+            String title = null;
+            int pages = 0;
+            while (reader.hasNext()) {
+                switch (reader.nextName()) {
+                    case "title" -> title = reader.nextString();
+                    case "pages" -> pages = reader.nextInt();
+                    default -> reader.skipValue();
+                }
+            }
+            reader.endObject();
+            return new Book(title, pages);
+        }
+    };
+
+    @Override
+    public <T> TypeAdapter<T> create(Gson gson, TypeToken<T> typeToken) {
+        if (typeToken.getRawType().isAssignableFrom(Book.class)) {
+            return (TypeAdapter<T>) instance;
+        }
+        return null;
+    }
+}

http/media/gson/src/test/resources/META-INF/services/com.google.gson.TypeAdapterFactory

@@ -0,0 +1 @@
+io.helidon.http.media.gson.GsonSupportTestBookTypeAdapterFactory