Add --dry-run option to simplify CI system integrations

Fixes: #105.

Test Plan:

  $ git show --name-only HEAD | grep java$ | xargs -r gjf --dry-run
  Need Formatting:
    [gerrit-common/src/main/java/com/google/gerrit/common/IoUtil.java,\
     gerrit-server/src/main/java/com/google/gerrit/server/account/Emails.java]

Author: davido

core/src/main/java/com/google/googlejavaformat/java/CommandLineOptions.java

@@ -26,6 +26,7 @@
 final class CommandLineOptions {
 
   private final ImmutableList<String> files;
+  private final boolean dryRun;
   private final boolean inPlace;
   private final ImmutableRangeSet<Integer> lines;
   private final ImmutableList<Integer> offsets;
@@ -41,6 +42,7 @@ final class CommandLineOptions {
 
   CommandLineOptions(
       ImmutableList<String> files,
+      boolean dryRun,
       boolean inPlace,
       ImmutableRangeSet<Integer> lines,
       ImmutableList<Integer> offsets,
@@ -54,6 +56,7 @@ final class CommandLineOptions {
       boolean sortImports,
       boolean removeUnusedImports) {
     this.files = files;
+    this.dryRun = dryRun;
     this.inPlace = inPlace;
     this.lines = lines;
     this.offsets = offsets;
@@ -73,6 +76,11 @@ ImmutableList<String> files() {
     return files;
   }
 
+  /** Dry run. */
+  boolean dryRun() {
+    return dryRun;
+  }
+
   /** Format files in place. */
   boolean inPlace() {
     return inPlace;
@@ -151,6 +159,7 @@ static class Builder {
     private final ImmutableRangeSet.Builder<Integer> lines = ImmutableRangeSet.builder();
     private final ImmutableList.Builder<Integer> offsets = ImmutableList.builder();
     private final ImmutableList.Builder<Integer> lengths = ImmutableList.builder();
+    private Boolean dryRun = false;
     private Boolean inPlace = false;
     private Boolean aosp = false;
     private Boolean version = false;
@@ -165,6 +174,11 @@ ImmutableList.Builder<String> filesBuilder() {
       return files;
     }
 
+    Builder dryRun(boolean dryRun) {
+      this.dryRun = dryRun;
+      return this;
+    }
+
     Builder inPlace(boolean inPlace) {
       this.inPlace = inPlace;
       return this;
@@ -227,6 +241,7 @@ Builder removeUnusedImports(boolean removeUnusedImports) {
     CommandLineOptions build() {
       return new CommandLineOptions(
           this.files.build(),
+          this.dryRun,
           this.inPlace,
           this.lines.build(),
           this.offsets.build(),

core/src/main/java/com/google/googlejavaformat/java/CommandLineOptionsParser.java

@@ -48,6 +48,10 @@ static CommandLineOptions parse(Iterable<String> options) {
       }
       // NOTE: update usage information in UsageException when new flags are added
       switch (flag) {
+	case "-n":
+        case "--dry-run":
+	  optionsBuilder.dryRun(true);
+	  break;
         case "-i":
         case "-r":
         case "-replace":

core/src/main/java/com/google/googlejavaformat/java/Main.java

@@ -30,6 +30,8 @@
 import java.util.Arrays;
 import java.util.LinkedHashMap;
 import java.util.Map;
+import java.util.Set;
+import java.util.TreeSet;
 import java.util.concurrent.ExecutionException;
 import java.util.concurrent.ExecutorService;
 import java.util.concurrent.Executors;
@@ -131,6 +133,7 @@ private int formatFiles(CommandLineOptions parameters, JavaFormatterOptions opti
     }
 
     boolean allOk = true;
+    Set<Path> needFormatting = new TreeSet<>();
     for (Map.Entry<Path, Future<String>> result : results.entrySet()) {
       String formatted;
       try {
@@ -151,7 +154,13 @@ private int formatFiles(CommandLineOptions parameters, JavaFormatterOptions opti
         allOk = false;
         continue;
       }
-      if (parameters.inPlace()) {
+      if (parameters.dryRun()) {
+        if (formatted.equals(inputs.get(result.getKey()))) {
+          continue;
+        } else {
+          needFormatting.add(result.getKey());
+	}
+      } else if (parameters.inPlace()) {
         if (formatted.equals(inputs.get(result.getKey()))) {
           continue; // preserve original file
         }
@@ -166,6 +175,13 @@ private int formatFiles(CommandLineOptions parameters, JavaFormatterOptions opti
         outWriter.write(formatted);
       }
     }
+
+    if (!needFormatting.isEmpty()) {
+      errWriter.println("Need Formatting:");
+      errWriter.println(needFormatting);
+      return 1;
+    }
+
     return allOk ? 0 : 1;
   }
 

core/src/main/java/com/google/googlejavaformat/java/UsageException.java

@@ -32,6 +32,8 @@ public final class UsageException extends Exception {
     "Usage: google-java-format [options] file(s)",
     "",
     "Options:",
+    "  -n, --dry-run",
+    "    Don't change anything, only check; supposed to be used form the CI.",
     "  -i, -r, -replace, --replace",
     "    Send formatted output back to files, not stdout.",
     "  -",