Adopt JsonElementWriter in GSON.

Add setSerializeNulls() to JsonWriter, so nulls can be skipped from serialization. This does not yet impact JsonElementWriter.

One change in behavior: if the only value is skipped, we now emit "null" rather than "".

gson/GSON 2.0 NOTES.txt

@@ -54,8 +54,16 @@ GSON 1.x sometimes sets subclass fields when an InstanceCreator returns a subcla
 GSON 2.x sets fields of the requested type only
 com.google.gson.functional.InstanceCreatorTest.testInstanceCreatorReturnsSubTypeForField
 
+
 GSON 1.x applies different rules for versioning for classes vs fields. So, if you deserialize a
    JSON into a field that is supposed to be skipped, the field is set to null (or default value).
    However, if you deserialize it to a top-level class, a default instance is returned.
 GSON 2.x returns null for the top-level class.
 com.google.gson.functional.VersioningTest.testIgnoreLaterVersionClassDeserialization
+
+
+GSON 1.x creates the empty string "" if the only element is skipped
+GSON 2.x writes "null" if the only element is skipped
+com.google.gson.functional.ObjectTest.testAnonymousLocalClassesSerialization
+com.google.gson.functional.FieldExclusionTest.testInnerClassExclusion
+com.google.gson.functional.VersioningTest.testIgnoreLaterVersionClassSerialization

gson/src/main/java/com/google/gson/Gson.java

@@ -443,7 +443,7 @@ public final class Gson {
    */
   public String toJson(Object src, Type typeOfSrc) {
     StringWriter writer = new StringWriter();
-    toJson(toJsonTree(src, typeOfSrc), writer);
+    toJson(src, typeOfSrc, writer);
     return writer.toString();
   }
 
@@ -486,8 +486,12 @@ public final class Gson {
    * @since 1.2
    */
   public void toJson(Object src, Type typeOfSrc, Appendable writer) throws JsonIOException {
-    JsonElement jsonElement = toJsonTree(src, typeOfSrc);
-    toJson(jsonElement, writer);
+    try {
+      JsonWriter jsonWriter = newJsonWriter(Streams.writerForAppendable(writer));
+      toJson(src, typeOfSrc, jsonWriter);
+    } catch (IOException e) {
+      throw new JsonIOException(e);
+    }
   }
 
   /**
@@ -496,7 +500,22 @@ public final class Gson {
    * @throws JsonIOException if there was a problem writing to the writer
    */
   public void toJson(Object src, Type typeOfSrc, JsonWriter writer) throws JsonIOException {
-    toJson(toJsonTree(src, typeOfSrc), writer);
+    TypeAdapter<?> adapter = miniGson.getAdapter(TypeToken.get(typeOfSrc));
+    boolean oldLenient = writer.isLenient();
+    writer.setLenient(true);
+    boolean oldHtmlSafe = writer.isHtmlSafe();
+    writer.setHtmlSafe(htmlSafe);
+    boolean oldSerializeNulls = writer.getSerializeNulls();
+    writer.setSerializeNulls(serializeNulls);
+    try {
+      ((TypeAdapter<Object>) adapter).write(writer, src);
+    } catch (IOException e) {
+      throw new JsonIOException(e);
+    } finally {
+      writer.setLenient(oldLenient);
+      writer.setHtmlSafe(oldHtmlSafe);
+      writer.setSerializeNulls(oldSerializeNulls);
+    }
   }
 
   /**
@@ -522,19 +541,29 @@ public final class Gson {
    */
   public void toJson(JsonElement jsonElement, Appendable writer) throws JsonIOException {
     try {
-      if (generateNonExecutableJson) {
-        writer.append(JSON_NON_EXECUTABLE_PREFIX);
-      }
-      JsonWriter jsonWriter = new JsonWriter(Streams.writerForAppendable(writer));
-      if (prettyPrinting) {
-        jsonWriter.setIndent("  ");
-      }
+      JsonWriter jsonWriter = newJsonWriter(Streams.writerForAppendable(writer));
       toJson(jsonElement, jsonWriter);
     } catch (IOException e) {
       throw new RuntimeException(e);
     }
   }
 
+  /**
+   * Returns a new JSON writer configured for this GSON and with the non-execute
+   * prefix if that is configured.
+   */
+  private JsonWriter newJsonWriter(Writer writer) throws IOException {
+    if (generateNonExecutableJson) {
+      writer.write(JSON_NON_EXECUTABLE_PREFIX);
+    }
+    JsonWriter jsonWriter = new JsonWriter(writer);
+    if (prettyPrinting) {
+      jsonWriter.setIndent("  ");
+    }
+    jsonWriter.setSerializeNulls(serializeNulls);
+    return jsonWriter;
+  }
+
   /**
    * Writes the JSON for {@code jsonElement} to {@code writer}.
    * @throws JsonIOException if there was a problem writing to the writer
@@ -544,6 +573,8 @@ public final class Gson {
     writer.setLenient(true);
     boolean oldHtmlSafe = writer.isHtmlSafe();
     writer.setHtmlSafe(htmlSafe);
+    boolean oldSerializeNulls = writer.getSerializeNulls();
+    writer.setSerializeNulls(serializeNulls);
     try {
       Streams.write(jsonElement, serializeNulls, writer);
     } catch (IOException e) {
@@ -551,6 +582,7 @@ public final class Gson {
     } finally {
       writer.setLenient(oldLenient);
       writer.setHtmlSafe(oldHtmlSafe);
+      writer.setSerializeNulls(oldSerializeNulls);
     }
   }
 

gson/src/main/java/com/google/gson/stream/JsonWriter.java

@@ -145,6 +145,10 @@ public class JsonWriter implements Closeable {
 
   private boolean htmlSafe;
 
+  private String deferredName;
+
+  private boolean serializeNulls = true;
+
   /**
    * Creates a new instance that writes a JSON-encoded stream to {@code out}.
    * For best performance, ensure {@link Writer} is buffered; wrapping in
@@ -217,6 +221,22 @@ public class JsonWriter implements Closeable {
     return htmlSafe;
   }
 
+  /**
+   * Sets whether object members are serialized when their value is null.
+   * This has no impact on array elements. The default is true.
+   */
+  public final void setSerializeNulls(boolean serializeNulls) {
+    this.serializeNulls = serializeNulls;
+  }
+
+  /**
+   * Returns true if object members are serialized when their value is null.
+   * This has no impact on array elements. The default is true.
+   */
+  public final boolean getSerializeNulls() {
+    return serializeNulls;
+  }
+
   /**
    * Begins encoding a new array. Each call to this method must be paired with
    * a call to {@link #endArray}.
@@ -224,6 +244,7 @@ public class JsonWriter implements Closeable {
    * @return this writer.
    */
   public JsonWriter beginArray() throws IOException {
+    writeDeferredName();
     return open(JsonScope.EMPTY_ARRAY, "[");
   }
 
@@ -243,6 +264,7 @@ public class JsonWriter implements Closeable {
    * @return this writer.
    */
   public JsonWriter beginObject() throws IOException {
+    writeDeferredName();
     return open(JsonScope.EMPTY_OBJECT, "{");
   }
 
@@ -276,6 +298,9 @@ public class JsonWriter implements Closeable {
     if (context != nonempty && context != empty) {
       throw new IllegalStateException("Nesting problem: " + stack);
     }
+    if (deferredName != null) {
+      throw new IllegalStateException("Dangling name: " + deferredName);
+    }
 
     stack.remove(stack.size() - 1);
     if (context == nonempty) {
@@ -309,11 +334,21 @@ public class JsonWriter implements Closeable {
     if (name == null) {
       throw new NullPointerException("name == null");
     }
-    beforeName();
-    string(name);
+    if (deferredName != null) {
+      throw new IllegalStateException();
+    }
+    deferredName = name;
     return this;
   }
 
+  private void writeDeferredName() throws IOException {
+    if (deferredName != null) {
+      beforeName();
+      string(deferredName);
+      deferredName = null;
+    }
+  }
+
   /**
    * Encodes {@code value}.
    *
@@ -324,6 +359,7 @@ public class JsonWriter implements Closeable {
     if (value == null) {
       return nullValue();
     }
+    writeDeferredName();
     beforeValue(false);
     string(value);
     return this;
@@ -335,6 +371,14 @@ public class JsonWriter implements Closeable {
    * @return this writer.
    */
   public JsonWriter nullValue() throws IOException {
+    if (deferredName != null) {
+      if (serializeNulls) {
+        writeDeferredName();
+      } else {
+        deferredName = null;
+        return this; // skip the name and the value
+      }
+    }
     beforeValue(false);
     out.write("null");
     return this;
@@ -346,6 +390,7 @@ public class JsonWriter implements Closeable {
    * @return this writer.
    */
   public JsonWriter value(boolean value) throws IOException {
+    writeDeferredName();
     beforeValue(false);
     out.write(value ? "true" : "false");
     return this;
@@ -362,6 +407,7 @@ public class JsonWriter implements Closeable {
     if (Double.isNaN(value) || Double.isInfinite(value)) {
       throw new IllegalArgumentException("Numeric values must be finite, but was " + value);
     }
+    writeDeferredName();
     beforeValue(false);
     out.append(Double.toString(value));
     return this;
@@ -373,6 +419,7 @@ public class JsonWriter implements Closeable {
    * @return this writer.
    */
   public JsonWriter value(long value) throws IOException {
+    writeDeferredName();
     beforeValue(false);
     out.write(Long.toString(value));
     return this;
@@ -390,6 +437,7 @@ public class JsonWriter implements Closeable {
       return nullValue();
     }
 
+    writeDeferredName();
     String string = value.toString();
     if (!lenient
         && (string.equals("-Infinity") || string.equals("Infinity") || string.equals("NaN"))) {

gson/src/test/java/com/google/gson/functional/FieldExclusionTest.java

@@ -55,7 +55,7 @@ public class FieldExclusionTest extends TestCase {
     Gson gson = new GsonBuilder().disableInnerClassSerialization().create();
     Outer.Inner target = outer.new Inner(VALUE);
     String result = gson.toJson(target);
-    assertEquals("", result);
+    assertEquals("null", result);
   }
 
   public void testDefaultNestedStaticClassIncluded() throws Exception {

gson/src/test/java/com/google/gson/functional/ObjectTest.java

@@ -267,7 +267,7 @@ public class ObjectTest extends TestCase {
   }
 
   public void testAnonymousLocalClassesSerialization() throws Exception {
-    assertEquals("", gson.toJson(new ClassWithNoFields() {
+    assertEquals("null", gson.toJson(new ClassWithNoFields() {
       // empty anonymous class
     }));
   }

gson/src/test/java/com/google/gson/functional/VersioningTest.java

@@ -82,7 +82,7 @@ public class VersioningTest extends TestCase {
 
   public void testIgnoreLaterVersionClassSerialization() {
     Gson gson = builder.setVersion(1.0).create();
-    assertEquals("", gson.toJson(new Version1_2()));
+    assertEquals("null", gson.toJson(new Version1_2()));
   }
 
   public void testIgnoreLaterVersionClassDeserialization() {

gson/src/test/java/com/google/gson/internal/bind/JsonElementWriterTest.java

@@ -24,6 +24,7 @@ public final class JsonElementWriterTest extends TestCase {
   // TODO: more tests
   // TODO: close support
   // TODO: figure out what should be returned by an empty writer
+  // TODO: test when serialize nulls is false
 
   public void testArray() throws IOException {
     JsonElementWriter writer = new JsonElementWriter();