Add a type adapter for JsonElement, so it serializes just like everything else.

Fixes issue 362.

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

@@ -248,6 +248,7 @@ public final class Gson {
         .typeAdapter(BigDecimal.class, new BigDecimalTypeAdapter())
         .typeAdapter(BigInteger.class, new BigIntegerTypeAdapter())
         .factory(new CollectionTypeAdapterFactory(constructorConstructor))
+        .factory(TypeAdapters.JSON_ELEMENT_FACTORY)
         .factory(ObjectTypeAdapter.FACTORY);
 
     for (TypeAdapter.Factory factory : typeAdapterFactories) {

gson/src/main/java/com/google/gson/internal/Streams.java

@@ -16,27 +16,23 @@
 
 package com.google.gson.internal;
 
-import com.google.gson.JsonArray;
 import com.google.gson.JsonElement;
 import com.google.gson.JsonIOException;
 import com.google.gson.JsonNull;
-import com.google.gson.JsonObject;
 import com.google.gson.JsonParseException;
-import com.google.gson.JsonPrimitive;
 import com.google.gson.JsonSyntaxException;
+import com.google.gson.internal.bind.TypeAdapters;
 import com.google.gson.stream.JsonReader;
 import com.google.gson.stream.JsonWriter;
 import com.google.gson.stream.MalformedJsonException;
 import java.io.EOFException;
 import java.io.IOException;
 import java.io.Writer;
-import java.util.Map;
 
 /**
  * Reads and writes GSON parse trees over streams.
  */
 public final class Streams {
-
   /**
    * Takes a reader in any state and returns the next value as a JsonElement.
    */
@@ -45,7 +41,7 @@ public final class Streams {
     try {
       reader.peek();
       isEmpty = false;
-      return parseRecursive(reader);
+      return TypeAdapters.JSON_ELEMENT.read(reader);
     } catch (EOFException e) {
       /*
        * For compatibility with JSON 1.5 and earlier, we return a JsonNull for
@@ -64,79 +60,11 @@ public final class Streams {
     }
   }
 
-  private static JsonElement parseRecursive(JsonReader reader) throws IOException {
-    switch (reader.peek()) {
-    case STRING:
-      return new JsonPrimitive(reader.nextString());
-    case NUMBER:
-      String number = reader.nextString();
-      return new JsonPrimitive(new LazilyParsedNumber(number));
-    case BOOLEAN:
-      return new JsonPrimitive(reader.nextBoolean());
-    case NULL:
-      reader.nextNull();
-      return JsonNull.INSTANCE;
-    case BEGIN_ARRAY:
-      JsonArray array = new JsonArray();
-      reader.beginArray();
-      while (reader.hasNext()) {
-        array.add(parseRecursive(reader));
-      }
-      reader.endArray();
-      return array;
-    case BEGIN_OBJECT:
-      JsonObject object = new JsonObject();
-      reader.beginObject();
-      while (reader.hasNext()) {
-        object.add(reader.nextName(), parseRecursive(reader));
-      }
-      reader.endObject();
-      return object;
-    case END_DOCUMENT:
-    case NAME:
-    case END_OBJECT:
-    case END_ARRAY:
-    default:
-      throw new IllegalArgumentException();
-    }
-  }
-
   /**
    * Writes the JSON element to the writer, recursively.
    */
-  public static void write(JsonElement element, JsonWriter writer)
+  public static void write(JsonElement element, JsonWriter writer) throws IOException {
-      throws IOException {
+    TypeAdapters.JSON_ELEMENT.write(writer, element);
-    if (element == null || element.isJsonNull()) {
-      writer.nullValue();
-    } else if (element.isJsonPrimitive()) {
-      JsonPrimitive primitive = element.getAsJsonPrimitive();
-      if (primitive.isNumber()) {
-        writer.value(primitive.getAsNumber());
-      } else if (primitive.isBoolean()) {
-        writer.value(primitive.getAsBoolean());
-      } else {
-        writer.value(primitive.getAsString());
-      }
-
-    } else if (element.isJsonArray()) {
-      writer.beginArray();
-      for (JsonElement e : element.getAsJsonArray()) {
-        write(e, writer);
-      }
-      writer.endArray();
-
-    } else if (element.isJsonObject()) {
-      writer.beginObject();
-      for (Map.Entry<String, JsonElement> e : element.getAsJsonObject().entrySet()) {
-        JsonElement value = e.getValue();
-        writer.name(e.getKey());
-        write(value, writer);
-      }
-      writer.endObject();
-
-    } else {
-      throw new IllegalArgumentException("Couldn't write " + element.getClass());
-    }
   }
 
   public static Writer writerForAppendable(Appendable appendable) {
@@ -183,4 +111,5 @@ public final class Streams {
       }
     }
   }
+
 }

gson/src/main/java/com/google/gson/internal/bind/TypeAdapters.java

@@ -16,7 +16,12 @@
 
 package com.google.gson.internal.bind;
 
+import com.google.gson.JsonArray;
+import com.google.gson.JsonElement;
 import com.google.gson.JsonIOException;
+import com.google.gson.JsonNull;
+import com.google.gson.JsonObject;
+import com.google.gson.JsonPrimitive;
 import com.google.gson.JsonSyntaxException;
 import com.google.gson.internal.LazilyParsedNumber;
 import com.google.gson.reflect.TypeToken;
@@ -34,6 +39,7 @@ import java.util.Calendar;
 import java.util.Date;
 import java.util.GregorianCalendar;
 import java.util.Locale;
+import java.util.Map;
 import java.util.StringTokenizer;
 import java.util.UUID;
 
@@ -526,6 +532,81 @@ public final class TypeAdapters {
 
   public static final TypeAdapter.Factory LOCALE_FACTORY = newFactory(Locale.class, LOCALE);
 
+  public static final TypeAdapter<JsonElement> JSON_ELEMENT = new TypeAdapter<JsonElement>() {
+    @Override public JsonElement read(JsonReader reader) throws IOException {
+      switch (reader.peek()) {
+      case STRING:
+        return new JsonPrimitive(reader.nextString());
+      case NUMBER:
+        String number = reader.nextString();
+        return new JsonPrimitive(new LazilyParsedNumber(number));
+      case BOOLEAN:
+        return new JsonPrimitive(reader.nextBoolean());
+      case NULL:
+        reader.nextNull();
+        return JsonNull.INSTANCE;
+      case BEGIN_ARRAY:
+        JsonArray array = new JsonArray();
+        reader.beginArray();
+        while (reader.hasNext()) {
+          array.add(read(reader));
+        }
+        reader.endArray();
+        return array;
+      case BEGIN_OBJECT:
+        JsonObject object = new JsonObject();
+        reader.beginObject();
+        while (reader.hasNext()) {
+          object.add(reader.nextName(), read(reader));
+        }
+        reader.endObject();
+        return object;
+      case END_DOCUMENT:
+      case NAME:
+      case END_OBJECT:
+      case END_ARRAY:
+      default:
+        throw new IllegalArgumentException();
+      }
+    }
+
+    @Override public void write(JsonWriter writer, JsonElement value) throws IOException {
+      if (value == null || value.isJsonNull()) {
+        writer.nullValue();
+      } else if (value.isJsonPrimitive()) {
+        JsonPrimitive primitive = value.getAsJsonPrimitive();
+        if (primitive.isNumber()) {
+          writer.value(primitive.getAsNumber());
+        } else if (primitive.isBoolean()) {
+          writer.value(primitive.getAsBoolean());
+        } else {
+          writer.value(primitive.getAsString());
+        }
+
+      } else if (value.isJsonArray()) {
+        writer.beginArray();
+        for (JsonElement e : value.getAsJsonArray()) {
+          write(writer, e);
+        }
+        writer.endArray();
+
+      } else if (value.isJsonObject()) {
+        writer.beginObject();
+        for (Map.Entry<String, JsonElement> e : value.getAsJsonObject().entrySet()) {
+          writer.name(e.getKey());
+          write(writer, e.getValue());
+        }
+        writer.endObject();
+
+      } else {
+        throw new IllegalArgumentException("Couldn't write " + value.getClass());
+      }
+    }
+  };
+
+  public static final TypeAdapter.Factory JSON_ELEMENT_FACTORY
+      = newFactory(JsonElement.class, JSON_ELEMENT);
+
   private static final class EnumTypeAdapter<T extends Enum<T>> extends TypeAdapter<T> {
     private final Class<T> classOfT;
 

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

@@ -17,10 +17,14 @@ package com.google.gson.functional;
 
 import com.google.gson.Gson;
 import com.google.gson.GsonBuilder;
+import com.google.gson.JsonArray;
 import com.google.gson.JsonDeserializationContext;
 import com.google.gson.JsonDeserializer;
 import com.google.gson.JsonElement;
+import com.google.gson.JsonNull;
+import com.google.gson.JsonObject;
 import com.google.gson.JsonParseException;
+import com.google.gson.JsonPrimitive;
 import com.google.gson.reflect.TypeToken;
 import java.lang.reflect.Type;
 import java.math.BigDecimal;
@@ -459,6 +463,66 @@ public class DefaultTypeAdaptersTest extends TestCase {
     }
   }
 
+  public void testJsonPrimitiveSerialization() {
+    assertEquals("5", gson.toJson(new JsonPrimitive(5), JsonElement.class));
+    assertEquals("true", gson.toJson(new JsonPrimitive(true), JsonElement.class));
+    assertEquals("\"foo\"", gson.toJson(new JsonPrimitive("foo"), JsonElement.class));
+    assertEquals("\"a\"", gson.toJson(new JsonPrimitive('a'), JsonElement.class));
+  }
+
+  public void testJsonPrimitiveDeserialization() {
+    assertEquals(new JsonPrimitive(5), gson.fromJson("5", JsonElement.class));
+    assertEquals(new JsonPrimitive(true), gson.fromJson("true", JsonElement.class));
+    assertEquals(new JsonPrimitive("foo"), gson.fromJson("\"foo\"", JsonElement.class));
+    assertEquals(new JsonPrimitive('a'), gson.fromJson("\"a\"", JsonElement.class));
+  }
+
+  public void testJsonNullSerialization() {
+    assertEquals("null", gson.toJson(JsonNull.INSTANCE, JsonElement.class));
+  }
+
+  public void testNullJsonElementSerialization() {
+    assertEquals("null", gson.toJson(null, JsonElement.class));
+  }
+
+  public void testJsonArraySerialization() {
+    JsonArray array = new JsonArray();
+    array.add(new JsonPrimitive(1));
+    array.add(new JsonPrimitive(2));
+    array.add(new JsonPrimitive(3));
+    assertEquals("[1,2,3]", gson.toJson(array, JsonElement.class));
+  }
+
+  public void testJsonArrayDeerialization() {
+    JsonArray array = new JsonArray();
+    array.add(new JsonPrimitive(1));
+    array.add(new JsonPrimitive(2));
+    array.add(new JsonPrimitive(3));
+    assertEquals(array, gson.fromJson("[1,2,3]", JsonElement.class));
+  }
+
+  public void testJsonObjectSerialization() {
+    JsonObject object = new JsonObject();
+    object.add("foo", new JsonPrimitive(1));
+    object.add("bar", new JsonPrimitive(2));
+    assertEquals("{\"foo\":1,\"bar\":2}", gson.toJson(object, JsonElement.class));
+  }
+
+  public void testJsonObjectDeerialization() {
+    JsonObject object = new JsonObject();
+    object.add("foo", new JsonPrimitive(1));
+    object.add("bar", new JsonPrimitive(2));
+    assertEquals(object, gson.fromJson("{\"foo\":1,\"bar\":2}", JsonElement.class));
+  }
+
+  public void testJsonNullDeerialization() {
+    assertEquals(JsonNull.INSTANCE, gson.fromJson("null", JsonElement.class));
+  }
+
+  public void testNullJsonElementDeserialization() {
+    assertEquals(JsonNull.INSTANCE, gson.fromJson("null", JsonElement.class));
+  }
+
   private static class ClassWithBigDecimal {
     BigDecimal value;
     ClassWithBigDecimal(String value) {