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Explore the source code of the class TypeAdapter.java

/*
 * Copyright (C) 2011 Google Inc.
 *
 * 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 com.google.gson;

import com.google.gson.internal.bind.JsonTreeWriter;
import com.google.gson.internal.bind.JsonTreeReader;
import com.google.gson.stream.JsonReader;
import com.google.gson.stream.JsonToken;
import com.google.gson.stream.JsonWriter;
import java.io.IOException;
import java.io.Reader;
import java.io.StringReader;
import java.io.StringWriter;
import java.io.Writer;

/**
 * Converts Java objects to and from JSON.
 *
 * 

Defining a type's JSON form

* By default Gson converts application classes to JSON using its built-in type * adapters. If Gson's default JSON conversion isn't appropriate for a type, * extend this class to customize the conversion. Here's an example of a type * adapter for an (X,Y) coordinate point:
   {@code
 *
 *   public class PointAdapter extends TypeAdapter {
 *     public Point read(JsonReader reader) throws IOException {
 *       if (reader.peek() == JsonToken.NULL) {
 *         reader.nextNull();
 *         return null;
 *       }
 *       String xy = reader.nextString();
 *       String[] parts = xy.split(",");
 *       int x = Integer.parseInt(parts[0]);
 *       int y = Integer.parseInt(parts[1]);
 *       return new Point(x, y);
 *     }
 *     public void write(JsonWriter writer, Point value) throws IOException {
 *       if (value == null) {
 *         writer.nullValue();
 *         return;
 *       }
 *       String xy = value.getX() + "," + value.getY();
 *       writer.value(xy);
 *     }
 *   }}
* With this type adapter installed, Gson will convert {@code Points} to JSON as * strings like {@code "5,8"} rather than objects like {@code {"x":5,"y":8}}. In * this case the type adapter binds a rich Java class to a compact JSON value. * *

The {@link #read(JsonReader) read()} method must read exactly one value * and {@link #write(JsonWriter,Object) write()} must write exactly one value. * For primitive types this is means readers should make exactly one call to * {@code nextBoolean()}, {@code nextDouble()}, {@code nextInt()}, {@code * nextLong()}, {@code nextString()} or {@code nextNull()}. Writers should make * exactly one call to one of value() or nullValue(). * For arrays, type adapters should start with a call to {@code beginArray()}, * convert all elements, and finish with a call to {@code endArray()}. For * objects, they should start with {@code beginObject()}, convert the object, * and finish with {@code endObject()}. Failing to convert a value or converting * too many values may cause the application to crash. * *

Type adapters should be prepared to read null from the stream and write it * to the stream. Alternatively, they should use {@link #nullSafe()} method while * registering the type adapter with Gson. If your {@code Gson} instance * has been configured to {@link GsonBuilder#serializeNulls()}, these nulls will be * written to the final document. Otherwise the value (and the corresponding name * when writing to a JSON object) will be omitted automatically. In either case * your type adapter must handle null. * *

To use a custom type adapter with Gson, you must register it with a * {@link GsonBuilder}:

   {@code
 *
 *   GsonBuilder builder = new GsonBuilder();
 *   builder.registerTypeAdapter(Point.class, new PointAdapter());
 *   // if PointAdapter didn't check for nulls in its read/write methods, you should instead use
 *   // builder.registerTypeAdapter(Point.class, new PointAdapter().nullSafe());
 *   ...
 *   Gson gson = builder.create();
 * }
* * @since 2.1 */ // non-Javadoc: // //

JSON Conversion

//

A type adapter registered with Gson is automatically invoked while serializing // or deserializing JSON. However, you can also use type adapters directly to serialize // and deserialize JSON. Here is an example for deserialization:

   {@code
//
//   String json = "{'origin':'0,0','points':['1,2','3,4']}";
//   TypeAdapter graphAdapter = gson.getAdapter(Graph.class);
//   Graph graph = graphAdapter.fromJson(json);
// }
// And an example for serialization:
   {@code
//
//   Graph graph = new Graph(...);
//   TypeAdapter graphAdapter = gson.getAdapter(Graph.class);
//   String json = graphAdapter.toJson(graph);
// }
// //

Type adapters are type-specific. For example, a {@code // TypeAdapter} can convert {@code Date} instances to JSON and JSON to // instances of {@code Date}, but cannot convert any other types. // public abstract class TypeAdapter { /** * Writes one JSON value (an array, object, string, number, boolean or null) * for {@code value}. * * @param value the Java object to write. May be null. */ public abstract void write(JsonWriter out, T value) throws IOException; /** * Converts {@code value} to a JSON document and writes it to {@code out}. * Unlike Gson's similar {@link Gson#toJson(JsonElement, Appendable) toJson} * method, this write is strict. Create a {@link * JsonWriter#setLenient(boolean) lenient} {@code JsonWriter} and call * {@link #write(com.google.gson.stream.JsonWriter, Object)} for lenient * writing. * * @param value the Java object to convert. May be null. * @since 2.2 */ public final void toJson(Writer out, T value) throws IOException { JsonWriter writer = new JsonWriter(out); write(writer, value); } /** * This wrapper method is used to make a type adapter null tolerant. In general, a * type adapter is required to handle nulls in write and read methods. Here is how this * is typically done:
*

   {@code
   *
   * Gson gson = new GsonBuilder().registerTypeAdapter(Foo.class,
   *   new TypeAdapter() {
   *     public Foo read(JsonReader in) throws IOException {
   *       if (in.peek() == JsonToken.NULL) {
   *         in.nextNull();
   *         return null;
   *       }
   *       // read a Foo from in and return it
   *     }
   *     public void write(JsonWriter out, Foo src) throws IOException {
   *       if (src == null) {
   *         out.nullValue();
   *         return;
   *       }
   *       // write src as JSON to out
   *     }
   *   }).create();
   * }
* You can avoid this boilerplate handling of nulls by wrapping your type adapter with * this method. Here is how we will rewrite the above example: *
   {@code
   *
   * Gson gson = new GsonBuilder().registerTypeAdapter(Foo.class,
   *   new TypeAdapter() {
   *     public Foo read(JsonReader in) throws IOException {
   *       // read a Foo from in and return it
   *     }
   *     public void write(JsonWriter out, Foo src) throws IOException {
   *       // write src as JSON to out
   *     }
   *   }.nullSafe()).create();
   * }
* Note that we didn't need to check for nulls in our type adapter after we used nullSafe. */ public final TypeAdapter nullSafe() { return new TypeAdapter() { @Override public void write(JsonWriter out, T value) throws IOException { if (value == null) { out.nullValue(); } else { TypeAdapter.this.write(out, value); } } @Override public T read(JsonReader reader) throws IOException { if (reader.peek() == JsonToken.NULL) { reader.nextNull(); return null; } return TypeAdapter.this.read(reader); } }; } /** * Converts {@code value} to a JSON document. Unlike Gson's similar {@link * Gson#toJson(Object) toJson} method, this write is strict. Create a {@link * JsonWriter#setLenient(boolean) lenient} {@code JsonWriter} and call * {@link #write(com.google.gson.stream.JsonWriter, Object)} for lenient * writing. * * @param value the Java object to convert. May be null. * @since 2.2 */ public final String toJson(T value) { StringWriter stringWriter = new StringWriter(); try { toJson(stringWriter, value); } catch (IOException e) { throw new AssertionError(e); // No I/O writing to a StringWriter. } return stringWriter.toString(); } /** * Converts {@code value} to a JSON tree. * * @param value the Java object to convert. May be null. * @return the converted JSON tree. May be {@link JsonNull}. * @since 2.2 */ public final JsonElement toJsonTree(T value) { try { JsonTreeWriter jsonWriter = new JsonTreeWriter(); write(jsonWriter, value); return jsonWriter.get(); } catch (IOException e) { throw new JsonIOException(e); } } /** * Reads one JSON value (an array, object, string, number, boolean or null) * and converts it to a Java object. Returns the converted object. * * @return the converted Java object. May be null. */ public abstract T read(JsonReader in) throws IOException; /** * Converts the JSON document in {@code in} to a Java object. Unlike Gson's * similar {@link Gson#fromJson(java.io.Reader, Class) fromJson} method, this * read is strict. Create a {@link JsonReader#setLenient(boolean) lenient} * {@code JsonReader} and call {@link #read(JsonReader)} for lenient reading. * * @return the converted Java object. May be null. * @since 2.2 */ public final T fromJson(Reader in) throws IOException { JsonReader reader = new JsonReader(in); return read(reader); } /** * Converts the JSON document in {@code json} to a Java object. Unlike Gson's * similar {@link Gson#fromJson(String, Class) fromJson} method, this read is * strict. Create a {@link JsonReader#setLenient(boolean) lenient} {@code * JsonReader} and call {@link #read(JsonReader)} for lenient reading. * * @return the converted Java object. May be null. * @since 2.2 */ public final T fromJson(String json) throws IOException { return fromJson(new StringReader(json)); } /** * Converts {@code jsonTree} to a Java object. * * @param jsonTree the Java object to convert. May be {@link JsonNull}. * @since 2.2 */ public final T fromJsonTree(JsonElement jsonTree) { try { JsonReader jsonReader = new JsonTreeReader(jsonTree); return read(jsonReader); } catch (IOException e) { throw new JsonIOException(e); } } }