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.JsonTreeReader;
import com.google.gson.internal.bind.JsonTreeWriter;
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.
*
* <h2>Defining a type's JSON form</h2>
*
* 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:
*
* <pre>{@code
* public class PointAdapter extends TypeAdapter<Point> {
* 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);
* }
* }
* }</pre>
*
* 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.
*
* <p>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 {@code value()} or {@code 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.
*
* <p>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.
*
* <p>Type adapters should be stateless and thread-safe, otherwise the thread-safety guarantees of
* {@link Gson} might not apply.
*
* <p>To use a custom type adapter with Gson, you must <i>register</i> it with a {@link
* GsonBuilder}:
*
* <pre>{@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();
* }</pre>
*
* @since 2.1
*/
// non-Javadoc:
//
// <h2>JSON Conversion</h2>
// <p>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: <pre>{@code
// String json = "{'origin':'0,0','points':['1,2','3,4']}";
// TypeAdapter<Graph> graphAdapter = gson.getAdapter(Graph.class);
// Graph graph = graphAdapter.fromJson(json);
// }</pre>
// And an example for serialization: <pre>{@code
// Graph graph = new Graph(...);
// TypeAdapter<Graph> graphAdapter = gson.getAdapter(Graph.class);
// String json = graphAdapter.toJson(graph);
// }</pre>
//
// <p>Type adapters are <strong>type-specific</strong>. For example, a {@code
// TypeAdapter<Date>} can convert {@code Date} instances to JSON and JSON to
// instances of {@code Date}, but cannot convert any other types.
//
public abstract class TypeAdapter<T> {
public 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}.
*
* <p>A {@link JsonWriter} with default configuration is used for writing the JSON data. To
* customize this behavior, create a {@link JsonWriter}, configure it and then use {@link
* #write(JsonWriter, Object)} instead.
*
* @param value the Java object to convert. May be {@code null}.
* @since 2.2
*/
public final void toJson(Writer out, T value) throws IOException {
JsonWriter writer = new JsonWriter(out);
write(writer, value);
}
/**
* Converts {@code value} to a JSON document.
*
* <p>A {@link JsonWriter} with default configuration is used for writing the JSON data. To
* customize this behavior, create a {@link JsonWriter}, configure it and then use {@link
* #write(JsonWriter, Object)} instead.
*
* @throws JsonIOException wrapping {@code IOException}s thrown by {@link #write(JsonWriter,
* Object)}
* @param value the Java object to convert. May be {@code null}.
* @since 2.2
*/
public final String toJson(T value) {
StringWriter stringWriter = new StringWriter();
try {
toJson(stringWriter, value);
} catch (IOException e) {
throw new JsonIOException(e);
}
return stringWriter.toString();
}
/**
* Converts {@code value} to a JSON tree.
*
* @param value the Java object to convert. May be {@code null}.
* @return the converted JSON tree. May be {@link JsonNull}.
* @throws JsonIOException wrapping {@code IOException}s thrown by {@link #write(JsonWriter,
* Object)}
* @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 {@code null}.
*/
public abstract T read(JsonReader in) throws IOException;
/**
* Converts the JSON document in {@code in} to a Java object.
*
* <p>A {@link JsonReader} with default configuration (that is with {@link
* Strictness#LEGACY_STRICT} as strictness) is used for reading the JSON data. To customize this
* behavior, create a {@link JsonReader}, configure it and then use {@link #read(JsonReader)}
* instead.
*
* <p>No exception is thrown if the JSON data has multiple top-level JSON elements, or if there is
* trailing data.
*
* @return the converted Java object. May be {@code 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.
*
* <p>A {@link JsonReader} with default configuration (that is with {@link
* Strictness#LEGACY_STRICT} as strictness) is used for reading the JSON data. To customize this
* behavior, create a {@link JsonReader}, configure it and then use {@link #read(JsonReader)}
* instead.
*
* <p>No exception is thrown if the JSON data has multiple top-level JSON elements, or if there is
* trailing data.
*
* @return the converted Java object. May be {@code 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 JSON element to convert. May be {@link JsonNull}.
* @return the converted Java object. May be {@code null}.
* @throws JsonIOException wrapping {@code IOException}s thrown by {@link #read(JsonReader)}
* @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);
}
}
/**
* 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:<br>
*
* <pre>{@code
* Gson gson = new GsonBuilder().registerTypeAdapter(Foo.class,
* new TypeAdapter<Foo>() {
* 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();
* }</pre>
*
* 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:
*
* <pre>{@code
* Gson gson = new GsonBuilder().registerTypeAdapter(Foo.class,
* new TypeAdapter<Foo>() {
* 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();
* }</pre>
*
* Note that we didn't need to check for nulls in our type adapter after we used nullSafe.
*/
public final TypeAdapter<T> nullSafe() {
return new TypeAdapter<T>() {
@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);
}
};
}
}