com.google.gson

Class TypeAdapter<T>

java.lang.Object

com.google.gson.TypeAdapter<T>

public abstract class TypeAdapter<T>
extends Object

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:

   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);
     }
   }

With this type adapter installed, Gson will convert Points to JSON as strings like "5,8" rather than objects like {"x":5,"y":8}. In this case the type adapter binds a rich Java class to a compact JSON value.

The read() method must read exactly one value and write() must write exactly one value. For primitive types this is means readers should make exactly one call to nextBoolean(), nextDouble(), nextInt(), nextLong(), nextString() or nextNull(). Writers should make exactly one call to one of value() or nullValue(). For arrays, type adapters should start with a call to beginArray(), convert all elements, and finish with a call to endArray(). For objects, they should start with beginObject(), convert the object, and finish with 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 nullSafe() method while registering the type adapter with Gson. If your Gson instance has been configured to 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 GsonBuilder:

   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

Constructor Summary

Constructors

Constructor and Description
TypeAdapter()

Method Summary

Methods

Modifier and Type	Method and Description
T	fromJson(Reader in) Converts the JSON document in in to a Java object.
T	fromJson(String json) Converts the JSON document in json to a Java object.
T	fromJsonTree(JsonElement jsonTree) Converts jsonTree to a Java object.
TypeAdapter<T>	nullSafe() This wrapper method is used to make a type adapter null tolerant.
abstract T	read(JsonReader in) Reads one JSON value (an array, object, string, number, boolean or null) and converts it to a Java object.
String	toJson(T value) Converts value to a JSON document.
void	toJson(Writer out, T value) Converts value to a JSON document and writes it to out.
JsonElement	toJsonTree(T value) Converts value to a JSON tree.
abstract void	write(JsonWriter out, T value) Writes one JSON value (an array, object, string, number, boolean or null) for value.

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Constructor Detail

TypeAdapter

public TypeAdapter()

Method Detail

write

public abstract void write(JsonWriter out,
         T value)
                    throws IOException

Writes one JSON value (an array, object, string, number, boolean or null) for value.

Parameters:

value - the Java object to write. May be null.

Throws:

IOException

toJson

public final void toJson(Writer out,
          T value)
                  throws IOException

Converts value to a JSON document and writes it to out. Unlike Gson's similar toJson method, this write is strict. Create a lenient JsonWriter and call write(com.google.gson.stream.JsonWriter, Object) for lenient writing.

Parameters:

value - the Java object to convert. May be null.

Throws:

IOException

Since:

2.2

nullSafe

public final TypeAdapter<T> nullSafe()

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:

   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();
 

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:

   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();
 

Note that we didn't need to check for nulls in our type adapter after we used nullSafe.

toJson

public final String toJson(T value)

Converts value to a JSON document. Unlike Gson's similar toJson method, this write is strict. Create a lenient JsonWriter and call write(com.google.gson.stream.JsonWriter, Object) for lenient writing.

Parameters:

value - the Java object to convert. May be null.

Since:

2.2

toJsonTree

public final JsonElement toJsonTree(T value)

Converts value to a JSON tree.

Parameters:

value - the Java object to convert. May be null.

Returns:

the converted JSON tree. May be JsonNull.

Since:

2.2

read

public abstract T read(JsonReader in)
                throws IOException

Reads one JSON value (an array, object, string, number, boolean or null) and converts it to a Java object. Returns the converted object.

Returns:

the converted Java object. May be null.

Throws:

IOException

fromJson

public final T fromJson(Reader in)
                 throws IOException

Converts the JSON document in in to a Java object. Unlike Gson's similar fromJson method, this read is strict. Create a lenient JsonReader and call read(JsonReader) for lenient reading.

Returns:

the converted Java object. May be null.

Throws:

IOException

Since:

2.2

fromJson

public final T fromJson(String json)
                 throws IOException

Converts the JSON document in json to a Java object. Unlike Gson's similar fromJson method, this read is strict. Create a lenient JsonReader and call read(JsonReader) for lenient reading.

Returns:

the converted Java object. May be null.

Throws:

IOException

Since:

2.2

fromJsonTree

public final T fromJsonTree(JsonElement jsonTree)

Converts jsonTree to a Java object.

Parameters:

jsonTree - the Java object to convert. May be JsonNull.

Since:

2.2