Module com.google.gson
Package com.google.gson

Class 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.

    Type adapters should be stateless and thread-safe, otherwise the thread-safety guarantees of Gson might not apply.

    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 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.

        A JsonWriter with default configuration is used for writing the JSON data. To customize this behavior, create a JsonWriter, configure it and then use write(JsonWriter, Object) instead.

        Parameters:
        value - the Java object to convert. May be null.
        Throws:
        IOException
        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.
        Throws:
        JsonIOException - wrapping IOExceptions thrown by write(JsonWriter, Object)
        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.

        A JsonReader with default configuration (that is with Strictness.LEGACY_STRICT as strictness) is used for reading the JSON data. To customize this behavior, create a JsonReader, configure it and then use read(JsonReader) instead.

        No exception is thrown if the JSON data has multiple top-level JSON elements, or if there is trailing data.

        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.

        A JsonReader with default configuration (that is with Strictness.LEGACY_STRICT as strictness) is used for reading the JSON data. To customize this behavior, create a JsonReader, configure it and then use read(JsonReader) instead.

        No exception is thrown if the JSON data has multiple top-level JSON elements, or if there is trailing data.

        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 JSON element to convert. May be JsonNull.
        Returns:
        the converted Java object. May be null.
        Throws:
        JsonIOException - wrapping IOExceptions thrown by read(JsonReader)
        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.