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

Annotation Type JsonAdapter

@Retention(RUNTIME) @Target({TYPE,FIELD}) public @interface JsonAdapter
An annotation that indicates the Gson TypeAdapter to use with a class or field.

Here is an example of how this annotation is used:

 @JsonAdapter(UserJsonAdapter.class)
 public class User {
   public final String firstName, lastName;
   private User(String firstName, String lastName) {
     this.firstName = firstName;
     this.lastName = lastName;
   }
 }
 public class UserJsonAdapter extends TypeAdapter<User> {
   @Override public void write(JsonWriter out, User user) throws IOException {
     // implement write: combine firstName and lastName into name
     out.beginObject();
     out.name("name");
     out.value(user.firstName + " " + user.lastName);
     out.endObject();
     // implement the write method
   }
   @Override public User read(JsonReader in) throws IOException {
     // implement read: split name into firstName and lastName
     in.beginObject();
     in.nextName();
     String[] nameParts = in.nextString().split(" ");
     in.endObject();
     return new User(nameParts[0], nameParts[1]);
   }
 }
Since User class specified UserJsonAdapter.class in @JsonAdapter annotation, it will automatically be invoked to serialize/deserialize User instances.

Here is an example of how to apply this annotation to a field. 

 private static final class Gadget {
   @JsonAdapter(UserJsonAdapter2.class)
   final User user;
   Gadget(User user) {
     this.user = user;
   }
 }
It's possible to specify different type adapters on a field, that field's type, and in the GsonBuilder. Field annotations take precedence over GsonBuilder-registered type adapters, which in turn take precedence over annotated types.

The class referenced by this annotation must be either a TypeAdapter or a TypeAdapterFactory, or must implement one or both of JsonDeserializer or JsonSerializer. Using TypeAdapterFactory makes it possible to delegate to the enclosing Gson instance.

Gson instances might cache the adapter they create for a @JsonAdapter annotation. It is not guaranteed that a new adapter is created every time the annotated class or field is serialized or deserialized.

Since:
2.3
Author:
Inderjeet Singh, Joel Leitch, Jesse Wilson

  • Required Element Summary

    Required Elements
    Modifier and Type
    Required Element
    Description
    Class<?>
    value
    Either a TypeAdapter or TypeAdapterFactory, or one or both of JsonDeserializer or JsonSerializer.

  • Optional Element Summary

    Optional Elements
    Modifier and Type
    Optional Element
    Description
    boolean
    nullSafe
    false, to be able to handle null values within the adapter, default value is true.