com.google.gson

Interface TypeAdapterFactory

public interface TypeAdapterFactory

Creates type adapters for set of related types. Type adapter factories are most useful when several types share similar structure in their JSON form.

Example: Converting enums to lowercase

In this example, we implement a factory that creates type adapters for all enums. The type adapters will write enums in lowercase, despite the fact that they're defined in CONSTANT_CASE in the corresponding Java model:

   public class LowercaseEnumTypeAdapterFactory implements TypeAdapter.Factory {
     public <T> TypeAdapter<T> create(Gson gson, TypeToken<T> type) {
       Class<T> rawType = (Class<T>) type.getRawType();
       if (!rawType.isEnum()) {
         return null;
       }

       final Map<String, T> lowercaseToConstant = new HashMap<String, T>();
       for (T constant : rawType.getEnumConstants()) {
         lowercaseToConstant.put(toLowercase(constant), constant);
       }

       return new TypeAdapter<T>() {
         public void write(JsonWriter out, T value) throws IOException {
           if (value == null) {
             out.nullValue();
           } else {
             out.value(toLowercase(value));
           }
         }

         public T read(JsonReader reader) throws IOException {
           if (reader.peek() == JsonToken.NULL) {
             reader.nextNull();
             return null;
           } else {
             return lowercaseToConstant.get(reader.nextString());
           }
         }
       };
     }

     private String toLowercase(Object o) {
       return o.toString().toLowerCase(Locale.US);
     }
   }
 

Type adapter factories select which types they provide type adapters for. If a factory cannot support a given type, it must return null when that type is passed to create(com.google.gson.Gson, com.google.gson.reflect.TypeToken<T>). Factories should expect create() to be called on them for many types and should return null for most of those types. In the above example the factory returns null for calls to create() where type is not an enum.

A factory is typically called once per type, but the returned type adapter may be used many times. It is most efficient to do expensive work like reflection in create() so that the type adapter's read() and write() methods can be very fast. In this example the mapping from lowercase name to enum value is computed eagerly.

As with type adapters, factories must be registered with a GsonBuilder for them to take effect:

   GsonBuilder builder = new GsonBuilder();
  builder.registerTypeAdapterFactory(new LowercaseEnumTypeAdapterFactory());
  ...
  Gson gson = builder.create();
 

If multiple factories support the same type, the factory registered earlier takes precedence.

Example: composing other type adapters

In this example we implement a factory for Guava's Multiset collection type. The factory can be used to create type adapters for multisets of any element type: the type adapter for Multiset<String> is different from the type adapter for Multiset<URL>.

The type adapter delegates to another type adapter for the multiset elements. It figures out the element type by reflecting on the multiset's type token. A Gson is passed in to create for just this purpose:

   public class MultisetTypeAdapterFactory implements TypeAdapter.Factory {
     public <T> TypeAdapter<T> create(Gson gson, TypeToken<T> typeToken) {
       Type type = typeToken.getType();
       if (typeToken.getRawType() != Multiset.class
           || !(type instanceof ParameterizedType)) {
         return null;
       }

       Type elementType = ((ParameterizedType) type).getActualTypeArguments()[0];
       TypeAdapter<?> elementAdapter = gson.getAdapter(TypeToken.get(elementType));
       return (TypeAdapter<T>) newMultisetAdapter(elementAdapter);
     }

     private <E> TypeAdapter<Multiset<E>> newMultisetAdapter(
         final TypeAdapter<E> elementAdapter) {
       return new TypeAdapter<Multiset<E>>() {
         public void write(JsonWriter out, Multiset<E> value) throws IOException {
           if (value == null) {
             out.nullValue();
             return;
           }

           out.beginArray();
           for (Multiset.Entry<E> entry : value.entrySet()) {
             out.value(entry.getCount());
             elementAdapter.write(out, entry.getElement());
           }
           out.endArray();
         }

         public Multiset<E> read(JsonReader in) throws IOException {
           if (in.peek() == JsonToken.NULL) {
             in.nextNull();
             return null;
           }

           Multiset<E> result = LinkedHashMultiset.create();
           in.beginArray();
           while (in.hasNext()) {
             int count = in.nextInt();
             E element = elementAdapter.read(in);
             result.add(element, count);
           }
           in.endArray();
           return result;
         }
       };
     }
   }
 

Delegating from one type adapter to another is extremely powerful; it's the foundation of how Gson converts Java objects and collections. Whenever possible your factory should retrieve its delegate type adapter in the create() method; this ensures potentially-expensive type adapter creation happens only once.

Since:

2.1

Method Summary

Methods

Modifier and Type	Method and Description
<T> TypeAdapter<T>	create(Gson gson, TypeToken<T> type) Returns a type adapter for type, or null if this factory doesn't support type.

Method Detail

create

<T> TypeAdapter<T> create(Gson gson,
                        TypeToken<T> type)

Returns a type adapter for type, or null if this factory doesn't support type.