/*
* 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.reflect.TypeToken;
/**
* Creates type adapters for set of related types. Type adapter factories are most useful when
* several types share similar structure in their JSON form.
*
* <h3>Example: Converting enums to lowercase</h3>
* 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
* {@code CONSTANT_CASE} in the corresponding Java model:
* <pre> {@code
*
* 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);
* }
* }
* }</pre>
*
* <p>
* 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 {@link #create}. Factories
* should expect {@code
* 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 {@code create()} where {@code type} is
* not an enum.
*
* <p>
* 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 {@code create()} so that the
* type adapter's {@code
* read()} and {@code write()} methods can be very fast. In this example the mapping from lowercase
* name to enum value is computed eagerly.
*
* <p>
* As with type adapters, factories must be <i>registered</i> with a {@link
* com.google.gson.GsonBuilder} for them to take effect:
* <pre> {@code
*
* GsonBuilder builder = new GsonBuilder();
* builder.registerTypeAdapterFactory(new LowercaseEnumTypeAdapterFactory());
* ...
* Gson gson = builder.create();
* }</pre> If multiple factories support the same type, the factory registered earlier takes
* precedence.
*
* <h3>Example: composing other type adapters</h3>
* In this example we implement a factory for Guava's {@code Multiset} collection type. The factory
* can be used to create type adapters for multisets of any element type: the type adapter for {@code
* Multiset<String>} is different from the type adapter for {@code
* Multiset<URL>}.
*
* <p>
* The type adapter <i>delegates</i> to another type adapter for the multiset elements. It figures
* out the element type by reflecting on the multiset's type token. A {@code Gson} is passed in to
* {@code create} for just this purpose:
* <pre> {@code
*
* 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;
* }
* };
* }
* }
* }</pre> 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 {@code create()} method; this ensures potentially-expensive type
* adapter creation happens only once.
*
* @since 2.1
*/
public interface TypeAdapterFactory {
/**
* Returns a type adapter for {@code type}, or null if this factory doesn't support
* {@code type}.
*/
<T> TypeAdapter<T> create(Gson gson, TypeToken<T> type);
}