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+/*
+ * 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);
+}
projects / unical.git / blobdiff