Gson之TypeAdapterFactory

在Gson之TypeAdapter的工作原理分析(1) 中分析了一下TypeAdapter的原理：先把TypeAdapter封装成TypeAdapterFactory然后通过TypeAdapterFactory的create创建TypeAdapter使用。在Gson中其实充斥了大量的TypeAdapterFactory,比如基本类型的TypeAdapter都有一个TypeAdapterFactory：

  public static final TypeAdapterFactory BYTE_FACTORY
      = newFactory(byte.class, Byte.class, BYTE);

 public static final TypeAdapterFactory BOOLEAN_FACTORY
      = newFactory(boolean.class, Boolean.class, BOOLEAN);

 public static final TypeAdapterFactory SHORT_FACTORY
      = newFactory(short.class, Short.class, SHORT);
    //等等，还有其他的一些TypeAdapterFactory就不一一列举

 public static <TT> TypeAdapterFactory newFactory(final Class<TT> unboxed, final Class<TT> boxed, final  TypeAdapter<? super TT> typeAdapter) {
    return new 
    () {
      @SuppressWarnings("unchecked") 
      //TypeAdapterFactory接口的一定义的唯一的一个方法
      public <T> TypeAdapter<T> create(Gson gson, TypeToken<T> typeToken) {
        Class<? super T> rawType = typeToken.getRawType();
        return (rawType == unboxed || rawType == boxed) ? (TypeAdapter<T>) typeAdapter : null;
      }

    };
  }

我们在自定义TypeAdapter的时候通过注册使用实际上就是先转换成TypeAdapterFactory然后在Gson的getAdapter方法中遍历TyperAdapterFactory，并调用create方法然后使用我们的TypeAdapter。能不能直接传我们自定义的TypeAdapterFactory呢？答案是确定的，因为GsonBuilder正好提供了这个方法：

  public GsonBuilder registerTypeAdapterFactory(TypeAdapterFactory factory) {
    factories.add(factory);
    return this;
  }

其实在我们用new Gson()这个Gson默认构造器的时候，其实调用了另外一个带参数的构造器进行初始化：

public Gson() {
    this(Excluder.DEFAULT, FieldNamingPolicy.IDENTITY,
        Collections.<Type, InstanceCreator<?>>emptyMap(), false, false, DEFAULT_JSON_NON_EXECUTABLE,
        true, false, false, LongSerializationPolicy.DEFAULT,
        Collections.<TypeAdapterFactory>emptyList());
  }

带有参数的构造器的的其中一个作用就是为Gson对象的List factories 添加大量的Gson自定义的TypeAdapterFactory:

 factories.addAll(typeAdapterFactories);

    // type adapters for basic platform types
    factories.add(TypeAdapters.STRING_FACTORY);
    factories.add(TypeAdapters.INTEGER_FACTORY);
    factories.add(TypeAdapters.BOOLEAN_FACTORY);
    factories.add(TypeAdapters.BYTE_FACTORY);
    factories.add(TypeAdapters.SHORT_FACTORY);
    factories.add(TypeAdapters.newFactory(long.class, Long.class,
            longAdapter(longSerializationPolicy)));
    factories.add(TypeAdapters.newFactory(double.class, Double.class,
            doubleAdapter(serializeSpecialFloatingPointValues)));
    factories.add(TypeAdapters.newFactory(float.class, Float.class,
            floatAdapter(serializeSpecialFloatingPointValues)));
    factories.add(TypeAdapters.NUMBER_FACTORY);
    factories.add(TypeAdapters.CHARACTER_FACTORY);
    factories.add(TypeAdapters.STRING_BUILDER_FACTORY);
    factories.add(TypeAdapters.STRING_BUFFER_FACTORY);
    factories.add(TypeAdapters.newFactory(BigDecimal.class, TypeAdapters.BIG_DECIMAL));
    factories.add(TypeAdapters.newFactory(BigInteger.class, TypeAdapters.BIG_INTEGER));
    factories.add(TypeAdapters.URL_FACTORY);
    factories.add(TypeAdapters.URI_FACTORY);
    factories.add(TypeAdapters.UUID_FACTORY);
    factories.add(TypeAdapters.LOCALE_FACTORY);
    factories.add(TypeAdapters.INET_ADDRESS_FACTORY);
    factories.add(TypeAdapters.BIT_SET_FACTORY);
    factories.add(DateTypeAdapter.FACTORY);
    factories.add(TypeAdapters.CALENDAR_FACTORY);
    factories.add(TimeTypeAdapter.FACTORY);
    factories.add(SqlDateTypeAdapter.FACTORY);
    factories.add(TypeAdapters.TIMESTAMP_FACTORY);
    factories.add(ArrayTypeAdapter.FACTORY);
    factories.add(TypeAdapters.ENUM_FACTORY);
    factories.add(TypeAdapters.CLASS_FACTORY);

    // type adapters for composite and user-defined types
    factories.add(new CollectionTypeAdapterFactory(constructorConstructor));
    factories.add(new MapTypeAdapterFactory(constructorConstructor, complexMapKeySerialization));
    factories.add(new ReflectiveTypeAdapterFactory(
        constructorConstructor, fieldNamingPolicy, excluder));

    this.factories = Collections.unmodifiableList(factories);

其实你也许有个小疑问，既然自己定义了TypeAdapter为什么还需要添加上面这些Factory,为此我专门修改了一下Gson的构造器：

 public Gson(final Excluder excluder, final FieldNamingStrategy fieldNamingPolicy,
          final Map<Type, InstanceCreator<?>> instanceCreators, boolean serializeNulls,
          boolean complexMapKeySerialization, boolean generateNonExecutableGson, boolean htmlSafe,
          boolean prettyPrinting, boolean serializeSpecialFloatingPointValues,
          LongSerializationPolicy longSerializationPolicy,
          List<TypeAdapterFactory> typeAdapterFactories,boolean myGosn) {
        this.constructorConstructor = new ConstructorConstructor(instanceCreators);
        this.serializeNulls = serializeNulls;
        this.generateNonExecutableJson = generateNonExecutableGson;
        this.htmlSafe = htmlSafe;
        this.prettyPrinting = prettyPrinting;

        List<TypeAdapterFactory> factories = new ArrayList<TypeAdapterFactory>();
        //省略了上面添加Gson自定的Factory的方法

        //直接添加自己的TypeAdapter封装的TypeFactory
        factories.addAll(typeAdapterFactories);
        this.factories = Collections.unmodifiableList(factories);


  }

通过这种方式，也是可以直接调用你的TypeAdapter来完成你的任务的。具体为什么在GsonBuilder注册过自定义的Adapter的时候，还需要添加这些Gson自己的TypeAdapterFactory？关于这个问题在下一篇博客中将详细分析说明。

这篇就写到这儿，最后写一个自定义的TypeAdapter来解析一下一个json串（来源）：

{
  "isbn": "978-0321336781",
  "title": "Java Puzzlers: Traps, Pitfalls, and Corner Cases",
  "authors": [ { "id": 1, "name": "Joshua Bloch" }, { "id": 2, "name": "Neal Gafter" } ] }

对应的JavaBean:

Book {

   private Author[] authors;
   private String isbn;
   private String title;
   //getter and setter方法
}
Author {
    private int id;
    private String name;
     //getter and setter方法
}

对应的TypeAdapter:

public class BookTypeAdapter extends TypeAdapter<Book> { @Override public Book read(final JsonReader in) throws IOException { final Book book = new Book(); in.beginObject(); while(in.hasNext()) {//遍历当前的jsonObject switch (in.nextName()) { case "isbn": book.setIsbn(in.nextString()); break; case "title": book.setTitle(in.nextString()); break; case "authors": final List<Author> list = new ArrayList<Author>(); in.beginArray(); while(in.hasNext()) {//这个是遍历json数组中是否还有下一个jsonObject对象 in.beginObject(); final Author author = new Author(); //这个循环是遍历当前JsonObject串的内容 while(in.hasNext()) { String authorObjNextName = in.nextName(); switch (authorObjNextName) { case "id": int id = in.nextInt(); author.setId(id); break; case "name": String name = in.nextString(); author.setName(name); break; } }//遍历完当前jsonObject对象 list.add(author); in.endObject(); }//循环遍历完当前的array book.setAuthors(list.toArray(new Author[list.size()])); in.endArray(); break; } } in.endObject(); return book; } @Override public void write(final JsonWriter out, final Book2 book) throws IOException { out.beginObject(); out.name("isbn").value(book.getIsbn()); out.name("title").value(book.getTitle()); out.name("authors").beginArray(); for(Author author:book.getAuthors()) { //开始写对象 out.beginObject(); out.name("id").value(author.getId()); out.name("name").value(author.getName()); out.endObject(); } out.endArray(); out.endObject(); } @Override public String toString() { return "BookTypeAdapter4"; } }