Gson解析原理概述

通过前几篇的关于Gson的博客我们知道Gson的解析原理：
1）如果是通过GsonBuilder创建的Gson对象，那么就用自定义的TyperAdapter来完成json的解析
2）如果是通过new Gson()创建的Gson对象，那么就用Java反射机制来完成json的解析。
那么Gson在是怎么判断是上述两种方式的哪一种呢？查看Gson源码，并没有提供一个特殊的字段来辨别？
其实很简单，通过Gson的factories(List) 这个对象add TypeAdapterFactory的顺序来控制的！当是用第一种方式来创建Gson对象的时候，把自定一个TypeAdapter或者TypeAdapterFactory添加在factories靠前的位置，然后在getAdapter方法中遍历factories的每一个TypeAdapterFactory，如果当前循环中的TypeAdapterFactory.create 返回的TypeAdapter！=null,就返回该Adapter退出了factories的遍历.因为自定义的TypeAdapter位置在factories中靠前这样就优先与ReflectiveTypeAdapterFactory 遍历到，所以就避免了用Gson的反射机制来解析json了。
Gson的有参构造器中的代码有如下添加TypeAdapterFactory的方法：

    // user's type adapters
    //优先添加自定义的TypeAdapter
    factories.addAll(typeAdapterFactories);

    // type adapters for basic platform types
    //添加Gson自带的TypeAdapterFactory
    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
    //到最后才把反射机制用的Factory添加到factories中
    factories.add(new CollectionTypeAdapterFactory(constructorConstructor));
    factories.add(new MapTypeAdapterFactory(constructorConstructor, complexMapKeySerialization));
    factories.add(new ReflectiveTypeAdapterFactory(
        constructorConstructor, fieldNamingPolicy, excluder));

    this.factories = Collections.unmodifiableList(factories);

getAAdpater遍历factories的代码如下：

 for (TypeAdapterFactory factory : factories) {
        TypeAdapter<T> candidate = factory.create(this, type);
        /** 如果是自定义的TypeAdapter就优先返回，否则就返回了 *ReflectiveTypeAdapterFactory **/
        if (candidate != null) {        
          call.setDelegate(candidate);
          typeTokenCache.put(type, candidate);
          return candidate;
        }
      }

因此Gson的整体解析原理可以用如下图来概述：

至于Gson的typeAdapter以及反射解析的具体，请参考之前写的博客