Json解析一直是网络通信中重要的数据解析框架。而Gson和FastJson是最为常用的两个Json解析框架。这一章节主要介绍Gson的原理。Gson的一个典型使用例子如下：

        String jsonString = "{\"name\":\"renyiguang\"}";
        Gson gson = new Gson();
        TestModel testModel = gson.fromJson(jsonString, TestModel.class);

Gson的fromJson()方法源码如下：

  public <T> T fromJson(String json, Class<T> classOfT) throws JsonSyntaxException {
    Object object = fromJson(json, (Type) classOfT);
    return Primitives.wrap(classOfT).cast(object);
  }

  public <T> T fromJson(String json, Type typeOfT) throws JsonSyntaxException {
    if (json == null) {
      return null;
    }
    StringReader reader = new StringReader(json);
    T target = (T) fromJson(reader, typeOfT);
    return target;
  }

  public <T> T fromJson(Reader json, Type typeOfT) throws JsonIOException, JsonSyntaxException {
    JsonReader jsonReader = newJsonReader(json);
    T object = (T) fromJson(jsonReader, typeOfT);
    assertFullConsumption(object, jsonReader);
    return object;
  }

  public <T> T fromJson(JsonReader reader, Type typeOfT) throws JsonIOException, JsonSyntaxException {
    boolean isEmpty = true;
    boolean oldLenient = reader.isLenient();
    reader.setLenient(true);
    try {
      reader.peek();
      isEmpty = false;
      TypeToken<T> typeToken = (TypeToken<T>) TypeToken.get(typeOfT);//1获取TypeToken
      TypeAdapter<T> typeAdapter = getAdapter(typeToken);//2获取TypeAdapter
      T object = typeAdapter.read(reader);//3解析
      return object;
    } catch (EOFException e) {
      /*
       * For compatibility with JSON 1.5 and earlier, we return null for empty
       * documents instead of throwing.
       */
      if (isEmpty) {
        return null;
      }
      throw new JsonSyntaxException(e);
    } catch (IllegalStateException e) {
      throw new JsonSyntaxException(e);
    } catch (IOException e) {
      // TODO(inder): Figure out whether it is indeed right to rethrow this as JsonSyntaxException
      throw new JsonSyntaxException(e);
    } finally {
      reader.setLenient(oldLenient);
    }
  }

如上，fromJson()方法主要分三个步骤：1获取TypeToken，2获取TypeAdapter，3利用TypeAdapter解析。
下面就按照这三个步骤分析。
（1）TypeToken
TypeToken的get()方法源码如下：

  public static TypeToken<?> get(Type type) {
    return new TypeToken<Object>(type);
  }

  TypeToken(Type type) {
    this.type = $Gson$Types.canonicalize($Gson$Preconditions.checkNotNull(type));
    this.rawType = (Class<? super T>) $Gson$Types.getRawType(this.type);
    this.hashCode = this.type.hashCode();
  }

TypeToken保存了待解析数据的Type类型。
（2）TypeAdapter
TypeAdapter是通过getAdapter()方法获取。getAdapter()方法源码如下：

  public <T> TypeAdapter<T> getAdapter(TypeToken<T> type) {
    TypeAdapter<?> cached = typeTokenCache.get(type == null ? NULL_KEY_SURROGATE : type);//缓存获取TypeAdapter
    if (cached != null) {
      return (TypeAdapter<T>) cached;
    }

    Map<TypeToken<?>, FutureTypeAdapter<?>> threadCalls = calls.get();
    boolean requiresThreadLocalCleanup = false;
    if (threadCalls == null) {
      threadCalls = new HashMap<TypeToken<?>, FutureTypeAdapter<?>>();
      calls.set(threadCalls);
      requiresThreadLocalCleanup = true;
    }

    // the key and value type parameters always agree
    FutureTypeAdapter<T> ongoingCall = (FutureTypeAdapter<T>) threadCalls.get(type);
    if (ongoingCall != null) {
      return ongoingCall;
    }

    try {
      FutureTypeAdapter<T> call = new FutureTypeAdapter<T>();
      threadCalls.put(type, call);

      for (TypeAdapterFactory factory : factories) {//从factories中查找
        TypeAdapter<T> candidate = factory.create(this, type);
        if (candidate != null) {
          call.setDelegate(candidate);
          typeTokenCache.put(type, candidate);
          return candidate;
        }
      }
      throw new IllegalArgumentException("GSON cannot handle " + type);
    } finally {
      threadCalls.remove(type);

      if (requiresThreadLocalCleanup) {
        calls.remove();
      }
    }
  }

当typeTokenCache中没有TypeToken对应的TypeAdapter时，就从factories中查找对应的TypeAdapter。factories查找TypeAdapter的标准是TypeAdapterFactory的create()方法返回非空。
factories的初始化在Gson的构造方法中，如下：

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

    List<TypeAdapterFactory> factories = new ArrayList<TypeAdapterFactory>();

    // built-in type adapters that cannot be overridden
    factories.add(TypeAdapters.JSON_ELEMENT_FACTORY);
    factories.add(ObjectTypeAdapter.FACTORY);

    // the excluder must precede all adapters that handle user-defined types
    factories.add(excluder);

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

    // 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);
    TypeAdapter<Number> longAdapter = longAdapter(longSerializationPolicy);
    factories.add(TypeAdapters.newFactory(long.class, Long.class, longAdapter));
    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.ATOMIC_INTEGER_FACTORY);
    factories.add(TypeAdapters.ATOMIC_BOOLEAN_FACTORY);
    factories.add(TypeAdapters.newFactory(AtomicLong.class, atomicLongAdapter(longAdapter)));
    factories.add(TypeAdapters.newFactory(AtomicLongArray.class, atomicLongArrayAdapter(longAdapter)));
    factories.add(TypeAdapters.ATOMIC_INTEGER_ARRAY_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.CURRENCY_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.CLASS_FACTORY);

    // type adapters for composite and user-defined types
    factories.add(new CollectionTypeAdapterFactory(constructorConstructor));
    factories.add(new MapTypeAdapterFactory(constructorConstructor, complexMapKeySerialization));
    this.jsonAdapterFactory = new JsonAdapterAnnotationTypeAdapterFactory(constructorConstructor);
    factories.add(jsonAdapterFactory);
    factories.add(TypeAdapters.ENUM_FACTORY);
    factories.add(new ReflectiveTypeAdapterFactory(
        constructorConstructor, fieldNamingStrategy, excluder, jsonAdapterFactory));

    this.factories = Collections.unmodifiableList(factories);
  }

如上factories里面包含了Gson能解析的所有的数据格式。下面以STRING_FACTORY为例来说明。

  public static final TypeAdapterFactory STRING_FACTORY = newFactory(String.class, STRING);

  public static <TT> TypeAdapterFactory newFactory(
      final Class<TT> type, final TypeAdapter<TT> typeAdapter) {
    return new TypeAdapterFactory() {
      @SuppressWarnings("unchecked") // we use a runtime check to make sure the 'T's equal
      @Override public <T> TypeAdapter<T> create(Gson gson, TypeToken<T> typeToken) {
        return typeToken.getRawType() == type ? (TypeAdapter<T>) typeAdapter : null;
      }
      @Override public String toString() {
        return "Factory[type=" + type.getName() + ",adapter=" + typeAdapter + "]";
      }
    };
  }

newFactory()方法返回的TypeAdapterFactory的create()方法就做了一件事，验证type的一致性。即如果需要处理的数据类型为String，则返回TypeAdapter，否则返回null。String类型对应的TypeAdapter源码如下：

  public static final TypeAdapter<String> STRING = new TypeAdapter<String>() {
    @Override
    public String read(JsonReader in) throws IOException {
      JsonToken peek = in.peek();
      if (peek == JsonToken.NULL) {
        in.nextNull();
        return null;
      }
      /* coerce booleans to strings for backwards compatibility */
      if (peek == JsonToken.BOOLEAN) {
        return Boolean.toString(in.nextBoolean());
      }
      return in.nextString();
    }
    @Override
    public void write(JsonWriter out, String value) throws IOException {
      out.value(value);
    }
  };

关于Gson解析，简单的流程已经介绍完毕。下面就从解析的角度注重分析。