GSON源码解析（一）

把之前做的笔记又重新整理了一下，发现简单只看了GSON的解析流程；

1、GSON的两种解析使用方式：

1）直接利用GSON中默认的反射机制来解析：

Gson gson1 = new Gson();
gson1.fromJson(str, Data.class);

2）利用自定义的TypeAdapter来解析：

// Builder模式
GsonBuilder gsonBuilder = new GsonBuilder();
// 注册自定义的Adapter;
gsonBuilder.registerTypeAdapter(Data.class, new TypeAdapter<Data>() {
    // 由对象组装成json
    @Override
    public void write(JsonWriter out, Data value) throws IOException {
    }

    // 解析成对应类对象
    @Override
    public Data read(JsonReader in) throws IOException {
        return null;
    }
}.nullSafe());
gson = gsonBuilder.create();

两种方式的区别前者主要使用 ReflectiveTypeAdapterFactory通过反射机制来getField，setField；后者自定义了一个TypeAdapter，重写了read方法，该方法会根据自己定义返回一个解析好的类对象，因此可以自己控制解析的输出；

2、首先来看GSON是怎样区别使用两种解析TypeAdapter的：

GsonBuilder#registerTypeAdapter:

private final List<TypeAdapterFactory> factories = new ArrayList<TypeAdapterFactory>();
// GSONBuilder.java
public GsonBuilder registerTypeAdapter(Type type, Object typeAdapter) {
    $Gson$Preconditions.checkArgument(typeAdapter instanceof JsonSerializer<?>
            || typeAdapter instanceof JsonDeserializer<?>
            || typeAdapter instanceof InstanceCreator<?>
            || typeAdapter instanceof TypeAdapter<?>);
    if (typeAdapter instanceof InstanceCreator<?>) {
        instanceCreators.put(type, (InstanceCreator) typeAdapter);
    }
    // 解析原生类型
    if (typeAdapter instanceof JsonSerializer<?> || typeAdapter instanceof JsonDeserializer<?>) {
        TypeToken<?> typeToken = TypeToken.get(type);
        factories.add(TreeTypeAdapter.newFactoryWithMatchRawType(typeToken, typeAdapter));
    }
    // 解析自定义的类
    if (typeAdapter instanceof TypeAdapter<?>) {
        // 创建一个TypeAdapterFactory，添加到一个List中保存
        factories.add(TypeAdapters.newFactory(TypeToken.get(type), (TypeAdapter) typeAdapter));
    }
    return this;
}

将自定义的adapter的封装成TypeAdapterFactory对象，添加到一个List -- 中保存factories；后面将会看到TypeAdapterFactory的主要作用提供一个create方法，判断Type对应的TypeAdapter对象。

GsonBuilder#create:

public Gson create() {
    List<TypeAdapterFactory> factories = new ArrayList<TypeAdapterFactory>();
    // 将所有的factorie添加到factories中传递给新建的Gson对象
    factories.addAll(this.factories);
    // 进行reverse，可以看到后添加的自定义Adapter将会放到最前面
    Collections.reverse(factories);
    factories.addAll(this.hierarchyFactories);
    addTypeAdaptersForDate(datePattern, dateStyle, timeStyle, factories);

    return new Gson(excluder, fieldNamingPolicy, instanceCreators,
            serializeNulls, complexMapKeySerialization,
            generateNonExecutableJson, escapeHtmlChars, prettyPrinting,
            serializeSpecialFloatingPointValues, longSerializationPolicy, factories);
}

Builder模式，创建一个GSON对象，可以看到这里将GsonBuilder中定义的factories添加到Gson的factories形参中；添加之前进行一次reverse，所以最后添加到TypeAdapter会反转到链表的最前面；

再来看Gson的构造函数：

3、Gson:

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) {
    .......

    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);

    // 自定义的TypeAdapter
    factories.addAll(typeAdapterFactories);

    // type adapters for basic platform types
    // 这里一次性添加所有系统中可能用到的所有types对应的Adapter
    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.ENUM_FACTORY);
    factories.add(TypeAdapters.CLASS_FACTORY);

    // 这里便对应着自定义类时默认使用的ReflectiveTypeAdapterFactory
    factories.add(new CollectionTypeAdapterFactory(constructorConstructor));
    factories.add(new MapTypeAdapterFactory(constructorConstructor, complexMapKeySerialization));
    factories.add(new ReflectiveTypeAdapterFactory(
            constructorConstructor, fieldNamingPolicy, excluder));

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

这里添加了用户定义的TypeAdapter（注意是放在链表的前面）；然后添加系统中可能会用到的基本类型对应的TypeAdpater（如String，Integer等）；对于用户自定义的类，则使用 ReflectiveTypeAdapterFactory ，放在链表的最后；

来看解析过程：

4、Gson.fromJson:

public <T> T fromJson(String json, Class<T> classOfT) throws JsonSyntaxException {
    Object object = fromJson(json, (Type) classOfT);
    // 这里进行类型转换
    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 = new JsonReader(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;
        // 根据Type类型获取对应的解析Adapter
        TypeToken<T> typeToken = (TypeToken<T>) TypeToken.get(typeOfT);
        TypeAdapter<T> typeAdapter = getAdapter(typeToken);
        
        // 执行TypeAdapter的read方法进行解析；可以看到如果是自定义的TypeAdapter，则会直接返回重写read方法中的返回值
        T object = typeAdapter.read(reader);
        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);
    }
}

最后来到的解析函数中可以看到，基本的解析逻辑就是，根据Type类型获取对应的解析TypeAdapter，然后调用该 TypeAdapter的read方法进行解析，得到返回值。

先来看如何获得对应TypeAdapter:

5、Gson#getAdapter:

private final ThreadLocal<Map<TypeToken<?>, FutureTypeAdapter<?>>> calls
        = new ThreadLocal<Map<TypeToken<?>, FutureTypeAdapter<?>>>();
public <T> TypeAdapter<T> getAdapter(TypeToken<T> type) {
    TypeAdapter<?> cached = typeTokenCache.get(type);
    if (cached != null) {
        return (TypeAdapter<T>) cached;
    }

    Map<TypeToken<?>, FutureTypeAdapter<?>> threadCalls = calls.get();
    boolean requiresThreadLocalCleanup = false;
    if (threadCalls == null) {
        // 如果当前线程不存在，则创建一个新的
        threadCalls = new HashMap<TypeToken<?>, FutureTypeAdapter<?>>();
        calls.set(threadCalls);
        requiresThreadLocalCleanup = true;
    }

    // the key and value type parameters always agree
    // 判断该Type类型是否已经保存
    FutureTypeAdapter<T> ongoingCall = (FutureTypeAdapter<T>) threadCalls.get(type);
    if (ongoingCall != null) {
        return ongoingCall;
    }

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

        // 遍历factories中保存的所有factory
        for (TypeAdapterFactory factory : factories) {
            // TypeAdapterFactory的作用是将TypeAdapter同TypeToken组合到一起；
            // 当type匹配时，返回已创建对象，否则返回null;所以可以通过判断返回值是否为null，来判断类型是否匹配
            // 见附二
            TypeAdapter<T> candidate = factory.create(this, type);
            if (candidate != null) {
                // 设置Delegate
                call.setDelegate(candidate);
                typeTokenCache.put(type, candidate);
                return candidate;
            }
        }
        throw new IllegalArgumentException("GSON cannot handle " + type);
    } finally {
        threadCalls.remove(type);

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

附一、 FutureTypeAdapter

// FutureTypeAdapter是对TypeAdapter的一个封装类，其实际工作委托给内部的delegate对象进行操作
static class FutureTypeAdapter<T> extends TypeAdapter<T> {
    private TypeAdapter<T> delegate;

    public void setDelegate(TypeAdapter<T> typeAdapter) {
        if (delegate != null) {
            throw new AssertionError();
        }
        delegate = typeAdapter;
    }

    @Override public T read(JsonReader in) throws IOException {
        if (delegate == null) {
            throw new IllegalStateException();
        }
        return delegate.read(in);
    }

    @Override public void write(JsonWriter out, T value) throws IOException {
        if (delegate == null) {
            throw new IllegalStateException();
        }
        delegate.write(out, value);
    }
}

附二、TypeAdapterFactory：

public static <TT> TypeAdapterFactory newFactory(
        final TypeToken<TT> type, final TypeAdapter<TT> typeAdapter) {
    return new TypeAdapterFactory() {
        @SuppressWarnings("unchecked") // we use a runtime check to make sure the 'T's equal
        public <T> TypeAdapter<T> create(Gson gson, TypeToken<T> typeToken) {
            return typeToken.equals(type) ? (TypeAdapter<T>) typeAdapter : null;
        }
    };
}

自定义的TypeAdapter直接调用其read方法即可；来看ReflectiveTypeAdapterFactory的使用原理：

6、ReflectiveTypeAdapterFactory：

public final class ReflectiveTypeAdapterFactory implements TypeAdapterFactory {
    private final ConstructorConstructor constructorConstructor;
    private final FieldNamingStrategy fieldNamingPolicy;
    private final Excluder excluder;

    public ReflectiveTypeAdapterFactory(ConstructorConstructor constructorConstructor,
                                        FieldNamingStrategy fieldNamingPolicy, Excluder excluder) {
        this.constructorConstructor = constructorConstructor;
        this.fieldNamingPolicy = fieldNamingPolicy;
        this.excluder = excluder;
    }

    // 匹配Adapter的时候，返回的Adapter
    public <T> TypeAdapter<T> create(Gson gson, final TypeToken<T> type) {
        Class<? super T> raw = type.getRawType();

        if (!Object.class.isAssignableFrom(raw)) {
            return null; // it's a primitive!
        }

        // 创建一个Adapter，用来对应之前的通过type查找对应的TypeAdapter操作
        ObjectConstructor<T> constructor = constructorConstructor.get(type);
        return new Adapter<T>(constructor, getBoundFields(gson, type, raw));
    }
}

截取 ReflectiveTypeAdapterFactory中的其中一部分，其中constructorConstructor用来通过反射机制构造实例；getBoundFields用来获取一个BoundFields，BoundFields将Field的name和一个自定义的BoundFields联系到一起，其实创建该Type的实例是通过BoundFields中重写的read方法来实现的。

static abstract class BoundField {
    final String name;
    final boolean serialized;
    final boolean deserialized;

    protected BoundField(String name, boolean serialized, boolean deserialized) {
        this.name = name;
        this.serialized = serialized;
        this.deserialized = deserialized;
    }

    abstract void write(JsonWriter writer, Object value) throws IOException, IllegalAccessException;
    abstract void read(JsonReader reader, Object value) throws IOException, IllegalAccessException;
}

1）先来看constructorConstructor：

public <T> ObjectConstructor<T> get(TypeToken<T> typeToken) {
    final Type type = typeToken.getType();
    final Class<? super T> rawType = typeToken.getRawType();

    // first try an instance creator

    @SuppressWarnings("unchecked") // types must agree
    final InstanceCreator<T> typeCreator = (InstanceCreator<T>) instanceCreators.get(type);
    if (typeCreator != null) {
        return new ObjectConstructor<T>() {
            public T construct() {
                return typeCreator.createInstance(type);
            }
        };
    }

    // Next try raw type match for instance creators
    @SuppressWarnings("unchecked") // types must agree
    final InstanceCreator<T> rawTypeCreator =
            (InstanceCreator<T>) instanceCreators.get(rawType);
    if (rawTypeCreator != null) {
        return new ObjectConstructor<T>() {
            public T construct() {
                return rawTypeCreator.createInstance(type);
            }
        };
    }

    // 创建一个默认构造器，用来创建实例
    ObjectConstructor<T> defaultConstructor = newDefaultConstructor(rawType);
    if (defaultConstructor != null) {
        return defaultConstructor;
    }

    ObjectConstructor<T> defaultImplementation = newDefaultImplementationConstructor(type, rawType);
    if (defaultImplementation != null) {
        return defaultImplementation;
    }

    // finally try unsafe
    return newUnsafeAllocator(type, rawType);
}

// 获取该Class的默认构造器，通过newInstance创建一个实例对象
private <T> ObjectConstructor<T> newDefaultConstructor(Class<? super T> rawType) {
    try {
        final Constructor<? super T> constructor = rawType.getDeclaredConstructor();
        if (!constructor.isAccessible()) {
            constructor.setAccessible(true);
        }
        return new ObjectConstructor<T>() {
            @SuppressWarnings("unchecked") // T is the same raw type as is requested
            public T construct() {
                try {
                    Object[] args = null;
                    return (T) constructor.newInstance(args);
                } catch (InstantiationException e) {
                    ..........
                }
            }
        };
    } catch (NoSuchMethodException e) {
        return null;
    }
}

重点看下面的 newDefaultConstructor，它提供了一个ObjectConstructor实例，该对象中有个construct方法，在解析过程中将会调用来创建一个T实例instance；由上可以，通过获取该Class的构造函数，然后调用默认构造函数的newInstance来创建一个实例。

2）创建BoundFields:

private Map<String, BoundField> getBoundFields(Gson context, TypeToken<?> type, Class<?> raw) {
    Map<String, BoundField> result = new LinkedHashMap<String, BoundField>();
    // 如果数据类型是Interface的话，直接返回
    if (raw.isInterface()) {
        return result;
    }

    Type declaredType = type.getType();
    while (raw != Object.class) {
        // 获得该类的所有Field
        Field[] fields = raw.getDeclaredFields();
        // 遍历所有Field
        for (Field field : fields) {
            // 判断是否能够序列化
            boolean serialize = excludeField(field, true);
            boolean deserialize = excludeField(field, false);
            if (!serialize && !deserialize) {
                continue;
            }
            // 访问Field
            field.setAccessible(true);
            Type fieldType = $Gson$Types.resolve(type.getType(), raw, field.getGenericType());
            // 创建BoundField，将Field的name和boundField绑定到一起
            BoundField boundField = createBoundField(context, field, getFieldName(field),
                    TypeToken.get(fieldType), serialize, deserialize);
            //
            BoundField previous = result.put(boundField.name, boundField);
            if (previous != null) {
                throw new IllegalArgumentException(declaredType
                        + " declares multiple JSON fields named " + previous.name);
            }
        }
        // 获取父类，重复上述操作
        type = TypeToken.get($Gson$Types.resolve(type.getType(), raw, raw.getGenericSuperclass()));
        raw = type.getRawType();
    }
    return result;
}

通过getField获得该类的所有Field,然后由Field信息创建（createBoundField）一个BoundField，并通过HashMap将两者绑定到一起，便于查询；

来看 createBoundField：

private ReflectiveTypeAdapterFactory.BoundField createBoundField(
        final Gson context, final Field field, final String name,
        final TypeToken<?> fieldType, boolean serialize, boolean deserialize) {
    final boolean isPrimitive = Primitives.isPrimitive(fieldType.getRawType());

    // special casing primitives here saves ~5% on Android...
    return new ReflectiveTypeAdapterFactory.BoundField(name, serialize, deserialize) {
        // 根据fieldType获得相应的TypeAdapter
        final TypeAdapter<?> typeAdapter = context.getAdapter(fieldType);

        @SuppressWarnings({"unchecked", "rawtypes"}) // the type adapter and field type always agree
        @Override
        void write(JsonWriter writer, Object value) throws IOException, IllegalAccessException {
            Object fieldValue = field.get(value);
            TypeAdapter t = new TypeAdapterRuntimeTypeWrapper(context, this.typeAdapter, fieldType.getType());
            t.write(writer, fieldValue);
        }

        @Override
        void read(JsonReader reader, Object value) throws IOException, IllegalAccessException {
            // 进而调用对应Type的TypeAdapter去实现
            Object fieldValue = typeAdapter.read(reader);
            if (fieldValue != null || !isPrimitive) {
                field.set(value, fieldValue);
            }
        }
    };
}

这里创建一个BoundField实例，并且重写了read，write方法；从后面的TypeAdapter，可以看到这些的作用；

3） ReflectiveTypeAdapterFactory的内部类Adapter:

public static final class Adapter<T> extends TypeAdapter<T> {
    private final ObjectConstructor<T> constructor;
    private final Map<String, BoundField> boundFields;

    private Adapter(ObjectConstructor<T> constructor, Map<String, BoundField> boundFields) {
        this.constructor = constructor;
        this.boundFields = boundFields;
    }

    @Override
    public T read(JsonReader in) throws IOException {
        // 为空值，直接返回
        if (in.peek() == JsonToken.NULL) {
            in.nextNull();
            return null;
        }

        T instance = constructor.construct();

        try {
            in.beginObject();
            while (in.hasNext()) {
                String name = in.nextName();
                // boundFields是个HashMap，这里通过Field的name获取对应的boundFields
                BoundField field = boundFields.get(name);
                if (field == null || !field.deserialized) {
                    in.skipValue();
                } else {
                    // 最终的对象创建逻辑在boundField中的read中实现；
                    field.read(in, instance);
                }
            }
        } catch (IllegalStateException e) {
            throw new JsonSyntaxException(e);
        } catch (IllegalAccessException e) {
            throw new AssertionError(e);
        }
        in.endObject();
        return instance;
    }

    @Override
    public void write(JsonWriter out, T value) throws IOException {
        if (value == null) {
            out.nullValue();
            return;
        }

        out.beginObject();
        try {
            for (BoundField boundField : boundFields.values()) {
                if (boundField.serialized) {
                    out.name(boundField.name);
                    boundField.write(out, value);
                }
            }
        } catch (IllegalAccessException e) {
            throw new AssertionError();
        }
        out.endObject();
    }
}

可以看到这里解析逻辑，即调用前面所述的constructor创建一个instance，然后遍历jsonReader，通过读取其中的name，查找对应的FieldBounds，然后调用FieldBounds中的read方法；再回到FieldBounds中的read方法，可以看到，其可以看做一个递归实现了，继续查找该Field的类型对应的Adapter，然后调用该TypeAdpter去创建相应对象，最后进行返回。

具体的解析逻辑分析完毕，主要通过Type找到对应的TypeAdapter，如果有自定义的，则直接调用自定义Adpter的read方法；否则，使用可以看到，均是反射来实现，通过newInstance来获取实例，通过getDeclaredFields来获取Field信息；其主要工作解析原理还没有具体分析：

其主要的解析原理在JsonReader中，因为GSON中涉及到蛮多泛型、数组的打包处理，所以还要理清楚Type,、ParameterizedType、GenericArrayType、TypeVariable、WildcardType这些类的关系和概念。