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。
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