概述
Represents a generic type {@code T}. 代表泛型类型T
Java doesn't yet provide a way to represent generic types, so this class does. Java未提供一种方式表示 泛型类型
Forces clients to create a subclass of this class which enables retrieval the type information even at runtime. 强制client创建一个该类的子类,以便运行时可以检索到类型
For example, to create a type literal for {@code List<String>}, you can create an empty anonymous inner class: 比如创建List<String>,可以创建一个空的匿名内部类
TypeReference<List<String>> list = new TypeReference<List<String>>() {};
This syntax cannot be used to create type literals that have wildcard parameters, such as {@code Class <?>} or {@code List <? extends CharSequence>}.
******不能创建 带有 通配符 的泛型类型
why
使用 fastjson 反序列化一个 泛型类 时,执行时会发现反序列化出来的列表元素的是 JSONObject,而不是期望的 User;
String userString = "[{\"userId\":\"111\",\"userName\":\"Tom\"},{\"userId\":\"222\",\"userName\":\"Jerry\"}]";
List<User> userInfoList = JSON.parseObject(userString, List.class);
为了处理这种问题,fastjson提供了 TypeReference,一个用于处理 泛型反序列化的类;
String userString = "[{\"userId\":\"111\",\"userName\":\"Tom\"},{\"userId\":\"222\",\"userName\":\"Jerry\"}]";
List<User> userInfoList = JSON.parseObject(userString, new TypeReference<List<User>>() {});
How
匿名内部类
// 基于接口 new Runnable(){ @Override public void run() { } }; // 基于抽象类 new AbstractList<String>() { @Override public int size() { return 0; } @Override public String get(int index) { return null; } }; // 基于 非抽象类 new ArrayList<String>(){ }
new TypeReference<List<User>>() {}
com.alibaba.fastjson.TypeReference#TypeReference()
public class TypeReference<T> { static ConcurrentMap<Type, Type> classTypeCache = new ConcurrentHashMap<Type, Type>(16, 0.75f, 1); protected final Type type; /** * Constructs a new type literal. Derives represented class from type * parameter. 构建一个新的类型,从 类型参数 派生; * * <p>Clients create an empty anonymous subclass. Doing so embeds the type * parameter in the anonymous class's type hierarchy so we can reconstitute it * at runtime despite erasure. 客户端创建一个空的匿名子类 */ protected TypeReference(){ // 获取代表 当前class的父类 的type Type superClass = getClass().getGenericSuperclass(); // 获取 该type的真实 类型参数 Type type = ((ParameterizedType) superClass).getActualTypeArguments()[0]; Type cachedType = classTypeCache.get(type); if (cachedType == null) { classTypeCache.putIfAbsent(type, type); cachedType = classTypeCache.get(type); } this.type = cachedType; } }