Java中TypeReference用法说明

在使用fastJson时,对于泛型的反序列化很多场景下都会使用到TypeReference，例如：

void testTypeReference() {
List<Integer> list = new ArrayList<>();
list.add(1);
list.add(9);
list.add(4);
list.add(8);
JSONObject jsonObj = new JSONObject();
jsonObj.put("a", list);
System.out.println(jsonObj);

List<String> list2 = jsonObj.getObject("a", new TypeReference<List<Integer>>(){});

System.out.println(list2);
}
输出

1{"a":[1,9,4,8]}
2[1, 9, 4, 8]
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18

{"msg":"","code":"0","data":[{"adjEq":"","details":[],"imr":"","isoEq":"","mgnRatio":"","mmr":"","notionalUsd":"","ordFroz":"","totalEq":"0","uTime":"1658332171773"}]}
//拿code的值
        String result1 = JSON.toJSONString(result);
        System.out.println("result1=="+result1);

        Result result2 = JSON.parseObject(result1, new TypeReference<Result>() {});
        System.out.println(result2.getCode());
//拿data的值   Balance是data中数据的实体类
		Result<List<Balance>> listResult = JSON.parseObject(result1, new TypeReference<Result<List<Balance>>>() {});
        System.out.println("result2=="+listResult.getData());
1
2
3
4
5
6
7
8
9
10

使用TypeReference可以明确的指定反序列化的类型，具体实现逻辑参考TypeReference的构造函数

protected TypeReference(){
Type superClass = getClass().getGenericSuperclass();

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;
}
解说
其中核心的方法是：getActualTypeArguments，它可以得到父类的反省类型

ParameterizedType是一个记录类型泛型的接口, 继承自Type,一共三方法:

Type[] getActualTypeArguments(); //返回泛型类型数组
Type getRawType(); //返回原始类型Type
Type getOwnerType(); //返回 Type 对象，表示此类型是其成员之一的类型。
例如 Map 对应的ParameterizedType三个方法分别取值如下:

[class java.lang.String, class java.lang.String]
interface java.util.Map
null
例证
package JsonLearn;

import java.lang.reflect.ParameterizedType;
import java.lang.reflect.Type;
import java.util.HashMap;
import java.util.Map;

public class TypeReferencBaseLearn {
public static class IntMap extends HashMap {}

void test1() {
IntMap intMap = new IntMap();
System.out.println(“getSuperclass:” + intMap.getClass().getSuperclass());
System.out.println(“getGenericSuperclass:” + intMap.getClass().getGenericSuperclass());
Type type = intMap.getClass().getGenericSuperclass();
if (type instanceof ParameterizedType) {
ParameterizedType p = (ParameterizedType)type;
for (Type t : p.getActualTypeArguments()) {
System.out.println(t);
}
}
}

void test2() {
Map intMap = new HashMap<>();
System.out.println(“\ngetSuperclass:” + intMap.getClass().getSuperclass());
System.out.println(“getGenericSuperclass:” + intMap.getClass().getGenericSuperclass());
Type type = intMap.getClass().getGenericSuperclass();
if (type instanceof ParameterizedType) {
ParameterizedType p = (ParameterizedType)type;
for (Type t : p.getActualTypeArguments()) {
System.out.println(t);
}
}
}

void test3() {
Map intMap = new HashMap(){};
System.out.println(“\ngetSuperclass:” + intMap.getClass().getSuperclass());
System.out.println(“getGenericSuperclass:” + intMap.getClass().getGenericSuperclass());
Type type = intMap.getClass().getGenericSuperclass();
if (type instanceof ParameterizedType) {
ParameterizedType p = (ParameterizedType)type;
for (Type t : p.getActualTypeArguments()) {
System.out.println(t);
}
}
}

public static void main(String[] args) {
TypeReferencBaseLearn obj = new TypeReferencBaseLearn();
obj.test1();
obj.test2();
obj.test3();
}
}
输出

getSuperclass:class java.util.HashMap
getGenericSuperclass:java.util.HashMap
class java.lang.String
class java.lang.Integer

getSuperclass:class java.util.AbstractMap
getGenericSuperclass:java.util.AbstractMap
K
V

getSuperclass:class java.util.HashMap
getGenericSuperclass:java.util.HashMap
class java.lang.String
class java.lang.Integer