Gson TypeToken 是如何工作的? [英] How does Gson TypeToken work?

问题描述

我知道在 Java 中，例如，C# 泛型是编译时特性，并且通过类型擦除被删除.那么，Gson 的 TypeToken 究竟是如何工作的呢?它如何获取对象的泛型类型?

I understand that in Java contrary to, for example, C# generics are compile-time feature and is removed via type erasure. So, how does Gson's TypeToken really work? How does it get the generic type of an object?

推荐答案

来自 JLS 的 §4.6(强调我的):

类型擦除是从类型(可能包括参数化类型和类型变量)到类型(从不参数化类型或类型变量)的映射.我们写|T|对于T类型的擦除，擦除映射定义如下:

Type erasure is a mapping from types (possibly including parameterized types and type variables) to types (that are never parameterized types or type variables). We write |T| for the erasure of type T. The erasure mapping is defined as follows:

参数化类型 (§4.5) G 的擦除是 |G|.

The erasure of a parameterized type (§4.5) G is |G|.

嵌套类型 T.C 的擦除是 |T|.C.

The erasure of a nested type T.C is |T|.C.

数组类型 T[] 的擦除是 |T|[].

The erasure of an array type T[] is |T|[].

类型变量的擦除(第 4.4 节)是擦除其最左边的边界.

The erasure of a type variable (§4.4) is the erasure of its leftmost bound.

所有其他类型的擦除是类型本身.

因此，如果您声明一个类具有其自身的匿名子类，它会保持它的参数化类型；它没有被删除.因此，请考虑以下代码:

Therefore, if you declare a class with an anonymous subclass of itself, it keeps it's parameterized type; it's not erased. Therefore, consider the following code:

import java.lang.reflect.ParameterizedType;
import java.util.Arrays;
import java.util.HashMap;

public class Erasure<T>
{
    public static void main(String...strings) {
      Class<?> foo = new Erasure<HashMap<Integer, String>>() {}.getClass();
      ParameterizedType t = (ParameterizedType) foo.getGenericSuperclass();
      System.out.println(t.getOwnerType());
      System.out.println(t.getRawType());
      System.out.println(Arrays.toString(t.getActualTypeArguments()));
    }
}

输出:

null
class Erasure
[java.util.HashMap<java.lang.Integer, java.lang.String>]

注意如果你没有匿名声明类，你会得到一个ClassCastException，因为擦除；超类不是参数化类型，而是 Object.

Notice that you would get a ClassCastException if you did not declare the class anonymously, because of erasure; the superclass would not be a parameterized type, it would be an Object.

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