java - Gson TypeToken 是如何工作的？

Question

我知道在 Java 中与 C# 泛型相反，例如，C# 泛型是编译时特性，并且通过类型删除被删除。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？

Answer 1

From §4.6 of the JLS (强调我的):

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:

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

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

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

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

The erasure of every other type is the type itself.

因此，如果您声明一个具有其自身匿名子类的类，它会保持其参数化类型；它没有被删除。因此，请考虑以下代码:

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()));
    }
}

这个输出:

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，因为删除；父类(super class)不是参数化类型，而是 Object。