Java泛型之类型擦除

类型擦除

学过C++模板的,在使用Java泛型的时候,会感觉到有点不疑问,例如:(1)无法定义一个泛型数组、无法调用泛型参数对象中对应的方法(当然,通过extends关键字是可以做到,只是比较麻烦);(2)ArrayList<Integer>和ArrayList<String>在运行时的类型是相同的。Java中的泛型有这些问题,是它的实现机制决定的,即“类型擦除”。

  1. 类型擦除的定义:编译通过后,准备进入JVM运行时,就不再有类型参数的概念,换句话说:每定义一个泛型类型,JVM会自动提供一个对应的原生类;
public class Holder4<T> {

private T a;
private T b;
private T c; public Holder4(T a, T b, T c) {
this.a = a;
this.b = b;
this.c = c;
} public T getA() {
return a;
} public T getB() {
return b;
} public T getC() {
return c;
} public void setA(T a) {
this.a = a;
} public void setB(T b) {
this.b = b;
} public void setC(T c) {
this.c = c;
} public static void main(String[] args) {
Holder4<Automobile> holder4 = new Holder4<>(new Automobile(),new Automobile(), new Automobile()); Automobile a = holder4.getA(); //编译器帮忙转型,不需要显式转型
Automobile b = holder4.getB();
Automobile c = holder4.getC();
}
}

在Java中,每定义一个泛型类型,就会自动提供一个对应的原始类型,例如:

public class Holder4Raw {

       private Object a;
private Object b;
private Object c; public Holder4Raw(Object a, Object b, Object c) {
this.a = a;
this.b = b;
this.c = c;
} public Object getA() {
return a;
} public Object getB() {
return b;
} public Object getC() {
return c;
} public void setA(Object a) {
this.a = a;
} public void setB(Object b) {
this.b = b;
} public void setC(Object c) {
this.c = c;
} public static void main(String[] args) {
Holder4Raw holder4Raw = new Holder4Raw(new Automobile(),new Automobile(), new Automobile()); Automobile a = (Automobile) holder4Raw.getA(); //显示的转型
Automobile b = (Automobile) holder4Raw.getB();
Automobile c = (Automobile) holder4Raw.getC();
}
}
  1. 为什么选择这种实现机制?
  • 在Java诞生10年后,才想实现类似于C++模板的概念,即泛型;
  • Java的类库是Java生态中非常宝贵的财富,必须保证向后兼容(即现有的代码和类文件依旧合法)和迁移兼容(泛化的代码和非泛化的代码可互相调用)基于上面这两个背景和考虑,Java设计者采取了“类型擦除”这种折中的实现方式。
  1. Java泛型依赖编译器实现,只存在于编译期,JVM中没有泛型的概念;那么,编译器做了什么工作呢?(1)set方法是编译期检查;(2)get方法的返回值进行转型,编译器插入了一个checkcast语句。

我们通过字节码进行观察,可以看出:(1)Holder4和Holder4Raw两个类的字节码完全相同;(2)在main函数的33、41和49行就是编译器插入的checkcast语句;

public class org.java.learn.generics.Holder4<T> {
public org.java.learn.generics.Holder4(T, T, T);
Code:
0: aload_0
1: invokespecial #1 // Method java/lang/Object."<init>":()V
4: aload_0
5: aload_1
6: putfield #2 // Field a:Ljava/lang/Object;
9: aload_0
10: aload_2
11: putfield #3 // Field b:Ljava/lang/Object;
14: aload_0
15: aload_3
16: putfield #4 // Field c:Ljava/lang/Object;
19: return public T getA();
Code:
0: aload_0
1: getfield #2 // Field a:Ljava/lang/Object;
4: areturn public T getB();
Code:
0: aload_0
1: getfield #3 // Field b:Ljava/lang/Object;
4: areturn public T getC();
Code:
0: aload_0
1: getfield #4 // Field c:Ljava/lang/Object;
4: areturn public void setA(T);
Code:
0: aload_0
1: aload_1
2: putfield #2 // Field a:Ljava/lang/Object;
5: return public void setB(T);
Code:
0: aload_0
1: aload_1
2: putfield #3 // Field b:Ljava/lang/Object;
5: return public void setC(T);
Code:
0: aload_0
1: aload_1
2: putfield #4 // Field c:Ljava/lang/Object;
5: return public static void main(java.lang.String[]);
Code:
0: new #5 // class org/java/learn/generics/Holder4
3: dup
4: new #6 // class org/java/learn/generics/Automobile
7: dup
8: invokespecial #7 // Method org/java/learn/generics/Automobile."<init>":()V
11: new #6 // class org/java/learn/generics/Automobile
14: dup
15: invokespecial #7 // Method org/java/learn/generics/Automobile."<init>":()V
18: new #6 // class org/java/learn/generics/Automobile
21: dup
22: invokespecial #7 // Method org/java/learn/generics/Automobile."<init>":()V
25: invokespecial #8 // Method "<init>":(Ljava/lang/Object;Ljava/lang/Object;Ljava/lang/Object;)V
28: astore_1
29: aload_1
30: invokevirtual #9 // Method getA:()Ljava/lang/Object;
33: checkcast #6 // class org/java/learn/generics/Automobile,get方法的转型
36: astore_2
37: aload_1
38: invokevirtual #10 // Method getB:()Ljava/lang/Object;
41: checkcast #6 // class org/java/learn/generics/Automobile,get方法的转型
44: astore_3
45: aload_1
46: invokevirtual #11 // Method getC:()Ljava/lang/Object;
49: checkcast #6 // class org/java/learn/generics/Automobile,get方法的转型
52: astore 4
54: return
}

参考资料

  1. 《Java编程思想》
  2. 《Effective Java》
  3. 《Java核心技术》

原文作者:duqicauc
转载地址:Java泛型之类型擦除 | Spring For All

from: https://zhuanlan.zhihu.com/p/31741402

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