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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();
       }
   }
   

2. 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。

3. 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》