javap和泛型'类型擦除 [英] javap and generics' type erasure

问题描述

我正在阅读Herbert Schilds关于java中的泛型类型擦除。假设在一个类上运行javap应该给我关于在类型擦除之后的公共，包保护和保护字段和方法的字节码信息。然而，我写了下面的类：

  class Ambiguity< T，V extends String> {
 T ob1; 
 V ob2; 
 
 void set（T o）{
 ob1 = o; 
} 
 
 void set（V o）{
 ob2 = o; 
} 
} 
  

并在生成的类文件上运行javap，得到了以下输出：

从Test.java编译

 类歧义< T，V extends java.lang.String> {
 T ob1; 
 V ob2; 
 Ambiguity（）; 
 void set（T）; 
 void set（V）; 
} 
  

根据我读的内容，我期待这样的输出。 / p>

 编译自Test.java
类歧义< java.lang.Object，java.lang.String> {
 java.lang.Object ob1; 
 java.lang.String ob2; 
 Ambiguity（）; 
 void set（java.lang.Object）; 
 void set（java.lang.String）; 
} 
  

我在这里错过了什么吗？我应该补充一点，我明白，以上述方式重载方法并不是一个好习惯。

编辑：这似乎是由于新的修复程序的结果javap的。
http://bugs.sun.com/bugdatabase/view_bug.do ？bug_id = 4870651

如果我从JDK 1.6运行javap，我会得到我期望的结果。如果我从最初使用的JDK 1.7 b30运行javap，我会得到带有通用信息的结果。

解决方案

在编译时间 - 在生成的字节码中，类会保留所有的通用类型信息。你看到的正是你所期望的。

不同之处在于类型在运行时处被擦除：例如，类的实例 不会知道它的类型参数是 Integer 和字符串。

I am reading Herbert Schilds about type erasure in generics in java. Supposedly running javap on a class should give me the bytecode information about public, package protected and protected fields and methods after type erasure. However, I wrote the following class:

    class Ambiguity<T, V extends String>{
    T ob1;
    V ob2;

    void set(T o){
        ob1 = o;
    }

    void set(V o){
        ob2 = o;
    }
}

and ran javap on the class file that was generated and got the following output

Compiled from "Test.java"

class Ambiguity<T, V extends java.lang.String> {
  T ob1;
  V ob2;
  Ambiguity();
  void set(T);
  void set(V);
}

I was expecting an output that looked like this based on what I read.

Compiled from "Test.java"
class Ambiguity<java.lang.Object, java.lang.String> {
  java.lang.Object ob1;
  java.lang.String ob2;
  Ambiguity();
  void set(java.lang.Object);
  void set(java.lang.String);
}

Am I missing something here? I should add that I understand that it is not a good practice to overload methods in the above manner. I was just seeing interested in seeing the results of javap under this ambiguity.

EDIT: This seems to be a result of a new fix in javap. http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=4870651

If I run javap from JDK 1.6 I get the results as I was expecting. If I run javap from JDK 1.7 b30 which was what I was using initially, I get the result with the generic information.

解决方案

At compile time -- in the generated bytecode -- classes will, yes, retain all of their generic type information. What you've seen is exactly what you should expect.

The difference is that types are erased at runtime: for example, an instance of the class Ambiguity<Integer, String> will not know that its type arguments are Integer and String respectively.

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