为什么Java 8对派生类应用注释不同？

发布时间：2020-12-14 16:17:33 所属栏目：Java 来源：网络整理

导读：如果我有以下两个类： // Base.javapublic abstract class BaseT { abstract void method(T t);} 和 // Derived.javapublic class Derived extends BaseNumber { @Deprecated void method(Number n) {}} 然后我使用javac Base.java Derived.java编译它们,然

如果我有以下两个类：

// Base.java
public abstract class Base<T> {
  abstract void method(T t);
}

和

// Derived.java
public class Derived extends Base<Number> {
  @Deprecated
  void method(Number n) {}
}

然后我使用javac Base.java Derived.java编译它们,然后使用javap -v Derived.如果我使用Java 7,我得到

public class Derived extends Base<java.lang.Number>
  Signature: #17                          // LBase<Ljava/lang/Number;>;
  SourceFile: "Derived.java"
  minor version: 0
  major version: 51
  flags: ACC_PUBLIC,ACC_SUPER
Constant pool:
   #1 = Methodref          #5.#20         //  Base."<init>":()V
   #2 = Class              #21            //  java/lang/Number
   #3 = Methodref          #4.#22         //  Derived.method:(Ljava/lang/Number;)V
   #4 = Class              #23            //  Derived
   #5 = Class              #24            //  Base
   #6 = Utf8               <init>
   #7 = Utf8               ()V
   #8 = Utf8               Code
   #9 = Utf8               LineNumberTable
  #10 = Utf8               method
  #11 = Utf8               (Ljava/lang/Number;)V
  #12 = Utf8               Deprecated
  #13 = Utf8               RuntimeVisibleAnnotations
  #14 = Utf8               Ljava/lang/Deprecated;
  #15 = Utf8               (Ljava/lang/Object;)V
  #16 = Utf8               Signature
  #17 = Utf8               LBase<Ljava/lang/Number;>;
  #18 = Utf8               SourceFile
  #19 = Utf8               Derived.java
  #20 = NameAndType        #6:#7          //  "<init>":()V
  #21 = Utf8               java/lang/Number
  #22 = NameAndType        #10:#11        //  method:(Ljava/lang/Number;)V
  #23 = Utf8               Derived
  #24 = Utf8               Base
{
  public Derived();
    flags: ACC_PUBLIC
    Code:
      stack=1,locals=1,args_size=1
         0: aload_0
         1: invokespecial #1                  // Method Base."<init>":()V
         4: return
      LineNumberTable:
        line 1: 0

  void method(java.lang.Number);
    flags:
    Code:
      stack=0,locals=2,args_size=2
         0: return
      LineNumberTable:
        line 7: 0
    Deprecated: true
    RuntimeVisibleAnnotations:
      0: #14()

  void method(java.lang.Object);
    flags: ACC_BRIDGE,ACC_SYNTHETIC
    Code:
      stack=2,args_size=2
         0: aload_0
         1: aload_1
         2: checkcast     #2                  // class java/lang/Number
         5: invokevirtual #3                  // Method method:(Ljava/lang/Number;)V
         8: return
      LineNumberTable:
        line 1: 0
}

如果我用Java 8做同样的事情,我会得到

public class Derived extends Base<java.lang.Number>
  minor version: 0
  major version: 52
  flags: ACC_PUBLIC,ACC_SUPER
Constant pool:
   #1 = Methodref          #5.#20         // Base."<init>":()V
   #2 = Class              #21            // java/lang/Number
   #3 = Methodref          #4.#22         // Derived.method:(Ljava/lang/Number;)V
   #4 = Class              #23            // Derived
   #5 = Class              #24            // Base
   #6 = Utf8               <init>
   #7 = Utf8               ()V
   #8 = Utf8               Code
   #9 = Utf8               LineNumberTable
  #10 = Utf8               method
  #11 = Utf8               (Ljava/lang/Number;)V
  #12 = Utf8               Deprecated
  #13 = Utf8               RuntimeVisibleAnnotations
  #14 = Utf8               Ljava/lang/Deprecated;
  #15 = Utf8               (Ljava/lang/Object;)V
  #16 = Utf8               Signature
  #17 = Utf8               LBase<Ljava/lang/Number;>;
  #18 = Utf8               SourceFile
  #19 = Utf8               Derived.java
  #20 = NameAndType        #6:#7          // "<init>":()V
  #21 = Utf8               java/lang/Number
  #22 = NameAndType        #10:#11        // method:(Ljava/lang/Number;)V
  #23 = Utf8               Derived
  #24 = Utf8               Base
{
  public Derived();
    descriptor: ()V
    flags: ACC_PUBLIC
    Code:
      stack=1,args_size=1
         0: aload_0
         1: invokespecial #1                  // Method Base."<init>":()V
         4: return
      LineNumberTable:
        line 1: 0

  void method(java.lang.Number);
    descriptor: (Ljava/lang/Number;)V
    flags:
    Code:
      stack=0,args_size=2
         0: return
      LineNumberTable:
        line 5: 0
    Deprecated: true
    RuntimeVisibleAnnotations:
      0: #14()

  void method(java.lang.Object);
    descriptor: (Ljava/lang/Object;)V
    flags: ACC_BRIDGE,args_size=2
         0: aload_0
         1: aload_1
         2: checkcast     #2                  // class java/lang/Number
         5: invokevirtual #3                  // Method method:(Ljava/lang/Number;)V
         8: return
      LineNumberTable:
        line 1: 0
    RuntimeVisibleAnnotations:
      0: #14()
}
Signature: #17                          // LBase<Ljava/lang/Number;>;
SourceFile: "Derived.java"

这里要注意的是,在Java 7版本中不存在的Java 8版本中的void方法(java.lang.Object)存根上有一个注释.这不仅仅是javap错误 – 如果您使用反射来检查运行时存在的注释,则Java 7版本仅在void方法(java.lang.Number)上具有注释,并且Java 8版本都具有这两个注释.这是怎么回事？

解决方法

这是因为它是固定的,因为这种行为似乎更加一致.有关详细信息,请参阅 JDK-6695379问题.而且这不仅仅是Java 8,它也被转载到Java 7u80：

C:Test>"C:Program FilesJavajdk1.7.0_79binjavac.exe" Derived.java

C:Test>javap -v Derived.class >javac7_79

C:Test>"C:Program FilesJavajdk1.7.0_80binjavac.exe" Derived.java

C:Test>javap -v Derived.class >javac7_80

C:Test>diff javac7_79 javac7_80
2,3c2,3
<   Last modified 18.05.2015; size 484 bytes
<   MD5 checksum bd5e729c8eda30f72f3dc5301fa9bfc2
---
>   Last modified 18.05.2015; size 496 bytes
>   MD5 checksum 728d9e30b9aab2381e711b3edd008000
69a70,71
>     RuntimeVisibleAnnotations:
>       0: #14()

（编辑：李大同）

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