为什么Java 8对派生类应用注释不同？
如果我有以下两个类：
// Base.java public abstract class Base { abstract void method(T t); } 和
// Derived.java public class Derived extends Base { @Deprecated void method(Number n) {} }
然后我用javac Base.java Derived.java编译它们然后使用javap -v Derived 。 如果我使用Java 7，我会得到
public class Derived extends Base Signature: #17 // LBase; SourceFile: "Derived.java" minor version: 0 major version: 51 flags: ACC_PUBLIC, ACC_SUPER Constant pool: #1 = Methodref #5.#20 // Base."":()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 #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; #18 = Utf8 SourceFile #19 = Utf8 Derived.java #20 = NameAndType #6:#7 // "":()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."":()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, locals=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 minor version: 0 major version: 52 flags: ACC_PUBLIC, ACC_SUPER Constant pool: #1 = Methodref #5.#20 // Base."":()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 #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; #18 = Utf8 SourceFile #19 = Utf8 Derived.java #20 = NameAndType #6:#7 // "":()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, locals=1, args_size=1 0: aload_0 1: invokespecial #1 // Method Base."":()V 4: return LineNumberTable: line 1: 0 void method(java.lang.Number); descriptor: (Ljava/lang/Number;)V flags: Code: stack=0, locals=2, 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, ACC_SYNTHETIC Code: stack=2, locals=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 RuntimeVisibleAnnotations: 0: #14() } Signature: #17 // LBase; SourceFile: "Derived.java"
这里需要注意的是Java 8版本中的void method(java.lang.Object)存根上有一个可见的注释，Java 7版本中没有该注释。 这不仅仅是javap犯了一个错误 – 如果你使用reflection来检查运行时出现的注释，那么Java 7版本只有一个关于void method(java.lang.Number)的注释，而Java 8版本在它们上面都有它。 这是怎么回事？
那是因为它被修复了，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。 而且这不仅是Java 8，它还被反向移植到Java 7u80：
C:\Test>"C:\Program Files\Java\jdk1.7.0_79\bin\javac.exe" Derived.java C:\Test>javap -v Derived.class >javac7_79 C:\Test>"C:\Program Files\Java\jdk1.7.0_80\bin\javac.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()