解密JDK8 枚举

写一个枚举类

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public enum Season { SPRING, SUMMER, AUTUMN, WINTER }

然后我们使用javac编译上面的类,得到class文件

然后,我们利用反编译的方法来看看字节码文件究竟是什么.这里使用的工具是javap的简单命令,先列举一下这个Season下的全部元素.

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Classfile /C:/Season.class Last modified 2016-12-8; size 956 bytes MD5 checksum 7f6dfb988d182327a1a73ee986a9d3fa Compiled from "Season.java" public final class cn.redis.model.Season extends java.lang.Enum<cn.redis.model.S eason> minor version: 0 major version: 52 flags: ACC_PUBLIC, ACC_FINAL, ACC_SUPER, ACC_ENUM Constant pool: #1 = Fieldref #4.#38 // cn/redis/model/Season.$VALUES:[Lcn/ redis/model/Season; #2 = Methodref #39.#40 // "[Lcn/redis/model/Season;".clone:() Ljava/lang/Object; #3 = Class #23 // "[Lcn/redis/model/Season;" #4 = Class #41 // cn/redis/model/Season #5 = Methodref #16.#42 // java/lang/Enum.valueOf:(Ljava/lang/ Class;Ljava/lang/String;)Ljava/lang/Enum; #6 = Methodref #16.#43 // java/lang/Enum."<init>":(Ljava/lang /String;I)V #7 = String #17 // SPRING #8 = Methodref #4.#43 // cn/redis/model/Season."<init>":(Lja va/lang/String;I)V #9 = Fieldref #4.#44 // cn/redis/model/Season.SPRING:Lcn/re dis/model/Season; #10 = String #19 // SUMMER #11 = Fieldref #4.#45 // cn/redis/model/Season.SUMMER:Lcn/re dis/model/Season; #12 = String #20 // AUTUMN #13 = Fieldref #4.#46 // cn/redis/model/Season.AUTUMN:Lcn/re dis/model/Season; #14 = String #21 // WINTER #15 = Fieldref #4.#47 // cn/redis/model/Season.WINTER:Lcn/re dis/model/Season; #16 = Class #48 // java/lang/Enum #17 = Utf8 SPRING #18 = Utf8 Lcn/redis/model/Season; #19 = Utf8 SUMMER #20 = Utf8 AUTUMN #21 = Utf8 WINTER #22 = Utf8 $VALUES #23 = Utf8 [Lcn/redis/model/Season; #24 = Utf8 values #25 = Utf8 ()[Lcn/redis/model/Season; #26 = Utf8 Code 大专栏  解密JDK8 枚举 #27 = Utf8 LineNumberTable #28 = Utf8 valueOf #29 = Utf8 (Ljava/lang/String;)Lcn/redis/model/Season; #30 = Utf8 <init> #31 = Utf8 (Ljava/lang/String;I)V #32 = Utf8 Signature #33 = Utf8 ()V #34 = Utf8 <clinit> #35 = Utf8 Ljava/lang/Enum<Lcn/redis/model/Season;>; #36 = Utf8 SourceFile #37 = Utf8 Season.java #38 = NameAndType #22:#23 // $VALUES:[Lcn/redis/model/Season; #39 = Class #23 // "[Lcn/redis/model/Season;" #40 = NameAndType #49:#50 // clone:()Ljava/lang/Object; #41 = Utf8 cn/redis/model/Season #42 = NameAndType #28:#51 // valueOf:(Ljava/lang/Class;Ljava/lan g/String;)Ljava/lang/Enum; #43 = NameAndType #30:#31 // "<init>":(Ljava/lang/String;I)V #44 = NameAndType #17:#18 // SPRING:Lcn/redis/model/Season; #45 = NameAndType #19:#18 // SUMMER:Lcn/redis/model/Season; #46 = NameAndType #20:#18 // AUTUMN:Lcn/redis/model/Season; #47 = NameAndType #21:#18 // WINTER:Lcn/redis/model/Season; #48 = Utf8 java/lang/Enum #49 = Utf8 clone #50 = Utf8 ()Ljava/lang/Object; #51 = Utf8 (Ljava/lang/Class;Ljava/lang/String;)Ljava/lang/Enum; { public static final cn.redis.model.Season SPRING; descriptor: Lcn/redis/model/Season; flags: ACC_PUBLIC, ACC_STATIC, ACC_FINAL, ACC_ENUM public static final cn.redis.model.Season SUMMER; descriptor: Lcn/redis/model/Season; flags: ACC_PUBLIC, ACC_STATIC, ACC_FINAL, ACC_ENUM public static final cn.redis.model.Season AUTUMN; descriptor: Lcn/redis/model/Season; flags: ACC_PUBLIC, ACC_STATIC, ACC_FINAL, ACC_ENUM public static final cn.redis.model.Season WINTER; descriptor: Lcn/redis/model/Season; flags: ACC_PUBLIC, ACC_STATIC, ACC_FINAL, ACC_ENUM public static cn.redis.model.Season[] values(); descriptor: ()[Lcn/redis/model/Season; flags: ACC_PUBLIC, ACC_STATIC Code: stack=1, locals=0, args_size=0 0: getstatic #1 // Field $VALUES:[Lcn/redis/model/ Season; 3: invokevirtual #2 // Method "[Lcn/redis/model/Season ;".clone:()Ljava/lang/Object; 6: checkcast #3 // class "[Lcn/redis/model/Season; " 9: areturn LineNumberTable: line 3: 0 public static cn.redis.model.Season valueOf(java.lang.String); descriptor: (Ljava/lang/String;)Lcn/redis/model/Season; flags: ACC_PUBLIC, ACC_STATIC Code: stack=2, locals=1, args_size=1 0: ldc #4 // class cn/redis/model/Season 2: aload_0 3: invokestatic #5 // Method java/lang/Enum.valueOf:( Ljava/lang/Class;Ljava/lang/String;)Ljava/lang/Enum; 6: checkcast #4 // class cn/redis/model/Season 9: areturn LineNumberTable: line 3: 0 static {}; descriptor: ()V flags: ACC_STATIC Code: stack=4, locals=0, args_size=0 0: new #4 // class cn/redis/model/Season 3: dup 4: ldc #7 // String SPRING 6: iconst_0 7: invokespecial #8 // Method "<init>":(Ljava/lang/Str ing;I)V 10: putstatic #9 // Field SPRING:Lcn/redis/model/Se ason; 13: new #4 // class cn/redis/model/Season 16: dup 17: ldc #10 // String SUMMER 19: iconst_1 20: invokespecial #8 // Method "<init>":(Ljava/lang/Str ing;I)V 23: putstatic #11 // Field SUMMER:Lcn/redis/model/Se ason; 26: new #4 // class cn/redis/model/Season 29: dup 30: ldc #12 // String AUTUMN 32: iconst_2 33: invokespecial #8 // Method "<init>":(Ljava/lang/Str ing;I)V 36: putstatic #13 // Field AUTUMN:Lcn/redis/model/Se ason; 39: new #4 // class cn/redis/model/Season 42: dup 43: ldc #14 // String WINTER 45: iconst_3 46: invokespecial #8 // Method "<init>":(Ljava/lang/Str ing;I)V 49: putstatic #15 // Field WINTER:Lcn/redis/model/Se ason; 52: iconst_4 53: anewarray #4 // class cn/redis/model/Season 56: dup 57: iconst_0 58: getstatic #9 // Field SPRING:Lcn/redis/model/Se ason; 61: aastore 62: dup 63: iconst_1 64: getstatic #11 // Field SUMMER:Lcn/redis/model/Se ason; 67: aastore 68: dup 69: iconst_2 70: getstatic #13 // Field AUTUMN:Lcn/redis/model/Se ason; 73: aastore 74: dup 75: iconst_3 76: getstatic #15 // Field WINTER:Lcn/redis/model/Se ason; 79: aastore 80: putstatic #1 // Field $VALUES:[Lcn/redis/model/ Season; 83: return LineNumberTable: line 4: 0 line 5: 13 line 6: 26 line 7: 39 line 3: 52 } Signature: #35 // Ljava/lang/Enum<Lcn/redis/model/Seaso n;>; SourceFile: "Season.java"

从上反编译结果可知

• java代码中的Season转换成了继承自的java.lang.enum的类
• 既然隐式继承自java.lang.enum,也就意味java代码中,Season不能再继承其他的类
• Season被标记成了final,意味着它不能被继承
• 静态块static {}; 0~52为实例化SPRING, SUMMER, AUTUMN, WINTER
• 53~83为创建Season[]数组$VALUES,并将上面的四个对象放入数组的操作.
• 8: invokevirtual #4 在switch-case中,还是将Enum转成了int值(通过调用Enum.oridinal()方法)