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  • 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)
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  • 原文地址：https://www.cnblogs.com/lijianming180/p/12326281.html
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