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