原理

那我们定义枚举类型后，到底发生了什么呢？我们对枚举的实现原理进行探究。

我们来解析下Color.class文件，命令javap Color

public final class Color extends java.lang.Enum<Color> {

  public static final Color Red;

  public static final Color Blue;

  public static final Color Green;

  public static final Color[] $VALUES;

  public static Color[] values();

  public static Color valueOf(java.lang.String);

  static {};

}

从解析后的文件中我们可以知道

1.枚举类是final 的，不能被继承

2.枚举类在经过编译后生成一个继承java.lang.Enum 的类 Color

3.编译后的枚举值，是该类的Color类型的成员变量，如 Red、Bule、Green ，并且是final static 修饰

4.枚举编译后的类添加了静态的values() 和valueOf(java.lang.String) 方法

5.静态代码块static {}

进一步细化Color.class

命令javap -c Color

public final class Color extends java.lang.Enum<Color> {

  public static final Color Red;

  public static final Color Blue;

  public static final Color Green;

  public static Color[] values();

    Code:

       0: getstatic     #1                  // Field $VALUES:[LColor;

       3: invokevirtual #2                  // Method "[LColor;".clone:()Ljava/lang/Object;

       6: checkcast     #3                  // class "[LColor;"

       9: areturn



  public static Color valueOf(java.lang.String);

    Code:

       0: ldc           #4                  // class Color

       2: aload_0

       3: invokestatic  #5                  // Method java/lang/Enum.valueOf:(Ljava/lang/Class;Ljava/lang/String;)Ljav

a/lang/Enum;

       6: checkcast     #4                  // class Color

       9: areturn



  static {};

    Code:

       0: new           #4                  // class Color

       3: dup

       4: ldc           #7                  // String Red

       6: iconst_0

       7: invokespecial #8                  // Method "<init>":(Ljava/lang/String;I)V

      10: putstatic     #9                  // Field Red:LColor;

      13: new           #4                  // class Color

      16: dup

      17: ldc           #10                 // String Blue

      19: iconst_1

      20: invokespecial #8                  // Method "<init>":(Ljava/lang/String;I)V

      23: putstatic     #11                 // Field Blue:LColor;

      26: new           #4                  // class Color

      29: dup

      30: ldc           #12                 // String Green

      32: iconst_2

      33: invokespecial #8                  // Method "<init>":(Ljava/lang/String;I)V

      36: putstatic     #13                 // Field Green:LColor;

      

      39: iconst_3

      40: anewarray     #4                  // class Color

      43: dup

      

      44: iconst_0

      45: getstatic     #9                  // Field Red:LColor;

      48: aastore

      49: dup

      50: iconst_1

      51: getstatic     #11                 // Field Blue:LColor;

      54: aastore

      55: dup

      56: iconst_2

      57: getstatic     #13                 // Field Green:LColor;

      60: aastore

      61: putstatic     #1                  // Field $VALUES:[LColor;

      64: return

}

还原后的代码如下：

public final class Color extends java.lang.Enum<Color> {

    //定义的枚举成员

    public static final Color Red;

    public static final Color Blue;

    public static final Color Green;

    //编译器自动生成的 javap -c 还查不出来，疑惑

    public static final /* synthetic */ Color[] $VALUES;//编译器自动生成的

    public static Color[] values(){

        /**

         * 0: getstatic     #1                  // Field $VALUES:[LColor;

         * 3: invokevirtual #2                  // Method "[LColor;".clone:()Ljava/lang/Object;

         * 6: checkcast     #3                  // class "[LColor;"

         * 9: areturn

         */

        return $VALUES.clone();

    }

    public static Color valueOf(java.lang.String s){

        /**

         * 0: ldc           #4                  // class Color

         * 2: aload_0

         * 3: invokestatic  #5                  // Method java/lang/Enum.valueOf:(Ljava/lang/Class;Ljava/lang/String;)Ljav

         * a/lang/Enum;

         * 6: checkcast     #4                  // class Color

         * 9: areturn

         */

        return Enum.valueOf(Color.class,s);

    }

    protected Color(String name, int ordinal) {

        super(name, ordinal);

    }

    static {

        /**

         * 0: new           #4                  // class Color

         * 3: dup

         * 4: ldc           #7                  // String Red

         * 6: iconst_0

         * 7: invokespecial #8                  // Method "<init>":(Ljava/lang/String;I)V

         * 10: putstatic     #9                  // Field Red:LColor;

         */

        Red=new Color("Red",0);

        /**

         * 13: new           #4                  // class Color

         * 16: dup

         * 17: ldc           #10                 // String Blue

         * 19: iconst_1

         * 20: invokespecial #8                  // Method "<init>":(Ljava/lang/String;I)V

         * 23: putstatic     #11                 // Field Blue:LColor;

         */

        Blue=new Color("Blue",1);

        /**

         * 26: new           #4                  // class Color

         * 29: dup

         * 30: ldc           #12                 // String Green

         * 32: iconst_2

         * 33: invokespecial #8                  // Method "<init>":(Ljava/lang/String;I)V

         * 36: putstatic     #13                 // Field Green:LColor;

         */

        Green=new Color("Green",2);

        /**

         * 39: iconst_3

         * 40: anewarray     #4                  // class Color

         * 43: dup

         * 44: iconst_0

         * 45: getstatic     #9                  // Field Red:LColor;

         * 48: aastore

         * 49: dup

         * 50: iconst_1

         * 51: getstatic     #11                 // Field Blue:LColor;

         * 54: aastore

         * 55: dup

         * 56: iconst_2

         * 57: getstatic     #13                 // Field Green:LColor;

         * 60: aastore

         * 61: putstatic     #1                  // Field $VALUES:[LColor;

         */

        $VALUES=new Color[]{Red,Blue,Green};

    };