java枚举类

java枚举类：

public enum Animal {
    Dog,
    Pig,
    Fish
}

javap -p Animal.class:

public final class main.com.example.test.Animal extends java.lang.Enum<main.com.example.test.Animal> {
  public static final main.com.example.test.Animal Dog;
  public static final main.com.example.test.Animal Pig;
  public static final main.com.example.test.Animal Fish;
  private static final main.com.example.test.Animal[] $VALUES;
  public static main.com.example.test.Animal[] values();
  public static main.com.example.test.Animal valueOf(java.lang.String);
  private main.com.example.test.Animal();
  static {};
}

1. Animal继承Enum
2. 每个枚举值都是静态变量

javap -c Animal.class

public final class main.com.example.test.Animal extends java.lang.Enum<main.com.example.test.Animal> {
  public static final main.com.example.test.Animal Dog;

  public static final main.com.example.test.Animal Pig;

  public static final main.com.example.test.Animal Fish;

  public static main.com.example.test.Animal[] values();
    Code:
       0: getstatic     #1                  // Field $VALUES:[Lmain/com/example/test/Animal;
       3: invokevirtual #2                  // Method "[Lmain/com/example/test/Animal;".clone:()Ljava/lang/Object;
       6: checkcast     #3                  // class "[Lmain/com/example/test/Animal;"
       9: areturn

  public static main.com.example.test.Animal valueOf(java.lang.String);
    Code:
       0: ldc           #4                  // class main/com/example/test/Animal
       2: aload_0
       3: invokestatic  #5                  // Method java/lang/Enum.valueOf:(Ljava/lang/Class;Ljava/lang/String;)Ljava/lang/Enum;
       6: checkcast     #4                  // class main/com/example/test/Animal
       9: areturn

  static {};
    Code:
       0: new           #4                  // class main/com/example/test/Animal
       3: dup
       4: ldc           #7                  // String Dog
       6: iconst_0
       7: invokespecial #8                  // Method "<init>":(Ljava/lang/String;I)V
      10: putstatic     #9                  // Field Dog:Lmain/com/example/test/Animal;
      13: new           #4                  // class main/com/example/test/Animal
      16: dup
      17: ldc           #10                 // String Pig
      19: iconst_1
      20: invokespecial #8                  // Method "<init>":(Ljava/lang/String;I)V
      23: putstatic     #11                 // Field Pig:Lmain/com/example/test/Animal;
      26: new           #4                  // class main/com/example/test/Animal
      29: dup
      30: ldc           #12                 // String Fish
      32: iconst_2
      33: invokespecial #8                  // Method "<init>":(Ljava/lang/String;I)V
      36: putstatic     #13                 // Field Fish:Lmain/com/example/test/Animal;
      39: iconst_3
      40: anewarray     #4                  // class main/com/example/test/Animal
      43: dup
      44: iconst_0
      45: getstatic     #9                  // Field Dog:Lmain/com/example/test/Animal;
      48: aastore
      49: dup
      50: iconst_1
      51: getstatic     #11                 // Field Pig:Lmain/com/example/test/Animal;
      54: aastore
      55: dup
      56: iconst_2
      57: getstatic     #13                 // Field Fish:Lmain/com/example/test/Animal;
      60: aastore
      61: putstatic     #1                  // Field $VALUES:[Lmain/com/example/test/Animal;
      64: return
}

1. 在static代码块中会对每个枚举值进行初始化

2. $VALUES是一个静态变量，是枚举值数组，在static代码块中进行初始化和添加

3. values()方法返回的就是$VALUES数组的拷贝，浅拷贝

4. valuesOf()方法根据字符串返回对应的枚举值，调用了Enum.valueOf()方法：

   //Enum.java
   public static <T extends Enum<T>> T valueOf(Class<T> enumType,
                                                   String name) {
       T result = enumType.enumConstantDirectory().get(name);
       if (result != null)
           return result;
       if (name == null)
           throw new NullPointerException("Name is null");
       throw new IllegalArgumentException(
           "No enum constant " + enumType.getCanonicalName() + "." + name);
   }
   

   enumConstantDirectory的来源可以看Class的源码：

   Map<String, T> enumConstantDirectory() {
       Map<String, T> directory = enumConstantDirectory;
       if (directory == null) {
           T[] universe = getEnumConstantsShared();
           if (universe == null)
               throw new IllegalArgumentException(
                   getName() + " is not an enum type");
           directory = new HashMap<>((int)(universe.length / 0.75f) + 1);
           for (T constant : universe) {
               directory.put(((Enum<?>)constant).name(), constant);
           }
           enumConstantDirectory = directory;
       }
       return directory;
   }
   

   getEnumConstantsShared()方法也在Class.java中：

   T[] getEnumConstantsShared() {
       T[] constants = enumConstants;
       if (constants == null) {
           if (!isEnum()) return null;
           try {
               final Method values = getMethod("values");
               java.security.AccessController.doPrivileged(
                   new java.security.PrivilegedAction<>() {
                       public Void run() {
                               values.setAccessible(true);
                               return null;
                           }
                       });
               @SuppressWarnings("unchecked")
               T[] temporaryConstants = (T[])values.invoke(null);
               enumConstants = constants = temporaryConstants;
           }
           // These can happen when users concoct enum-like classes
           // that don't comply with the enum spec.
           catch (InvocationTargetException | NoSuchMethodException |
                  IllegalAccessException ex) { return null; }
       }
       return constants;
   }
   

   所以可以看到是通过反射调用枚举类中的values()方法得到枚举值的数组，然后生成一个HashMap映射表