java中枚举声明需用到enum关键字简单的如：
public enum TestEnum {
Mon,
Tue,
Wen,
Fri
}
但是我们查看一下编译后得到的汇编就能知道，enum编译后得到的还是一个class，现在用javap -verbose TestEnum命令查看编译得到的class文件：
Classfile /F:/TestEnum.class
Last modified 2017-5-8; size 868 bytes
MD5 checksum 485a9071bf44ca9406beceea0dbca950
Compiled from "TestEnum.java"
public final class TestEnum extends java.lang.Enum<TestEnum>
minor version: 0
major version: 52
flags: ACC_PUBLIC, ACC_FINAL, ACC_SUPER, ACC_ENUM
Constant pool:
#1 = Fieldref #4.#38 // TestEnum.$VALUES:[LTestEnum;
#2 = Methodref #39.#40 // "[LTestEnum;".clone:()Ljava/lang/Object;
#3 = Class #23 // "[LTestEnum;"
#4 = Class #41 // TestEnum
#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 // Mon
#8 = Methodref #4.#43 // TestEnum."<init>":(Ljava/lang/String;I)V
#9 = Fieldref #4.#44 // TestEnum.Mon:LTestEnum;
#10 = String #19 // Tue
#11 = Fieldref #4.#45 // TestEnum.Tue:LTestEnum;
#12 = String #20 // Wen
#13 = Fieldref #4.#46 // TestEnum.Wen:LTestEnum;
#14 = String #21 // Fri
#15 = Fieldref #4.#47 // TestEnum.Fri:LTestEnum;
#16 = Class #48 // java/lang/Enum
#17 = Utf8 Mon
#18 = Utf8 LTestEnum;
#19 = Utf8 Tue
#20 = Utf8 Wen
#21 = Utf8 Fri
#22 = Utf8 $VALUES
#23 = Utf8 [LTestEnum;
#24 = Utf8 values
#25 = Utf8 ()[LTestEnum;
#26 = Utf8 Code
#27 = Utf8 LineNumberTable
#28 = Utf8 valueOf
#29 = Utf8 (Ljava/lang/String;)LTestEnum;
#30 = Utf8 <init>
#31 = Utf8 (Ljava/lang/String;I)V
#32 = Utf8 Signature
#33 = Utf8 ()V
#34 = Utf8 <clinit>
#35 = Utf8 Ljava/lang/Enum<LTestEnum;>;
#36 = Utf8 SourceFile
#37 = Utf8 TestEnum.java
#38 = NameAndType #22:#23 // $VALUES:[LTestEnum;
#39 = Class #23 // "[LTestEnum;"
#40 = NameAndType #49:#50 // clone:()Ljava/lang/Object;
#41 = Utf8 TestEnum
#42 = NameAndType #28:#51 // valueOf:(Ljava/lang/Class;Ljava/lang/String;)Ljava/lang/Enum;
#43 = NameAndType #30:#31 // "<init>":(Ljava/lang/String;I)V
#44 = NameAndType #17:#18 // Mon:LTestEnum;
#45 = NameAndType #19:#18 // Tue:LTestEnum;
#46 = NameAndType #20:#18 // Wen:LTestEnum;
#47 = NameAndType #21:#18 // Fri:LTestEnum;
#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 TestEnum Mon;
descriptor: LTestEnum;
flags: ACC_PUBLIC, ACC_STATIC, ACC_FINAL, ACC_ENUM
public static final TestEnum Tue;
descriptor: LTestEnum;
flags: ACC_PUBLIC, ACC_STATIC, ACC_FINAL, ACC_ENUM
public static final TestEnum Wen;
descriptor: LTestEnum;
flags: ACC_PUBLIC, ACC_STATIC, ACC_FINAL, ACC_ENUM
public static final TestEnum Fri;
descriptor: LTestEnum;
flags: ACC_PUBLIC, ACC_STATIC, ACC_FINAL, ACC_ENUM
private static final TestEnum[] $VALUES;
descriptor: [LTestEnum;
flags: ACC_PRIVATE, ACC_STATIC, ACC_FINAL, ACC_SYNTHETIC
public static TestEnum[] values();
descriptor: ()[LTestEnum;
flags: ACC_PUBLIC, ACC_STATIC
Code:
stack=1, locals=0, args_size=0
0: getstatic #1 // Field $VALUES:[LTestEnum;
3: invokevirtual #2 // Method "[LTestEnum;".clone:()Ljava/lang/Object;
6: checkcast #3 // class "[LTestEnum;"
9: areturn
LineNumberTable:
line 1: 0
public static TestEnum valueOf(java.lang.String);
descriptor: (Ljava/lang/String;)LTestEnum;
flags: ACC_PUBLIC, ACC_STATIC
Code:
stack=2, locals=1, args_size=1
0: ldc #4 // class TestEnum
2: aload_0
3: invokestatic #5 // Method java/lang/Enum.valueOf:(Ljava/lang/Class;Ljava/lang/String;)Ljava/lang/Enum;
6: checkcast #4 // class TestEnum
9: areturn
LineNumberTable:
line 1: 0
private TestEnum();
descriptor: (Ljava/lang/String;I)V
flags: ACC_PRIVATE
Code:
stack=3, locals=3, args_size=3
0: aload_0
1: aload_1
2: iload_2
3: invokespecial #6 // Method java/lang/Enum."<init>":(Ljava/lang/String;I)V
6: return
LineNumberTable:
line 1: 0
Signature: #33 // ()V
static {};
descriptor: ()V
flags: ACC_STATIC
Code:
stack=4, locals=0, args_size=0
0: new #4 // class TestEnum
3: dup
4: ldc #7 // String Mon
6: iconst_0
7: invokespecial #8 // Method "<init>":(Ljava/lang/String;I)V
10: putstatic #9 // Field Mon:LTestEnum;
13: new #4 // class TestEnum
16: dup
17: ldc #10 // String Tue
19: iconst_1
20: invokespecial #8 // Method "<init>":(Ljava/lang/String;I)V
23: putstatic #11 // Field Tue:LTestEnum;
26: new #4 // class TestEnum
29: dup
30: ldc #12 // String Wen
32: iconst_2
33: invokespecial #8 // Method "<init>":(Ljava/lang/String;I)V
36: putstatic #13 // Field Wen:LTestEnum;
39: new #4 // class TestEnum
42: dup
43: ldc #14 // String Fri
45: iconst_3
46: invokespecial #8 // Method "<init>":(Ljava/lang/String;I)V
49: putstatic #15 // Field Fri:LTestEnum;
52: iconst_4
53: anewarray #4 // class TestEnum
56: dup
57: iconst_0
58: getstatic #9 // Field Mon:LTestEnum;
61: aastore
62: dup
63: iconst_1
64: getstatic #11 // Field Tue:LTestEnum;
67: aastore
68: dup
69: iconst_2
70: getstatic #13 // Field Wen:LTestEnum;
73: aastore
74: dup
75: iconst_3
76: getstatic #15 // Field Fri:LTestEnum;
79: aastore
80: putstatic #1 // Field $VALUES:[LTestEnum;
83: return
LineNumberTable:
line 2: 0
line 3: 13
line 4: 26
line 5: 39
line 1: 52
}
Signature: #35 // Ljava/lang/Enum<LTestEnum;>;
SourceFile: "TestEnum.java"
现在一点一点解释
1.enum最终编译得到一个继承java.lang.Enum的final class（Enum的源码这里不做讲解，很简单）
2.这个类TestEnum类，有几个类型为自身的静态最终变量Mon、Tue、Wen、Fri也就是在enum中的几个声明，以及一个TestEnum[]数组变量$VALUES
3.静态方法values()，作用是返回该类的静态变量数组$VALUES
4.静态方法valueOf()，内部调用Enum类的方法
public static <T extends Enum<T>> T valueOf(Class<T> enumType,
String name) {
//enumConstantDirectory返回一个第一次使用时懒加载的，简单名称与Enum实例 键值对的map
T result = enumType.enumConstantDirectory().get(name);
if (result != null)
return result;
if (name == null)
throw new NullPointerException("Name is null");
throw new IllegalArgumentException(
"No enum constant " + enumType.getCanonicalName() + "." + name);
}
5.私有构造方法private TestEnum()；内部就是调用enum的protected构造方法
6.静态代码块，它的作用是以枚举各个元素对应的String值和从0开始的int值为参数，依次初始化TestEnum对象并赋值给对应的静态最终变量（int值跟enum的ordinal相对应，String值跟enum的name相对应），接下来就是把这些静态变量值放到$VALUES数组中
所有枚举就是含有自身静态变量实例的类