from:https://blog.csdn.net/mhmyqn/article/details/48087247
Java从JDK1.5开始支持枚举,也就是说,Java一开始是不支持枚举的,就像泛型一样,都是JDK1.5才加入的新特性。通常一个特性如果在一开始没有提供,在语言发展后期才添加,会遇到一个问题,就是向后兼容性的问题。像Java在1.5中引入的很多特性,为了向后兼容,编译器会帮我们写的源代码做很多事情,比如泛型为什么会擦除类型,为什么会生成桥接方法,foreach迭代,自动装箱/拆箱等,这有个术语叫“语法糖”,而编译器的特殊处理叫“解语法糖”。那么像枚举也是在JDK1.5中才引入的,又是怎么实现的呢?
Java在1.5中添加了java.lang.Enum抽象类,它是所有枚举类型基类。提供了一些基础属性和基础方法。同时,对把枚举用作Set和Map也提供了支持,即java.util.EnumSet和java.util.EnumMap。
如何定义枚举类型
比如表示加减乘除操作,我们可以定义如下枚举:
- package com.mikan;
- /**
- * @author Mikan
- * @date 2015-08-29 12:06
- */
- public enum Operator {
- ADD,
- SUBTRACT,
- MULTIPLY,
- DIVIDE
- }
- package com.mikan;
- /**
- * @author Mikan
- * @date 2015-08-29 12:06
- */
- public enum Operator {
- ADD {
- @Override
- public int calculate(int a, int b) {
- return a + b;
- }
- },
- SUBTRACT {
- @Override
- public int calculate(int a, int b) {
- return a - b;
- }
- },
- MULTIPLY {
- @Override
- public int calculate(int a, int b) {
- return a * b;
- }
- },
- DIVIDE {
- @Override
- public int calculate(int a, int b) {
- if (b == 0) {
- throw new IllegalArgumentException("divisor must not be 0");
- }
- return a / b;
- }
- };
- public abstract int calculate(int a, int b);
- }
- package com.mikan;
- /**
- * @author Mikan
- * @date 2015-08-29 12:06
- */
- public enum Operator {
- ADD ("+") {
- @Override
- public int calculate(int a, int b) {
- return a + b;
- }
- },
- SUBTRACT ("-") {
- @Override
- public int calculate(int a, int b) {
- return a - b;
- }
- },
- MULTIPLY ("*") {
- @Override
- public int calculate(int a, int b) {
- return a * b;
- }
- },
- DIVIDE ("/") {
- @Override
- public int calculate(int a, int b) {
- if (b == 0) {
- throw new IllegalArgumentException("divisor must not be 0");
- }
- return a / b;
- }
- };
- Operator (String operator) {
- this.operator = operator;
- }
- private String operator;
- public abstract int calculate(int a, int b);
- public String getOperator() {
- return operator;
- }
- }
实现原理分析
既然可以像使用普通的类一样使用枚举,编译器究竟为我们做了些什么事呢?要想知道这其中的秘密,最有效的途径就是查看生成的字节码。下面就来看看上面定义的枚举生成的字节码是怎么样的。首先来看看反编译的基本信息:
- localhost:mikan mikan$ javap Operator.class
- Compiled from "Operator.java"
- public abstract class com.mikan.Operator extends java.lang.Enum<com.mikan.Operator> {
- public static final com.mikan.Operator ADD;
- public static final com.mikan.Operator SUBTRACT;
- public static final com.mikan.Operator MULTIPLY;
- public static final com.mikan.Operator DIVIDE;
- public static com.mikan.Operator[] values();
- public static com.mikan.Operator valueOf(java.lang.String);
- public abstract int calculate(int, int);
- public java.lang.String getOperator();
- com.mikan.Operator(java.lang.String, int, java.lang.String, com.mikan.Operator$1);
- static {};
- }
看下面详细的反编译信息:
- localhost:mikan mikan$ javap -c -v Operator.class
- Classfile /Users/mikan/Documents/workspace/project/algorithm/target/classes/com/mikan/Operator.class
- Last modified 2015-8-29; size 1720 bytes
- MD5 checksum 478439554cb827fec3c36cf51c8d36da
- Compiled from "Operator.java"
- public abstract class com.mikan.Operator extends java.lang.Enum<com.mikan.Operator>
- Signature: #67 // Ljava/lang/Enum<Lcom/mikan/Operator;>;
- SourceFile: "Operator.java"
- InnerClasses:
- static #24; //class com/mikan/Operator$4
- static #19; //class com/mikan/Operator$3
- static #14; //class com/mikan/Operator$2
- static #9; //class com/mikan/Operator$1
- minor version: 0
- major version: 51
- flags: ACC_PUBLIC, ACC_SUPER, ACC_ABSTRACT, ACC_ENUM
- Constant pool:
- // 省略常量池信息
- {
- public static final com.mikan.Operator ADD;
- flags: ACC_PUBLIC, ACC_STATIC, ACC_FINAL, ACC_ENUM
- public static final com.mikan.Operator SUBTRACT;
- flags: ACC_PUBLIC, ACC_STATIC, ACC_FINAL, ACC_ENUM
- public static final com.mikan.Operator MULTIPLY;
- flags: ACC_PUBLIC, ACC_STATIC, ACC_FINAL, ACC_ENUM
- public static final com.mikan.Operator DIVIDE;
- flags: ACC_PUBLIC, ACC_STATIC, ACC_FINAL, ACC_ENUM
- public static com.mikan.Operator[] values();
- flags: ACC_PUBLIC, ACC_STATIC
- Code:
- stack=1, locals=0, args_size=0
- 0: getstatic #2 // Field $VALUES:[Lcom/mikan/Operator;
- 3: invokevirtual #3 // Method "[Lcom/mikan/Operator;".clone:()Ljava/lang/Object;
- 6: checkcast #4 // class "[Lcom/mikan/Operator;"
- 9: areturn
- LineNumberTable:
- line 7: 0
- public static com.mikan.Operator valueOf(java.lang.String);
- flags: ACC_PUBLIC, ACC_STATIC
- Code:
- stack=2, locals=1, args_size=1
- 0: ldc_w #5 // class com/mikan/Operator
- 3: aload_0
- 4: invokestatic #6 // Method java/lang/Enum.valueOf:(Ljava/lang/Class;Ljava/lang/String;)Ljava/lang/Enum;
- 7: checkcast #5 // class com/mikan/Operator
- 10: areturn
- LineNumberTable:
- line 7: 0
- LocalVariableTable:
- Start Length Slot Name Signature
- 0 11 0 name Ljava/lang/String;
- public abstract int calculate(int, int);
- flags: ACC_PUBLIC, ACC_ABSTRACT
- public java.lang.String getOperator();
- flags: ACC_PUBLIC
- Code:
- stack=1, locals=1, args_size=1
- 0: aload_0
- 1: getfield #8 // Field operator:Ljava/lang/String;
- 4: areturn
- LineNumberTable:
- line 46: 0
- LocalVariableTable:
- Start Length Slot Name Signature
- 0 5 0 this Lcom/mikan/Operator;
- com.mikan.Operator(java.lang.String, int, java.lang.String, com.mikan.Operator$1);
- flags: ACC_SYNTHETIC
- Code:
- stack=4, locals=5, args_size=5
- 0: aload_0
- 1: aload_1
- 2: iload_2
- 3: aload_3
- 4: invokespecial #1 // Method "<init>":(Ljava/lang/String;ILjava/lang/String;)V
- 7: return
- LineNumberTable:
- line 7: 0
- LocalVariableTable:
- Start Length Slot Name Signature
- 0 8 0 this Lcom/mikan/Operator;
- 0 8 1 x0 Ljava/lang/String;
- 0 8 2 x1 I
- 0 8 3 x2 Ljava/lang/String;
- 0 8 4 x3 Lcom/mikan/Operator$1;
- static {};
- flags: ACC_STATIC
- Code:
- stack=5, locals=0, args_size=0
- 0: new #9 // class com/mikan/Operator$1
- 3: dup
- 4: ldc #10 // String ADD
- 6: iconst_0
- 7: ldc #11 // String +
- 9: invokespecial #12 // Method com/mikan/Operator$1."<init>":(Ljava/lang/String;ILjava/lang/String;)V
- 12: putstatic #13 // Field ADD:Lcom/mikan/Operator;
- 15: new #14 // class com/mikan/Operator$2
- 18: dup
- 19: ldc #15 // String SUBTRACT
- 21: iconst_1
- 22: ldc #16 // String -
- 24: invokespecial #17 // Method com/mikan/Operator$2."<init>":(Ljava/lang/String;ILjava/lang/String;)V
- 27: putstatic #18 // Field SUBTRACT:Lcom/mikan/Operator;
- 30: new #19 // class com/mikan/Operator$3
- 33: dup
- 34: ldc #20 // String MULTIPLY
- 36: iconst_2
- 37: ldc #21 // String *
- 39: invokespecial #22 // Method com/mikan/Operator$3."<init>":(Ljava/lang/String;ILjava/lang/String;)V
- 42: putstatic #23 // Field MULTIPLY:Lcom/mikan/Operator;
- 45: new #24 // class com/mikan/Operator$4
- 48: dup
- 49: ldc #25 // String DIVIDE
- 51: iconst_3
- 52: ldc #26 // String /
- 54: invokespecial #27 // Method com/mikan/Operator$4."<init>":(Ljava/lang/String;ILjava/lang/String;)V
- 57: putstatic #28 // Field DIVIDE:Lcom/mikan/Operator;
- 60: iconst_4
- 61: anewarray #5 // class com/mikan/Operator
- 64: dup
- 65: iconst_0
- 66: getstatic #13 // Field ADD:Lcom/mikan/Operator;
- 69: aastore
- 70: dup
- 71: iconst_1
- 72: getstatic #18 // Field SUBTRACT:Lcom/mikan/Operator;
- 75: aastore
- 76: dup
- 77: iconst_2
- 78: getstatic #23 // Field MULTIPLY:Lcom/mikan/Operator;
- 81: aastore
- 82: dup
- 83: iconst_3
- 84: getstatic #28 // Field DIVIDE:Lcom/mikan/Operator;
- 87: aastore
- 88: putstatic #2 // Field $VALUES:[Lcom/mikan/Operator;
- 91: return
- LineNumberTable:
- line 9: 0
- line 15: 15
- line 21: 30
- line 27: 45
- line 7: 60
- }
- localhost:mikan mikan$
- InnerClasses:
- static #24; //class com/mikan/Operator$4
- static #19; //class com/mikan/Operator$3
- static #14; //class com/mikan/Operator$2
- static #9; //class com/mikan/Operator$1
静态代码块:
- static {};
- flags: ACC_STATIC
- Code:
- stack=5, locals=0, args_size=0
- // 创建一个Operator$1的内部类对象
- 0: new #9 // class com/mikan/Operator$1
- 3: dup
- // 接下来的三条指令分别是把三个参数推送到栈顶,然后调用Operator$1的编译器生成的<init>方法
- 4: ldc #10 // String ADD
- 6: iconst_0
- 7: ldc #11 // String +
- // 调用<init>方法
- 9: invokespecial #12 // Method com/mikan/Operator$1."<init>":(Ljava/lang/String;ILjava/lang/String;)V
- // 设置ADD属性的值为新创建的对象
- 12: putstatic #13 // Field ADD:Lcom/mikan/Operator;
- // 接下来是分别初始化另外三个属性SUBTRACT、MULTIPLY、DIVIDE,这里就不再重复
- 15: new #14 // class com/mikan/Operator$2
- 18: dup
- 19: ldc #15 // String SUBTRACT
- 21: iconst_1
- 22: ldc #16 // String -
- 24: invokespecial #17 // Method com/mikan/Operator$2."<init>":(Ljava/lang/String;ILjava/lang/String;)V
- 27: putstatic #18 // Field SUBTRACT:Lcom/mikan/Operator;
- 30: new #19 // class com/mikan/Operator$3
- 33: dup
- 34: ldc #20 // String MULTIPLY
- 36: iconst_2
- 37: ldc #21 // String *
- 39: invokespecial #22 // Method com/mikan/Operator$3."<init>":(Ljava/lang/String;ILjava/lang/String;)V
- 42: putstatic #23 // Field MULTIPLY:Lcom/mikan/Operator;
- 45: new #24 // class com/mikan/Operator$4
- 48: dup
- 49: ldc #25 // String DIVIDE
- 51: iconst_3
- 52: ldc #26 // String /
- 54: invokespecial #27 // Method com/mikan/Operator$4."<init>":(Ljava/lang/String;ILjava/lang/String;)V
- 57: putstatic #28 // Field DIVIDE:Lcom/mikan/Operator;
- // 下面是new了一个长度为4的Operator类型的数组,并分别设置数组中各元素的值为上面的四个属性的值
- 60: iconst_4
- 61: anewarray #5 // class com/mikan/Operator
- 64: dup
- 65: iconst_0
- 66: getstatic #13 // Field ADD:Lcom/mikan/Operator;
- 69: aastore
- 70: dup
- 71: iconst_1
- 72: getstatic #18 // Field SUBTRACT:Lcom/mikan/Operator;
- 75: aastore
- 76: dup
- 77: iconst_2
- 78: getstatic #23 // Field MULTIPLY:Lcom/mikan/Operator;
- 81: aastore
- 82: dup
- 83: iconst_3
- 84: getstatic #28 // Field DIVIDE:Lcom/mikan/Operator;
- 87: aastore
- //下面是设置属性$VALUES的值为刚创建的数组
- 88: putstatic #2 // Field $VALUES:[Lcom/mikan/Operator;
- 91: return
- Operator ADD = new Operator1();
- Operator SUBTRACT = new Operator1();
- Operator MULTIPLY = new Operator1();
- Operator DIVIDE = new Operator1();
- Operator[] $VALUES = new Operator[4];
- $VALUES[0] = ADD;
- $VALUES[1] = SUBTRACT;
- $VALUES[2] = MULTIPLY;
- $VALUES[3] = DIVIDE;
- public static com.mikan.Operator[] values();
- flags: ACC_PUBLIC, ACC_STATIC
- Code:
- stack=1, locals=0, args_size=0
- 0: getstatic #2 // Field $VALUES:[Lcom/mikan/Operator;
- 3: invokevirtual #3 // Method "[Lcom/mikan/Operator;".clone:()Ljava/lang/Object;
- 6: checkcast #4 // class "[Lcom/mikan/Operator;"
- 9: areturn
- public static com.mikan.Operator[] values() {
- return (Operator[])$VALUES.clone();
- }
- public static com.mikan.Operator valueOf(java.lang.String);
- flags: ACC_PUBLIC, ACC_STATIC
- Code:
- stack=2, locals=1, args_size=1
- 0: ldc_w #5 // class com/mikan/Operator
- 3: aload_0
- 4: invokestatic #6 // Method java/lang/Enum.valueOf:(Ljava/lang/Class;Ljava/lang/String;)Ljava/lang/Enum;
- 7: checkcast #5 // class com/mikan/Operator
- 10: areturn
- public static com.mikan.Operator valueOf(String name) {
- return (Operator)Enum.valueOf(Operator.class, name);
- }
下面看看生成的内部类Operator$1:
- localhost:mikan mikan$ javap Operator\$1.class
- Compiled from "Operator.java"
- final class com.mikan.Operator$1 extends com.mikan.Operator {
- com.mikan.Operator$1(java.lang.String, int, java.lang.String);
- public int calculate(int, int);
- }
- localhost:mikan mikan$
- ADD {
- @Override
- public int calculate(int a, int b) {
- return a + b;
- }
- },
构造方法为什么增加了两个参数?
有一个问题,构造方法我们明明只定义了一个参数,为什么生成的构造方法是三个参数呢?
从Enum类中我们可以看到,为每个枚举都定义了两个属性,name和ordinal,name表示我们定义的枚举常量的名称,如ADD、SUBTRACT等,而ordinal是一个顺序号,根据定义的顺序分别赋予一个整形值,从0开始。在枚举常量初始化时,会自动为初始化这两个字段,设置相应的值,所以才在构造方法中添加了两个参数。即:
- com.mikan.Operator$1(String name, int ordinal, String operator);
我们可以从Enum类的代码中看到,定义的name和ordinal属性都是final的,而且大部分方法也都是final的,特别是clone、readObject、writeObject这三个方法,这三个方法和枚举通过静态代码块来进行初始化一起,它保证了枚举类型的不可变性,不能通过克隆,不能通过序列化和反序列化来复制枚举,这能保证一个枚举常量只是一个实例,即是单例的,所以在effective java中推荐使用枚举来实现单例。
总结
枚举本质上是通过普通的类来实现的,只是编译器为我们进行了处理。每个枚举类型都继承自java.lang.Enum,并自动添加了values和valueOf方法。而每个枚举常量是一个静态常量字段,使用内部类实现,该内部类继承了枚举类。所有枚举常量都通过静态代码块来进行初始化,即在类加载期间就初始化。另外通过把clone、readObject、writeObject这三个方法定义为final的,同时实现是抛出相应的异常。这样保证了每个枚举类型及枚举常量都是不可变的。可以利用枚举的这两个特性来实现线程安全的单例。