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JDK1.5新特性.Enum深度解析

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  • JDK
iBATISDAOJavaJVM 
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Enum

一般用来表示一组相同类型的常量.比如性别，月份，颜色等.
1.声名Enum 
Public enum Sex{
MAN,
WOMAN
}
Public enum Color{
RED,BLUE,GREEN,BLACK
}

2. 使用Enum
JavaEnum简单的用法一般用于代表一组常量，可用来代码一类相同类型的常量值.[常量一般大写，注意规范] 
package org.yclframework.auth.test.dao.ibatis;

public enum Sex {
	MAN("男"),WOMAN("女");

	private final String sex;

	private Sex(String sex){
		this.sex=sex;
	}

	public String getSex(){
		return sex;
	}

	public static void print(String sex){
		if(sex.equals("MAN")){
			System.out.println(Sex.MAN.getSex());
		}else if(sex.equals("WOMAN")){
			System.out.println(Sex.WOMAN.getSex());
		}

	}

}

建立测试类如下: 
package org.yclframework.auth.test.dao.ibatis;

public class Testenum {

	/**
	 * @param args
	 */
	public static void main(String[] args) {
		for(Sex sex:Sex.values()){
			System.out.println(sex.getSex());
		}
		Sex.print("MAN");

	}

}

这个Enum类到底和正常的类有何区别，请看以下分解: 
// Compiled from Sex.java (version 1.5 : 49.0, super bit)
// Signature: Ljava/lang/Enum<Lorg/yclframework/auth/test/dao/ibatis/Sex;>;
public final enum org.yclframework.auth.test.dao.ibatis.Sex {
  //enum是最终类，不可改变的.
  // Field descriptor #6 Lorg/yclframework/auth/test/dao/ibatis/Sex;
  public static final enum org.yclframework.auth.test.dao.ibatis.Sex MAN;//声名自身变量
  
  // Field descriptor #6 Lorg/yclframework/auth/test/dao/ibatis/Sex;
  public static final enum org.yclframework.auth.test.dao.ibatis.Sex WOMAN;//声名自身变量
  
  // Field descriptor #9 Ljava/lang/String;
  private final java.lang.String sex;//声名一个string类型
  
  // Field descriptor #11 [Lorg/yclframework/auth/test/dao/ibatis/Sex;
  private static final synthetic org.yclframework.auth.test.dao.ibatis.Sex[] ENUM$VALUES;//声名一个数组
  
  // Method descriptor #13 ()V
  // Stack: 5, Locals: 0
  static {};
     0  new org.yclframework.auth.test.dao.ibatis.Sex [1]//创建一个sex对象
     3  dup制栈顶数值并将复制值压入栈顶
     4  ldc <String "MAN"> [15] 将String型常量值从常量池中推送至栈顶
     6  iconst_0 将int型0推送至栈顶
     7  ldc <String "男"> [16] 将String型常量值从常量池中推送至栈顶
     9  invokespecial org.yclframework.auth.test.dao.ibatis.Sex(java.lang.String, int, java.lang.String) [18]调用Sext的方法，这个方法参数为String ,int,String
12  putstatic org.yclframework.auth.test.dao.ibatis.Sex.MAN : org.yclframework.auth.test.dao.ibatis.Sex [22]
为指定的类的静态域赋值
    15  new org.yclframework.auth.test.dao.ibatis.Sex [1]
    18  dup
    19  ldc <String "WOMAN"> [24]
    21  iconst_1
    22  ldc <String "女"> [25]
    24  invokespecial org.yclframework.auth.test.dao.ibatis.Sex(java.lang.String, int, java.lang.String) [18]
    27  putstatic org.yclframework.auth.test.dao.ibatis.Sex.WOMAN : org.yclframework.auth.test.dao.ibatis.Sex [27]
    30  iconst_2
    31  anewarray org.yclframework.auth.test.dao.ibatis.Sex [1]
    34  dup
    35  iconst_0
36  getstatic org.yclframework.auth.test.dao.ibatis.Sex.MAN : org.yclframework.auth.test.dao.ibatis.Sex [22]
获取指定类的静态域，并将其值压入栈顶
    39  aastore
    40  dup
    41  iconst_1
    42  getstatic org.yclframework.auth.test.dao.ibatis.Sex.WOMAN : org.yclframework.auth.test.dao.ibatis.Sex [27]
    45  aastore
    46  putstatic org.yclframework.auth.test.dao.ibatis.Sex.ENUM$VALUES : org.yclframework.auth.test.dao.ibatis.Sex[] [29]
    49  return
      Line numbers:
        [pc: 0, line: 4]
        [pc: 30, line: 3]
  
  // Method descriptor #21 (Ljava/lang/String;ILjava/lang/String;)V
  // Stack: 3, Locals: 4
  private Sex(java.lang.String arg0, int arg1, java.lang.String sex);
     0  aload_0 [this]
     1  aload_1
     2  iload_2
     3  invokespecial java.lang.Enum(java.lang.String int,) [33]
     6  aload_0 [this]
     7  aload_3 [sex]
     8  putfield org.yclframework.auth.test.dao.ibatis.Sex.sex : java.lang.String [36]
    11  return
      Line numbers:
        [pc: 0, line: 8]
        [pc: 6, line: 9]
        [pc: 11, line: 10]
      Local variable table:
        [pc: 0, pc: 12] local: this index: 0 type: org.yclframework.auth.test.dao.ibatis.Sex
        [pc: 0, pc: 12] local: sex index: 3 type: java.lang.String
  
  // Method descriptor #40 ()Ljava/lang/String;
  // Stack: 1, Locals: 1
  public java.lang.String getSex();
    0  aload_0 [this]
    1  getfield org.yclframework.auth.test.dao.ibatis.Sex.sex : java.lang.String [36]
    4  areturn
      Line numbers:
        [pc: 0, line: 13]
      Local variable table:
        [pc: 0, pc: 5] local: this index: 0 type: org.yclframework.auth.test.dao.ibatis.Sex
  
  // Method descriptor #42 (Ljava/lang/String;)V
  // Stack: 2, Locals: 1
  public static void print(java.lang.String sex);
     0  aload_0 [sex]
     1  ldc <String "MAN"> [15]
     3  invokevirtual java.lang.String.equals(java.lang.Object) : boolean [43]
     6  ifeq 24
     9  getstatic java.lang.System.out : java.io.PrintStream [49]
    12  getstatic org.yclframework.auth.test.dao.ibatis.Sex.MAN : org.yclframework.auth.test.dao.ibatis.Sex [22]
    15  invokevirtual org.yclframework.auth.test.dao.ibatis.Sex.getSex() : java.lang.String [55]
    18  invokevirtual java.io.PrintStream.println(java.lang.String) : void [57]
    21  goto 45
    24  aload_0 [sex]
    25  ldc <String "WOMAN"> [24]
    27  invokevirtual java.lang.String.equals(java.lang.Object) : boolean [43]
    30  ifeq 45
    33  getstatic java.lang.System.out : java.io.PrintStream [49]
    36  getstatic org.yclframework.auth.test.dao.ibatis.Sex.WOMAN : org.yclframework.auth.test.dao.ibatis.Sex [27]
    39  invokevirtual org.yclframework.auth.test.dao.ibatis.Sex.getSex() : java.lang.String [55]
    42  invokevirtual java.io.PrintStream.println(java.lang.String) : void [57]
    45  return
      Line numbers:
        [pc: 0, line: 17]
        [pc: 9, line: 18]
        [pc: 24, line: 19]
        [pc: 33, line: 20]
        [pc: 45, line: 23]
      Local variable table:
        [pc: 0, pc: 46] local: sex index: 0 type: java.lang.String
  
  // Method descriptor #63 ()[Lorg/yclframework/auth/test/dao/ibatis/Sex;
  // Stack: 5, Locals: 3
  public static org.yclframework.auth.test.dao.ibatis.Sex[] values();//声名values数组
     0  getstatic org.yclframework.auth.test.dao.ibatis.Sex.ENUM$VALUES : org.yclframework.auth.test.dao.ibatis.Sex[] [29]
     3  dup
     4  astore_0
     5  iconst_0
     6  aload_0
     7  arraylength
     8  dup
     9  istore_1
    10  anewarray org.yclframework.auth.test.dao.ibatis.Sex [1]
    13  dup
    14  astore_2
    15  iconst_0
    16  iload_1
    17  invokestatic java.lang.System.arraycopy(java.lang.Object, int, java.lang.Object, int, int) : void [64]
    20  aload_2
    21  areturn
      Line numbers:
        [pc: 0, line: 1]
  
  // Method descriptor #69 (Ljava/lang/String;)Lorg/yclframework/auth/test/dao/ibatis/Sex;
  // Stack: 2, Locals: 1
  public static org.yclframework.auth.test.dao.ibatis.Sex valueOf(java.lang.String arg0);
     0  ldc <Class org.yclframework.auth.test.dao.ibatis.Sex> [1]
     2  aload_0
     3  invokestatic java.lang.Enum.valueOf(java.lang.Class, java.lang.String) : java.lang.Enum [70]
     6  checkcast org.yclframework.auth.test.dao.ibatis.Sex [1]
     9  areturn
      Line numbers:
        [pc: 0, line: 1]
}

看这个Sex类，我明明很简单的几个方法，为什么会有这么多的解析过程： 
private static final synthetic org.yclframework.auth.test.dao.ibatis.Sex[] ENUM$VALUES;//声名一个数组
MAN("男")既然解析成:
  new org.yclframework.auth.test.dao.ibatis.Sex [1]//创建一个sex对象
  dup制栈顶数值并将复制值压入栈顶
  ldc <String "MAN"> [15] 将String型常量值从常量池中推送至栈顶
  iconst_0 将int型0推送至栈顶
  ldc <String "男"> [16] 将String型常量值从常量池中推送至栈顶

  invokespecial org.yclframework.auth.test.dao.ibatis.Sex(java.lang.String, int, java.lang.String) [18]调用Sex的方法，这个方法参数为String ,int,String
  putstatic org.yclframework.auth.test.dao.ibatis.Sex.MAN : org.yclframework.auth.test.dao.ibatis.Sex [22]
为指定的类的静态域赋值

JVM自动将MAN("男")解析为字符串”MAN”,int下标 0,字符串”男”,再自动invoke Sex(String,int,String)Sex(“MAN”,0,”男”);
   然后在后面又声名了一个数组，并把他们添加进去. 
31  anewarray org.yclframework.auth.test.dao.ibatis.Sex [1]
    34  dup
    35  iconst_0
36  getstatic org.yclframework.auth.test.dao.ibatis.Sex.MAN : org.yclframework.auth.test.dao.ibatis.Sex [22]
获取指定类的静态域，并将其值压入栈顶
    39  aastore
    40  dup
    41  iconst_1
    42  getstatic org.yclframework.auth.test.dao.ibatis.Sex.WOMAN : org.yclframework.auth.test.dao.ibatis.Sex [27]
    45  aastore
    46  putstatic org.yclframework.auth.test.dao.ibatis.Sex.ENUM$VALUES : org.yclframework.auth.test.dao.ibatis.Sex[] [29]
    49  return

在类的最后又添加了一个变量，名为values(),并有一个方法把上面的数组复制过来 
public static org.yclframework.auth.test.dao.ibatis.Sex[] values();//声名values数组
invokestatic java.lang.System.arraycopy(java.lang.Object, int, java.lang.Object, int, int) : void [64]
public static org.yclframework.auth.test.dao.ibatis.Sex valueOf(java.lang.String arg0);

这些方法是后来添加的,因为这个类用了很多的static final 不可改变的固定地址，JVM调用的时候用比较快。让我们反编译这个类来看看固定实现. 
// Decompiled by Jad v1.5.8e2. Copyright 2001 Pavel Kouznetsov.
// Jad home page: http://kpdus.tripod.com/jad.html
// Decompiler options: packimports(3) fieldsfirst ansi space 
// Source File Name:   Sex.java

package org.yclframework.auth.test.dao.ibatis;

import java.io.PrintStream;

public final class Sex extends Enum
{

	public static final Sex MAN;
	public static final Sex WOMAN;
	private final String sex;
	private static final Sex ENUM$VALUES[];

	private Sex(String s, int i, String sex)
	{
		super(s, i);
		this.sex = sex;
	}

	public String getSex()
	{
		return sex;
	}

	public static void print(String sex)
	{
		if (sex.equals("MAN"))
			System.out.println(MAN.getSex());
		else
		if (sex.equals("WOMAN"))
			System.out.println(WOMAN.getSex());
	}

	public static Sex[] values()
	{
		Sex asex[];
		int i;
		Sex asex1[];
		System.arraycopy(asex = ENUM$VALUES, 0, asex1 = new Sex[i = asex.length], 0, i);
		return asex1;
	}

	public static Sex valueOf(String s)
	{
		return (Sex)Enum.valueOf(org/yclframework/auth/test/dao/ibatis/Sex, s);
	}

	static 
	{
		MAN = new Sex("MAN", 0, "男");
		WOMAN = new Sex("WOMAN", 1, "女");
		ENUM$VALUES = (new Sex[] {
			MAN, WOMAN
		});
	}
}

一看代码就清楚了，原来这个类继承了enum,并调用父菜单，而我们可以直接声名为enum类型，添加了源代码，实现的功能却很丰富，这就是JDK5的魅力。
如果你把这个类用javac进行编译，对不起通不过。
Enum类不支持显示继承.
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