Singleton

问题：编写一个Singleton类

　简单的方式

package cn.changb.singleton;

/**
* 简单的方式:弊端是在并发执行时，14行处可能存在多个实例
*/
public class SingletonDemo1 {
private static SingletonDemo1 INSTANCE;

private SingletonDemo1() {
}

public static SingletonDemo1 getInstance() {
if (INSTANCE == null) {
INSTANCE = new SingletonDemo1();
}
return INSTANCE;
}
}

　公共域方式

package cn.changb.singleton;

/**
* 公共域方式：简单，不存在并发出现多实例的问题，但弊端是返回实例时增加一些额外的逻辑
*/
public class SingletonDemo2 {
public static final SingletonDemo2 INSTANCE = new SingletonDemo2();

private SingletonDemo2() {
}
}

　工厂化方式

package cn.changb.singleton;

/**
* 工厂化方式：可以在实例化过程中(11行处)添加逻辑
*/
public class SingletonDemo3 {
private static final SingletonDemo3 INSTANCE = new SingletonDemo3();

private SingletonDemo3() {
}

public static SingletonDemo3 getInstance() {
return INSTANCE;
}
}

　枚举类方式

package cn.changb.singleton;

/**
* 枚举类方式：最佳实践，推荐方式
*
* 不存在多个实例问题、反序列化问题！
*/
public enum SingletonDemo4 {
INSTANCE;
}

　SingletonTest

package cn.changb.singleton;

public class SingletonTest {

public static void main(String[] args) {
// TODO Auto-generated method stub
System.out.println("SingletonDemo1" + SingletonDemo1.getInstance());
System.out.println("SingletonDemo1" + SingletonDemo1.getInstance());

System.out.println("SingletonDemo2" + SingletonDemo2.INSTANCE);
System.out.println("SingletonDemo2" + SingletonDemo2.INSTANCE);

System.out.println("SingletonDemo3" + SingletonDemo3.getInstance());
System.out.println("SingletonDemo3" + SingletonDemo3.getInstance());

System.out.println("SingletonDemo4" + SingletonDemo4.INSTANCE);
System.out.println("SingletonDemo4" + SingletonDemo4.INSTANCE);
}

}