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);
    }

}