懒汉式实现单例模式

package pattern;

/**
 * 懒汉式实现单例模式
 * 1、创建一个静态变量
 * 2、构造方法私有化
 * 3、创建一个静态方法获取实例对象，如果对象为空则创建对象
 * @author Administrator
 *
 */
public class Singleton {

    private static volatile Singleton singleton = null;

    private Singleton(){}

    //线程安全且高效
    public static Singleton getInstance(){
        if(singleton == null){
            synchronized (Singleton.class) {
                if(singleton == null){
                    singleton = new Singleton();
                }
            }
        }
        return singleton;
    }

    //线程安全，不高效
    public static Singleton getInstance3(){
        synchronized (Singleton.class) {
            if(singleton == null){
                singleton = new Singleton();
            }
            return singleton;
        }
    }

    //线程安全，不高效
    public static synchronized Singleton getInstance2(){
        if(singleton == null){
            singleton = new Singleton();
        }
        return singleton;
    }

    //线程不安全
    public static Singleton getInstance1(){
        if(singleton == null){
            singleton = new Singleton();
        }
        return singleton;
    }


}