单例模式--懒汉

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1、线程不安全

/**
 * 线程不安全
 */
public class SingleApplicationUnsafe {

    private static SingleApplicationUnsafe singleApplication = null;

    private SingleApplicationUnsafe(){

    }

    /**
     * 双重检测+锁
     * @return
     */
    public static SingleApplicationUnsafe getInstance(){
        if(singleApplication ==null){
            synchronized (SingleApplicationUnsafe.class){
                if(singleApplication == null){
                    singleApplication = new SingleApplicationUnsafe();
                }
            }
        }
        return singleApplication;
    }

}

2、线程安全

/**
 * 双重检测+锁+volatile  线程安全
 */
public class SingleApplicationSafe {

    private static volatile SingleApplicationSafe singleApplication = null;

    private SingleApplicationSafe(){

    }

    /**
     * 双重检测+锁+volatile
     * @return
     */

    /**
     *  singleApplication = new SingleApplicationSafe();这个步骤是三步
     *  1、第一步  分配内存空间
     *  2、第二步 初始化singleApplication
     *  3、第三步  singleApplication 指向内存空间
     *  jvm和cpu会进行优化，造成指令重排，所以第二步和第三步有可能发生颠倒，所以可能造成返回的实例没有具体的信息
     *
     *  解决办法就是：利用volatile,这个关键字可以禁止指令重排
     * @return
     */
    public static SingleApplicationSafe getInstance(){
        if(singleApplication ==null){
            synchronized (SingleApplicationSafe.class){
                if(singleApplication == null){
                    singleApplication = new SingleApplicationSafe();
                }
            }
        }
        return singleApplication;
    }

}