懒汉单例的多线程安全

单例模式有不少的变种，饿汉模式变化不大很少说

非线程安全的懒汉模式

public class LazySingletonDemo {

    private static LazySingletonDemo instance;

    public static LazySingletonDemo getInstance() {
        if (instance == null) {
            instance = new LazySingletonDemo();
        }
        return instance;
    }
}

线程安全的懒汉模式

这种状况使用场景不是不少，除非特殊的状况下，假如只是单纯的做为工具类或者存放常量这种做用的话，考虑多线程安全没有太大意义，多new出来的对象失去引用很快会被GC掉，除非一瞬间并发量爆炸，同时new了超多的对象(真遇到了那得放鞭炮)

1.使用synchronized

public class LazySingletonDemo {

    private static LazySingletonDemo instance;

    public static LazySingletonDemo getInstance() {
        System.out.println(Thread.currentThread().getName() + " 1");
        if (instance == null) {
            System.out.println(Thread.currentThread().getName() + " 2");
            synchronized (LazySingletonDemo.class) {
                System.out.println(Thread.currentThread().getName() + " 3");
                if (instance == null) {
                    System.out.println(Thread.currentThread().getName() + " 4");
                    instance = new LazySingletonDemo();
                }
            }
        }
        return instance;
    }

    public static void main(String[] args) {
        Runnable r = new Runnable() {
            public void run() {
                LazySingletonDemo.getInstance();
            }
        };
        Thread t1 = new Thread(r, "t1");
        Thread t2 = new Thread(r, "t2");
        Thread t3 = new Thread(r, "t3");
        Thread t4 = new Thread(r, "t4");
        Thread t5 = new Thread(r, "t5");
        Thread t6 = new Thread(r, "t6");
        t1.start();
        t2.start();
        t3.start();
        t4.start();
        t5.start();
        t6.start();
    }
}

输出

t1 1
t6 1
t6 2
t3 1
t3 2
t4 1
t4 2
t2 1
t6 3
t5 1
t1 2
t5 2
t6 4
t2 2
t5 3
t2 3
t1 3
t4 3
t3 3

能够看出，6个线程都进入了第1,2,3步骤，可是进入到new的步骤4只有一个线程

这里不把锁放到方法上的缘由是，每次拿实例的时候都要锁，很是影响性能，而只有开头new的时候才须要锁

2.使用Lock

public class LazySingletonDemo {

    private static LazySingletonDemo instance;
    private static ReentrantLock lock = new ReentrantLock();

    public static LazySingletonDemo getInstance() {
        System.out.println(Thread.currentThread().getName() + " 1");
        if (instance == null) {
            System.out.println(Thread.currentThread().getName() + " 2");
            try {
                lock.lock();
                System.out.println(Thread.currentThread().getName() + " 3");
                if (instance == null) {
                    System.out.println(Thread.currentThread().getName() + " 4");
                    instance = new LazySingletonDemo();
                }
            } finally {
                System.out.println(Thread.currentThread().getName() + " 5");
                lock.unlock();
            }

        }
        return instance;
    }

    public static void main(String[] args) throws InterruptedException {
        Runnable r = new Runnable() {
            public void run() {
                LazySingletonDemo.getInstance();
            }
        };
        Thread t1 = new Thread(r, "t1");
        Thread t2 = new Thread(r, "t2");
        Thread t3 = new Thread(r, "t3");
        Thread t4 = new Thread(r, "t4");
        Thread t5 = new Thread(r, "t5");
        Thread t6 = new Thread(r, "t6");
        t1.start();
        t2.start();
        t3.start();
        t4.start();
        t5.start();
        t6.start();
    }
}

输出

t1 1
t3 1
t6 1
t6 2
t4 1
t4 2
t2 1
t6 3
t3 2
t5 1
t5 2
t1 2
t6 4
t2 2
t6 5
t4 3
t4 5
t3 3
t3 5
t5 3
t5 5
t1 3
t1 5
t2 3
t2 5

同上

至于Synchronized和Lock的区别就不说了，网上都有