懶漢單例的多線程安全

單例模式有不少的變種，餓漢模式變化不大很少說

非線程安全的懶漢模式

public class LazySingletonDemo {

    private static LazySingletonDemo instance;

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

線程安全的懶漢模式

這種狀況使用場景不是不少，除非特殊的狀況下，假如只是單純的做爲工具類或者存放常量這種做用的話，考慮多線程安全沒有太大意義，多new出來的對象失去引用很快會被GC掉，除非一瞬間併發量爆炸，同時new了超多的對象(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();
    }
}

輸出

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

能夠看出，6個線程都進入了第1,2,3步驟，可是進入到new的步驟4只有一個線程

這裏不把鎖放到方法上的緣由是，每次拿實例的時候都要鎖，很是影響性能，而只有開頭new的時候才須要鎖

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();
    }
}

輸出

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

至於Synchronized和Lock的區別就不說了，網上都有