不使用synchronized和lock 鎖實現線程安全單例

單例實現方式一，鎖機制

public class Singleton {
    private static Singleton singleton=null;

    public Singleton() {
    }
    public static Singleton getIntance(){
        if(singleton==null){
            synchronized (Singleton.class){
                if(singleton==null){
                    singleton=new Singleton();
                }
            }
        }
        return singleton;
    }
}
單例實現方式二，靜態內部類

public class Singleton {
    private static class SingletonHolder {
        private static final Singleton singleton = new Singleton();

    }

    public Singleton() {
    }

    public static Singleton getIntance() {
        return SingletonHolder.singleton;
    }
}
靜態內部類雖然沒有顯示調用synchronized，但是借助ClassLoad的線程安全機制，隱式調用了synchronized

單例實現方式三，CAS操作

public class Singleton {
    private static final AtomicReference<Singleton> instance = new AtomicReference<>();

    public Singleton() {
    }

    public static Singleton getIntance() {
        for (; ; ) {
            Singleton singleton = instance.get();
            if (singleton == null) {
                singleton = new Singleton();
                instance.compareAndSet(null, singleton);
            }
            return singleton;
        }
    }

    public static class SingletonHold implements Runnable {

        @Override
        public void run() {
            System.out.println(Thread.currentThread().getName() + ":" + Singleton.getIntance());
        }

    }

    public static void main(String[] args) {
        SingletonHold singletonHold = new SingletonHold();
        Thread thread1 = new Thread(singletonHold);
        Thread thread2 = new Thread(singletonHold);
        Thread thread3 = new Thread(singletonHold);
        thread1.start();
        thread2.start();
        thread3.start();
    }
}
運行結果：

Thread-0:com.rongke.web.Singleton@7249381e
Thread-1:com.rongke.web.Singleton@7249381e
Thread-2:com.rongke.web.Singleton@7249381e

使用cas好處：不需要使用鎖來實現線程安全，而是依賴底層硬件實現，減少了因為鎖導致線程切換和阻塞的性能開銷，可以支持較大的並行度

使用cas缺點：如果線程一直處於for無限循環中，對cpu性能影響很大，如果多個線程執行singleton=new Singleton()，會產出堆內存浪費。