线程安全的两种单例模式

两种常见的单例模式

静态内部类单例模式

/*
 *  静态内部类单例模式
 */
public class Singleton {
    private Singleton() {
    }

    private static final class SingletonHolder {
        private static final Singleton INSTANCE = new Singleton();
    }

    public static Singleton getInstance() {
        return SingletonHolder.INSTANCE;
    }

    public static void main(String[] args) {
        Thread t1 = new Thread(new Runnable() {

            @Override
            public void run() {
                System.out.println(Singleton.getInstance().hashCode());
            }

        }, "t1");

        Thread t2 = new Thread(new Runnable() {

            @Override
            public void run() {
                System.out.println(Singleton.getInstance().hashCode());
            }

        }, "t2");

        Thread t3 = new Thread(new Runnable() {

            @Override
            public void run() {
                System.out.println(Singleton.getInstance().hashCode());
            }

        }, "t3");

        t1.start();
        t2.start();
        t3.start();
    }
}

import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

public class InnerSingleton {
    private InnerSingleton() {
    }

    private static final class InnerSingletonHandle {
        private static final InnerSingleton INSTANCE = new InnerSingleton();
    }

    public static InnerSingleton getInstance() {
        return InnerSingletonHandle.INSTANCE;
    }

    public static void main(String[] args) {
        int num = 10;
        ExecutorService executorService = Executors.newCachedThreadPool();

        for (int i = 0; i < num; i++) {
            executorService.execute(new Runnable() {
                @Override
                public void run() {
                    System.out.println(Thread.currentThread().getName() + ":	" + InnerSingleton.getInstance().hashCode());
                }
            });
        }
        executorService.shutdown();
    }
}

运行结果

``` pool-1-thread-1: 991873903 pool-1-thread-2: 991873903 pool-1-thread-3: 991873903 pool-1-thread-4: 991873903 pool-1-thread-6: 991873903 pool-1-thread-7: 991873903 pool-1-thread-8: 991873903 pool-1-thread-10: 991873903 pool-1-thread-5: 991873903 pool-1-thread-9: 991873903

Process finished with exit code 0

</div>

### 双重校验单例模式
```java
/*
 * 双重校验单例模式
 */
public class Singleton {
    private static volatile Singleton instance;

    private Singleton() {
    }

    public static Singleton getInstance() {
        if (instance == null) {
            synchronized (Singleton.class) {
                if (instance == null) {
                    instance = new Singleton();
                }
                // instance = new Singleton();
            }
        }
        return instance;
    }

    public static void main(String[] args) {
        Thread t1 = new Thread(new Runnable() {

            @Override
            public void run() {
                System.out.println(Singleton.getInstance().hashCode());
            }

        }, "t1");

        Thread t2 = new Thread(new Runnable() {

            @Override
            public void run() {
                System.out.println(Singleton.getInstance().hashCode());
            }

        }, "t2");

        Thread t3 = new Thread(new Runnable() {

            @Override
            public void run() {
                System.out.println(Singleton.getInstance().hashCode());
            }

        }, "t3");

        t1.start();
        t2.start();
        t3.start();
    }
}

import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

public class DoubleCheckSingleton {
    private DoubleCheckSingleton() {
    }

    private static volatile DoubleCheckSingleton INSTANCE;

    public static DoubleCheckSingleton getInstance() {
        if (INSTANCE == null) {
            try {
                Thread.sleep(3000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            synchronized (DoubleCheckSingleton.class) {
                if (INSTANCE == null) {
                    INSTANCE = new DoubleCheckSingleton();
                }
                // INSTANCE = new DoubleCheckSingleton();
            }
        }
        return INSTANCE;
    }

    public static void main(String[] args) {
        int num = 10;
        ExecutorService executorService = Executors.newCachedThreadPool();

        for (int i = 0; i < num; i++) {
            executorService.execute(new Runnable() {
                @Override
                public void run() {
                    System.out.println(Thread.currentThread().getName() + ":	" + DoubleCheckSingleton.getInstance().hashCode());
                }
            });
        }
        executorService.shutdown();
    }
}

运行结果

``` pool-1-thread-1: 405652733 pool-1-thread-4: 405652733 pool-1-thread-7: 405652733 pool-1-thread-2: 405652733 pool-1-thread-6: 405652733 pool-1-thread-5: 405652733 pool-1-thread-3: 405652733 pool-1-thread-8: 405652733 pool-1-thread-9: 405652733 pool-1-thread-10: 405652733

Process finished with exit code 0

</div>

### 最后总结
![](https://img2018.cnblogs.com/blog/1306719/201811/1306719-20181107155022590-664214116.png)
![](https://img2018.cnblogs.com/blog/1306719/201811/1306719-20181107155028084-1715481635.png)