单例模式之懒汉模式，懒汉模式之高效模式，DLC双判断模式

import java.util.concurrent.LinkedBlockingQueue;
import java.util.concurrent.ThreadPoolExecutor;
import java.util.concurrent.TimeUnit;

/**
 * 单例模式之懒汉模式
 * 懒汉模式之高效模式
 * DLC双判断模式
 */
public class SingleClassLazySync {
    private static SingleClassLazySync singleClassLazySync;

    private SingleClassLazySync() {
    }

    public static SingleClassLazySync getInstance() {
        if (singleClassLazySync == null) {
            synchronized (SingleClassLazySync.class) {
                if (singleClassLazySync == null) {
                    System.out.println("i m in new instance!");
                    singleClassLazySync = new SingleClassLazySync();
                }
            }
        }
        return singleClassLazySync;
    }

    static class A implements Runnable {

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

    public static void main(String[] args) {
        ThreadPoolExecutor executor = new ThreadPoolExecutor(5, 5, 0l, TimeUnit.MILLISECONDS, new LinkedBlockingQueue<Runnable>());
        boolean flag = true;
        for (int i = 0; i < 5; i++)
            executor.execute(new A());
        executor.shutdown();
    }
}

　　两个判断的位置

第一个为了日后获取实例的线程准备

第二个为了刚刚获得锁的线程准备，如果没有这个，将会有同步问题。

输出结果：