1

package com.example.demo.thread;

/**
 * @date 2018/3/4
 */
public class ThreadPool {
    private static ThreadPool threadPool = new ThreadPool();
    private ThreadPool() {
    }
    /**
     * 单例模式的实现方式
     *
     * @return
     */
    public static ThreadPool getThreadPool() {
        return threadPool;
    }

}

2

package com.example.demo.thread;

/**
 * @date 2018/3/4
 */
public class ThreadPool {
    private static ThreadPool threadPool;

    private ThreadPool() {
    }

    /**
     * 单例模式的实现方式
     *
     * @return
     */
    public static ThreadPool getThreadPool() {
        if (threadPool == null) {
            return new ThreadPool();
        }
        return threadPool;
    }

}

3

package com.example.demo.thread;

/**
 * @date 2018/3/4
 */
public class ThreadPool {
    private static ThreadPool threadPool;

    private ThreadPool() {
    }

    /**
     * 单例模式的实现方式
     *
     * @return
     */
    public static synchronized ThreadPool getThreadPool() {
        if (threadPool == null) {
            return new ThreadPool();
        }
        return threadPool;
    }

}

4

package com.example.demo.thread;

/**
 * @date 2018/3/4
 */
public class ThreadPool {
    private static ThreadPool threadPool;

    private ThreadPool() {
    }

    /**
     * 单例模式的实现方式
     *
     * @return
     */
    public static ThreadPool getThreadPool() {
        if (threadPool == null) {
            synchronized (ThreadPool.class) {
                if (threadPool == null) {
                    return new ThreadPool();
                }
            }
        }
        return threadPool;
    }

}

5

package com.example.demo.thread;

/**
 * @date 2018/3/4
 */
public class ThreadPool {
    private static volatile ThreadPool threadPool;

    private ThreadPool() {
    }

    /**
     * 单例模式的实现方式
     *
     * @return
     */
    public static ThreadPool getThreadPool() {
        if (threadPool == null) {
            synchronized (ThreadPool.class) {
                if (threadPool == null) {
                    return new ThreadPool();
                }
            }
        }
        return threadPool;
    }

}

6

参考