Java多线程之单例模式（线程安全）

package org.study2.javabase.ThreadsDemo.sync;

/**
 * @Auther:GongXingRui
 * @Date:2018/9/20
 * @Description: 单例模式 - 二次确认，提高效率
 **/
public class DanDemo {
    public static void main(String args[]) {
        Dan dan1 = new Dan();
        Dan dan2 = new Dan();
        dan1.start();
        dan2.start();
    }
}

class Dan extends Thread {
    @Override
    public void run() {
        System.out.println(JVM.getInstance());
    }

}

/**
 * 单例模式 - 懒汉式
 */
class JVM {
    private static JVM instance = null;

    private JVM() {

    }

    // 二次确认提高效率
    public static JVM getInstance() {
        if (null == instance) { // 提高效率
            synchronized (JVM.class) {
                if (null == instance) { // 安全
                    instance = new JVM();
                }
            }
        }
        return instance;
    }

    public static JVM getInstance2() {
        synchronized (JVM.class) {
            if (null == instance) {
                instance = new JVM();
            }
        }
        return instance;
    }

    public static synchronized JVM getInstance1() {
        if (null == instance) {
            instance = new JVM();
        }
        return instance;
    }
}

/**
 * 单例模式 - 饿汉式
 * 线程安全
 */
class JVM2 {
    private static JVM2 instance = new JVM2();

    private JVM2() {

    }

    public static JVM2 getInstance() {
        return instance;
    }
}