Singleton7.java

package com.cheng.bigtalkdesignpatterns.singleton;

/**
 * 第七种（双重校验锁）
 */
public class Singleton7 {
    private volatile static Singleton7 instance;

    private Singleton7() {}

    public static Singleton7 getInstance() {
        if (instance == null) {
            synchronized (Singleton7.class) {
                if (instance == null) {
                    instance = new Singleton7();
                }
            }
        }
        return instance;
    }
    /**
     * 这个是第二种方式的升级版，俗称双重检查锁定
     */
}