【面試】代碼默寫-DCL單例

1、雙重鎖檢查（DCL）單例模式

public class Singleton {
    
    private static volatile Singleton instance = null;//防止指令重排序，拿到半成品對象
 
    private Singleton() {}
 
    public static Singleton getInstance() {
        if(null == instance) {//避免每次都進行鎖競爭，提高執行效率
            synchronized(Singleton.class) {//加鎖
                if(null == instance) {//防止多個線程越過第一個if後多次創建對象
                    instance = new Singleton();
                }
            }
        }
        return instance;
    }
}

2、靜態內部類單例

public class Singleton {
 
    private Singleton() {}
 
    pirvate static class SingletonHolder {
        public final static Singleton instance = new Singleton();
    }
 
    public static Singleton getInstance() {
        return SingletonHolder.instance;
    }
}