單例的幾點概念。

1、構造函數必須設置爲private

避免被new 出來，只能通過暴露的方法獲取實例

2、懶漢和惡漢

懶漢就是用的時候才加載

double check

public class SingletonWithDoubleChecked {
    private SingletonWithDoubleChecked(){};
    private static volatile SingletonWithDoubleChecked instance = null;
    private static final Object lock = new Object();
    public static SingletonWithDoubleChecked getInstance(){
        if (instance == null) {
            synchronized(lock) {
                if (instance == null) {
                    instance = new SingletonWithDoubleChecked();
                }
            }
        }
        return instance;
    }
}

類似餓漢，但是用的時候才加載

public class SingletonTest {
    private SingletonTest(){};
    private static class SingletonGet{
        private static final SingletonTest instance = new SingletonTest();
    }
    public static SingletonTest getInstance(){
        return  SingletonGet.instance;
    }
}

餓漢就是項目啓動就加載

public class SingletonTest {
    private SingletonTest(){};
    private static final SingletonTest instance = new SingletonTest();
    public static SingletonTest getInstance(){
        return  instance;
    }
}

爲什麼double check 需要volatile？

禁止指令重排

線程可見

參考