單例模式幾種不同實作方式？

      單例模式，是一種常用的軟體設計模式。在它的核心結構中隻包含一個被稱為單例的特殊類。通過單例模式可以保證系統中，應用該模式的一個類隻有一個執行個體。即一個類隻有一個對象執行個體。

目錄

​​第一種（懶漢模式，線程不安全）​​

​​第二種（懶漢模式，線程安全）​​

​​第三種（餓漢模式）​​

​​第四種（餓漢模式，變種）​​

第一種（懶漢模式，線程不安全）

public class SingletonDemo1 {
    private static SingletonDemo1 instance;
    private SingletonDemo1(){}
    public static SingletonDemo1 getInstance(){
        if (instance == null) {
            instance = new SingletonDemo1();
        }
        return instance;
    }
}      

第二種（懶漢模式，線程安全）

public class SingletonDemo2 {
    private static SingletonDemo2 instance;
    private SingletonDemo2(){}
    public static synchronized SingletonDemo2 getInstance(){
        if (instance == null) {
            instance = new SingletonDemo2();
        }
        return instance;
    }
}      

第三種（餓漢模式）

public class SingletonDemo3 {
    private static SingletonDemo3 instance = new SingletonDemo3();
    private SingletonDemo3(){}
    public static SingletonDemo3 getInstance(){
        return instance;
    }
}      

第四種（餓漢模式，變種）

public class SingletonDemo4 {
    private static SingletonDemo4 instance = null;
    static{
        instance = new SingletonDemo4();
    }
    private SingletonDemo4(){}
    public static SingletonDemo4 getInstance(){
        return instance;
    }
}      

第五種（靜态内部類）

public class SingletonDemo5 {
    private static class SingletonHolder{
        private static final SingletonDemo5 instance = new SingletonDemo5();
    }
    private SingletonDemo5(){}
    public static final SingletonDemo5 getInsatance(){
        return SingletonHolder.instance;
    }
}      

第六種（枚舉）

public class SingletonDemo6 {

    private SingletonDemo6() {}

    public static SingletonDemo6 getInstance() {
        return Singleton.INSTANCE.getInstance();
    }

    private static enum Singleton {
        INSTANCE;

        private SingletonDemo6 singleton;

        private Singleton() {
            singleton = new SingletonDemo6();
        }

        public SingletonDemo6 getInstance() {
            return singleton;
        }
    }
}      

第七種（雙重校驗鎖） 

public class SingletonDemo7 {
    private volatile static SingletonDemo7 singletonDemo7;
    private SingletonDemo7(){}
    public static SingletonDemo7 getSingletonDemo7(){
        if (singletonDemo7 == null) {
            synchronized (SingletonDemo7.class) {
                if (singletonDemo7 == null) {
                    singletonDemo7 = new SingletonDemo7();
                }
            }
        }
        return singletonDemo7;
    }
}