饿汉式

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

 

 

 

懒汉式（线程不安全）

public class Singleton{
    private static Singleton instace;
    private Singleton(){}

    public static Singleton getInstance(){
        if (instance == null){
            instance = new Singleton();
        }
        return instace;
    }
}

 

 

 

懒汉式（线程安全）

效率低，每次都要加锁

public class Singleton{
    private static Singleton instace;
    private Singleton(){}

    public static synchronized Singleton getInstance(){
        if (instance == null){
            instance = new Singleton();
        }
        return instace;
    }
}

 

 

 

双重校验锁 DCL（Double Checked Locking）

先判断，若对象已创建，则不进入synchronized代码块。提升了性能且保证安全。
Volate一般用于保证多线程可见性。这里用来防止指令重排序。

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

 

 

 

静态内部类

调用getInstance()才初始化，更加灵活。

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