通过单例模式可以保证系统中，应用该模式的类一个类只有一个实例。即一个类只有一个对象实例

1.懒汉式

//懒汉式 ， 线程安全public class Single {    //创建实例对象并且初始化    private static Single instance = null;    //构造器私有化    private Single() {            }    //获取唯一的可用的对象    public static synchronized Single getInstance() {        if(instance == null) {            instance = new Single();        }        return instance;    }        public static void main(String[] args) {        Single s1 = Single.getInstance();        Single s2 = Single.getInstance();        System.out.println(s1 == s2);    }}

 

2.饿汉式

//饿汉式public class Single {        private static final Single instance = new Single();    //构造器私有化    private Single() {            }    //获取唯一的可用的对象    public static Single getInstance() {        return instance;    }        public static void main(String[] args) {        Single s1 = Single.getInstance();        Single s2 = Single.getInstance();        System.out.println(s1 == s2);    }}

 

3.双重锁形式:

//双重锁形式public class Single {        private static volatile Single instance = null;    //构造器私有化    private Single() {            }    //获取唯一的可用的对象    public static Single getInstance() {        if(instance == null) {            synchronized (Single.class) {                if(instance == null) {                    instance = new Single();                }            }        }        return instance;    }        public static void main(String[] args) {        Single s1 = Single.getInstance();        Single s2 = Single.getInstance();        System.out.println(s1 == s2);    }}

 

一般用第三种