Java 单例模式

在单例模式中分为懒汉式和饿汉式，所谓懒汉式就是延迟加载，在需要获取单例实例对象的时候再去进行初始化，

饿汉式就是在类加载的时候就已完成实例的初始化。

饿汉式是线程安全的。

在懒汉式中又分为线程安全和非线程安全的。

/**
 * Created by
 */
public class Singleton {
    //单例模式  饿汉式 线程安全
    public static class Singleton1{
        private final static Singleton1 instance = new Singleton1();
        private Singleton1(){}
        public static Singleton1 getInstance(){
            return instance;
        }
    }
    //单例模式 懒汉式 非线程安全
    public static class Singleton2{
        private static Singleton2 instance = null;
        private Singleton2(){}
        public static Singleton2 getInstance(){
            if(instance == null){
                instance = new Singleton2();
            }
            return instance;
        }
    }
    //单例模式  懒汉  线程安全 但是 效率不高
    public static class Singleton3{
        private static Singleton3 instance = null;
        private Singleton3(){}
        public static synchronized Singleton3 getInstance(){
            if(instance == null){
                instance = new Singleton3();
            }
            return instance;
        }
    }
    //单例模式  懒汉 线程安全 高效
    public static  class Singleton4{
        private static Singleton4 instance = null;
        private Singleton4(){}
        public static Singleton4 getInstance(){
            if(instance == null){
                synchronized (Singleton4.class){
                    if(instance == null){
                        instance = new Singleton4();
                    }
                }
            }
            return instance;
        }
    }
    //静态内部类实现单例模式
    public static class Singleton5{
        private static class Single{
            private static final Singleton5 instance = new Singleton5();
        }
        private Singleton5(){}
        public static Singleton5 getInstance(){
            return Single.instance;
        }

    }
}