简单总结单例模式的4种写法

tangjin

作者

一、单例模式

属于创建者模式的一种, 单例模式的目的是使该类只有一个实例，同一个类的不同对象有不同的hashCode()

单例模式是由该类自行创建唯一个向外暴露的全局的对象

二、写法

三、饿汉式

/** * 饿汉式: * 不适用new创建对象而是使用静态的getInstance()方法创建对象 * jvm保证线程安全 */ public class SingleTest{ public static void main(String[] args) { Single single1 = Single.getInstance(); Single single2 = Single.getInstance(); System.out.println(single1.hashCode()); System.out.println(single2.hashCode()); System.out.println(single1 == single2); } } class Single { final static Single INSTANCE = new Single(); public static Single getInstance(){return INSTANCE;} }

四、懒汉式

//懒汉式的线程安全的写法又称为双检索懒汉模式 public class SingleTest { public static void main(String[] args) { for(int i = 0;i < 10; i++){ new Thread(new Runnable() { @Override public void run() { System.out.println(Single.getInstance().hashCode()); } }).start(); } } } class Single{ private static Single instance = null; Single(){} public static Single getInstance(){ if (instance == null){ synchronized (Single.class) { if(instance == null){ try{ Thread.sleep(1); }catch (Exception e){ e.printStackTrace(); } instance = new Single(); } } } return instance; } }

五、内部类

/** * 静态内部类:完美写法 */ public class SingleTest { public static void main(String[] args) { Single single1 = Single.getInstance(); Single single2 = Single.getInstance(); System.out.println(single1.hashCode()); System.out.println(single2.hashCode()); System.out.println(single1 == single2); } } class Single{ private static Single instance; public static Single getInstance(){ return InnerSingle.INSTANCE; } private static class InnerSingle{ private final static Single INSTANCE = new Single(); } }

六、枚举

//枚举：完美写法 public class SingleTest { public static void main(String[] args) { Single single1 = Single.INSTANCE; Single single2 = Single.INSTANCE; System.out.println(single1.hashCode()); System.out.println(single2.hashCode()); System.out.println(single1 == single2); } } enum Single{ INSTANCE; }