1、饿汉式:
public class Singleton { private final static Singleton INSTANCE = new Singleton(); private Singleton() { } public static Singleton getInstance() { return INSTANCE; } }
2、懒加载之占位符模式:(推荐)
属于懒汉式单例,因为Java机制规定,内部类SingletonHolder只有在getInstance()方法第一次调用的时候才会被加载(实现了lazy),而且其加载过程是线程安全的。内部类加载的时候实例化一次instance。
public class Singleton { private Singleton() { } private static class SingletonHolder { private final static Singleton INSTANCE = new Singleton(); } public static Singleton getInstance() { return SingletonHolder.INSTANCE; } }
3、懒加载之普通加锁模式:
public class Singleton { private static Singleton instance = null; private Singleton() { } public static synchronized Singleton getInstance() { if(instance == null) { instance = new Singleton(); } return instance; } }
虽然解决了线程安全问题,但是每个线程调用getInstance都要加锁,我们想要只在第一次调用getInstance时加锁,请看下面的双重检测方案
4、懒加载之双重检测模式:(推荐)
public class Singleton { private static Singleton instance = null; private Singleton() { } public static Singleton getInstance() { if(instance == null) { synchronzied(Singleton.class) { if(instance == null) { instance = new Singleton(); } } } return instance; } }
特别注意这种方式会有指令重排问题(关于指令重排可参考这篇博文),针对instance实例变量用volatile修饰就可以了,volatile修饰的话就可以确保instance= new Singleton();对应的指令不会重排序:
public class Singleton { private static volatile Singleton instance = null; //以volatile关键字修饰防止指令重排 private Singleton() { } //构造函数为私有,防止被实例化 public static Singleton getInstance() { if(instance == null) { //双重检测 synchronzied(Singleton.class) { if(instance == null) { instance = new Singleton(); } } } return instance; } }