线程安全的单例模式的几种实现方法分享
1、饿汉式单例
1 public class Singleton { 2 private final static Singleton INSTANCE = new Singleton(); 3 4 5 private Singleton() { } 6 7 public static Singleton getInstance() { 8 return INSTANCE; 9 } 10 }
2、借助内部类
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。
1 public class Singleton { 2 3 4 private Singleton() { } 5 6 7 private static class SingletonHolder { 8 private final static Singleton INSTANCE = new Singleton(); 9 } 10 11 public static Singleton getInstance() { 12 return SingletonHolder.INSTANCE; 13 } 14 }
3、普通加锁解决
1 public class Singleton { 2 3 4 private Singleton() { } 5 6 7 private static class SingletonHolder { 8 private final static Singleton INSTANCE = new Singleton(); 9 } 10 11 public static Singleton getInstance() { 12 return SingletonHolder.INSTANCE; 13 } 14 }
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、双重检测,但要注意写法
public class Singleton { private static Singleton instance = null; private Singleton() { } public static Singleton getInstance() { if(instance == null) { synchronzied(Singleton.class) { Singleton temp = instance; if(temp == null) { temp = new Singleton(); instance = temp } } } return instance; } }
由于指令重排序问题,所以不可以直接写成下面这样:
1 public class Singleton { 2 private static Singleton instance = null; 3 4 private Singleton() { } 5 6 public static Singleton getInstance() { 7 if(instance == null) { 8 synchronzied(Singleton.class) { 9 if(instance == null) { 10 instance = new Singleton(); 11 } 12 } 13 } 14 15 return instance; 16 } 17 }
但是如果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,如下的单例代码也是线程安全的:
1 public class Singleton { 2 private static volatile Singleton instance = null; 3 4 private Singleton() { } 5 6 public static Singleton getInstance() { 7 if(instance == null) { 8 synchronzied(Singleton.class) { 9 if(instance == null) { 10 instance = new Singleton(); 11 } 12 } 13 } 14 15 return instance; 16 } 17 }