线程安全的单例模式

来源：互联网发布：数据库数据如何查找编辑：程序博客网时间：2024/06/06 09:19

1、饿汉式单例

public class Singleton {   private final static Singleton INSTANCE = new Singleton();   private Singleton() { }   public static Singleton getInstance() {      return INSTANCE;   }}

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。

public class Singleton {   private Singleton() { }   private static class SingletonHolder {      private final static Singleton INSTANCE = new Singleton();   }   public static Singleton getInstance() {      return SingletonHolder.INSTANCE;   }}

3、普通加锁解决

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、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：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;   }}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：public class Singleton {   private static volatile Singleton instance = null;   private Singleton() { }   public static Singleton getInstance() {      if(instance == null) {         synchronzied(Singleton.class) {            if(instance == null) {               instance = new Singleton();            }         }      }      return instance;   }}

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