深入Singleton

Singleton: ensure a class only has one instance and provide a global point of access to it.

java 代码

1)
// 非线程安全
public class Singleton{
           private static Singleton instance ;
           private Singleton(){
             super();
           }
           public static Singleton getInstance(){
              if(instance==null)   // 延迟加载,但导致了多线程同步的问题
                 instance = new Singleton();
              return instance ;
           }
 }
// 以下3种方式是线程安全的
2)
public class Singleton{
           private static Singleton instance = new Singleton();  //eagerly load
           private Singleton(){
             super();
           }
           public static Singleton getInstance(){
              return instance ;
           }
 }
3)
public class Singleton{
           private static Singleton instance ;
           private Singleton(){
             super();
           }
           public static  synchronized Singleton getInstance(){ //普通的同步操作,但效率低
              if(instance==null)
                 instance = new Singleton();
              return instance ;
           }
    }
4)
public class Singleton{
           private volatile static Singleton instance ;//指定instance不放入CPU缓存
           private Singleton(){
             super();
           }
           public static Singleton getInstance(){
                  if(instance==null){
                     synchonized(Singleton.class){// 即double-checked locking
                          if(instance==null)
                             instance = new Singleton();
                      }
                  }
               return instance ;
           }
 }

PS: #你不能继承Singleton类
       #如果有多个classLoader分别加载这个Singleton,则仍能new出多个对象实例,因为每个ClassLoader都会定义一个命名空间.
       #JDK1.2之前的环境垃圾回收器不能很好的支持自引用