singleton 单例模式

单线程中的单例模式:

public class Sinleton{

                   private volatile static Singleton instance;       

                        private Singleton()   {}

                        public static Singleton getInstance(){           

                                   if(instance == null){                                                 

                                    instance  = new Singleton();

                }

                           return instance;

   }

}

在多线程中使用单例模式:

     public class Sinleton{

                   private volatile static Singleton instance;       

                        private Singleton()   {}

                        public static synchronized Singleton getInstance(){            //只需要把getInstance方法编程同步的就可以了,可是这样

                                   if(instance == null){                                                   //会降低性能,所以可以使用下面的双重检查锁

                                    instance  = new Singleton();

                }

                           return instance;

   }

}

一个合格的单例模式

    利用双重检查加锁:当创建instance实例为空时需要加锁,当instance创建出来之后则无需加锁,这样做的目可以大大的减小消耗.整个过程只加了一次锁.(因为同步锁非常耗时)

             public class Sinleton{

                   private volatile static Singleton instance;       

                        private Singleton()   {}

                        public static Singleton getInstance(){

                                   if(instance == null){

                                          synchronized(Singleton.class){

                                          if(instance == null){

                                                instance  = new Singleton();

                               }

                      }

                }

                           return instance;

   }

}

      volatile关键字  

简单地说就是防止编译器对代码进行优化.比如如下程序：

1 2 3 4

XBYTE[2]=0x55; XBYTE[2]=0x56; XBYTE[2]=0x57; XBYTE[2]=0x58;

对外部硬件而言，上述四条语句分别表示不同的操作，会产生四种不同的动作，但是编译器却会对上述四条语句进行优化，认为只有XBYTE[2]=0x58（即忽略前三条语句，只产生一条机器代码）。如果键入volatile，则编译器会逐一的进行编译并产生相应的机器代码（产生四条代码）