单例模式

public class Singleton
{
         private static Singleton singleton = new Singleton();//类加载了便初始化该单例对象
                                                                                                                                   //之所以称之为饿汉式，因为他太急了
         private Singleton()
         {
         }
         public static Singleton getInstance()
         {
                   return singleton;
         }
}

public class Singleton
{
         private static Singleton singleton;
 
         private Singleton()
         {
         }
         //注意这里synchronized，保证了线程安全,意思就是每次只有一个线程能够访问该方法
         public static synchronized Singleton getInstance()
         {
                   if(singleton == null)
                   {//当singleton要用的时候才初始化，加判断避免重复初始化，保证只new了一次
                            singleton= new Singleton();
                   }
                   return singleton;
         }
}

public class Singleton
{
         private static Singleton instance = null;
 
         private Singleton()
         {
                   //dosomething
         }
 
         public static Singleton getInstance()
         {
                   if(instance == null)
                   {
                            synchronized(Singleton.class)//可以思考一下，为什么这样提高效率
                            {
                                     if(null == instance)
                                     {
                                               instance= new Singleton();
                                     }
                            }
                   }
                   return instance;
         }
}