java单例五种实现模式解析

public class ImageLoader{ 
   private static ImageLoader instance = new ImageLoader; 
   private ImageLoader(){} 
   public static ImageLoader getInstance(){ 
     return instance; 
   } 
}

public class SingletonDemo2 {   
  //类初始化时，不初始化这个对象(延时加载，真正用的时候再创建)
  private static SingletonDemo2 instance;  
  //构造器私有化
  private SingletonDemo2(){}   
  //方法同步，调用效率低
  public static synchronized SingletonDemo2 getInstance(){
    if(instance==null){
      instance=new SingletonDemo2();
    }
    return instance;
  }
}

 public class SingletonDemo3 {   
   private static class SingletonClassInstance{
     private static final SingletonDemo3 instance=new SingletonDemo3();
   }   
   private SingletonDemo3(){}   
   public static SingletonDemo3 getInstance(){
     return SingletonClassInstance.instance;
   }   
 }

 public enum SingletonDemo4 {   
   //枚举元素本身就是单例
   INSTANCE;   
   //添加自己需要的操作
   public void singletonOperation(){   
   }
 }