public Class Singleton{
 //在类加载时便实例化自身，饿汉模式
  private static Class Singletom instance = new Singleton();
  private Singleton(){};
  
  public getInstance(){
   return this.instance 
  }
 } 

public Class Singleton{
  private static Class Singletom instance = null;
  private Singleton(){};
  
  public static Singleton getInstance(){
   if(instance==null){
   //双重检测锁定懒汉模式，如果是直接在getInsance上加锁，因为有99%的情况是线程安全的，会增加性能消耗，故才有双重检测锁定，优化锁，让锁只在1%的情况才执行 
   synchronized (Singleton.class) {

if (singleton == null) 
{ 
    singleton = new Singleton(); 
    } 
    } 
   return this.instance 
  } 
}

public Class Singleton{
  //静态内部类的方式，应该是因为classLoader的机制使得内部类不会在类  
  //装载时便实例化，所以可行
  private static Class SingletomLoader(){
   private static Singleton instance = new Singleton();
  }
  private Singleton(){};
  
  public static final Singleton getInstance(){
   
   return SingletomLoader.instance 
  }
 }