单例推荐的几种创建方法

第一种（静态内部类）：

  public class Singleton { 
      private Singleton (){}
      public static final Singleton getInstance() {  
          return SingletonHolder.INSTANCE;  
      }  
      private static class SingletonHolder {  
          private static final Singleton INSTANCE = new Singleton();  
      }  
  }

第二种(枚举)

public class EnumSingleton{
    private EnumSingleton(){}
    public static EnumSingleton getInstance(){
        return Singleton.INSTANCE.getInstance();
    }
    
    private static enum Singleton{
        INSTANCE;
        private EnumSingleton singleton;
        //JVM会保证此方法绝对只调用一次
        private Singleton(){
            singleton = new EnumSingleton();
        }
        public EnumSingleton getInstance(){
            return singleton;
        }
    }
}

第三种(双重校验锁) //JDK 1.5之后,双重检查锁才能达到正常的单例效果(1.5之前volatile 关键字使用有问题)

public class Singleton {
    private volatile static Singleton singleton;
    private Singleton() {}
    public static Singleton getIntance() {
          if(singleton == null) {
              synchronized(Singleton.class) {
                    if(singleton == null) {
                          singleton = new Singleton();
                    }
              }
          }
     return singleton;
    }
}