java单例的七种写法

懒汉

线程不安全

 public class Singleton {  
     private static Singleton instance;  
     private Singleton (){}   
     public static Singleton getInstance() {  
       if (instance == null) {  
           instance = new Singleton();  
       }  
     return instance;  
     }  
} 

线程安全

public class Singleton {  
  private static Singleton instance;  
  private Singleton (){}
  public static synchronized Singleton getInstance() {  
  if (instance == null) {  
      instance = new Singleton();  
  }  
  return instance;  
  }  
}  

饿汉

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

饿汉变种

public class Singleton {  
  private static Singleton instance = null;  
  static {  
  instance = new Singleton();  
  }  
  private Singleton (){}
  public static Singleton getInstance() {  
  return this.instance;  
  }  
 }  

静态内部类

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

枚举

public enum Singleton {  
     INSTANCE;  
     public void whateverMethod() {  
     }  
 } 

双重校验锁

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