java单例模式

饿汉模式:

public class HungrySingleton {
  private HungrySingleton() {} ❶

  private static HungrySingleton instance = new HungrySingleton();❷

  public static HungrySingleton getInstance() {
    return instance;
  }
}

懒汉模式:

public class LazySingleton {
   private static LazySingleton instance = null;	❶
     
	 private LazySingleton() {	
 	 }	
  public static LazySingleton getInstance() {	❷
    if(instance == null){ ❸
      instance = new LazySingleton();
    }
    return instance;
  }
}

线程安全懒汉模式:

public class LazySingleton {
  private LazySingleton() {	
  }	
  private static volatile LazySingleton instance = null;	❶

  public static synchronized LazySingleton getInstance() { ❷
    if(instance == null){
      instance = new LazySingleton();
    }
    return instance;
  }
}

双重检测锁:

public class LazySingleton {
    private LazySingleton() {
    }	
    
    private static volatile LazySingleton instance = null; ❷
    public static LazySingleton getInstance(){ ❶
      if(instance == null){ ❸	
        synchronized (LazySingleton.class) { ❺
          if(instance == null)	❹
          instance = new LazySingleton();❻
        }
      }
      return instance;
    }
  }