单例(Singleton)模式

如果要保证系统里一个类最多只能存在一个实例时,我们就需要单例模式.

public class Singleton{
   public static Singleton instance=new Singleton();
  //other fields
  private Singeton(){}
  public static Singleton getInstance(){
       return instance;
}
  //other methods
}

public class ThreadSafeSingleton{
	private volatile static ThreadSafeSingleton instance;
	private ThreadSafeSingleton(){};
	public static  ThreadSafeSingleton getInstance(){
		if(instance==null){

		synchronisezd(ThreadSafeSingleton.class){
		if(instance==null)
		instance =new ThreadSafeSingleton();
		}

	}
	return instance;
}
}