三種單例模式實際都是有運用的。

優點：延遲載入

缺點：不加同步的懶漢式是執行緒不安全的，加了synchronized之後就變成執行緒安全的了

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

優點：執行緒安全

缺點：浪費記憶體空間

例項應用：java.lang.Runtime

public class Singleton {

    private static final Singleton singleton = new Singleton();

    private Singleton(){

    }

    public static Singleton getInstance(){

       return singleton ;

    }

}

特點：效能又保證執行緒安全

public class Singleton{

    private static Singleton singleton;   

    private Singleton(){

    }

    public static Singleton getInstance(){

       if(singleton == null){

           synchronized(Singleton.class){

              if(singleton == null){

                  singleton = new Singleton();

              }
           }
       }

    return singleton;
    }
}