三种单例模式实际都是有运用的。

懒汉式单例模式：

优点：延迟加载

缺点：不加同步的懒汉式是线程不安全的，加了synchronized之后就变成线程安全的了

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

饿汉式单例模式：

优点：线程安全

缺点：浪费内存空间

实例应用：java.lang.Runtime

public class Singleton {

    private static final Singleton singleton = new Singleton();

    private Singleton(){

    }

    public static Singleton getInstance(){

       return singleton ;

    }

}

双重检查加锁模式：

特点：性能又保证线程安全

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;
    }
}