singleton实例

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

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