singleton实例

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

改进版

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;
}
}