线程安全的实现代码

public class Singleton {  

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