多线程中的单例模式可能具有重复代码

Question

我看到有一个针对Singleton的在线教程，以实现多线程并确保实例只实例化一次。本教程下面有代码，但在代码中，有重复代码，因为他们想确保实例不为null。

if(instance == null){} 

但是，这个条件检查出现两次，这是重复吗？

    public class Singleton {
        private static Singleton instance;


        private Singleton(){
            //Here runs a lot code, that's why we don't want to instantiate here
        }

        // Using synchronized to ensure Singleton (only instantiate once) when 
        //implement multi threading.
        public static Singleton getInstance(){
            if(instance == null){
                synchronized(Singleton.class){
                    if(instance == null){
                        instance = new Singleton();
                    }    
                }
            } 
            return instance;
        }

    }

Answer 1

它没有重复。它再次被检查，因为在第一次检查if(instance == null)和锁定synchronized(Singleton.class)之间，另一个线程可以实例化它并打破单例。检查第一个代码块和第二个代码块之间的差异。

    public static Singleton getInstance(){
        if(instance == null){
            // Another thread could instantiate in this moment.
            synchronized(Singleton.class){
                instance = new Singleton(); // If another thread did already instantiate it, this would be the second call to the constructor.
            }
        } 
        return instance;
    }

锁定完成后再次检查可防止该竞争状况。

    public static Singleton getInstance(){
        if(instance == null){
            // Another thread could instantiate in this moment.
            synchronized(Singleton.class){
                // Now this code is thread safe
                if(instance == null){
                    // No other thread made a race condition before the first check and the locking. We can safely do the heavy work.
                    instance = new Singleton();
                }    
            }
        } 
        return instance;
    }