通过 this vs dummy 对象同步

2022-01-22 00:00:00 multithreading thread-safety synchronization synchronized java

到目前为止，我已经看到了同步块的用法，但最近我了解到使用虚拟对象更可取.我找到了与此相关的以下主题.

I have seen usage of synchronized block by this so far but recently I learned that using dummy object is preferable. I found the following topic related to this.

Java 同步方法锁定对象或方法?

总而言之，在下面的代码中，两个不同的对象不能同时运行 addA 和 addB，因为两者都使用 this 来锁定.

As a summary, in the code below, two different object can not run addA and addB concurrently as both uses this for lock. 

private int a;
private int b;

public synchronized void addA(){
    a++;
}

public synchronized void addB(){
    b++;
}

如果我使用虚拟对象进行锁定，我会感到困惑，如果我在两种方法中使用下面的行来同步会有什么不同?因为他们仍然会有相同的锁.

I am confused if I use dummy object for lock, what will be different if I use the line below in both method to synchronize? Because still they would have same lock.

synchronized(dummyObject){
    ...
}

那么，我应该为每种方法使用两个不同的虚拟对象以与 sycnhronized as 一起使用，这意味着什么?

So what it means that I should have two different dummy object for each method to use with sycnhronized as?

 public void addA(){
    synchronized(dummyObj1){
       a++;
    }
}

public void addB(){
    synchronized(dummyObj2){
       b++;
    }
}

推荐答案

这正是锁对象的重点——你可以为不同的操作使用不同的锁.假设同时运行 addA 和 addB 是有意义的(并且从外观上看 - 它确实如此)，您确实应该有两个单独的锁，每个方法一个.

That is exactly the point of lock objects - you can use different locks for different operations. Assuming it makes sense to run addA and addB concurrently (and from the looks of it - it definitely does), you should indeed have two separate locks, one for each method.

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