Java 并发与集合

Question

For the below function, I am trying to return a new Set mHashSet that is a copy of another set iGraphicSectors :

public Set<String> getGraphics() {
    synchronized (iGraphicSectors) {  // class variable of type Set<String>
        LinkedHashSet<String> mHashSet = new LinkedHashSet<String>();
        synchronized (mHashSet) {
            mHashSet.addAll(iGraphicSectors);
        }
        return mHashSet;
    }
}

However line 5 mHashSet.addAll(iGraphicSectors); is throwing a ConcurrentModificationException (I am not sure how this is possible). Is there a way I can accomplish the above task in a thread-safe manner?

Answer 1

What you need to do is to use a Set that is thread-safe for iGraphicSectors as you obviously read and modify it concurrently, the simplest way can be to use the decorator Collections.synchronizedSet(Set<T> s) to make your current Set thread-safe, any read and write accesses will then be automatically protected with a synchronized block thanks to the decorator, however you still need to protect iterations over it explicitly with a synchronized block.

Here is an example of how to create it:

Set<String> iGraphicSectors = Collections.synchronizedSet(new HashSet<String>());

Here is how your code would then look like:

public Set<String> getGraphics() {
    // Still needed as the constructor of LinkedHashSet will iterate
    // over iGraphicSectors 
    synchronized (iGraphicSectors) {  
        return new LinkedHashSet<String>(iGraphicSectors);
    }
}

Answer 2

From your latest comments, it sounds like you simply want to make the Set immutable - which you can do without using any synchronization primitives with :

return Collections.unmodifiableSet(iGraphicSectors);

at the end of this function (See the docs ).

As a side note, it's fairly obvious that you don't want to use synchronization, as the Set is constructed locally within the function. It does not have any visibility to other threads executing in your program. If any synchronization is to take place, it's not in this method .

The real question is, will the Set returned by this method change? If so, you can return a synchronized Set from this function with:

return Collections.synchronizedSet(...)

at the end of this function (again, see the docs ).