在线程中同步Hashmap

Question

I am facing some issues with synchronizing my idMap . This map is being used in two run() methods which are run concurrently. In the 1st run() method i'm simply mapping event id value) to response id (key). in the 2nd run() method I wish to obtain the event id (value) with that same response id (key). However, at times some event id is there and at times they can't be obtained. The program compiles just fine, but i'm no expert at threading and i believe threading is causing this idMap to be out of sync. My question is simply, how can I make idMap work smoothly and obtain the event ids as I intend to?

ConcurrentHashMap<String, String> idMap = new ConcurrentHashMap<String, String>();
ConcurrentHashMap<String, ExecutorService> executors = new ConcurrentHashMap<String, ExecutorService>();

private final class ResponderTask implements Runnable {
    private ResponderTask(Event event) {
        this.event = event;
    }
    // 1st run()    
    public void run() {
        idMap.put(response.getId(), event.getId()); 
    }
}//end ResponderTask 

private final class QuoteTask implements Runnable {
    //constructor
    //2nd run() 
    public void run() {
        IdMap.get(response.getId());
    }
}//end QuoteTask

public void onResponse(final Response response) {

    ExecutorService quoteExecutor = executors.get(response.getId());
    if (quoteExecutor == null) {
        quoteExecutor = Executors.newSingleThreadExecutor();                
        executors.put(event.getId(), quoteExecutor);            
    }
    quoteExecutor.execute(new ResponderTask(event));
}

Answer 1

However, at times some event id is there and at times they can't be obtained. The program compiles just fine, but i'm no expert at threading and i believe threading is causing this idMap to be out of sync.

idMap is a ConcurrentHashMap which is properly synchronized and highly used and tested by many folks. If the id is not in the map when you look it up then it has not been put in there. If you explain a bit more about your code we may be able to find your problem.

For example, I don't see where the response object originates from. Is it possible that the ResponseTask is processing a different response then you expect? Is response and event supposed to be the same argument?

My question is simply, how can I make idMap work smoothly and obtain the event ids as I intend to?

It's a little hard to figure out what the proper operation of the program is supposed to be. If you are looking for the other thread to get the event you could use a BlockingQueue . Then your other thread can do a queue.take() which will wait until there is an event to process. But I'm not sure what the goal is here.

One thing that is very strange is the use of a map of ExecutorService . Do you really need multiple of them? I suspect that you really should use a single Executors.newCachedThreadPool() . Maybe, however, you want a single thread working on all requests with the same id in which case your code should work. I assume you are doing something like the following when you want to shutdown you application:

for (ExecutorService executor : executors.values()) {
    executor.shutdown();
}