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POHow can I implement or find the equivalent of a thread-safe CompletionService?
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  1. POHow can I implement or find the equivalent of a thread-safe CompletionService?
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    copied!<p>I have a simple web service running inside a Tomcat container, which by nature is multi-threaded. In each request that comes into the service, I want to make concurrent calls to an external service. The ExecutorCompletionService in java.util.concurrent gets me partly there. I can provide it a thread pool, and it will take care of executing my concurrent calls and I will be notified when any of the results are ready.</p> <p>The code to process a particular incoming request might look like:</p> <pre><code>void handleRequest(Integer[] input) { // Submit tasks CompletionService&lt;Integer&gt; completionService = new ExecutorCompletionService&lt;Integer&gt;(Executors.newCachedThreadPool()); for (final Integer i : input) { completionService.submit(new Callable&lt;Integer&gt;() { public Integer call() { return -1 * i; } }); } // Do other stuff... // Get task results try { for (int i = 0; i &lt; input.size; i++) { Future&lt;Integer&gt; future = completionService.take(); Integer result = future.get(); // Do something with the result... } } catch (Exception e) { // Handle exception } } </code></pre> <p>This should work fine and dandy, but is quite inefficient since a new thread pool is being allocated for each incoming request. If I move the CompletionService out as a shared instance, I will run into thread-safety problems with multiple requests sharing the same CompletionService and thread pool. As requests submit tasks and get results, the results they get not be the ones they submitted.</p> <p>Thus, what I need is a thread-safe CompletionService that allows me to share a common thread pool across all incoming requests. As each thread completes a task, the appropriate thread for the incoming request should be notified so that it can gather the results.</p> <p>What's the most straightforward way to implement this sort of functionality? I'm sure this pattern has been applied many times; I'm just not sure if this is something provided by the Java concurrency library, or if can be easily built using some of the Java concurrency building blocks.</p> <p><strong>UPDATE:</strong> one caveat I forgot to mention is that I would like to be notified as soon as any of my submitted tasks complete. That's the primary advantage of using a CompletionService, as it decouples the production and consumption of the tasks and results. I don't actually care about the order in which I get the results back, and I'd like to avoid unnecessarily blocking while waiting for the results to be returned in order.</p>
 

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