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POThread Pool handling 'duplicate' tasks
 

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    <p>I want to be executing some different tasks in parallel, but have the concept that if a task is already queued or is currently processing, it will not get re-queued. I have read up a little on the Java API and have come up with the code below, which seems to work. Can anybody shed light on whether the method I am using is the best approach. Any dangers (thread safety?) or better ways to do this? Code is as below:</p> <pre><code>import java.util.HashMap; import java.util.concurrent.Future; import java.util.concurrent.LinkedBlockingQueue; import java.util.concurrent.ThreadPoolExecutor; import java.util.concurrent.TimeUnit; public class TestExecution implements Runnable { String key1; String key2; static HashMap&lt;TestExecution, Future&lt;?&gt;&gt; executions = new HashMap&lt;TestExecution, Future&lt;?&gt;&gt;(); static LinkedBlockingQueue&lt;Runnable&gt; q = new LinkedBlockingQueue&lt;Runnable&gt;(); static ThreadPoolExecutor tpe = new ThreadPoolExecutor(2, 5, 1, TimeUnit.MINUTES, q); public static void main(String[] args) { try { execute(new TestExecution("A", "A")); execute(new TestExecution("A", "A")); execute(new TestExecution("B", "B")); Thread.sleep(8000); execute(new TestExecution("B", "B")); } catch (InterruptedException e) { e.printStackTrace(); } } static boolean execute(TestExecution e) { System.out.println("Handling "+e.key1+":"+e.key2); if (executions.containsKey(e)) { Future&lt;?&gt; f = (Future&lt;?&gt;) executions.get(e); if (f.isDone()) { System.out.println("Previous execution has completed"); executions.remove(e); } else { System.out.println("Previous execution still running"); return false; } } else { System.out.println("No previous execution"); } Future&lt;?&gt; f = tpe.submit(e); executions.put(e, f); return true; } public TestExecution(String key1, String key2) { this.key1 = key1; this.key2 = key2; } public boolean equals(Object obj) { if (obj instanceof TestExecution) { TestExecution t = (TestExecution) obj; return (key1.equals(t.key1) &amp;&amp; key2.equals(t.key2)); } return false; } public int hashCode () { return key1.hashCode()+key2.hashCode(); } public void run() { try { System.out.println("Start processing "+key1+":"+key2); Thread.sleep(4000); System.out.println("Finish processing "+key1+":"+key2); } catch (InterruptedException e) { e.printStackTrace(); } } } </code></pre> <p>Follow up to comment below:<br> The plan is that triggering the tasks to execute will be handled by cron calling RESTful web service. For example below is the setup for one task triggered at 9:30 every day, plus another scheduled every two minutes. </p> <pre><code>0/2 * * * * restclient.pl key11 key12 30 09 * * * restclient.pl key21 key22 </code></pre> <p>In this case, if task key11:key12 is running, or already queued to run, I don't want to queue another instance. I understand we have other options for scheduling, however we tend to use cron for other tasks, so I want to try to keep this.</p> <p>Second Update. In response to comments so far I have re-written the code, could you comment on any issues with the following updated solution?</p> <pre><code>import java.util.concurrent.LinkedBlockingQueue; public class TestExecution implements Runnable { String key1; String key2; static TestThreadPoolExecutor tpe = new TestThreadPoolExecutor(new LinkedBlockingQueue&lt;Runnable&gt;()); public static void main(String[] args) { try { tpe.execute(new TestExecution("A", "A")); tpe.execute(new TestExecution("A", "A")); tpe.execute(new TestExecution("B", "B")); Thread.sleep(8000); tpe.execute(new TestExecution("B", "B")); } catch (InterruptedException e) { e.printStackTrace(); } } public TestExecution(String key1, String key2) { this.key1 = key1; this.key2 = key2; } public boolean equals(Object obj) { if (obj instanceof TestExecution) { TestExecution t = (TestExecution) obj; return (key1.equals(t.key1) &amp;&amp; key2.equals(t.key2)); } return false; } public int hashCode () { return key1.hashCode()+key2.hashCode(); } public void run() { try { System.out.println("Start processing "+key1+":"+key2); Thread.sleep(4000); System.out.println("Finish processing "+key1+":"+key2); } catch (InterruptedException e) { e.printStackTrace(); } } } import java.util.Collections; import java.util.HashSet; import java.util.Set; import java.util.concurrent.LinkedBlockingQueue; import java.util.concurrent.ThreadPoolExecutor; import java.util.concurrent.TimeUnit; public class TestThreadPoolExecutor extends ThreadPoolExecutor { Set&lt;Runnable&gt; executions = Collections.synchronizedSet(new HashSet&lt;Runnable&gt;()); public TestThreadPoolExecutor(LinkedBlockingQueue&lt;Runnable&gt; q) { super(2, 5, 1, TimeUnit.MINUTES, q); } public void execute(Runnable command) { if (executions.contains(command)) { System.out.println("Previous execution still running"); return; } else { System.out.println("No previous execution"); } super.execute(command); executions.add(command); } protected void afterExecute(Runnable r, Throwable t) { super.afterExecute(r, t); executions.remove(r); } } </code></pre>
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