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POMaking Spring beans behave like ThreadLocal instances for an ExecutorService
 

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  1. POMaking Spring beans behave like ThreadLocal instances for an ExecutorService
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    <p>In my web application, I have a background service. This service uses Generator class that contains an Engine class and an <code>ExecutorService</code> configured to use multiple threads and that accepts GeneratorTasks.</p> <pre><code>@Component public class Generator { @Autowired private Engine heavyEngine; private ExecutorService exec = Executors.newFixedThreadPool(3); //I actually pass the singleton instance Generator class into the task. public void submitTask(TaskModel model, TaskCallback callback) { this.exec.submit(new GeneratorTask(model, this, callback)); } } @Component public class Engine { public Engine() { //time-consuming initialization code here } } public class GeneratorTask implements Callable&lt;String&gt; { public GeneratorTask(TaskModel m, Generator g, ReceiptCallback c) { this.m = m; this.generator = g; this.c = c; } public String call() throws Exception { //This actually calls the Engine class of the generator. //Maybe I should have passed the Engine itself? this.generator.runEngine(c); } } </code></pre> <p>The Engine class takes a long time to initialize so I ideally want to initialize it only once per thread. I can't just make it a singleton instance because the instance can't be shared across multiple threads (it relies on sequential processing). It's perfectly fine to reuse the instance though, after a processing task has completed.</p> <p>I was thinking of making the <code>private Engine heavyEngine</code> variable a ThreadLocal variable. However, I'm also new to Spring so I was wondering if there might be another way to inject ThreadLocal variables using Spring annotations. I've looked at scoping the bean to <code>request</code> scope, but I'm not sure how I should go about it given my design.</p> <p>Any guidance on how to improve my design would be appreciated.</p>
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    1. USJensen Ching
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    1. COSince `Engine` is autowired I assume you have declared it as a bean. By default Spring beans are singleton, so there is a good chance that you `Engine` is already a singleton...
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    2. COSorry I might have been unclear. My problem is, I can't make Engine a singleton because due to its processing logic, it can't be accessed by multiple concurrent threads. It's safe to reuse the Engine once a complete 'job' has been done, which is why I was thinking of making an instance per thread that uses it.
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    3. COI understand, I was saying that with your current code Engine is already a singleton, even if it's not what you want it's what your code is actually doing.
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