java 40 lines · 7 steps

Bounded parallel thumbnail rendering in Java

A fixed thread pool renders thumbnails concurrently while each task's result is collected with a timeout and per-item error handling.

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Intermediate concurrency thread-pool futures timeout error-handling
1public class ThumbnailProcessor {
2 
3 private static final int MAX_CONCURRENCY = 4;
4 
5 private final ExecutorService executor = Executors.newFixedThreadPool(MAX_CONCURRENCY);
6 private final ThumbnailGenerator generator;
7 
8 public ThumbnailProcessor(ThumbnailGenerator generator) {
9 this.generator = generator;
10 }
11 
12 public Map<String, byte[]> processAll(List<URI> sources) throws InterruptedException {
13 Map<URI, Future<byte[]>> pending = new LinkedHashMap<>();
14 
15 for (URI source : sources) {
16 pending.put(source, executor.submit(() -> generator.render(source)));
17 }
18 
19 Map<String, byte[]> results = new LinkedHashMap<>();
20 for (Map.Entry<URI, Future<byte[]>> entry : pending.entrySet()) {
21 String key = entry.getKey().toString();
22 try {
23 results.put(key, entry.getValue().get(30, TimeUnit.SECONDS));
24 } catch (TimeoutException e) {
25 entry.getValue().cancel(true);
26 log.warn("Thumbnail timed out for {}", key);
27 } catch (ExecutionException e) {
28 log.error("Thumbnail failed for {}", key, e.getCause());
29 }
30 }
31 return results;
32 }
33 
34 public void shutdown() throws InterruptedException {
35 executor.shutdown();
36 if (!executor.awaitTermination(1, TimeUnit.MINUTES)) {
37 executor.shutdownNow();
38 }
39 }
40}
01 / 01
STEP 01

Walkthrough

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Three takeaways
  1. 1A fixed thread pool caps how many tasks run at once, protecting the machine from unbounded parallelism.
  2. 2Submitting all work first and then collecting Futures lets independent tasks overlap instead of running one at a time.
  3. 3Per-future timeouts and try/catch keep one slow or failing task from sinking the entire batch.

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