BoxStore: actually print thread stacks if pool isn't shut down in time #258

Previously Thread.dumpStack() was used which only prints the stack of
the current thread. Also *all* existing threads were printed, not just
those from the internal thread pool.

Author: greenrobot-team

CHANGELOG.md

@@ -10,6 +10,8 @@ For more insights into what changed in the ObjectBox C++ core, [check the Object
   an "Another BoxStore is still open for this directory" exception. Note that calling `close()` *is recommended* before 
   creating a new instance. [#1201](https://github.com/objectbox/objectbox-java/issues/1201)
 - When using `BoxStoreBuilder.buildDefault()`, don't leak Store when setting as default fails.
+- To help diagnose, print stacks of all threads in the internal thread pool if shutting it down takes too long when
+  closing `BoxStore`.
 
 ## 4.3.1 - 2025-08-12
 

objectbox-java/src/main/java/io/objectbox/BoxStore.java

@@ -743,18 +743,31 @@ public void close() {
         }
     }
 
-    /** dump thread stacks if pool does not terminate promptly. */
+    /**
+     * Waits briefly for the internal {@link #threadPool} to terminate. If it does not terminate in time, prints stack
+     * traces of the pool threads.
+     */
     private void checkThreadTermination() {
         try {
             if (!threadPool.awaitTermination(1, TimeUnit.SECONDS)) {
-                int activeCount = Thread.activeCount();
-                getErrorOutput().println("Thread pool not terminated in time; printing stack traces...");
-                Thread[] threads = new Thread[activeCount + 2];
+                getErrorOutput().println("ObjectBox thread pool not terminated in time." +
+                        " Ensure all async calls have completed and subscriptions are cancelled before closing the Store." +
+                        "\nPrinting pool threads...");
+                // Note: this may not print any pool threads if other threads are started while enumerating
+                // (and the pool threads do not make it into the threads array).
+                Thread[] threads = new Thread[Thread.activeCount()];
                 int count = Thread.enumerate(threads);
                 for (int i = 0; i < count; i++) {
-                    getErrorOutput().println("Thread: " + threads[i].getName());
-                    Thread.dumpStack();
+                    Thread thread = threads[i];
+                    if (thread.getName().startsWith(ObjectBoxThreadPool.THREAD_NAME_PREFIX)) {
+                        getErrorOutput().println("Thread: " + thread.getName());
+                        StackTraceElement[] trace = thread.getStackTrace();
+                        for (StackTraceElement traceElement : trace) {
+                            getErrorOutput().println("\tat " + traceElement);
+                        }
+                    }
                 }
+                getErrorOutput().println("Printing pool threads...DONE");
             }
         } catch (InterruptedException e) {
             e.printStackTrace(getErrorOutput());

objectbox-java/src/main/java/io/objectbox/internal/ObjectBoxThreadPool.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2017 ObjectBox Ltd.
+ * Copyright 2017-2024 ObjectBox Ltd.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -36,6 +36,8 @@
  */
 @Internal
 public class ObjectBoxThreadPool extends ThreadPoolExecutor {
+
+    public static String THREAD_NAME_PREFIX = "ObjectBox-";
     private final BoxStore boxStore;
 
     public ObjectBoxThreadPool(BoxStore boxStore) {
@@ -54,7 +56,7 @@ static class ObjectBoxThreadFactory implements ThreadFactory {
         private static final AtomicInteger POOL_COUNT = new AtomicInteger();
 
         private final ThreadGroup group;
-        private final String namePrefix = "ObjectBox-" + POOL_COUNT.incrementAndGet() + "-Thread-";
+        private final String namePrefix = THREAD_NAME_PREFIX + POOL_COUNT.incrementAndGet() + "-Thread-";
         private final AtomicInteger threadCount = new AtomicInteger();
 
         ObjectBoxThreadFactory() {