Use explicit ExecutorService for Java scenario event listening

Avoid exhausting the ForkJoin.commonPool() in constrained environments, which can cause deadlocks.

Signed-off-by: Mark S. Lewis <[email protected]>

java/src/test/java/scenario/BasicEventListener.java

@@ -9,27 +9,33 @@
 import java.util.Objects;
 import java.util.concurrent.BlockingQueue;
 import java.util.concurrent.CompletableFuture;
+import java.util.concurrent.ExecutorService;
+import java.util.concurrent.Executors;
 import java.util.concurrent.SynchronousQueue;
 import java.util.concurrent.TimeUnit;
 
 import org.hyperledger.fabric.client.CloseableIterator;
 
 public final class BasicEventListener<T> implements EventListener<T> {
     private final BlockingQueue<T> eventQueue = new SynchronousQueue<>();
+    private final ExecutorService executor = Executors.newSingleThreadExecutor();
     private final Runnable close;
 
     public BasicEventListener(final CloseableIterator<T> iterator) {
         close = iterator::close;
 
         // Start reading events immediately as Java gRPC implementation may not invoke the gRPC service until the first
         // read attempt occurs.
-        CompletableFuture.runAsync(() -> iterator.forEachRemaining(event -> {
-            try {
-                eventQueue.put(event);
-            } catch (InterruptedException e) {
-                iterator.close();
-            }
-        }));
+        CompletableFuture.runAsync(
+                () -> iterator.forEachRemaining(event -> {
+                    try {
+                        eventQueue.put(event);
+                    } catch (InterruptedException e) {
+                        iterator.close();
+                    }
+                }),
+                executor
+        );
     }
 
     public T next() throws InterruptedException {
@@ -40,5 +46,6 @@ public T next() throws InterruptedException {
 
     public void close() {
         close.run();
+        executor.shutdownNow();
     }
 }