Stop AsyncHttpDelivery from indefinitely blocking exit

The executor used by AsyncHttpDelivery indefinitely blocks application exit unless a shutdown is manually triggered. This replaces the executor with one which times out when idle and so doesn't block an exit.

Author: foygl

src/main/java/com/bugsnag/delivery/AsyncHttpDelivery.java

@@ -7,15 +7,23 @@
 
 import java.net.Proxy;
 import java.util.concurrent.ExecutorService;
-import java.util.concurrent.Executors;
+import java.util.concurrent.LinkedBlockingQueue;
+import java.util.concurrent.ThreadPoolExecutor;
 import java.util.concurrent.TimeUnit;
 
 public class AsyncHttpDelivery implements HttpDelivery {
     private static final Logger logger = LoggerFactory.getLogger(AsyncHttpDelivery.class);
     private static final int SHUTDOWN_TIMEOUT = 5000;
 
-    protected HttpDelivery baseDelivery = new SyncHttpDelivery();
-    protected ExecutorService executorService = Executors.newSingleThreadExecutor();
+    private HttpDelivery baseDelivery = new SyncHttpDelivery();
+
+    // Create an exector service which keeps idle threads alive for a maximum of SHUTDOWN_TIMEOUT.
+    // This should avoid blocking an application that doesn't call shutdown from exiting.
+    private ExecutorService executorService =
+            new ThreadPoolExecutor(0, 1,
+                    SHUTDOWN_TIMEOUT, TimeUnit.MILLISECONDS,
+                    new LinkedBlockingQueue<Runnable>());
+
     private boolean shuttingDown = false;
 
     /**