Can I do a synchronous request with volley?

Question

It looks like it is possible with Volley's RequestFuture class. For example, to create a synchronous JSON HTTP GET request, you can do the following:

RequestFuture<JSONObject> future = RequestFuture.newFuture();
JsonObjectRequest request = new JsonObjectRequest(URL, new JSONObject(), future, future);
requestQueue.add(request);

try {
  JSONObject response = future.get(); // this will block
} catch (InterruptedException e) {
  // exception handling
} catch (ExecutionException e) {
  // exception handling
}

Note @Matthews answer is correct BUT if you are on another thread and you do a volley call when you have no internet, your error callback will be called on the main thread, but the thread you are on will be blocked FOREVER. (Therefore if that thread is an IntentService, you will never be able to send another message to it and your service will be basically dead).

Use the version of get() that has a timeout future.get(30, TimeUnit.SECONDS) and catch the error to exit your thread.

To match @Mathews answer:

        try {
            return future.get(30, TimeUnit.SECONDS);
        } catch (InterruptedException e) {
            // exception handling
        } catch (ExecutionException e) {
            // exception handling
        } catch (TimeoutException e) {
            // exception handling
        }

Below I wrapped it in a method & use a different request:

   /**
     * Runs a blocking Volley request
     *
     * @param method        get/put/post etc
     * @param url           endpoint
     * @param errorListener handles errors
     * @return the input stream result or exception: NOTE returns null once the onErrorResponse listener has been called
     */
    public InputStream runInputStreamRequest(int method, String url, Response.ErrorListener errorListener) {
        RequestFuture<InputStream> future = RequestFuture.newFuture();
        InputStreamRequest request = new InputStreamRequest(method, url, future, errorListener);
        getQueue().add(request);
        try {
            return future.get(REQUEST_TIMEOUT, TimeUnit.SECONDS);
        } catch (InterruptedException e) {
            Log.e("Retrieve cards api call interrupted.", e);
            errorListener.onErrorResponse(new VolleyError(e));
        } catch (ExecutionException e) {
            Log.e("Retrieve cards api call failed.", e);
            errorListener.onErrorResponse(new VolleyError(e));
        } catch (TimeoutException e) {
            Log.e("Retrieve cards api call timed out.", e);
            errorListener.onErrorResponse(new VolleyError(e));
        }
        return null;
    }

It is probably recommended to use the Futures, but if for whatever reason you don't want to, instead of cooking your own synchronized blocking thing you should use a java.util.concurrent.CountDownLatch. So that would work like this..

//I'm running this in an instrumentation test, in real life you'd ofc obtain the context differently...
final Context context = InstrumentationRegistry.getTargetContext();
final RequestQueue queue = Volley.newRequestQueue(context);
final CountDownLatch countDownLatch = new CountDownLatch(1);
final Object[] responseHolder = new Object[1];

final StringRequest stringRequest = new StringRequest(Request.Method.GET, "http://google.com", new Response.Listener<String>() {
    @Override
    public void onResponse(String response) {
        responseHolder[0] = response;
        countDownLatch.countDown();
    }
}, new Response.ErrorListener() {
    @Override
    public void onErrorResponse(VolleyError error) {
        responseHolder[0] = error;
        countDownLatch.countDown();
    }
});
queue.add(stringRequest);
try {
    countDownLatch.await();
} catch (InterruptedException e) {
    throw new RuntimeException(e);
}
if (responseHolder[0] instanceof VolleyError) {
    final VolleyError volleyError = (VolleyError) responseHolder[0];
    //TODO: Handle error...
} else {
    final String response = (String) responseHolder[0];
    //TODO: Handle response...
}

Since people seemed to actually try to do this and ran into some trouble I decided I'd actually provide a "real life" working sample of this in use. Here it is https://github.com/timolehto/SynchronousVolleySample

Now even though the solution works, it has some limitations. Most importantly, you can't call it on the main UI thread. Volley does execute the requests on the background, but by default Volley uses the main Looper of the application to dispatch the responses. This causes a deadlock as the main UI thread is waiting for the response, but the Looper is waiting for onCreate to finish before processing the delivery. If you really really want to do this you could, instead of the static helper methods, instantiate your own RequestQueue passing it your own ExecutorDelivery tied to a Handler using a Looper which is tied to different thread from the main UI thread.

As a complementary observation to both @Blundells and @Mathews answers, I'm not sure any call is delivered to anything but the main thread by Volley.

The Source

Having a look at the RequestQueue implementation it seems the RequestQueue is using a NetworkDispatcher to execute the request and a ResponseDelivery to deliver the result (the ResponseDelivery is injected into the NetworkDispatcher). The ResponseDelivery is in turn created with a Handler spawn from the main thread (somewhere around line 112 in the RequestQueue implementation).

Somewhere about line 135 in the NetworkDispatcher implementation it seems like also successful results are delivered through the same ResponseDelivery as any errors. Again; a ResponseDelivery based on a Handler spawn from the main thread.

Rationale

For the use-case where a request is to be made from an IntentService it's fair to assume that the thread of the service should block until we have a response from Volley (to guarantee a living runtime scope to handle the result in).

Suggested solutions

One approach would be to override the default way a RequestQueue is created, where an alternative constructor is used instead, injecting a ResponseDelivery which spawns from the current thread rather than the main thread. I haven't investigated the implications of this, however.

I want to add something to Matthew's accepted answer. While RequestFuture might seem to make a synchronous call from the thread you created it, it does not. Instead, the call is executed on a background thread.

From what I understand after going through the library, requests in the RequestQueue are dispatched in its start() method:

    public void start() {
        ....
        mCacheDispatcher = new CacheDispatcher(...);
        mCacheDispatcher.start();
        ....
           NetworkDispatcher networkDispatcher = new NetworkDispatcher(...);
           networkDispatcher.start();
        ....
    }

Now both CacheDispatcher and NetworkDispatcher classes extend thread. So effectively a new worker thread is spawned for dequeuing the request queue and the response is returned to the success and error listeners implemented internally by RequestFuture.

Although your second purpose is attained but you first purpose is not since a new thread is always spawned, no matter from which thread you execute RequestFuture.

In short, true synchronous request is not possible with default Volley library. Correct me if I am wrong.

You achieve this with kotlin Coroutines

implementation "org.jetbrains.kotlinx:kotlinx-coroutines-core:1.3.7"
implementation "org.jetbrains.kotlinx:kotlinx-coroutines-android:1.3.7"

private suspend fun request(context: Context, link : String) : String{
   return suspendCancellableCoroutine { continuation ->
      val queue = Volley.newRequestQueue(context)
      val stringRequest = StringRequest(Request.Method.GET, link,
         { response ->
            continuation.resumeWith(Result.success(response))
         },
          {
            continuation.cancel(Exception("Volley Error"))
         })

      queue.add(stringRequest)
   }
}

And call with

CoroutineScope(Dispatchers.IO).launch {
    val response = request(CONTEXT, "https://www.google.com")
    withContext(Dispatchers.Main) {
       Toast.makeText(CONTEXT, response,Toast.LENGTH_SHORT).show()
   }
}