a.u.r a.u.r - 5 months ago 10
Java Question

Schedule exceptions in Java

I'm developing a task that requires downloading files from given URLs, one of the requirement is that it could handle network failures and exceptions.

In my implementation I had a DonwloadManager that assigns a thread for every URL, so as not to be blocking.

The problem is that I am not sure how to mock failures and exceptions through the code, I tried using Quartz as scheduler and a job that only throws exception, but as far as I know the scheduler runs on different thread, so this won't affect the downloading thread at all.

Is there a way to mock the exceptions within the downloading thread ?

Here is my code for more insights:

public class DownloadManager {

int allocatedMemory = 0;

void validateURLs(String[] urls) {
if (urls.length == 0) {
throw new IllegalArgumentException("URLs List is empty");

public ArrayList<Status> downloadURLs(String[] urls, int memorySize) throws Exception {
ExecutorService executor = Executors.newWorkStealingPool();
CompletionService<Status> completionService = new ExecutorCompletionService<Status>(executor);
ArrayList<Status> downloadStatus = new ArrayList<Status>();
allocatedMemory = memorySize / urls.length;
for (String url : urls) {
completionService.submit(new DownloadWorker(url, allocatedMemory));
for(int i=0;i<urls.length;i++){
Future<Status> URLStatus = completionService.take();
return downloadStatus;

And this is the download Worker:

class DownloadWorker implements Callable<Status> {

static final int SUCCESS = 1, FAILURE = 0;

private int bufferSize;

private String assignedURL;

public DownloadWorker(String assignedURL, int bufferSize) {
this.bufferSize = bufferSize;
this.assignedURL = assignedURL;

public Status call() throws Exception {
URLConnection openConnection = new URL(assignedURL).openConnection();
"Mozilla/5.0 (Windows NT 6.1; WOW64; rv:25.0) Gecko/20100101 Firefox/39.0");
String outputFileName = assignedURL.substring(assignedURL.lastIndexOf("/") + 1);
RandomAccessFile outputFile = new RandomAccessFile(outputFileName, "rw");
ReadableByteChannel inputChannel = Channels.newChannel(openConnection.getInputStream());
FileChannel outputChannel = outputFile.getChannel();
ByteBuffer buffer = ByteBuffer.allocate(bufferSize);
while (inputChannel.read(buffer) > 0) {
for (int i = 0; i < buffer.limit(); i++) {
return new Status(assignedURL, true);


Any suggestions on how to mock/test the exception programmatically ?

Update: I was thinking, instead of scheduling exceptions, it would be easier to mock the resulting Future status, so that it returns an exception.


In your DownloadWorker, sleep for 6 seconds and when you are getting a Future in DownloadManager thread, set timeout as 5 seconds. Now you will get timeout exception.

In this way, list out Exceptions you want to catch in DownloadManager thread and throw them from your DownloadWorker.

Other way is overriding ThredPoolExecutor after execute method as per this one:

Handling exceptions from Java ExecutorService tasks

Note that FutureTasks swallows the exceptions (if you use submit() instead of execute() on ExecutorService. Have a look at source code. So raise exceptions properly from DownloadWorker.