Multithreading for array in Java

Question

So, the scenario is like:

//Some code...

public Map<String, String> someFunction() {
    for (final UserDetail user : userDetailList) {
        // the following (below) code runs in background thread
        // long running task
        new RecordPersonalisedDao().getPendingRecordsForUid(user.getuId(), new RecordPersonalisedDao.OnResultFetched() {
            @Override
            public void onResult(Result result) {
                // callback after the user is processed
                // put some parameter from result to map
                map.put(user.getName(), result.getTotal());
            }
        });
    }
    // return map only after all users are processed
    return map;
}

As mentioned in the comment of above piece of code, I want the final map to be returned only after the entire list of user is processed.

I cannot change the functionality of RecordPersonalisedDao#getPendingRecordsForUid so as to make it run in the main thread only.

How do I achieve this in java ?

Edit: This type of problem can be faced in general. So, I want to understand the solution for the same in java.

To put my question simply, I want behaviour like

Run this code in background for all members in the array, and once it's done, send a callback.

(Roughly like)

[list_of_some_data]
    .forEach( run this function )
    .after(after the function is run in background for  all members of list - return some value)

Possible duplicate of [thread safe map for java](https://stackoverflow.com/questions/1792023/thread-safe-map-for-java) — Scrambo, Sep 11 '17 at 13:25
Use join() method to wait until background thread is completed. — Supun Amarasinghe, Sep 11 '17 at 13:31
What you want is an ExecutorService and to use the Future that is returned by submitting a task as your callback. — bhspencer, Sep 11 '17 at 13:36
If I understand correctly then you want to use `Future` and use `CompletableFuture.allOf` in order to wait for all the `Future`s. See here https://stackoverflow.com/a/36261808/3883957. — yishaiz, Sep 11 '17 at 13:38

score 1 · Answer 1 · answered Sep 11 '17 at 13:39

1

Before the loop, create a CountdownLatch with a count equal to the user list length. Inside the result handler, count down after updating the map. After the loopawait() the latch to be counted down, then return.

answered Sep 11 '17 at 13:39

erickson

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The real question is how to do things in parallel. I really don't see how your "answer" answers that. Don't get me wrong ... but I guess you didn't collect your 200K reputation by giving such answers, did you? – GhostCat Sep 11 '17 at 15:03
@GhostCat The question very clearly states that `getPendingRecordsForUid()` runs in a background thread, and that the question, more generally, is how to block until all background tasks have completed—under the constraint that you can't change the task itself. – erickson Sep 11 '17 at 15:41
E.g., "the following (below) code runs in background thread," "I cannot change the functionality of RecordPersonalisedDao#getPendingRecordsForUid so as to make it run in the main thread only," and "return map only after all users are processed" – erickson Sep 11 '17 at 16:03
1

Thank you Sir :) Used ``CountDownLatch`` to solve the same. – Sumit Jha Sep 11 '17 at 18:09

score 1 · Answer 2 · answered Sep 11 '17 at 13:46

public Map<String, String> someFunction() {
    CountDownLatch cdl = new CountDownLatch(userDetailsList.size());
    for (final UserDetail user : userDetailList) {
        // the following (below) code runs in background thread
        // long running task
        new RecordPersonalisedDao().getPendingRecordsForUid(user.getuId(), new RecordPersonalisedDao.OnResultFetched() {
            @Override
            public void onResult(Result result) {
                // callback after the user is processed
                // put some parameter from result to map
                map.put(user.getName(), result.getTotal());

                //We're done grabbing the results.. count down.
                cdl.countDown();
            }
        });
    }

    //Block this thread until all the results are in.
    cdl.await();

    return map;
}

Multithreading for array in Java

2 Answers2