How to use Threads to run two Queries in Parallel

Question

How to make sure that the first iteration is run by One Thread and second iteration by Another Thread.

In the below code snippet, the runQuery method executes a Query and brings the resultSet as output.

I have two such queries and I want to run both in Parallel.

for (int i = 0; i < list.size(); i++) {
    String ProcessedRecord = list.get(i);
    String app_name = application.get(i);

    // ResultSet feedDetails = runQuery(ProcessedRecord,app_name); 
    try {
        System.out.println("Inside try block ... ");
        ResultSet feedDetails = dat.runQuery(ProcessedRecord, app_name);
        rs_feedDetails.add(feedDetails); 
    } catch(SQLException e) {
        e.printStackTrace();
    }
}

You posted a very close question few hours ago too. http://stackoverflow.com/questions/37811698/how-to-connect-to-3-different-databases-and-run-queries-parallel-through-jdbc — Sneh, Jun 14 '16 at 15:45

score 1 · Answer 1 · edited Jun 15 '16 at 10:23

You can do this with the help of ExecutorService, Callables and Futures.

Below is the modified code :

    ExecutorService pool = Executors.newFixedThreadPool(Runtime.getRuntime().availableProcessors()); //A thread pool to execute your tasks
    List<Future<ResultSet>> futures = new ArrayList<>(); //List to hold futures

    for (int i = 0; i < list.size(); i++) {
        String ProcessedRecord = list.get(i);
        String app_name = application.get(i);

        // ResultSet feedDetails = runQuery(ProcessedRecord,app_name); 
        try {
            Future<ResultSet> toAdd = pool.submit(new Callable<ResultSet>() { //submit a callable of resultset to the pool
                @Override
                public ResultSet call() throws Exception {
                    return dat.runQuery(ProcessedRecord, app_name);
                }
            });
            futures.add(toAdd);
        } catch(SQLException e) {
            e.printStackTrace();
        }
    }
    pool.shutdown(); // shut down the thread pool
    for(int i = 0; i < futures.size(); i++) {
        rs_feedDetails.add(futures.get(i).get()); //invoke get method of every future in your list. The get method will return you the result.
    }

EDIT

As mentioned in the comments, under certain circumstances tasks may be executed on the same thread in serial - it is possible to enforce parallelism with ExecutorService parameters like "parallelism" or "amountThreads"

I am being pedantic, but there is no guarantee that these tasks will be executed by two different threads. They may be executed in parallel by two different threads, or serially in a single thread. That being said, this is the correct answer to the question. — Sean Bright, Jun 14 '16 at 15:54
um ... that would pretty much **defeat the purpose of ExecutorServices** - as long as `availableProcessors` (in your example) is greater than 1, submitted tasks will **always** be executed in parallel - except for the rare case of ultra-fast task completion -- if task A completes **before** task B is *submitted* (astronomically unlikely) then the two tasks **may** be executed one after another on the same `Thread`, but being hit by a metor is much more likely. — specializt, Jun 14 '16 at 16:04

How to use Threads to run two Queries in Parallel

1 Answers1