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Restart thread once the threads inside it finished

I have thread x which I start like so:
ScheduledExecutorService exec = Executors.newSingleThreadScheduledExecutor();
exec.scheduleAtFixedRate(() -> {
Inside x I have a CyclicBarrier with another 10 threads:
final CyclicBarrier _threadGate = new CyclicBarrier(10);
ArrayList<Thread> _threadList = new ArrayList<>();
Then I add the thread to the list
for (int i = 0; i < 10; i++) {
_threadList.add(new Thread() {
#Override
public void run() {
_threadGate.await();
//long processing code
So after the threads are ready I start them, it is important for them to start at the same time (well almost, looping takes time, even if its 0,01ms):
for (int i = 0; i < _threadList.size(); i++) {
_threadList.get(i).start();
}
Now, the end of x, the main thread, is like this:
}, 0, repeatTimer, TimeUnit.SECONDS);
If repeatTimer is 300 this means that it starts again the 10 threads after 5 minutes.
The time for the 10 threads to finish is an UNKNOWN amount, but it is under 5 minutes. Somewhere between 2 and 4 minutes for sure.
What I want to achieve
Once the 10 threads finish, restart X but with a delay of 5 seconds.
For this I have been thinking of setting the repeatTimer value to the time elapsed by the 10 threads + 5 seconds (I dont know how to do it, I dont know w hen last thread finishes its task), but is this correct? or is there another way of doing it?

I don't see the necessity of having SchedulingExecutorService here. You can just wait until all threads finish their job using CountDownLatch.
Here's a simple exapmple:
while (!stopped) {
CountDownLatch latch = new CountDownLatch(N);
// create and start your threads
latch.await(); // this method blocks until each Thread calls countDown()
// wait here 5 seconds if you want
}
Decrement the latch in last action of each thread:
public void run() {
_threadGate.await();
// thread actions
latch.countDown();
}

Set thread number limitation [duplicate]

I want to launch a lot of tasks to run on a database of +-42Mio records. I want to run this in batches of 5000 records/time (results in 850 tasks).
I also want to limit the number of threads (to 16) java starts to do this for me and I am using the current code to accomplish this task:
ExecutorService executorService = Executors.newFixedThreadPool(16);
for (int j = 1; j < 900 + 1; j++) {
int start = (j - 1) * 5000;
int stop = (j) * 5000- 1;
FetcherRunner runner = new FetcherRunner(routes, start, stop);
executorService.submit(runner);
Thread t = new Thread(runner);
threadsList.add(t);
t.start();
}
Is this the correct way to do this? Particularly as I have the impression that java just fires away all tasks ...(FetcherRunner implements runnable)

The first part using ExecutorService looks good:
...
FetcherRunner runner = new FetcherRunner(routes, start, stop);
executorService.submit(runner);
The part with Thread should not be there, I am assuming you have it there just to show how you had it before?
Update:
Yes, you don't require the code after executorService.submit(runner), that is going to end up spawning a huge number of threads. If your objective is to wait for all submitted tasks to complete after the loop, then you can get a reference to Future when submitting tasks and wait on the Future, something like this:
ExecutorService executorService = Executors.newFixedThreadPool(16);
List<Future<Result>> futures = ..;
for (int j = 1; j < 900+ 1; j++) {
int start = (j - 1) * 5000;
int stop = (j) * 5000- 1;
FetcherRunner runner = new FetcherRunner(routes, start, stop);
futures.add(executorService.submit(runner));
}
for (Future<Result> future:futures){
future.get(); //Do something with the results..
}

Is this the correct way of working?
The first part is correct. But you shouldn't be creating and starting new Thread objects. When you submit the Runnable, the ExecutorService puts it on its queue, and then runs it when a worker thread becomes available.
.... I use the threadlist to detect when all my threads are finished so I can continue processing results.
Well if you do what you are currently doing, you are running each task twice. Worse still, the swarm of manually created threads will all try to run in parallel.
A simple way to make sure that all of the tasks have completed is to call awaitTermination(...) on the ExecutorService. (An orderly shutdown of the executor service will have the same effect ... if you don't intend to use it again.)
The other approach is to create a Future for each FetcherRunner's results, and attempt to get the result after all of the tasks have been submitted. That has the advantage that you can start processing early results before later ones have been produced. (However, if you don't need to ... or can't ... do that, using Futures won't achieve anything.)

You don't need to the part after the call to submit. The code you have that creates a Thread will result in 900 threads being created! Yowza. The ExecutorService has a pool of 16 threads and you can run 16 jobs at once. Any jobs submitted when all 16 threads are busy will be queued. From the docs:
Creates a thread pool that reuses a fixed number of threads operating
off a shared
unbounded queue. At any point, at most nThreads threads will be active processing tasks.
If additional tasks are submitted when all threads are active, they will wait in the
queue until a thread is available. If any thread terminates due to a failure during
execution prior to shutdown, a new one will take its place if needed to execute
subsequent tasks. The threads in the pool will exist until it is explicitly shutdown.
So there is no need for yet another thread. If you need to be notified after a task has finished you can have it call out. Other options are to cache all of the Future's returned from submit, and upon each task being finished you can check to see if all Future's are done. After all Future's are finished you can dispatch another function to run. But it will run ON one of the threads in the ExecutorService.

Changed from your code:
ExecutorService executorService = Executors.newFixedThreadPool(16);
for (int j = 1; j < 900 + 1; j++) {
int start = (j - 1) * 5000;
int stop = (j) * 5000 - 1;
FetcherRunner runner = new FetcherRunner(routes, start, stop);
executorService.submit(runner);
}

The best way would be to use countdownlatch as follows
ExecutorService executorService = Executors.newFixedThreadPool(16);
CountdownLatch latch = new CountdownLatch(900);
FetcherRunner runner = new FetcherRunner(routes, start, stop, latch);
latch.await();
in the FetcherRunner under finally block use latch.countDown(); code after await() will be executed only when all the tasks are completed.

Join()ing on multiple threads and handling leaked output

I'm solving a multi-threaded problem in Java and have the core problem itself resolved, but my output isn't what I'd expect.
I have a main thread that spins new threads that each perform their tasks. It looks like the following (pseudocode):
initializeThreads(); // Creates the new threads
startThreads(); // Starts the new threads
sleep( duration ); // Lets the threads run for `duration` time
shutdownThreads(); // interrupt the threads
printOutput(); // output the results to the console
// definition of shutdownThreads() is at the end of my question
My issue happens upon trying to join() on each thread in my list of threads. Every now and then my program gets caught in an endless loop because I'm guessing the interrupt() I call on my list of threads doesn't happen fast enough compared to the join() and not all the threads get interrupted.
My other issue occurs when the program does shutdown the threads correctly and print the termination output. While the program is running each thread has messages that it logs to the console, and after I interrupt the threads and display the final program message some of these thread-specific logging messages leak out to the console.
For example, let's say one of my threads outputs "Thread-1 waiting for lock on object..." while it runs, and the main program finally wakes itself up, terminates the thread, and outputs "Program finished". Sometimes, the thread-specific message will show up after the program termination message.
If anyone could help me figure out how to stop this from happening please let me know!
shutdownThreads()
private void shutdownThreads() {
Thread t;
for (int i = 0; i < threads.size(); i++) {
t = threads.get(i);
t.interrupt();
}
for (int i = 0; i < threads.size(); i++) {
t = threads.get(i);
try {
t.join();
} catch (InterruptedException e) {
System.out.println("Interrupted while waiting on thread to exit");
}
}
}
EDIT: One thing I thought of doing is rewriting shutdownThreads() to do this:
for (int i = 0; i < threads.size(); i++) {
Thread t = threads.get(i);
t.interrupt();
while (!t.isInterrupted()) ;
}
but this doesn't seem too elegant.

Your edited code at the bottom, that's gonna be a forever loop before it iterates the for() twice.

can the free threads be used in doing the work of other thread simultaneously in java?

I have 10 threads filling unique codes in 10 tables simultaneously. Each thread filling up million records. After sometimes 7 tables got filled up but the rest 3 are still filling up. I want to indulge the free 7 threads in filling up the tables simultaneously with the running 3 threads can this be done??
String noOfCodes = ""+((Integer.parseInt(totalOfCodes))/10);
ExecutorService executor = Executors.newFixedThreadPool(10);
for (int i = 0; i < 10; i++) {
String threadNo = ""+i;
Runnable worker = new CodeGeneratorDAO(pgmId, digits, points, validity, noOfCodes, product, threadNo);
executor.execute(worker);
resp.setSuccess(true);
}
executor.shutdown();
while (!executor.isTerminated()) {
}
System.out.println("Finished all threads");

A simple solution is to define a Runnable executing a smaller task that your current Runnable. Breaking down the tasks will smooth the overall execution time.
You say that your Runnable "fills up 1000 records", so define your Runnable as filling up 1 record and submit all your 10 * 1000 records to be updated to your ExecutorService:
ExecutorService executor = Executors.newFixedThreadPool(10);
for(Runnable oneRecordRunnable : allRunnables) {
executor.submit(oneRecordRunnable);
}
executor.shutdown();
executor.awaitTermination(1, TimeUnit.HOURS);
As a side note, I replaced your cpu-burning while(true) loop by the awaitTermination method.

How to correctly shutdown all threads?

I have 8 threads which each execute loops similar to this in their run method:
for (int i = 0; i < someVar; i++) {
for (int j = 0; j < someOtherVar; j++) {
if (Thread.currentThread().isInterrupted()) {
return;
}
// another loop and some calculations here
}
}
I start the Threads like this:
executor = Executors.newFixedThreadPool(threads);
// generate the threads and run them
for (int i = 0; i < threads; i++) {
MyClass cls = new MyClass(i);
executor.execute(cls);
}
Now I need to be able to pretty much instantly kill my threads, that's why I added the if statement above and use at another point executor.shutdownNow(). But it just isn't working fast enough. Some calculations might take a few seconds before they finish and the next iteration starts.
Is there any other reliable way to pretty much immediately shutdown threads?

You need an interrupt operation that calls into the thread and stops the current execution. Here's some documentation: http://docs.oracle.com/javase/tutorial/essential/concurrency/interrupt.html
An interrupt is an indication to a thread that it should stop what it is doing and do something else. It's up to the programmer to decide exactly how a thread responds to an interrupt, but it is very common for the thread to terminate.