We Keep Coding

Java unexpected concurrent result

While testing concurrency, I found something unexpected.
Concurrency was controlled using concurrentHashMap and AtomicLong.
public class HumanRepository {
private final static Map<Long, Human> STORE = new ConcurrentHashMap<>();
private AtomicLong sequence = new AtomicLong();
public void save(Human human) {
STORE.put(sequence.incrementAndGet(), human);
}
public int size() {
return STORE.size();
}
public Long getSeq() {
return sequence.get();
}
}
I tested saving in multiple threads.
#Test
void name() throws NoSuchMethodException, InterruptedException {
final int threads = 3_500;
final ExecutorService es = Executors.newFixedThreadPool(threads);
final CountDownLatch count = new CountDownLatch(threads);
final HumanRepository repository = new HumanRepository();
for (int i = 0; i < threads; i++) {
try {
es.execute(() -> repository.save(new Human("aa")));
} finally {
count.countDown();
}
}
count.await();
System.out.println("seq = " + repository.getSeq());
System.out.println("size = " + repository.size());
}
I tested it with 3500 threads simultaneously. The result I expected is 3500 for both seq and size.
But sometimes I get seq=3499, size=3500.
That's weird. It is strange that seq does not come out as 3500, and even though the size is 3500, it does not make sense that seq is 3499.
I don't know why the data number and seq in the map are not the same and 3500 is not coming out.
** If you do Thread.sleep(400L); after count.await();, surprisingly, the value of seq is 3500

You are not actually waiting for all tasks to complete. Which means that if you get the 3500/3500 output, it's by chance.
Specifically, you decrease the countdown latch on the main thread after scheduling the job, instead of inside of the job, once it's done. That means your countdownlatch is basically just another glorified loop variable that doesn't do any inter-thread communication. Try something like this instead:
for (int i = 0; i < threads; i++) {
es.execute(() -> {
repository.save(new Human("aa"));
count.countDown();
});
}

You are calling count.countDown() outside the thread executing the HumanRepository.save(). So its possible that the main thread is not synchronized for the completion of the threads.
So you may see the results of repository.getSeq() while one thread is running. Can you try with the following code?
final int threads = 3_500;
final ExecutorService es = Executors.newFixedThreadPool(threads);
final CountDownLatch count = new CountDownLatch(threads);
final HumanRepository repository = new HumanRepository();
for (int i = 0; i < threads; i++) {
try {
es.execute(() -> {
repository.save(new Human("aa"));
count.countDown();
});
} finally {
}
}
count.await();
System.out.println("seq = " + repository.getSeq());
System.out.println("size = " + repository.size());

Keeping a counter with ExecutorService?

I'd like to keep a counter of executed threads, to use in the same threads that I am executing.
The problem here is that although the counter increases, it increases unevenly and from the console output I got this (I have a for loop that executes 5 threads with ExecutorService):
This is a test. N:3
This is a test. N:4
This is a test. N:4
This is a test. N:4
This is a test. N:4
As you can see instead of getting 1,2,3,4,5 I got 3,4,4,4,4.
I assume this is because the for loop is running fast enough to execute the threads, and the threads are fast enough to execute the code requesting for the counter faster than the counter can update itself (does that even make sense?).
Here is the code (it is smaller and there is no meaningful use for the counter):
for (int i = 0; i < 5; i++)
{
Thread thread;
thread = new Thread()
{
public void run()
{
System.out.println("This is test. N: "+aldo );
//In here there is much more stuff, saying it because it might slow down the execution (if that is the culprit?)
return;
}
};
threadList.add(thread);
}
//later
for (int i = 0; i < threadList.size(); i++)
{
executor.execute(threadList.get(i));
aldo = aldo + 1;
}
executor.shutdown();
try
{
executor.awaitTermination(Long.MAX_VALUE, TimeUnit.NANOSECONDS);
}
catch (InterruptedException e)
{
}
Yes, aldo the counter ( with a few other lists, I think) are missing from the code (they are very simple).

The best way I know of doing this is by creating a custom thread class with a constructor that passes in a number. The variable holding the number can then be used later for any needed logging. Here is the code I came up with.
public static void main(String[] args) {
class NumberedThread implements Runnable {
private final int number;
public NumberedThread(int number) {
this.number = number;
}
#Override
public void run() {
System.out.println("This is test. N: " + number);
}
}
List<Thread> threadList = new ArrayList<>();
for (int i = 1; i < 6; i++) threadList.add(new Thread(new NumberedThread(i)));
ExecutorService executor = Executors.newFixedThreadPool(10);;
for (Thread thread : threadList) executor.execute(thread);
executor.shutdown();
try {
executor.awaitTermination(Long.MAX_VALUE, TimeUnit.NANOSECONDS);
}
catch (InterruptedException ignored) { }
}
You could also use a string object instead if you wanted to name the threads.

aldo is not modified by the tasks in the thread, but instead is modified in the main thread, here:
for (int i = 0; i < threadList.size(); i++) {
executor.execute(threadList.get(i));
//here...
aldo = aldo + 1;
}
Also, since you want a counter that can increase its value in several threads, then you may use an AtomicInteger rather than int.
Your code should look like this:
AtomicInteger aldo = new AtomicInteger(1);
for (int i = 0; i < 5; i++) {
executor.execute( () -> {
System.out.println("This is test. N: " + aldo.getAndIncrement());
});
}

How to remove manually created pauses in Main-thread?

Problem description:
We have a given matrix randomly filled with digits and have to create separate threads for each row of the matrix that count how many times the digits encounter in that row.
Without these sleeps in the main thread, it's not working correctly..
Here's my solution.
Also it's following here:
public class TestingMatrixThreads {
public static void main(String[] arr) throws InterruptedException {
int[][] a = new int[67][6];
// class.Count works with class.Matrix, that's why I've made it this way
Matrix m = new Matrix(a);
m.start();
Thread.sleep(1000); // Here comes the BIG question -> how to avoid these
// manually created pauses
Count c;
Thread t;
// Creating new threads for each row of the matrix
for (int i = 0; i < Matrix.matr.length; i++) {
c = new Count(i);
t = new Thread(c);
t.start();
}
//Again - the same question
System.out.println("Main - Sleep!");
Thread.sleep(50);
System.out.println("\t\t\t\t\tMain - Alive!");
int sum = 0;
for (int i = 0; i < Count.encounters.length; i++) {
System.out.println(i + "->" + Count.encounters[i]);
sum += Count.encounters[i];
}
System.out.println("Total numbers of digits: " + sum);
}
}
class Count implements Runnable {
int row;
public static int[] encounters = new int[10]; // here I store the number of each digit's(array's index) encounters
public Count(int row) {
this.row = row;
}
public synchronized static void increment(int number) {
encounters[number]++;
}
#Override
public void run() {
System.out.println(Thread.currentThread().getName() + ", searching in row " + row + " STARTED");
for (int col = 0; col < Matrix.matr[0].length; col++) {
increment(Matrix.matr[row][col]);
}
try {
Thread.sleep(1); // If it's missing threads are starting and stopping consequently
} catch (InterruptedException e) {
}
System.out.println(Thread.currentThread().getName() + " stopped!");
}
}
class Matrix extends Thread {
static int[][] matr;
public Matrix(int[][] matr) {
Matrix.matr = matr;
}
#Override
public void run() {
//print();
fill();
System.out.println("matrix filled");
print();
}
public static void fill() {
for (int i = 0; i < matr.length; i++) {
for (int j = 0; j < matr[0].length; j++) {
matr[i][j] = (int) (Math.random() * 10);
}
}
}
public static void print() {
for (int i = 0; i < matr.length; i++) {
for (int j = 0; j < matr[0].length; j++) {
System.out.print(matr[i][j] + " ");
}
System.out.println();
}
}
}
P.S. I'm sorry if this question is too stupid for you to answer, but I'm a newbie in Java programming, as well as it's my very first post in stackoverflow, so please excuse me for the bad formatting, too :)
Thank you in advance!

Change the Thread.sleep by m.join()
Doing this will make the main thread wait for the other to complete its work and then it will continu its execution.
Cheers

To answer your main question:
Thread.join();
For example:
public static void main(String[] args) throws Exception {
final Thread t = new Thread(new Runnable() {
#Override
public void run() {
System.out.println("Do stuff");
}
});
t.start();
t.join();
}
The start call, as you know, kicks off the other Thread and runs the Runnable. The join call then waits for that started thread to finish.
A more advanced way to deal with multiple threads is with an ExecutorService. This detaches the threads themselves from the tasks they do. You can have a pool of n threads and m > n tasks.
Example:
public static void main(String[] args) throws Exception {
final class PrintMe implements Callable<Void> {
final String toPrint;
public PrintMe(final String toPrint) {
this.toPrint = toPrint;
}
#Override
public Void call() throws Exception {
System.out.println(toPrint);
return null;
}
}
final List<Callable<Void>> callables = new LinkedList<>();
for (int i = 0; i < 10; ++i) {
callables.add(new PrintMe("I am " + i));
}
final ExecutorService es = Executors.newFixedThreadPool(4);
es.invokeAll(callables);
es.shutdown();
es.awaitTermination(1, TimeUnit.DAYS);
}
Here we have 4 threads and 10 tasks.
If you go down this route you probably need to look into the Future API to so that you can check whether the tasks completed successfully. You can also return a value from the task; in your case a Callable<Integer> would seem to be appropriate so that you can return the result of your calculation from the call method and gather up the results from the Future.

As other Answers have stated, you can do this simply using join; e.g.
Matrix m = new Matrix(a);
m.start();
m.join();
However, I just want to note that if you do that, you are not going to get any parallelism from the Matrix thread. You would be better of doing this:
Matrix m = new Matrix(a);
m.run();
i.e. executing the run() method on the main thread. You might get some parallelism by passing m to each "counter" thread, and having them all join the Matrix thread ... but I doubt that it will be worthwhile.
Frankly, I'd be surprised if you get a worthwhile speedup for any of the multi-threading you are trying here:
If the matrix is small, the overheads of creating the threads will dominate.
If the matrix is large, you are liable to run into memory contention issues.
The initialization phase takes O(N^2) computations compared with the parallelized 2nd phase that has N threads doing O(N) computations. Even if you can get a decent speedup in the 2nd phase, the 1st phase is likely to dominate.

MultiThread Java Only Utilize One Thread

I'm working on this project and I want to utilize multi-thread in my code. So I developed this little piece of code and tested it but it turned out that it only uses one of the threads in my computer. Can someone please tell me what's wrong with it and how I can improve it?
public static int choiceCount(List<Character> charlist) throws InterruptedException, ExecutionException {
int coreCount = 8;
ExecutorService e1 = Executors.newFixedThreadPool(coreCount);
Integer total = 0;
for (int i = 0; i < coreCount; i++) {
Future<Integer> result = e1.submit(new Count(coreCount, i, charlist));
total += result.get();
}
e1.shutdown();
return total;
}
And here's the Callable
class Count implements Callable<Integer> {
//where the processing code is
}
So when I run this program, it only uses 12.5% of my CPU which is one thread only... Ideas guys?
Thanks

The problem is in your loop:
for (int i = 0; i < coreCount; i++) {
Future<Integer> result = e1.submit(new Count(coreCount, i, charlist));
total += result.get();
}
What this does, is:
submit a calculation
call get() on the Future object, which waits for the calculation to finish
then do the next iteration of the loop
So, in each iteration, your code is waiting for the calculation to be finished before you submit the next one.
You should create two loops, one to submit the calculations, which stores all the Future objects in a collection, and then a second loop which calls get() on each of the Future objects.

You have to save the Future object rather than wait for each one before submitting the next.
public static int choiceCount(List<Character> charlist) throws InterruptedException, ExecutionException {
int coreCount = Runtime.getRuntime().availableProcessors();
ExecutorService e1 = Executors.newFixedThreadPool(coreCount);
int total = 0;
List<Future<Integer>> futures = new ArrayList<>();
// start all the tasks, before
for (int i = 0; i < coreCount; i++)
futures.add(e1.submit(new Count(coreCount, i, charlist)));
// notify the executor to stop when finished in case get() throws an exception
e1.shutdown();
// collecting the results.
for (Future<Integer> future: futures)
total += future.get();
return total;
}

You should create a list of List<Callable<Integer>> and then use invokeAll on the executor which will start all your computation threads.
List<Callable<Integer>> callables = new ArrayList<Callable<Integer>>();
for (int i = 0; i < coreCount; i++) {
callables.add(new Count(coreCount, i, charlist));
}
List<Future<Integer>> futures = executor.invokeAll(callables); //why is it e1?
//Then you can wait for all computations to finish by calling
for (Future<Integer> future : futures) {
Integer result = future.get(); //blocks for computation to finish.
//do something with result
}

comparison of code performance, threaded versus non-threaded

I have some thread-related questions, assuming the following code. Please ignore the possible inefficiency of the code, I'm only interested in the thread part.
//code without thread use
public static int getNextPrime(int from) {
int nextPrime = from+1;
boolean superPrime = false;
while(!superPrime) {
boolean prime = true;
for(int i = 2;i &lt nextPrime;i++) {
if(nextPrime % i == 0) {
prime = false;
}
}
if(prime) {
superPrime = true;
} else {
nextPrime++;
}
}
return nextPrime;
}
public static void main(String[] args) {
int primeStart = 5;
ArrayList list = new ArrayList();
for(int i = 0;i &lt 10000;i++) {
list.add(primeStart);
primeStart = getNextPrime(primeStart);
}
}
If I'm running the code like this and it takes about 56 seconds. If, however, I have the following code (as an alternative):
public class PrimeRunnable implements Runnable {
private int from;
private int lastPrime;
public PrimeRunnable(int from) {
this.from = from;
}
public boolean isPrime(int number) {
for(int i = 2;i &lt from;i++) {
if((number % i) == 0) {
return false;
}
}
lastPrime = number;
return true;
}
public int getLastPrime() {
return lastPrime;
}
public void run() {
while(!isPrime(++from))
;
}
}
public static void main(String[] args) {
int primeStart = 5;
ArrayList list = new ArrayList();
for(int i = 0;i &lt 10000;i++) {
PrimeRunnable pr = new PrimeRunnable(primeStart);
Thread t = new Thread(pr);
t.start();
t.join();
primeStart = pr.getLastPrime();
list.add(primeStart);
}
}
The whole operation takes about 7 seconds. I am almost certain that even though I only create one thread at a time, a thread doesn't always finish when another is created. Is that right? I am also curious: why is the operation ending so fast?
When I'm joining a thread, do other threads keep running in the background, or is the joined thread the only one that's running?

By putting the join() in the loop, you're starting a thread, then waiting for that thread to stop before running the next one. I think you probably want something more like this:
public static void main(String[] args) {
int primeStart = 5;
// Make thread-safe list for adding results to
List list = Collections.synchronizedList(new ArrayList());
// Pull thread pool count out into a value so you can easily change it
int threadCount = 10000;
Thread[] threads = new Thread[threadCount];
// Start all threads
for(int i = 0;i < threadCount;i++) {
// Pass list to each Runnable here
// Also, I added +i here as I think the intention is
// to test 10000 possible numbers>5 for primeness -
// was testing 5 in all loops
PrimeRunnable pr = new PrimeRunnable(primeStart+i, list);
Thread[i] threads = new Thread(pr);
threads[i].start(); // thread is now running in parallel
}
// All threads now running in parallel
// Then wait for all threads to complete
for(int i=0; i<threadCount; i++) {
threads[i].join();
}
}
By the way pr.getLastPrime() will return 0 in the case of no prime, so you might want to filter that out before adding it to your list. The PrimeRunnable has to absorb the work of adding to the final results list. Also, I think PrimeRunnable was actually broken by still having incrementing code in it. I think this is fixed, but I'm not actually compiling this.
public class PrimeRunnable implements Runnable {
private int from;
private List results; // shared but thread-safe
public PrimeRunnable(int from, List results) {
this.from = from;
this.results = results;
}
public void isPrime(int number) {
for(int i = 2;i < from;i++) {
if((number % i) == 0) {
return;
}
}
// found prime, add to shared results
this.results.add(number);
}
public void run() {
isPrime(from); // don't increment, just check one number
}
}
Running 10000 threads in parallel is not a good idea. It's a much better idea to create a reasonably sized fixed thread pool and have them pull work from a shared queue. Basically every worker pulls tasks from the same queue, works on them and saves the results somewhere. The closest port of this with Java 5+ is to use an ExecutorService backed by a thread pool. You could also use a CompletionService which combines an ExecutorService with a result queue.
An ExecutorService version would look like:
public static void main(String[] args) {
int primeStart = 5;
// Make thread-safe list for adding results to
List list = Collections.synchronizedList(new ArrayList());
int threadCount = 16; // Experiment with this to find best on your machine
ExecutorService exec = Executors.newFixedThreadPool(threadCount);
int workCount = 10000; // See how # of work is now separate from # of threads?
for(int i = 0;i < workCount;i++) {
// submit work to the svc for execution across the thread pool
exec.execute(new PrimeRunnable(primeStart+i, list));
}
// Wait for all tasks to be done or timeout to go off
exec.awaitTermination(1, TimeUnit.DAYS);
}
Hope that gave you some ideas. And I hope the last example seemed a lot better than the first.

You can test this better by making the exact code in your first example run with threads. Sub your main method with this:
private static int currentPrime;
public static void main(String[] args) throws InterruptedException {
for (currentPrime = 0; currentPrime < 10000; currentPrime++) {
Thread t = new Thread(new Runnable() {
public void run() {
getNextPrime(currentPrime);
}});
t.run();
t.join();
}
}
This will run in the same time as the original.
To answer your "join" question: yes, other threads can be running in the background when you use "join", but in this particular case you will only have one active thread at a time, because you are blocking the creation of new threads until the last thread is done executing.

JesperE is right, but I don't believe in only giving hints (at least outside a classroom):
Note this loop in the non-threaded version:
for(int i = 2;i < nextPrime;i++) {
if(nextPrime % i == 0) {
prime = false;
}
}
As opposed to this in the threaded version:
for(int i = 2;i < from;i++) {
if((number % i) == 0) {
return false;
}
}
The first loop will always run completely through, while the second will exit early if it finds a divisor.
You could make the first loop also exit early by adding a break statement like this:
for(int i = 2;i < nextPrime;i++) {
if(nextPrime % i == 0) {
prime = false;
break;
}
}

Read your code carefully. The two cases aren't doing the same thing, and it has nothing to do with threads.
When you join a thread, other threads will run in the background, yes.

Running a test, the second one doesn't seem to take 9 seconds--in fact, it takes at least as long as the first (which is to be expected, threding can't help the way it's implemented in your example.
Thread.join will only return when the thread.joined terminates, then the current thread will continue, the one you called join on will be dead.
For a quick reference--think threading when starting one iteration does not depend on the result of the previous one.