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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());
});
}

Notify seems to be waking up more than one thread

I am doing a sample program with wait() and notify(), but when notify() is called, more than one thread is wakes up instead of one.
The code is:
public class MyQueue<T> {
Object[] entryArr;
private volatile int addIndex;
private volatile int pending = -1;
private final Object lock = new Object();
private volatile long notifiedThreadId;
private int capacity;
public MyQueue(int capacity) {
entryArr = new Object[capacity];
this.capacity = capacity;
}
public void add(T t) {
synchronized (lock) {
if (pending >= 0) {
try {
pending++;
lock.wait();
System.out.println(notifiedThreadId + ":" + Thread.currentThread().getId());
} catch (InterruptedException e) {
e.printStackTrace();
}
} else if (pending == -1) {
pending++;
}
}
if (addIndex == capacity) { // its ok to replace existing value
addIndex = 0;
}
try {
entryArr[addIndex] = t;
} catch (ArrayIndexOutOfBoundsException e) {
System.out.println("ARRAYException:" + Thread.currentThread().getId() + ":" + pending + ":" + addIndex);
e.printStackTrace();
}
addIndex++;
synchronized (lock) {
if (pending > 0) {
pending--;
notifiedThreadId = Thread.currentThread().getId();
lock.notify();
} else if (pending == 0) {
pending--;
}
}
}
}
public class TestMyQueue {
public static void main(String args[]) {
final MyQueue<String> queue = new MyQueue<>(2);
for (int i = 0; i < 200; i++) {
Runnable r = new Runnable() {
#Override
public void run() {
for (int i = 0; i < Integer.MAX_VALUE; i++) {
queue.add(Thread.currentThread().getName() + ":" + i);
}
}
};
Thread t = new Thread(r);
t.start();
}
}
}
After some time, I see two threads being wake up by single thread. The output looks like:
91:114
114:124
124:198
198:106
106:202
202:121
121:40
40:42
42:83
83:81
81:17
17:189
189:73
73:66
66:95
95:199
199:68
68:201
201:70
70:110
110:204
204:171
171:87
87:64
64:205
205:115
Here I see 115 thread notified two threads, and 84 thread notified two threads; because of this we are seeing the ArrayIndexOutOfBoundsException.
115:84
115:111
84:203
84:200
ARRAYException:200:199:3
ARRAYException:203:199:3
What is the issue in the program?

What is the issue in the program?
You have a couple of problems with your code that may be causing this behavior. First, as #Holder commented on, there are a lot of code segments that can be run by multiple threads simultaneously that should be protected using synchronized blocks.
For example:
if (addIndex == capacity) {
addIndex = 0;
}
If multiple threads run this then multiple threads might see addIndex == capacity and multiple would be overwriting the 0th index. Another example is:
addIndex++;
This is a classic race condition if 2 threads try to execute this statement at the same time. If addIndex was 0 beforehand, after the 2 threads execute this statement, the value of addIndex might be 1 or 2 depending on the race conditions.
Any statements that could be executed at the same time by multiple threads have to be properly locked within a synchronized block or otherwise protected. Even though you have volatile fields, there can still be race conditions because there are multiple operations being executed.
Also, a classic mistake is to use if statements when checking for over or under flows on your array. They should be while statements to make sure you don't have the class consumer producer race conditions. See my docs here or take a look at the associated SO question: Why does java.util.concurrent.ArrayBlockingQueue use 'while' loops instead of 'if' around calls to await()?

Error in Two Process critical section solution

I have applied two process critical section solution to two threads instead of processes. My code is:
class Main
{
static boolean flag[];
static int turn;
static int count;
synchronized static void print(char ch,int n)
{
int i;
System.out.println(ch);
for(i=0;i<n;i++){
System.out.println(i);
}
}
public static void main(String[] args) throws IOException
{
flag = new boolean[2];
flag[0] = flag[1] = false;
turn = 0;
count = 0;
ThreadLevelOne t1 = new ThreadLevelOne('a');
ThreadLevelTwo t2 = new ThreadLevelTwo('b');
t1.start();
t2.start();
}
static class ThreadLevelOne extends Thread{
private char ch;
public ThreadLevelOne(char ch){
this.ch = ch;
}
public void run(){
while(true)
{
flag[0] = true;
turn = 1;
while(flag[1] && turn == 1);
print(ch,3);
count++;
System.out.println("Counter is : " + count);
flag[0] = false;
}
}
}
static class ThreadLevelTwo extends Thread{
private char ch;
public ThreadLevelTwo(char ch){
this.ch = ch;
}
public void run()
{
while(true)
{
flag[1] = true;
turn = 0;
while(flag[0] && turn == 0);
print( ch, 4);
count++;
System.out.println("Counter is : " + count);
flag[1] = false;
}
}
}
}
On executing the above code, it does not run infinitely but halts at arbitrary counter value on each execution. Is this a valid application of the two process solution to threads? If yes, then why is program halting at arbitrary counter value? If no, then how can this be achieved in threads?
Edit after the answer of codeBlind:
output: Program execution halts at this stage
Even if i dont increment the counter value, then also the program halts after a certain time

You're a victim of concurrently executing non-atomic operations, specifically count++, as well as the way you are using flags in each thread. But for simplicity's sake, let's talk about count++. The ++ operator actually executes three commands, each in their own clock-cycle:
read value of count
add 1 to value retrieved from count
store new value into count
The problem you're seeing is a result of these commands being interleaved across two threads. Thread A may not have stored the new count value by the time that Thread B attempts to read it.
A quick fix would be to use AtomicInteger for count instead of primitive int - AtomicInteger guarantees thread safety for integer operations.
EDIT
There are other race conditions in this code as well. Each thread's while loop argument (e.g. flag[0] && turn == 0) is non-atomic, but both threads are capable of modifying turn. You've left open the possibility that one thread could set turn before the other thread's while argument is fully evaluated, causing your threads to deadlock down the road.
If you only wish to guarantee that each thread must not be inside the while loop while the other thread is, then you should instead write each of your while loops to look something like this:
while(true){
synchronized(Main.class){
print( ch, 4);
count++;
System.out.println("Counter is : " + count);
}
}
If you want to guarantee that each thread must "take turns", you should look into using wait() and notify().

Ok so I figured it out, the issue is that each thread needs to pause in order for the other thread to run.
Instead of just spinning the cpu using:
while(flag[0] && turn == 0);
You need to pause the thread by calling the sleep method.
while(flag[0] && turn == 0){
try {
this.sleep(100);
} catch (InterruptedException e) {
e.printStackTrace();
}
}

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.

Thread Scheduling - Shared Array

I need two threads to write one a shared array of ints. Both threads need to write on all the elements of that array. Each thread will write either 1 or 7, and the result should be like 171717171 (or 71717171). To do that I have the first Thread1 write at position 0, then wait. Thread2 now writes at position 0 and 1, notifies Thread1, and waits. Thread1 writes at position 1 and 2, notifies Thread2 and waits, etc. With the following code I get correct output, although when run with JPF it finds a deadlock. Its become really frustrating since I can not find whats wrong with it. Any advice would be appreciated.
import java.util.logging.Level;
import java.util.logging.Logger;
public class WriterThreadManager {
private int[] array = {0, 0, 0, 0, 0, 0, 0, 0, 0, 0};
private Thread thread7;
private Thread thread1;
public static void main(String[] args) {
WriterThreadManager mng = new WriterThreadManager();
mng.exec();
}
public WriterThreadManager() {
thread7 = new Thread(new WriterRunnable(this, 7));
thread1 = new Thread(new WriterRunnable(this, 1));
}
public void overwriteArray(int pos, int num) {
array[pos] = num;
printArray();
}
private void printArray() {
for (int i = 0; i < array.length; i++) {
System.out.print(array[i]);
}
System.out.println("");
}
public synchronized void stopThread() {
try {
this.wait();
} catch (InterruptedException ex) {
Logger.getLogger(WriterThreadManager.class.getName()).log(Level.SEVERE, null, ex);
}
}
public synchronized void wakeUpThread() {
notifyAll();
}
private void exec() {
thread7.start();
thread1.start();
}
public int length() {
return array.length;
}
}
public class WriterRunnable implements Runnable {
private WriterThreadManager mng;
private int numberToWrite;
private static boolean flag = true;
#Override
public void run() {
int counter = 0;
int j = 0;
//first thread to get in should write only at
//position 0 and then wait.
synchronized (mng) {
if (flag) {
flag = false;
mng.overwriteArray(0, numberToWrite);
j = 1;
waitForOtherThread();
}
}
for (int i = j; i < mng.length(); i++) {
mng.overwriteArray(i, numberToWrite);
counter++;
if (i == mng.length() - 1) {
mng.wakeUpThread();
break;
}
if (counter == 2) {
waitForOtherThread();
counter = 0;
}
}
}
private void waitForOtherThread() {
mng.wakeUpThread();
mng.stopThread();
}
public WriterRunnable(WriterThreadManager ar, int num) {
mng = ar;
numberToWrite = num;
}
}
p.s: an example of the execution:
1000000000
7000000000
7700000000
7100000000
7110000000
7170000000
7177000000
7171000000
7171100000
7171700000
7171770000
7171710000
7171711000
7171717000
7171717700
7171717100
7171717110
7171717170
7171717177
7171717171
The error snapshot from JPF is the following:
thread java.lang.Thread:{id:1,name:Thread-1,status:WAITING,priority:5,lockCount:1,suspendCount:0}
waiting on: WriterThreadManager#152
call stack:
at java.lang.Object.wait(Object.java)
at WriterThreadManager.stopThread(WriterThreadManager.java:43)
at WriterRunnable.waitForOtherThread(WriterRunnable.java:53)
at WriterRunnable.run(WriterRunnable.java:45)
thread java.lang.Thread:{id:2,name:Thread-2,status:WAITING,priority:5,lockCount:1,suspendCount:0}
waiting on: WriterThreadManager#152
call stack:
at java.lang.Object.wait(Object.java)
at WriterThreadManager.stopThread(WriterThreadManager.java:43)
at WriterRunnable.waitForOtherThread(WriterRunnable.java:53)
at WriterRunnable.run(WriterRunnable.java:45)

I believe the race is due to this method:
private void waitForOtherThread() {
mng.wakeUpThread();
mng.stopThread();
}
While the individual wakeUpThread() and stopThread() methods are synchronized, you have the opportunity for unexpected thread scheduling between these calls.
Consider:
thread7 - notify thread1 to wakup
thread1 - wake up
thread1 - work to completion
thread1 - notify thread7 to wakeup
thread1 - wait to be notified to wakeup
thread7 - wait to be notified to wakeup
In this case you have deadlocked because thread1 sent its notifyAll() before thread7 had a chance to wait() for it.
Running in a different context can mess with your timing and cause these types of behaviors to appear.
To avoid this I suggestion doing this:
private void waitForOtherThread() {
synchronized(mng) {
mng.wakeUpThread();
mng.stopThread();
}
}
Or better yet, use a semaphore as #KumarVivekMitra suggested. Semaphores combine both the notification system and a counter so that the order of the notify and wait don't matter.

- I think a better approach here would be java.util.Semaphores, which will help you to decide the access over the objects resources by specific numbers of threads at a time.
- Well you can also use the SingleThreadExecutor to solve this, which starts and completes a task before moving on to the 2nd task, so there will be No need of synchronization needed here from your side.

I don't think you need any sort of coordination here. Just have one thread write the even locations and the other thread write the odd locations. Let them both go as fast as they can. Done!