Develop Reference

Executing some code only once in a Thread region

In a certain part of a Java code that i am working, i need to place a timer inside a run() method. Each thread will execute all code inside run(). But i need to start measuring after block (1) and before block of code (2) so the timer needs to be triggered there.
for (int i = 0; i < total_threads-1; i++){
final int id = i+1;
th[i] = new Thread(new Runnable() {
public final void run(){
/* ... block of code (1) executed by multiple threads ... */
/* How can i start this counter only once? */
final long begin = System.currentTimeMillis();
/* ... another block of code (2) executed by multiple threads i need to measure!!! ... */
}
});
th[i].start();
}
for(int i = 0 ; i < total_threads-1 ; i++) {
try {
th[i].join();
}
catch (InterruptedException e) {}
}
final long end = System.currentTimeMillis();
System.out.println((end-begin) / 1000.0);
But all the threads will have their own begin variable and start the counter which is a problem because System.currentTimeMillis() should be triggered once and not by many threads.
I probably could separate the code of run() in two different parallel regions but would imply creating the threads twice which would be unacceptable (in terms of performance).
There is a similar technique of OpenMP directive #pragma omp master for Java using Java threads?
How can i measure the time correctly here?

You can check the thread ID to execute prefer line once:
if (id == YOUR_THREAD_ID)
{
begin = System.currentTimeMillis();
}

The simplest way would just be to record the time in the master thread (i.e. the thread which creates all the children) instead of in the children themselves.
But if you really want to start the timer in the children (maybe there's some expensive setup?), you could use a Guava StopWatch object.
The code would look something like:
StopWatch timer = StopWatch.createUnstarted();
for (int i = 0; i < total_threads-1; i++){
final int id = i+1;
th[i] = new Thread(() -> {
/* ... block of code (1) executed by multiple threads ... */
try {
synchronized (timer) {
timer.start();
}
} catch (IllegalStateException e) {
// ignore; the watch is already started
}
/* ... another block of code (2) executed by multiple threads i need to measure!!! ... */
});
th[i].start();
}
for(int i = 0 ; i < total_threads-1 ; i++) {
try {
th[i].join();
} catch (InterruptedException e) {}
}
timer.stop();
System.out.println(timer.elapsed(TimeUnit.SECONDS));

Synchronise ArrayList over two threads

I'm having a difficult time understanding how to synchronise an ArrayList over two threads. Basically, I want one thread appending objects to the list and the other one reading from that list at the same time.
Here is the class that deploys the threads:
public class Main {
public static ArrayList<Good> goodList = new ArrayList();
public static void main(String[] args) {
Thread thread1 = new Thread(new GoodCreator());
Thread thread2 = new Thread(new WeightCounter());
thread1.start();
thread2.start();
}
}
Then the two Runnable classes:
This one reads lines of two values from a text file and appends new objects.
public class GoodCreator implements Runnable{
private ArrayList<Good> goodList = Main.goodList;
private static Scanner scan;
#Override
public void run() {
System.out.println("Thread 1 started");
int objCount = 0;
try {
scan = new Scanner(new File(System.getProperty("user.home") + "//Goods.txt"));
} catch (FileNotFoundException e) {
System.out.println("File not found!");
e.printStackTrace();
}
while(scan.hasNextLine()){
String line = scan.nextLine();
String[] words = line.split("\\s+");
synchronized(goodList){
goodList.add(new Good(Integer.parseInt(words[0]), Integer.parseInt(words[1])));
objCount++;
}
if(objCount % 200 == 0) System.out.println("created " + objCount + " objects");
}
}
}
This iterates over the arraylist and is supposed to sum up one of the fields.
public class WeightCounter implements Runnable{
private ArrayList<Good> goodList = Main.goodList;
#Override
public void run() {
System.out.println("Thread 2 started");
int weightSum = 0;
synchronized(goodList){
for(Good g : goodList){
weightSum += g.getWeight();
}
}
System.out.println(weightSum);
}
}
No matter the input, weightSum never gets incremented and stays 0
Thread 1 started
Thread 2 started
0
Any help is much appreciated

You are running two independently running threads. These thread can run in any order and if one stop e.g. to read from a file, the other thread doesn't assume it has to wait for it.
In short, your second thread completes before the first thread has added anything to the list.
There is no good fix as this is not a good example of why you would use multiple threads, however to get an outcome what you can do is this.
public class WeightCounter implements Runnable{
private ArrayList<Good> goodList = Main.goodList;
#Override
public void run() {
System.out.println("Thread 2 started");
for(int i = 0; i < 10; i++) {
try {
Thread.sleep(100);
} catch (InterruptedException ie) {
throw AssertionError(ie);
}
int weightSum = 0;
synchronized(goodList){
for (Good g : goodList)
weightSum += g.getWeight();
}
System.out.println(weightSum);
}
}
}
This will print the sum 10 times, 0.1 seconds apart. Depending on how long your file takes to load you will be able to see the sum for what has loaded so far.

This is something called a producer-consumer task. You can do it with arraylist, but it's honestly just not the right way to approach this problem.
Luckily, Java provides us with some collections, the BlockingQueue collections, which are designed specifically for this reason;
//the collection with the stuff in it
static BlockingQueue<Object> items = new BlockingQueue<Object>();
//(there are a few different types of blocking queues, check javadocs.
//you would want Linked or Array blocking queue
//what happens on the reader thread
public void producer()
{
//read the data into the collection
for (all the data in the file)
{
//add the next item
items.put(/* next item from file or w/e */);
//stop if necessary
if (atEndOfFile) stillReadingData = false;
//etc
}
}
Now you need to read the data out of the queue - luckily this is easy enough;
//what happens on the other threads
public void consumer()
{
//keep this thread alive so long as there is data to process
//or so long as there might be more data to process
while (stillReadingData || !items.isEmpty())
{
//get the next item from the list
//while the list is empty, we basically sleep for "timeout" timeunits,
//then the while-loop would repeat, and so on
Object o = items.poll(long timeout, int units);
if (o != null) //process it
}
}
In this way, you can continuously add items to the queue with the producer thread, and the items will be processed as soon as a consumer thread is free (this approach scales well with lots of consumer threads). If you still need a collection for the items, then you should make a second collection and add them to that after they have been processed.
As a side note, you may still need to synchronize oprations which occur while processing the items. For example, you would need to synchronize increments on "weightSum" (or alternately use AtomicInteger).

Try this change in the WeightCounter class.
public class WeightCounter implements Runnable{
private ArrayList<Good> goodList = Main.goodList;
#Override
public void run() {
System.out.println("Thread 2 started");
int weightSum = 0;
while(goodList.isEmpty()) {
Thread.sleep(1000);
}
synchronized(goodList){
for(Good g : goodList){
weightSum += g.getWeight();
}
}
System.out.println(weightSum);
}
}
This change will cause the WeightCounter thread to wait for the other thread to finish populating the goodList with data before attempting to read from it.

Prohibiting One Task when another is running

So let's say I'm creating and starting a bunch of threads in a for loop, that is being executed in the run method of a launcher thread. Let's also say that I want to be able to interrupt the launcher thread and all threads that the thread has created, and I do this through a button.
So something like this -
try{
for(int i = 0; i < n;i++){
Worker currThread = new Worker(someArgs);
workerThreads.add(currThread);
currThread.start();
}
} catch (InterruptedException e){
e.printStackTrace();
}
BUTTON-
public void actionPerformed(ActionEvent arg0) {
List<Worker> threads = launchThread.getWorkerThreads();
for(int i = 0; i < threads.size();i++){
threads.get(i).interrupt();
}
launchThread.interrupt();
}
Now, let's say that I want to make it so that the interrupts cannot occur at the same time as thread creation. I think a way to do this would be to construct a dummy object and put both pieces of code inside a lock
synchronized(dummyObject){
//thread creation or interruption code here (shown above)
}
Will this way work? I ask because I'm not sure how to test to see if it will.

Start the threads separately from creating them.
for(int i = 0; i < n; i++) {
Worker currThread = new Worker(someArgs);
workerThreads.add(currThread);
}
// later
for (Worker w : workerThreads) {
w.start();
}
If that's still not enough, your dummyObject synchronization should work just fine.
// You probably need to make this a (private final) field
Object lock = new Object();
// later
synchronized (lock) {
for(int i = 0; i < n; i++) {
Worker currThread = new Worker(someArgs);
workerThreads.add(currThread);
w.start();
}
}
// later still
public void actionPerformed(ActionEvent arg0) {
synchronized (lock) {
// interruption code here
}
}

The concept of synchronization remains the same however complicated are the underlying operations to be executed.
As you specified, there are two types of mutually exclusive tasks (thread creation and interruption). So locking is pretty much the canonical tool for the job.

Displaying busy status with thread in java

I'm writing a Java app which writes to excel sheet bunch of data, and it takes a while to do so.
I'd like to create something like writing out dots to screen like on Linux when you're installing something.
Is that possible in java?printing dots, while other thread actually does the writing to excel, then after its finished the one displaying dots also quits?
I'd like to print dots to console.

A variation to #John V. answer would be to use a ScheduledExecutorService:
// SETUP
Runnable notifier = new Runnable() {
public void run() {
System.out.print(".");
}
};
ScheduledExecutorService scheduler = Executors.newScheduledThreadPool(1);
// IN YOUR WORK THREAD
scheduler.scheduleAtFixedRate(notifier, 1, 1, TimeUnit.SECONDS);
// DO YOUR WORK
schedule.shutdownNow();
Modify the notifier object to suit your individual needs.

Its very possible. Use a newSingleThreadExecutor to print the dots while the other thread does the parsing. For example
ExecutorService e = Executors.newSingleThreadExecutor();
Future f = e.submit(new Runnable(){
public void run(){
while(!Thread.currentThread().isInterrupted()){
Thread.sleep(1000); //exclude try/catch for brevity
System.out.print(".");
}
}
});
//do excel work
f.cancel(true);
e.shutdownNow();

Yes, it is possible, you will want to have your working thread set a variable to indicate that it is working and when it is finished. Then create a thread by either extending the Thread class or implementing the Runnable interface. This thread should just infinitely loop and inside this loop it should do whatever printing you want it to do, and then check the variable to see if the work is done. When the variable value changes, break the loop and end the thread.
One note. Watch your processing speed. Use Thread.sleep() inside your loop if your processor usage goes way high. This thread should not be labour intensive. System.gc() is another popular way to make threads wait.

Not an elegant solution, but get's the job done. It prints 1, 2, 3, 1, 2... dots in a loop and terminates everything after 5 seconds.
public class Busy {
public Busy() {
Indicator i = new Indicator();
ExecutorService ex = Executors.newSingleThreadExecutor();
ex.submit(i);
try {
Thread.sleep(5000);
} catch (InterruptedException e) {
e.printStackTrace();
}
i.finished = true;
ex.shutdown();
}
public static void main(String[] args) {
new Busy();
}
private class Indicator implements Runnable {
private static final int DOTS_NO = 3;
private volatile boolean finished = false;
#Override
public void run() {
for (int i = 0; !finished; i = (i + 1) % (DOTS_NO + 1)) {
for (int j = 0; j < i; j++) {
System.out.print('.');
}
try {
Thread.sleep(500);
} catch (InterruptedException e) {
e.printStackTrace();
}
for (int j = 0; j < i; j++) {
System.out.print("\b \b");
}
}
}
}
}

How to wait for a number of threads to complete?

What is a way to simply wait for all threaded process to finish? For example, let's say I have:
public class DoSomethingInAThread implements Runnable{
public static void main(String[] args) {
for (int n=0; n<1000; n++) {
Thread t = new Thread(new DoSomethingInAThread());
t.start();
}
// wait for all threads' run() methods to complete before continuing
}
public void run() {
// do something here
}
}
How do I alter this so the main() method pauses at the comment until all threads' run() methods exit? Thanks!

You put all threads in an array, start them all, and then have a loop
for(i = 0; i < threads.length; i++)
threads[i].join();
Each join will block until the respective thread has completed. Threads may complete in a different order than you joining them, but that's not a problem: when the loop exits, all threads are completed.

One way would be to make a List of Threads, create and launch each thread, while adding it to the list. Once everything is launched, loop back through the list and call join() on each one. It doesn't matter what order the threads finish executing in, all you need to know is that by the time that second loop finishes executing, every thread will have completed.
A better approach is to use an ExecutorService and its associated methods:
List<Callable> callables = ... // assemble list of Callables here
// Like Runnable but can return a value
ExecutorService execSvc = Executors.newCachedThreadPool();
List<Future<?>> results = execSvc.invokeAll(callables);
// Note: You may not care about the return values, in which case don't
// bother saving them
Using an ExecutorService (and all of the new stuff from Java 5's concurrency utilities) is incredibly flexible, and the above example barely even scratches the surface.

import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Future;
public class DoSomethingInAThread implements Runnable
{
public static void main(String[] args) throws ExecutionException, InterruptedException
{
//limit the number of actual threads
int poolSize = 10;
ExecutorService service = Executors.newFixedThreadPool(poolSize);
List<Future<Runnable>> futures = new ArrayList<Future<Runnable>>();
for (int n = 0; n < 1000; n++)
{
Future f = service.submit(new DoSomethingInAThread());
futures.add(f);
}
// wait for all tasks to complete before continuing
for (Future<Runnable> f : futures)
{
f.get();
}
//shut down the executor service so that this thread can exit
service.shutdownNow();
}
public void run()
{
// do something here
}
}

instead of join(), which is an old API, you can use CountDownLatch. I have modified your code as below to fulfil your requirement.
import java.util.concurrent.*;
class DoSomethingInAThread implements Runnable{
CountDownLatch latch;
public DoSomethingInAThread(CountDownLatch latch){
this.latch = latch;
}
public void run() {
try{
System.out.println("Do some thing");
latch.countDown();
}catch(Exception err){
err.printStackTrace();
}
}
}
public class CountDownLatchDemo {
public static void main(String[] args) {
try{
CountDownLatch latch = new CountDownLatch(1000);
for (int n=0; n<1000; n++) {
Thread t = new Thread(new DoSomethingInAThread(latch));
t.start();
}
latch.await();
System.out.println("In Main thread after completion of 1000 threads");
}catch(Exception err){
err.printStackTrace();
}
}
}
Explanation:
CountDownLatch has been initialized with given count 1000 as per your requirement.
Each worker thread DoSomethingInAThread will decrement the CountDownLatch, which has been passed in constructor.
Main thread CountDownLatchDemo await() till the count has become zero. Once the count has become zero, you will get below line in output.
In Main thread after completion of 1000 threads
More info from oracle documentation page
public void await()
throws InterruptedException
Causes the current thread to wait until the latch has counted down to zero, unless the thread is interrupted.
Refer to related SE question for other options:
wait until all threads finish their work in java

Avoid the Thread class altogether and instead use the higher abstractions provided in java.util.concurrent
The ExecutorService class provides the method invokeAll that seems to do just what you want.

Consider using java.util.concurrent.CountDownLatch. Examples in javadocs

Depending on your needs, you may also want to check out the classes CountDownLatch and CyclicBarrier in the java.util.concurrent package. They can be useful if you want your threads to wait for each other, or if you want more fine-grained control over the way your threads execute (e.g., waiting in their internal execution for another thread to set some state). You could also use a CountDownLatch to signal all of your threads to start at the same time, instead of starting them one by one as you iterate through your loop. The standard API docs have an example of this, plus using another CountDownLatch to wait for all threads to complete their execution.

As Martin K suggested java.util.concurrent.CountDownLatch seems to be a better solution for this. Just adding an example for the same
public class CountDownLatchDemo
{
public static void main (String[] args)
{
int noOfThreads = 5;
// Declare the count down latch based on the number of threads you need
// to wait on
final CountDownLatch executionCompleted = new CountDownLatch(noOfThreads);
for (int i = 0; i < noOfThreads; i++)
{
new Thread()
{
#Override
public void run ()
{
System.out.println("I am executed by :" + Thread.currentThread().getName());
try
{
// Dummy sleep
Thread.sleep(3000);
// One thread has completed its job
executionCompleted.countDown();
}
catch (InterruptedException e)
{
// TODO Auto-generated catch block
e.printStackTrace();
}
}
}.start();
}
try
{
// Wait till the count down latch opens.In the given case till five
// times countDown method is invoked
executionCompleted.await();
System.out.println("All over");
}
catch (InterruptedException e)
{
e.printStackTrace();
}
}
}

If you make a list of the threads, you can loop through them and .join() against each, and your loop will finish when all the threads have. I haven't tried it though.
http://docs.oracle.com/javase/8/docs/api/java/lang/Thread.html#join()

Create the thread object inside the first for loop.
for (int i = 0; i < threads.length; i++) {
threads[i] = new Thread(new Runnable() {
public void run() {
// some code to run in parallel
}
});
threads[i].start();
}
And then so what everyone here is saying.
for(i = 0; i < threads.length; i++)
threads[i].join();

You can do it with the Object "ThreadGroup" and its parameter activeCount:

As an alternative to CountDownLatch you can also use CyclicBarrier e.g.
public class ThreadWaitEx {
static CyclicBarrier barrier = new CyclicBarrier(100, new Runnable(){
public void run(){
System.out.println("clean up job after all tasks are done.");
}
});
public static void main(String[] args) {
for (int i = 0; i < 100; i++) {
Thread t = new Thread(new MyCallable(barrier));
t.start();
}
}
}
class MyCallable implements Runnable{
private CyclicBarrier b = null;
public MyCallable(CyclicBarrier b){
this.b = b;
}
#Override
public void run(){
try {
//do something
System.out.println(Thread.currentThread().getName()+" is waiting for barrier after completing his job.");
b.await();
} catch (InterruptedException e) {
e.printStackTrace();
} catch (BrokenBarrierException e) {
e.printStackTrace();
}
}
}
To use CyclicBarrier in this case barrier.await() should be the last statement i.e. when your thread is done with its job. CyclicBarrier can be used again with its reset() method. To quote javadocs:
A CyclicBarrier supports an optional Runnable command that is run once per barrier point, after the last thread in the party arrives, but before any threads are released. This barrier action is useful for updating shared-state before any of the parties continue.

The join() was not helpful to me. see this sample in Kotlin:
val timeInMillis = System.currentTimeMillis()
ThreadUtils.startNewThread(Runnable {
for (i in 1..5) {
val t = Thread(Runnable {
Thread.sleep(50)
var a = i
kotlin.io.println(Thread.currentThread().name + "|" + "a=$a")
Thread.sleep(200)
for (j in 1..5) {
a *= j
Thread.sleep(100)
kotlin.io.println(Thread.currentThread().name + "|" + "$a*$j=$a")
}
kotlin.io.println(Thread.currentThread().name + "|TaskDurationInMillis = " + (System.currentTimeMillis() - timeInMillis))
})
t.start()
}
})
The result:
Thread-5|a=5
Thread-1|a=1
Thread-3|a=3
Thread-2|a=2
Thread-4|a=4
Thread-2|2*1=2
Thread-3|3*1=3
Thread-1|1*1=1
Thread-5|5*1=5
Thread-4|4*1=4
Thread-1|2*2=2
Thread-5|10*2=10
Thread-3|6*2=6
Thread-4|8*2=8
Thread-2|4*2=4
Thread-3|18*3=18
Thread-1|6*3=6
Thread-5|30*3=30
Thread-2|12*3=12
Thread-4|24*3=24
Thread-4|96*4=96
Thread-2|48*4=48
Thread-5|120*4=120
Thread-1|24*4=24
Thread-3|72*4=72
Thread-5|600*5=600
Thread-4|480*5=480
Thread-3|360*5=360
Thread-1|120*5=120
Thread-2|240*5=240
Thread-1|TaskDurationInMillis = 765
Thread-3|TaskDurationInMillis = 765
Thread-4|TaskDurationInMillis = 765
Thread-5|TaskDurationInMillis = 765
Thread-2|TaskDurationInMillis = 765
Now let me use the join() for threads:
val timeInMillis = System.currentTimeMillis()
ThreadUtils.startNewThread(Runnable {
for (i in 1..5) {
val t = Thread(Runnable {
Thread.sleep(50)
var a = i
kotlin.io.println(Thread.currentThread().name + "|" + "a=$a")
Thread.sleep(200)
for (j in 1..5) {
a *= j
Thread.sleep(100)
kotlin.io.println(Thread.currentThread().name + "|" + "$a*$j=$a")
}
kotlin.io.println(Thread.currentThread().name + "|TaskDurationInMillis = " + (System.currentTimeMillis() - timeInMillis))
})
t.start()
t.join()
}
})
And the result:
Thread-1|a=1
Thread-1|1*1=1
Thread-1|2*2=2
Thread-1|6*3=6
Thread-1|24*4=24
Thread-1|120*5=120
Thread-1|TaskDurationInMillis = 815
Thread-2|a=2
Thread-2|2*1=2
Thread-2|4*2=4
Thread-2|12*3=12
Thread-2|48*4=48
Thread-2|240*5=240
Thread-2|TaskDurationInMillis = 1568
Thread-3|a=3
Thread-3|3*1=3
Thread-3|6*2=6
Thread-3|18*3=18
Thread-3|72*4=72
Thread-3|360*5=360
Thread-3|TaskDurationInMillis = 2323
Thread-4|a=4
Thread-4|4*1=4
Thread-4|8*2=8
Thread-4|24*3=24
Thread-4|96*4=96
Thread-4|480*5=480
Thread-4|TaskDurationInMillis = 3078
Thread-5|a=5
Thread-5|5*1=5
Thread-5|10*2=10
Thread-5|30*3=30
Thread-5|120*4=120
Thread-5|600*5=600
Thread-5|TaskDurationInMillis = 3833
As it's clear when we use the join:
The threads are running sequentially.
The first sample takes 765 Milliseconds while the second sample takes 3833 Milliseconds.
Our solution to prevent blocking other threads was creating an ArrayList:
val threads = ArrayList<Thread>()
Now when we want to start a new thread we most add it to the ArrayList:
addThreadToArray(
ThreadUtils.startNewThread(Runnable {
...
})
)
The addThreadToArray function:
#Synchronized
fun addThreadToArray(th: Thread) {
threads.add(th)
}
The startNewThread funstion:
fun startNewThread(runnable: Runnable) : Thread {
val th = Thread(runnable)
th.isDaemon = false
th.priority = Thread.MAX_PRIORITY
th.start()
return th
}
Check the completion of the threads as below everywhere it's needed:
val notAliveThreads = ArrayList<Thread>()
for (t in threads)
if (!t.isAlive)
notAliveThreads.add(t)
threads.removeAll(notAliveThreads)
if (threads.size == 0){
// The size is 0 -> there is no alive threads.
}

The problem with:
for(i = 0; i < threads.length; i++)
threads[i].join();
...is, that threads[i + 1] never can join before threads[i].
Except the "latch"ed ones, all solutions have this lack.
No one here (yet) mentioned ExecutorCompletionService, it allows to join threads/tasks according to their completion order:
public class ExecutorCompletionService<V>
extends Object
implements CompletionService<V>
A CompletionService that uses a supplied Executor to execute tasks. This class arranges that submitted tasks are, upon completion, placed on a queue accessible using take. The class is lightweight enough to be suitable for transient use when processing groups of tasks.
Usage Examples.
Suppose you have a set of solvers for a certain problem, each returning a value of some type Result, and would like to run them concurrently, processing the results of each of them that return a non-null value, in some method use(Result r). You could write this as:
void solve(Executor e, Collection<Callable<Result>> solvers) throws InterruptedException, ExecutionException {
CompletionService<Result> cs = new ExecutorCompletionService<>(e);
solvers.forEach(cs::submit);
for (int i = solvers.size(); i > 0; i--) {
Result r = cs.take().get();
if (r != null)
use(r);
}
}
Suppose instead that you would like to use the first non-null result of the set of tasks, ignoring any that encounter exceptions, and cancelling all other tasks when the first one is ready:
void solve(Executor e, Collection<Callable<Result>> solvers) throws InterruptedException {
CompletionService<Result> cs = new ExecutorCompletionService<>(e);
int n = solvers.size();
List<Future<Result>> futures = new ArrayList<>(n);
Result result = null;
try {
solvers.forEach(solver -> futures.add(cs.submit(solver)));
for (int i = n; i > 0; i--) {
try {
Result r = cs.take().get();
if (r != null) {
result = r;
break;
}
} catch (ExecutionException ignore) {}
}
} finally {
futures.forEach(future -> future.cancel(true));
}
if (result != null)
use(result);
}
Since: 1.5 (!)
Assuming use(r) (of Example 1) also asynchronous, we had a big advantage. #