I need to implement a test that uses threads in Java that waits for a condition, then does something to make the condition be false and during that check the condition is false. The exact scenario is below. I am new to threads in general, and threads in Java in particular (never used threads in any coding language). The scenario is like this:
wait for a file in linux to have the value 1 (using cat on the file)
once the value 1 is found, run ping on the linux machine, during the ping I need to verify the value in the same file is 0 during the entire ping session
after ping ends I need to verify the value in the file is 1 again
Could you direct me to a simple example on how to do this in Java using threads?
Thanks..
The general approach is to start a thread and use some synchronisation mechanizm to identify when your thread compleate the job.
One of the methods can be using of ExecutorService and awaitTermination, second approach may be using of CountDownLatch:
CountDownLatch latch = new CountDownLatch(2);
Thread myThead1 = new MyThread(latch);
Thread myThead2 = new MyThread(latch);
myThread1.start();
myThread2.start();
// This will block execution until myThead1 and myThead2 will call countDown:
latch.await();
The thread implementation should call countDown method after job is complited:
public class MyThread extends Thread {
private final CountDownLatch latch;
public MyThread(CountDownLatch latch) {
this.latch = latch;
}
public void run() {
// do your job here
latch.countDown();
}
}
Please check java-doc for java.util.concurrent.locks.Condition. There is a good example that you can use as a base for your code.
Related
first what i am trying to do:
During the main thread execution i want to pause the main thread and start two parallel threads. As soon as both this parallel threads terminate, i'd like to start again with the main thread.
What i tried:
...
...
main thread is executing
...
...
CyclicBarrier barrier = new CyclicBarrier(2);
Thread child1 = new Thread(new ThreadBuilderTask(barrier,0));
Thread child2 = new Thread(new ThreadBuilderTask(barrier,1));
child1.start();
child2.start();
/* Now i'm expecting that child1 and child2 are running in parallel calling their fooFunction */
child1.join();
child2.join();
/*Now i'm expecting that main thread will wait for child1and also for child2 (that are running in parallel).*/
... main thread starts again after both child1 and child2 finished (reached the await of the barrier)
... (break point set here, never reached)
...
Thread builder custom class
public class ThreadBuilderTask implements Runnable{
private CyclicBarrier barrier;
private int index;
...setters and getters..
#Override
public void run() {
fooFunction(this.getIndex());
try {
this.getBarrier().await();
} catch (InterruptedException | BrokenBarrierException e) {
return;
}
}
public ThreadBuilderTask(CyclicBarrier barrier,int index){
this.barrier = barrier;
this.index = index;
}
public fooFunction(int index){
//Something taking some seconds to execute
}
It's not clear what is happening here but it is definetely not working. As soon as i call join everything stops and the main thread never restart. (I put a breakpoint after the joins to see when the main thread restarts).
Maybe there is a bit of confusion with these concepts and also i'm not sure if i need to use both the barrier and the joins or simply one of those techniques.
Thanks
Davide
As mentioned in the comments I'd also suggest to use CompletableFuture. A very basic example of your described requirements could look like this:
final Runnable runnable1 = ...;
final Runnable runnable2 = ...;
CompletableFuture<Void> future1 = CompletableFuture.runAsync(runnable1);
CompletableFuture<Void> future2 = CompletableFuture.runAsync(runnable2);
CompletableFuture.allOf(future1, future2).get(); // waits for both runnables to finish
You might want to add more/some exception handling to this example. But it should give an idea how this might work.
You may consider to use Java CompletableFuture to achieve the objective.
Using its functions like supplyAsync or runAsync you may start child threads and join their respective result in the end. Or you can simply let the main thread wait until the subsequent threads completes.
Recently I managed to implement a sample scatter-gather function using the same class.
Check Java Doc for more offerings and to find best available function: https://docs.oracle.com/javase/8/docs/api/java/util/concurrent/CompletableFuture.html
I want to run several threads and join them at the end of my main method, so I can know when they have finished and process some info.
I don't want to put my threads in an array and do a join() one by one as join is a blocking method and I stay would waiting in the main thread for some threads still running, while other threads may have already finished, without having a possibility of knowing.
I have thought on the possibility of implementing an observer pattern for my threads: An interface with a update() method, an abstract class extending from thread (or implementing runnable) with set and get methods for the listeners and a class starting all my threads and waiting them to finish.
If my understanding is right, an observer would not block in a specific join() for a thread. Instead it will wait somehow until an update() method is called by a thread to perform an action. In this case, the update() should be called right after the thread finishes.
I'm clueless on how to implement this. I've tried with similar models, but I don't know how to use the observer/listener to wake/block my main thread. I've used this old post as a template: How to know if other threads have finished? but I can't find a way to wake my main method once a thread calls the update() method. There will be only one observer object instantiated for all threads.
Could you think of a way to use an observer pattern to wait for all threads to finish without blocking main with one by one join() calls? Any other suggestion to solve this problem would be greatly appreciated. Thanks in advance.
Java already has an API to do that: a CompletionService.
A service that decouples the production of new asynchronous tasks from the consumption of the results of completed tasks. Producers submit tasks for execution. Consumers take completed tasks and process their results in the order they complete.
I think you don't need an observer pattern. Thread waiting for any results will have to block, otherwise it will finish or loop in infinity. You can use some kind of BlockingQueue - producers will add result of computation to the blocking queue (then finish) and main thread will just receive these results blocking when there's not any result yet..
Good news for you, it's already implemented :) Great mechanism of CompletionService and Executors framework. Try this:
private static final int NTHREADS = 5;
private static final int NTASKS = 100;
private static final ExecutorService exec = Executors.newFixedThreadPool(NTHREADS);
public static void main(String[] args) throws InterruptedException {
final CompletionService<Long> ecs = new ExecutorCompletionService<Long>(exec);
for (final int i = 0; i < NTASKS ; ++i) {
Callable<Long> task = new Callable<Long>() {
#Override
public Long call() throws Exception {
return i;
}
};
ecs.submit(task);
}
for (int i = 0; i < NTASKS; ++i) {
try {
long l = ecs.take().get();
System.out.print(l);
} catch (ExecutionException e) {
e.getCause().printStackTrace();
}
}
exec.shutdownNow();
exec.awaitTermination(50, TimeUnit.MILLISECONDS);
}
Sounds to me like you are looking for something like the Counting Completion Service recently discussed by Dr. Heinz M. Kabutz.
I have one "Runnable" threads which is initiating few "Callable" threads and I want to display results when all above threads has finished their jobs.
What is the best way to do it?
My code is as follows
Connector.java (Starting Runnable Thread)
public class Connector {
private static void anyFileConnector() {
// Starting searching Thread
ExecutorService executor = Executors.newFixedThreadPool(100);
executor.submit(traverse, executor);
//HERE I WANT MY ALL SEARCH RESULTS/OUTPUT : CURRENTLY IT IS STARTING OTHER THREADS AND NOT SHOWING ME ANY RESULTS BECAUSE NONE OF THEM WAS FINISHED.(IN CONSOLE, I WAS ABLE TO SEE RESULTS FROM ALL THE THREADS
setSearchResult(traverse.getResult());
executor.shutdown();
}
}
Traverse.java (Runnable Thread)
I am using ExecutorCompletionService to handle it...but it didn't create any difference.
:(
public class Traverse implements Runnable {
public void run() {
ExecutorService executor = Executors.newFixedThreadPool(100);
ExecutorCompletionService<List<ResultBean>> taskCompletionService =
new ExecutorCompletionService<List<ResultBean>>(executor);
try (DirectoryStream<Path> stream = Files
.newDirectoryStream(dir)) {
Search newSearch = new Search();
taskCompletionService.submit(newSearch);
}
list.addAll(taskCompletionService.take().get());
}
}
Search.java (Callable Thread)
public class Search implements Callable<List<ResultBean>> {
public List<ResultBean> call() {
synchronized (Search.class) {
// It will return results
return this.search();
}
}
}
Go for CyclicBarrier and you will be able to achieve this.
A cyclic barrier will perform a task as soon as all the threads are done with their work, this is where you can print the en result.
Check this lik for working of CyclicBarrier : http://javarevisited.blogspot.com/2012/07/cyclicbarrier-example-java-5-concurrency-tutorial.html
Easy - all the Callables will return Future objects which you can used to wait and get the result by calling Future.get() in a blocking wait way. So your problem is just a for loop waiting for each future on the callables blockingly.
After that, just aggregate the results to return to client.
The submit method of executor service can return a list of Future objects. What you can do for your case is call isDone() method of these Future objects in a while loop.
Whenever, any future task gets completed this method will return true. You can now call get() method on this to get the value returned by this task. In this way you could get hold of all the future task values without having to wait for any particular task to get complete (since your first future task could have the longest completion time)
I created some workflow how to wait for all thread which I created. This example works in 99 % of cases but sometimes method waitForAllDone is finished sooner then all thread are completed. I know it because after waitForAllDone I am closing stream which is using created thread so then occurs exception
Caused by: java.io.IOException: Stream closed
my thread start with:
#Override
public void run() {
try {
process();
} finally {
Factory.close(this);
}
}
closing:
protected static void close(final Client client) {
clientCount--;
}
when I creating thread I call this:
public RobWSClient getClient() {
clientCount++;
return new Client();
}
and clientCount variable inside factory:
private static volatile int clientCount = 0;
wait:
public void waitForAllDone() {
try {
while (clientCount > 0) {
Thread.sleep(10);
}
} catch (InterruptedException e) {
LOG.error("Error", e);
}
}
You need to protect the modification and reading of clientCount via synchronized. The main issue is that clientCount-- and clientCount++ are NOT an atomic operation and therefore two threads could execute clientCount-- / clientCount++ and end up with the wrong result.
Simply using volatile as you do above would ONLY work if ALL operations on the field were atomic. Since they are not, you need to use some locking mechanism. As Anton states, AtomicInteger is an excellent choice here. Note that it should be either final or volatile to ensure it is not thread-local.
That being said, the general rule post Java 1.5 is to use a ExecutorService instead of Threads. Using this in conjuction with Guava's Futures class could make waiting for all to complete to be as simple as:
Future<List<?>> future = Futures.successfulAsList(myFutureList);
future.get();
// all processes are complete
Futures.successfulAsList
I'm not sure that the rest of your your code has no issues, but you can't increment volatile variable like this - clientCount++; Use AtomicInteger instead
The best way to wait for threads to terminate, is to use one of the high-level concurrency facilities.
In this case, the easiest way would be to use an ExecutorService.
You would 'offer' a new task to the executor in this way:
...
ExecutorService executor = Executors.newFixedThreadPool(POOL_SIZE);
...
Client client = getClient(); //assuming Client implements runnable
executor.submit(client);
...
public void waitForAllDone() {
executor.awaitTermination(30, TimeUnit.SECOND) ; wait termination of all threads for 30 secs
...
}
In this way, you don't waste valuable CPU cycles in busy waits or sleep/awake cycles.
See ExecutorService docs for details.
I would like to ask basic question about Java threads. Let's consider a producer - consumer scenario. Say there is one producer, and n consumer. Consumer arrive at random time, and once they are served they go away, meaning each consumer runs on its own thread. Should I still use run forever condition for consumer ?
public class Consumer extends Thread {
public void run() {
while (true) {
}
}
}
Won't this keep thread running forever ?
I wouldn't extend Thread, instead I would implement Runnable.
If you want the thread to run forever, I would have it loop forever.
A common alternative is to use
while(!Thread.currentThread().isInterrupted()) {
or
while(!Thread.interrupted()) {
It will, so you might want to do something like
while(beingServed)
{
//check if the customer is done being served (set beingServed to false)
}
This way you'll escaped the loop when it's meant to die.
Why not use a boolean that represents the presence of the Consumer?
public class Consumer extends Thread {
private volatile boolean present;
public Consumer() {
present = true;
}
public void run() {
while (present) {
// Do Stuff
}
}
public void consumerLeft() {
present = false;
}
}
First, you can create for each consumer and after the consumer will finish it's job than the consumer will finish the run function and will die, so no need for infinite loop. however, creating thread for each consumer is not good idea since creation of thread is quite expensive in performance point of view. threads are very expensive resources. In addition, i agree with the answers above that it is better to implement runnable and not to extends thread. extend thread only when you wish to customize your thread.
I strongly suggest you will use thread pool and the consumer will be the runnable object that ran by the thread in the thread pool.
the code should look like this:
public class ConsumerMgr{
int poolSize = 2;
int maxPoolSize = 2;
long keepAliveTime = 10;
ThreadPoolExecutor threadPool = null;
final ArrayBlockingQueue<Runnable> queue = new ArrayBlockingQueue<Runnable>(
5);
public ConsumerMgr()
{
threadPool = new ThreadPoolExecutor(poolSize, maxPoolSize,
keepAliveTime, TimeUnit.SECONDS, queue);
}
public void runTask(Runnable task)
{
// System.out.println("Task count.."+threadPool.getTaskCount() );
// System.out.println("Queue Size before assigning the
// task.."+queue.size() );
threadPool.execute(task);
// System.out.println("Queue Size after assigning the
// task.."+queue.size() );
// System.out.println("Pool Size after assigning the
// task.."+threadPool.getActiveCount() );
// System.out.println("Task count.."+threadPool.getTaskCount() );
System.out.println("Task count.." + queue.size());
}
It is not a good idea to extend Thread (unless you are coding a new kind of thread - ie never).
The best approach is to pass a Runnable to the Thread's constructor, like this:
public class Consumer implements Runnable {
public void run() {
while (true) {
// Do something
}
}
}
new Thread(new Consumer()).start();
In general, while(true) is OK, but you have to handle being interrupted, either by normal wake or by spurious wakeup. There are many examples out there on the web.
I recommend reading Java Concurrency in Practice.
for producer-consumer pattern you better use wait() and notify(). See this tutorial. This is far more efficient than using while(true) loop.
If you want your thread to processes messages until you kill them (or they are killed in some way) inside while (true) there would be some synchronized call to your producer thread (or SynchronizedQueue, or queuing system) which would block until a message becomes available. Once a message is consumed, the loop restarts and waits again.
If you want to manually instantiate a bunch of thread which pull a message from a producer just once then die, don't use while (true).