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Concurrency Issue with ConcurrentLinkedQueue with multiple threads writing and multiple threads reading. #Fast directory scan

I have multiple threads writing and reading from the same queue. I am using ConcurrentLinkedQueue. Any thread can add an elements to the queue and any other thread can poll for an element and process it. My problem is if queue is empty I want to the thread that is polling on queue to wait until some element is added to it by some other thread and the whole process should exit if all threads are waiting that is no thread is writing anymore.
I am using this to list all files in a directory and its subdirectories #faster scan.
Following is code snippet 1:
{
private Queue dList = new ConcurrentLinkedQueue();
/* some processing code */
public void Run
{
/* some processing code */
while(dName == null)
{
try{
synchronized(dList){
dList.wait();
}
} catch(InterruptedException e){}
dName = dList.poll();
}
/* some processing code */
{
dList.add(item);
synchronized(dList){
dList.notifyAll();
}
}
}
Is above code right? Does it do what I want all the time. Though program is running fine, I do not understand how threads are exiting as I have not mentioned any exit condition.
Is it right to use synchronized on ConcurrentLinkedQueue?
Does this work with huge directories as well?
EDIT1:
Using LinkedBlockingQueue, but how to exit when all the threads are in waiting state that means no more directories to scan?
Will it work having one static variable, increment it and check it is equal to numberofthreads, then exit. Does it work or are there any better solutions?
EDIT 2:
Thank you all.
Exiting from threads is solved by AtomicInteger. If all threads are waiting on queue, then their AtomicInteger is incremented, if AtomicInteger is equal to number of threads then break;

What you are describing is blocking. You are far better off using a datastructure that specifically does that like ArrayBlockingQueue rather than roll up one of your own. It does concurrency, but also has blocking ways to access the queue that force the running thread to wait until there's an item to get. With all the required locking etc. You can also do unbounded things with LinkedBlockingQueue
https://docs.oracle.com/javase/7/docs/api/java/util/concurrent/ArrayBlockingQueue.html
https://docs.oracle.com/javase/7/docs/api/java/util/concurrent/LinkedBlockingQueue.html
This makes it so that you can have consumer threads and producer threads and you don't have to do any of the locking or syncronizing yourself. You just use take() or do a put(item) and the thread will wait if there's not room to put that item or if there's no item to take. It does the relevant locking and unlocking for you. And only waits when it can't really get the item or put it there. There's also operations that will timeout etc.
new Thread(new Runnable() {
#Override
public void run() {
while (true) {
if (Thread.currentThread().isInterrupted()) return;
try {
File file = blockingqueue.take();
} catch (InterruptedException ignored) {
return;
}
processFile(file);
}
}
}).start();
You don't really have to synchronize the ConcurrentLinkedQueue, it's thread safe. What you're doing is implementing blocking. There are a lot of things to explain how this should be done. It doesn't matter what object you wait and notify on (though must be the same object), the same rules apply.
You need to wait() in one thread and notify() in another (I can't
tell if this is the case in your code).
You need to do so in synchronized blocks synched on the same object. (correct)
But, you didn't seem to implement any of the other requirements. You need to check whether the queue is empty before you wait, there's no point in waiting just every time for no reason.

How can i wait on severals threads completed their code? [duplicate]

I'm making a Java application with an application-logic-thread and a database-access-thread.
Both of them persist for the entire lifetime of the application and both need to be running at the same time (one talks to the server, one talks to the user; when the app is fully started, I need both of them to work).
However, on startup, I need to make sure that initially the app thread waits until the db thread is ready (currently determined by polling a custom method dbthread.isReady()).
I wouldn't mind if app thread blocks until the db thread was ready.
Thread.join() doesn't look like a solution - the db thread only exits at app shutdown.
while (!dbthread.isReady()) {} kind of works, but the empty loop consumes a lot of processor cycles.
Any other ideas? Thanks.

Use a CountDownLatch with a counter of 1.
CountDownLatch latch = new CountDownLatch(1);
Now in the app thread do-
latch.await();
In the db thread, after you are done, do -
latch.countDown();

I would really recommend that you go through a tutorial like Sun's Java Concurrency before you commence in the magical world of multithreading.
There are also a number of good books out (google for "Concurrent Programming in Java", "Java Concurrency in Practice".
To get to your answer:
In your code that must wait for the dbThread, you must have something like this:
//do some work
synchronized(objectYouNeedToLockOn){
while (!dbThread.isReady()){
objectYouNeedToLockOn.wait();
}
}
//continue with work after dbThread is ready
In your dbThread's method, you would need to do something like this:
//do db work
synchronized(objectYouNeedToLockOn){
//set ready flag to true (so isReady returns true)
ready = true;
objectYouNeedToLockOn.notifyAll();
}
//end thread run method here
The objectYouNeedToLockOn I'm using in these examples is preferably the object that you need to manipulate concurrently from each thread, or you could create a separate Object for that purpose (I would not recommend making the methods themselves synchronized):
private final Object lock = new Object();
//now use lock in your synchronized blocks
To further your understanding:
There are other (sometimes better) ways to do the above, e.g. with CountdownLatches, etc. Since Java 5 there are a lot of nifty concurrency classes in the java.util.concurrent package and sub-packages. You really need to find material online to get to know concurrency, or get a good book.

Requirement ::
To wait execution of next thread until previous finished.
Next thread must not start until previous thread stops, irrespective of time consumption.
It must be simple and easy to use.
Answer ::
#See java.util.concurrent.Future.get() doc.
future.get() Waits if necessary for the computation to complete, and then retrieves its result.
Job Done!! See example below
import java.util.concurrent.Callable;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import org.junit.Test;
public class ThreadTest {
public void print(String m) {
System.out.println(m);
}
public class One implements Callable<Integer> {
public Integer call() throws Exception {
print("One...");
Thread.sleep(6000);
print("One!!");
return 100;
}
}
public class Two implements Callable<String> {
public String call() throws Exception {
print("Two...");
Thread.sleep(1000);
print("Two!!");
return "Done";
}
}
public class Three implements Callable<Boolean> {
public Boolean call() throws Exception {
print("Three...");
Thread.sleep(2000);
print("Three!!");
return true;
}
}
/**
* #See java.util.concurrent.Future.get() doc
* <p>
* Waits if necessary for the computation to complete, and then
* retrieves its result.
*/
#Test
public void poolRun() throws InterruptedException, ExecutionException {
int n = 3;
// Build a fixed number of thread pool
ExecutorService pool = Executors.newFixedThreadPool(n);
// Wait until One finishes it's task.
pool.submit(new One()).get();
// Wait until Two finishes it's task.
pool.submit(new Two()).get();
// Wait until Three finishes it's task.
pool.submit(new Three()).get();
pool.shutdown();
}
}
Output of this program ::
One...
One!!
Two...
Two!!
Three...
Three!!
You can see that takes 6sec before finishing its task which is greater than other thread. So Future.get() waits until the task is done.
If you don't use future.get() it doesn't wait to finish and executes based time consumption.
Good Luck with Java concurrency.

A lot of correct answers but without a simple example.. Here is an easy and simple way how to use CountDownLatch:
//inside your currentThread.. lets call it Thread_Main
//1
final CountDownLatch latch = new CountDownLatch(1);
//2
// launch thread#2
new Thread(new Runnable() {
#Override
public void run() {
//4
//do your logic here in thread#2
//then release the lock
//5
latch.countDown();
}
}).start();
try {
//3 this method will block the thread of latch untill its released later from thread#2
latch.await();
} catch (InterruptedException e) {
e.printStackTrace();
}
//6
// You reach here after latch.countDown() is called from thread#2

public class ThreadEvent {
private final Object lock = new Object();
public void signal() {
synchronized (lock) {
lock.notify();
}
}
public void await() throws InterruptedException {
synchronized (lock) {
lock.wait();
}
}
}
Use this class like this then:
Create a ThreadEvent:
ThreadEvent resultsReady = new ThreadEvent();
In the method this is waiting for results:
resultsReady.await();
And in the method that is creating the results after all the results have been created:
resultsReady.signal();
EDIT:
(Sorry for editing this post, but this code has a very bad race condition and I don't have enough reputation to comment)
You can only use this if you are 100% sure that signal() is called after await(). This is the one big reason why you cannot use Java object like e.g. Windows Events.
The if the code runs in this order:
Thread 1: resultsReady.signal();
Thread 2: resultsReady.await();
then thread 2 will wait forever. This is because Object.notify() only wakes up one of the currently running threads. A thread waiting later is not awoken. This is very different from how I expect events to work, where an event is signalled until a) waited for or b) explicitly reset.
Note: Most of the time, you should use notifyAll(), but this is not relevant to the "wait forever" problem above.

Try CountDownLatch class out of the java.util.concurrent package, which provides higher level synchronization mechanisms, that are far less error prone than any of the low level stuff.

You could do it using an Exchanger object shared between the two threads:
private Exchanger<String> myDataExchanger = new Exchanger<String>();
// Wait for thread's output
String data;
try {
data = myDataExchanger.exchange("");
} catch (InterruptedException e1) {
// Handle Exceptions
}
And in the second thread:
try {
myDataExchanger.exchange(data)
} catch (InterruptedException e) {
}
As others have said, do not take this light-hearted and just copy-paste code. Do some reading first.

The Future interface from the java.lang.concurrent package is designed to provide access to results calculated in another thread.
Take a look at FutureTask and ExecutorService for a ready-made way of doing this kind of thing.
I'd strongly recommend reading Java Concurrency In Practice to anyone interested in concurrency and multithreading. It obviously concentrates on Java, but there is plenty of meat for anybody working in other languages too.

If you want something quick and dirty, you can just add a Thread.sleep() call within your while loop. If the database library is something you can't change, then there is really no other easy solution. Polling the database until is ready with a wait period won't kill the performance.
while (!dbthread.isReady()) {
Thread.sleep(250);
}
Hardly something that you could call elegant code, but gets the work done.
In case you can modify the database code, then using a mutex as proposed in other answers is better.

This applies to all languages:
You want to have an event/listener model. You create a listener to wait for a particular event. The event would be created (or signaled) in your worker thread. This will block the thread until the signal is received instead of constantly polling to see if a condition is met, like the solution you currently have.
Your situation is one of the most common causes for deadlocks- make sure you signal the other thread regardless of errors that may have occurred. Example- if your application throws an exception- and never calls the method to signal the other that things have completed. This will make it so the other thread never 'wakes up'.
I suggest that you look into the concepts of using events and event handlers to better understand this paradigm before implementing your case.
Alternatively you can use a blocking function call using a mutex- which will cause the thread to wait for the resource to be free. To do this you need good thread synchronization- such as:
Thread-A Locks lock-a
Run thread-B
Thread-B waits for lock-a
Thread-A unlocks lock-a (causing Thread-B to continue)
Thread-A waits for lock-b
Thread-B completes and unlocks lock-b

You could read from a blocking queue in one thread and write to it in another thread.

Since
join() has been ruled out
you have already using CountDownLatch and
Future.get() is already proposed by other experts,
You can consider other alternatives:
invokeAll from ExecutorService
invokeAll(Collection<? extends Callable<T>> tasks)
Executes the given tasks, returning a list of Futures holding their status and results when all complete.
ForkJoinPool or newWorkStealingPool from Executors ( since Java 8 release)
Creates a work-stealing thread pool using all available processors as its target parallelism level.

This idea can apply?. If you use CountdownLatches or Semaphores works perfect but if u are looking for the easiest answer for an interview i think this can apply.

is this the correct way to 'stop' a thread gracefully?

instead of continuous checking of variable inside a loop:
class Tester {
public static void main() {
Try t = new Try();
Thread.sleep(10); //wait for 10 milliseconds
t.interrupt(); // 'interrupt' i.e stop the thread
}
}
public class Try extends Thread {
public void interrupt() {
//perform all cleanup code here
this.stop();
/*stop() is unsafe .but if we peform all cleanup code above it should be okay ???. since thread is calling stop itself?? */
}
}

In order to perform interrupt in a good manner you should poll for the "interrupted()" method inside the thread that is being interrupted.
Just be aware that calling interrupted() method resets the interruption flag (that is set when calling interrupt()).
I guess the bottom line is that you have to continuously poll inside the thread in order to perform a graceful interruption.

You should never ever call .stop() on a Thread, period. It's not enough for the thread to perform its own cleanup. Since calling .stop() immediately releases all monitors, other threads may see shared data in an inconsistent state which may result in almost impossible to track errors.

Use Thread.interrupt() method instead of Thread.stop(). In the interrupted thread you can catch the InterruptedException and do any cleanup required.
A similar questions has already been asked here, you can find a code sample there too.

How to kill a thread which has a while(true)?

I am trying to close all my thread in my threadpool.
Usually I try:
while(!Thread.currentThread().isInterrupted()) {...
To close the while loop...
But I have one Thread which only consists about
while(!Thread.currentThread().isInterrupted()) {//which is true
This is how I close the threads:
pool.shutdownNow();
So how would you close such a Thread?

You can add a volatile boolean flag.
public class Worker implements Runnable {
volatile boolean cancel = false;
#Override
public void run() {
while (!cancel) {
// Do Something here
}
}
public void cancel() {
cancel = true;
}
}
Now you can just call
worker.cancel();
Update:
From Java doc of shutdownNow()
Attempts to stop all actively executing tasks, halts the processing of waiting tasks, and returns a list of the tasks that were awaiting execution.
here are no guarantees beyond best-effort attempts to stop processing actively executing tasks. For example, typical implementations will cancel via Thread.interrupt(), so any task that fails to respond to interrupts may never terminate.
So either you will have to define your interruption policy by preserving the interrupts
catch (InterruptedException ie) {
// Preserve interrupt status
Thread.currentThread().interrupt();
}

Instead of that you might use a self created flag as condition for the while loop.
public class MyClass implements Runnable
{
private volatile boolean running = true;
public void stopRunning()
{
running = false;
}
public void run()
{
while (running)
{
}
// shutdown stuff here
}
}
Now, to stop it, just call:
myClassObject.stopRunning();
This will let the code finish normally.

If you have implemented this as you have described, it should just work.
When you call pool.shutdownNow() it is supposed to interrupt all worker threads that are currently active. Assuming that the application specific run() methods check the interrupted flag and terminate themselves when they find it set, your threads should shutdown.
There is really no need to add a different mechanism using an ad hoc cancel flag ... or some such.
Incidentally, there are a couple reasons why interrupt() is better than ad hoc cancellation:
Standard APIs like ExecutorService use it.
Various low-level API methods like sleep, wait, join and some I/O methods are sensitive to it.

If you are using a java.util.concurrent ExecutorService implementation, then it will definitely send an interrupt signal to all the threads in its thread pool. The problem with your rogue task may be that the loop doesn't in fact iterate, but blocks somewhere within, so the interrupted status is not getting checked at all.
Yet another problem you may have: the while loop runs some code that catches InterruptedException without handling it properly, effectively swallowing the interrupt signal. This is a common coding mistake and in most cases due to the ugly truth that InterruptedException is checked.

Getting the output of a Thread

What do you think is the best way for obtaining the results of the work of a thread? Imagine a Thread which does some calculations, how do you warn the main program the calculations are done?
You could poll every X milliseconds for some public variable called "job finished" or something by the way, but then you'll receive the results later than when they would be available... the main code would be losing time waiting for them. On the other hand, if you use a lower X, the CPU would be wasted polling so many times.
So, what do you do to be aware that the Thread, or some Threads, have finished their work?
Sorry if it looks similar to this other question, that's probably the reason for the eben answer, I suppose. What I meant was running lots of threads and know when all of them have finished, without polling them.
I was thinking more in the line of sharing the CPU load between multiple CPU's using batches of Threads, and know when a batch has finished. I suppose it can be done with Futures objects, but that blocking get method looks a lot like a hidden lock, not something I like.
Thanks everybody for your support. Although I also liked the answer by erickson, I think saua's the most complete, and the one I'll use in my own code.

Don't use low-level constructs such as threads, unless you absolutely need the power and flexibility.
You can use a ExecutorService such as the ThreadPoolExecutor to submit() Callables. This will return a Future object.
Using that Future object you can easily check if it's done and get the result (including a blocking get() if it's not yet done).
Those constructs will greatly simplify the most common threaded operations.
I'd like to clarify about the blocking get():
The idea is that you want to run some tasks (the Callables) that do some work (calculation, resource access, ...) where you don't need the result right now. You can just depend on the Executor to run your code whenever it wants (if it's a ThreadPoolExecutor then it will run whenever a free Thread is available). Then at some point in time you probably need the result of the calculation to continue. At this point you're supposed to call get(). If the task already ran at that point, then get() will just return the value immediately. If the task didn't complete, then the get() call will wait until the task is completed. This is usually desired since you can't continue without the tasks result anyway.
When you don't need the value to continue, but would like to know about it if it's already available (possibly to show something in the UI), then you can easily call isDone() and only call get() if that returns true).

You could create a lister interface that the main program implements wich is called by the worker once it has finished executing it's work.
That way you do not need to poll at all.
Here is an example interface:
/**
* Listener interface to implement to be called when work has
* finished.
*/
public interface WorkerListener {
public void workDone(WorkerThread thread);
}
Here is an example of the actual thread which does some work and notifies it's listeners:
import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;
/**
* Thread to perform work
*/
public class WorkerThread implements Runnable {
private List listeners = new ArrayList();
private List results;
public void run() {
// Do some long running work here
try {
// Sleep to simulate long running task
Thread.sleep(5000);
} catch (InterruptedException e) {
e.printStackTrace();
}
results = new ArrayList();
results.add("Result 1");
// Work done, notify listeners
notifyListeners();
}
private void notifyListeners() {
for (Iterator iter = listeners.iterator(); iter.hasNext();) {
WorkerListener listener = (WorkerListener) iter.next();
listener.workDone(this);
}
}
public void registerWorkerListener(WorkerListener listener) {
listeners.add(listener);
}
public List getResults() {
return results;
}
}
And finally, the main program which starts up a worker thread and registers a listener to be notified once the work is done:
import java.util.Iterator;
import java.util.List;
/**
* Class to simulate a main program
*/
public class MainProg {
public MainProg() {
WorkerThread worker = new WorkerThread();
// Register anonymous listener class
worker.registerWorkerListener(new WorkerListener() {
public void workDone(WorkerThread thread) {
System.out.println("Work done");
List results = thread.getResults();
for (Iterator iter = results.iterator(); iter.hasNext();) {
String result = (String) iter.next();
System.out.println(result);
}
}
});
// Start the worker thread
Thread thread = new Thread(worker);
thread.start();
System.out.println("Main program started");
}
public static void main(String[] args) {
MainProg prog = new MainProg();
}
}

Polling a.k.a busy waiting is not a good idea. As you mentioned, busy waiting wastes CPU cycles and can cause your application to appear unresponsive.
My Java is rough, but you want something like the following:
If one thread has to wait for the output of another thread you should make use of a condition variable.
final Lock lock = new ReentrantLock();
final Condition cv = lock.newCondition();
The thread interested in the output of the other threat should call cv.wait(). This will cause the current thread to block. When the worker thread is finished working, it should call cv.signal(). This will cause the blocked thread to become unblocked, allowing it to inspect the output of the worker thread.

As an alternative to the concurrency API as described by Saua (and if the main thread doesn't need to know when a worker thread finishes) you could use the publish/subscribe pattern.
In this scenario the child Thread/Runnable is given a listener that knows how to process the result and which is called back to when child Thread/Runnable completes.

Your scenario is still a little unclear.
If you are running a batch job, you may want to use invokeAll. This will block your main thread until all the tasks are complete. There is no "busy waiting" with this approach, where the main thread would waste CPU polling the isDone method of a Future. While this method returns a list of Futures, they are already "done". (There's also an overloaded version that can timeout before completion, which might be safer to use with some tasks.) This can be a lot cleaner than trying to gather up a bunch of Future objects yourself and trying to check their status or block on their get methods individually.
If this is an interactive application, with tasks sporadically spun off to be executed in the background, using a callback as suggested by nick.holt is a great approach. Here, you use the submit a Runnable. The run method invokes the callback with the result when it's been computed. With this approach, you may discard the Future returned by submit, unless you want to be able to cancel running tasks without shutting down the whole ExecutorService.
If you want to be able to cancel tasks or use the timeout capabilities, an important thing to remember is that tasks are canceled by calling interrupt on their thread. So, your task needs to check its interrupted status periodically and abort as needed.

Subclass Thread, and give your class a method that returns the result. When the method is called, if the result hasn't been created, yet, then join() with the Thread. When join() returns, your Thread's work will be done and the result should be available; return it.
Use this only if you actually need to fire off an asynchronous activity, do some work while you're waiting, and then obtain the result. Otherwise, what's the point of a Thread? You might as well just write a class that does the work and returns the result in the main thread.
Another approach would be a callback: have your constructor take an argument that implements an interface with a callback method that will be called when the result is computed. This will make the work completely asynchronous. But if you at all need to wait for the result at some point, I think you're still going to need to call join() from the main thread.

As noted by saua: use the constructs offered by java.util.concurrent. If you're stuck with a pre 1.5 (or 5.0) JRE, you ,might resort to kind of rolling your own, but you're still better of by using a backport: http://backport-jsr166.sourceforge.net/