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thread is not interruptible

I'm trying to work on the famous dining philosophers and its quite finished but I'm having a quite hard time trying to interrupt threads.
so as you know in this problem we have 5 threads (philosophers) and the user set an experiment time at which the experiment will end.
worth noticing that I looked multiple answers on StackOverflow.
The first one is from #Konrad Reiche How do you kill a Thread in Java?
/Stackoverflow link
In that particular post, people have stated that using volatile boolean as a flag might work out but I'm afraid that it is stated in the exercise paper that I cant use the volatile boolean to interrupt a thread but I can use it for other purposes. (studying exercise).
The second one is Thread interrupt() does not interrupt thread/Stackoverflow link
yet nothing really helped!.
I will try to provide the necessary code and I hope someone would just point out my mistake/s.
the class Philosopher is public and extends Thread!.
1)First attempt:(might get rejected by the professor if he doesn't want us to use volatile boolean as flag!)
when using volatile boolean like this it works:
private volatile boolean isNotStopped=true;
#Override
public void stopPhilosopher() {
System.out.printf("\n%s will stop.\n",selfPhilosopher.getName());
selfPhilosopher.interrupt();
isNotStopped=false;
}
#Override
public void run() {
while (isNotStopped){//selfPhilosopher is a thread equals to this!.
try {
think();
eat();
} catch (InterruptedException e) {//somehow this was never triggered!.
System.out.printf("%s was interrupted.\n",selfPhilosopher.getName());
}finally {//in the finally block i always get RUNNER, FALSE
System.out.printf("the %s is %s and is interrupted %b.\n", selfPhilosopher.getName(),selfPhilosopher.getState(), selfPhilosopher.isInterrupted());
}
}
}
[UPDATE] on Second attempt:[WORKING]
replacing selfPhilosopher.isInterrupted() with Thread.currentThread().isInterrupted() didn't make any difference as selfPhilosopher=this;
yet I was getting "will stop" from the stopPhilosopher() method but the threads seem to be like zombies keep coming back to life :(
due to the fact that I'm pretty convinced by the opinion of #Konrad Reiche from the first reference provided and the answer of #Nathan Hughes I will stick with using the boolean flag provided by the java isInterrupted() instead of using a volatile flag.
#Override
public void stopPhilosopher() {
System.out.printf("\n%s will stop.\n",selfPhilosopher.getName());
selfPhilosopher.interrupt();
}
#Override
public void run() {
while (!selfPhilosopher.isInterrupted()){//selfPhilosopher is a thread equals to this!.
try {
think();
eat();
} catch (InterruptedException e) {//somehow this was never triggered!.Now it works.
System.out.printf("%s was interrupted from catch clause!..\n",selfPhilosopher.getName());
selfPhilosopher.interrupt();
}
}
}
OUTPUT:
Philosopher2 in seat nr: 2 was interrupted from catch clause!..

When an InterruptedException is thrown, the interrupt flag is cleared. That means the next check your loop makes will indicate the thread is not interrupted and the thread will keep on running. That's what you're seeing when your finally block prints out false for the interrupt flag.
This is described in the API doc for the sleep method:
InterruptedException - if any thread has interrupted the current thread. The interrupted status of the current thread is cleared when this exception is thrown.
Add this line to the block where you catch an InterruptedException:
Thread.currentThread().interrupt(); // restores interrupt flag

First of all: I favour the volatile boolean flag over isInterrupted(). It is clear, concise and idiomatically established. The necessary structure for isInterrupted() depends more on details (e.g. try/catch in your case because there seems to be a Thread.sleep (or something similar) declaring InterruptedException).
Since try/catch InterruptedException in your example is within the while-loop the interrupt is consumed and need to be resend to selfPhilosopher (which looks a bit dubious). To avoid this put the while-loop into a surrounding try-catch:
try {
while (!selfPhilosopher.isInterrupted()) {
think();
eat();
}
} catch (InterruptedException e) {
System.out.printf("%s was interrupted from catch clause!..\n",selfPhilosopher.getName());
// not necessary anymore: selfPhilosopher.interrupt();
}
The volatile boolean flag would not need resending and could be used in both constellations (while surrounding try/catch as well as while within try).
Additionally: Suppose your class implements Runnable and is not extending Thread then you also need to take care of setting selfPhilosopher at the beginning of run (and nowhere else).

Delaying 8 Seconds not working?

So, I am trying to send a packet (With Spigot) every 8 seconds.
I would post this on the Spigot forums but I always get the error on the wait. I have tried Scheduler but no luck.
Code:
Object obj = new Object();
try {
synchronized (obj) {
while (true) {
for (Player player : Bukkit.getOnlinePlayers()) {
System.out.println("Hi");
obj.wait(8000);
}
}
}
} catch (InterruptedException exception) {
}
Ignore the Player player thing it does nothing.
Help please. Any help is appreciated.

wait() is a method intended for asynchronous programming, where you let a chunk of code run while something else "needs time" to finish, as to not block the whole execution of the program. Think of it as a a mechanism that lets asynchronous things happen. As per the docs:
Causes the current thread to wait until another thread invokes the
notify() method or the notifyAll() method for this object.
Whereas sleep() is a method that makes the current thread stand by for a moment until the desired time has passed and so it will continue its execution:
Thread.sleep causes the current thread to suspend execution for a
specified period.
The former is useful in asynchronous programming, where you know something will potentially block execution of your program for a while and you want to do other stuff in that time, like connecting to a socket, probably all in the same thread. The latter is when you want to stop everything from happening for a moment on the same thread.
If what you want to do is just delay the execution of your method, then this would be the way to go:
try {
while (true) {
for (Player player : Bukkit.getOnlinePlayers()) {
System.out.println("Hi");
Thread.sleep(8000);
}
}
} catch (InterruptedException exception) {
// Catch something here
}

wait() - notify() mechanism in java malfunctioning in a strange way [duplicate]

This question already has answers here:
Why should wait() always be called inside a loop
(11 answers)
Closed 7 years ago.
I've tried reading some answers to similar questions here (I always do that) but did not find (or did not understand?) the answer to this particular issue.
I am implementing a fairly simple consumer-producer class, which receives elements to a list from a different thread and consumes them repeatedly. The class has the following code:
public class ProduceConsume implements Runnable
{
LinkedList<Integer> _list = new LinkedList<Integer>();
public synchronized void produce(Integer i)
{
_list.add(i);
notify();
}
public void run()
{
while(true)
{
Integer i = consume();
// Do something with the integer...
}
}
private synchronized Integer consume()
{
if(_list.size() == 0)
{
try
{
wait();
}
catch(InterruptedException e){}
return _list.poll();
}
}
}
The problem is - it usually works fine, but sometimes, the execution gets to
return _list.poll();
with the list still empty. I can't wrap my head around it - am I doing something terribly wrong? Shouldn't the runnable thread, which repeatedly tries to poll detect a zero length list, wait, and be awakened only after the producer method is done, hence making the list non-empty?
Nothing else "touches" the class from the outside, except for calls to produce. No other threads are synchronized on the runnable class.
By the way, for several reasons, I wish to use my own variant and not classes such as CopyOnWriteArrayList, etc.
Thanks! Any help would be greatly appreciated.
P.S - I have not used the wait-notify many times, but when I did, in the past, it worked. So if I apologize if I made some huge stupid error!

As the Javadoc for Object.wait states
As in the one argument version, interrupts and spurious wakeups are possible, and this method should always be used in a loop:
synchronized (obj) {
while (<condition does not hold>)
obj.wait();
... // Perform action appropriate to condition
}
Additionally, you shouldn't ignore an exception like InterruptedException. This will look like a spurious wake up and as you say produces an error.
private synchronized Integer consume() {
try {
while (_list.isEmpty())
wait();
return _list.poll();
} catch(InterruptedException e) {
throw new IllegalStateException("Interrupted");
}
}

Since wait releases the lock you can't reason based on conditions tested before it started waiting, assuming the condition must have changed once wait is exited is not valid. You need to call wait in a loop, so that once the thread ceases waiting and takes the lock again, it checks that the condition it's waiting for has the expected value:
private synchronized Integer consume()
{
try {
while (_list.size() == 0) {
wait();
}
} catch (InterruptedException e) {
Thread.currentThread().interrupt();
}
return _list.poll();
}
From the Oracle tutorial:
Note: Always invoke wait inside a loop that tests for the condition being waited for.
Also it's not safe to assume that just because wait returned that something sent a notification. wait can return even if there is no notification (the spurious wakeup).
It's hard to say what caused what you're seeing without a complete working example.
The linked Oracle tutorial page has a Producer Consumer example you might want to look at.

Have threads run indefinitely in a java application

I am trying to program a game in which I have a Table class and each person sitting at the table is a separate thread. The game involves the people passing tokens around and then stopping when the party chime sounds.
how do i program the run() method so that once I start the person threads, they do not die and are alive until the end of the game
One solution that I tried was having a while (true) {} loop in the run() method but that increases my CPU utilization to around 60-70 percent. Is there a better method?

While yes, you need a loop (while is only one way, but it is simplest) you also need to put something inside the loop that waits for things to happen and responds to them. You're aiming to have something like this pseudocode:
loop {
event = WaitForEvent();
RespondToEvent(event);
} until done;
OK, that's the view from 40,000 feet (where everything looks like ants!) but it's still the core of what you want. Oh, and you also need something to fire off the first event that starts the game, obviously.
So, the key then becomes the definition of WaitForEvent(). The classic there is to use a queue to hold the events, and to make blocking reads from the queue so that things wait until something else puts an event in the queue. This is really a Concurrency-101 data-structure, but an ArrayBlockingQueue is already defined correctly and so is what I'd use in my first implementation. You'll probably want to hide its use inside a subclass of Thread, perhaps like this:
public abstract class EventHandlingThread<Event> extends Thread {
private ArrayBlockingQueue<Event> queue = new ArrayBlockingQueue<Event>();
private boolean done;
protected abstract void respondToEvent(Event event);
public final void postEvent(Event event) throws InterruptedException {
queue.put(event);
}
protected final void done() {
done = true;
}
public final void run() {
try {
while (!done) {
respondToEvent(queue.take());
}
} catch (InterruptedException e) {
// Maybe log this, maybe not...
} catch (RuntimeException e) {
// Probably should log this!
}
}
}
Subclass that for each of your tasks and you should be able to get going nicely. The postEvent() method is called by other threads to send messages in, and you call done() on yourself when you've decided enough is enough. You should also make sure that you've always got some event that can be sent in which terminates things so that you can quit the game…

I would look into Locks and Conditions. This way you can write code that waits for a certain condition to happen. This won't take a lot of CPU power and is even much more efficient and better performing than sleeping .

To make a thread run for an infinite time:
final Object obj = new Object();
try {
Thread th = new Thread(new Runnable() {
public void run() {
synchronized(obj) {
try {
System.out.println("Waiting");
obj.wait();
System.out.println("Done waiting");
}catch(Exception ex) {
ex.printStackTrace();
}
}
}
});
th.start();
System.out.println("Waiting to join.");
// Dont notify; but wait for joining. This will ensure that main thread is running always.
th.join();
System.out.println("End of the Program");
} catch(Exception ex) {
ex.printStackTrace();
}

You may add Thread.sleep() with appropriate time to minimize useless loop iterations.
Another solution is using synchronization. While threads are not required to do anything, they enter into a sleeping state on a monitor using the wait() method, and then when the turn comes, required thread is woken up by the notify() method.

Actor model seems suitable for this scenario. Each person sitting on the table and the table itself can be modelled as actors and the event of passing the tokens and starting and stopping of the game can be modelled as messages to be passed between the actors.
As a bonus, by modelling the scenario as actors you get rid of explicit manipulation of threads, synchronization and locking.
On JVM I will prefer using Scala for modelling actors. For Java you can use libraries like Kilim. See this post for a comparison of Actor model related libraries in Java.

One Way is to use while loop but keep a check i.e
while(true){
if(condition!=true){
Thread.sleep(time);
}else{
break;
}
}
This way if your condition says game is not over it will keep person thread at sleep and memory consumption will be very low.

You should test for a condition in the while loop:
while (!gameOver)
{
do_intersting_stuff();
}
Heavy CPU load is typical for busy wait. Is your loop actually just checking a flag over and over, like
while (!gameOver)
{
if (actionNeeded)
{
do_something();
}
}
you might change to another notification system to sleep and wake up, as this just burns CPU time for nothing.

How can I wrap a method so that I can kill its execution if it exceeds a specified timeout?

I have a method that I would like to call. However, I'm looking for a clean, simple way to kill it or force it to return if it is taking too long to execute.
I'm using Java.
to illustrate:
logger.info("sequentially executing all batches...");
for (TestExecutor executor : builder.getExecutors()) {
logger.info("executing batch...");
executor.execute();
}
I figure the TestExecutor class should implement Callable and continue in that direction.
But all i want to be able to do is stop executor.execute() if it's taking too long.
Suggestions...?
EDIT
Many of the suggestions received assume that the method being executed that takes a long time contains some kind of loop and that a variable could periodically be checked.
However, this is not the case. So something that won't necessarily be clean and that will just stop the execution whereever it is is acceptable.

You should take a look at these classes :
FutureTask, Callable, Executors
Here is an example :
public class TimeoutExample {
public static Object myMethod() {
// does your thing and taking a long time to execute
return someResult;
}
public static void main(final String[] args) {
Callable<Object> callable = new Callable<Object>() {
public Object call() throws Exception {
return myMethod();
}
};
ExecutorService executorService = Executors.newCachedThreadPool();
Future<Object> task = executorService.submit(callable);
try {
// ok, wait for 30 seconds max
Object result = task.get(30, TimeUnit.SECONDS);
System.out.println("Finished with result: " + result);
} catch (ExecutionException e) {
throw new RuntimeException(e);
} catch (TimeoutException e) {
System.out.println("timeout...");
} catch (InterruptedException e) {
System.out.println("interrupted");
}
}
}

Java's interruption mechanism is intended for this kind of scenario. If the method that you wish to abort is executing a loop, just have it check the thread's interrupted status on every iteration. If it's interrupted, throw an InterruptedException.
Then, when you want to abort, you just have to invoke interrupt on the appropriate thread.
Alternatively, you can use the approach Sun suggest as an alternative to the deprecated stop method. This doesn't involve throwing any exceptions, the method would just return normally.

I'm assuming the use of multiple threads in the following statements.
I've done some reading in this area and most authors say that it's a bad idea to kill another thread.
If the function that you want to kill can be designed to periodically check a variable or synchronization primitive, and then terminate cleanly if that variable or synchronization primitive is set, that would be pretty clean. Then some sort of monitor thread can sleep for a number of milliseconds and then set the variable or synchronization primitive.

Really, you can't... The only way to do it is to either use thread.stop, agree on a 'cooperative' method (e.g. occassionally check for Thread.isInterrupted or call a method which throws an InterruptedException, e.g. Thread.sleep()), or somehow invoke the method in another JVM entirely.
For certain kinds of tests, calling stop() is okay, but it will probably damage the state of your test suite, so you'll have to relaunch the JVM after each call to stop() if you want to avoid interaction effects.
For a good description of how to implement the cooperative approach, check out Sun's FAQ on the deprecated Thread methods.
For an example of this approach in real life, Eclipse RCP's Job API's 'IProgressMonitor' object allows some management service to signal sub-processes (via the 'cancel' method) that they should stop. Of course, that relies on the methods to actually check the isCancelled method regularly, which they often fail to do.
A hybrid approach might be to ask the thread nicely with interrupt, then insist a couple of seconds later with stop. Again, you shouldn't use stop in production code, but it might be fine in this case, esp. if you exit the JVM soon after.
To test this approach, I wrote a simple harness, which takes a runnable and tries to execute it. Feel free to comment/edit.
public void testStop(Runnable r) {
Thread t = new Thread(r);
t.start();
try {
t.join(2000);
} catch (InterruptedException e) {
throw new RuntimeException(e);
}
if (!t.isAlive()) {
System.err.println("Finished on time.");
return;
}
try {
t.interrupt();
t.join(2000);
if (!t.isAlive()) {
System.err.println("cooperative stop");
return;
}
} catch (InterruptedException e) {
throw new RuntimeException(e);
}
System.err.println("non-cooperative stop");
StackTraceElement[] trace = Thread.getAllStackTraces().get(t);
if (null != trace) {
Throwable temp = new Throwable();
temp.setStackTrace(trace);
temp.printStackTrace();
}
t.stop();
System.err.println("stopped non-cooperative thread");
}
To test it, I wrote two competing infinite loops, one cooperative, and one that never checks its thread's interrupted bit.
public void cooperative() {
try {
for (;;) {
Thread.sleep(500);
}
} catch (InterruptedException e) {
System.err.println("cooperative() interrupted");
} finally {
System.err.println("cooperative() finally");
}
}
public void noncooperative() {
try {
for (;;) {
Thread.yield();
}
} finally {
System.err.println("noncooperative() finally");
}
}
Finally, I wrote the tests (JUnit 4) to exercise them:
#Test
public void testStopCooperative() {
testStop(new Runnable() {
#Override
public void run() {
cooperative();
}
});
}
#Test
public void testStopNoncooperative() {
testStop(new Runnable() {
#Override
public void run() {
noncooperative();
}
});
}
I had never used Thread.stop() before, so I was unaware of its operation. It works by throwing a ThreadDeath object from whereever the target thread is currently running. This extends Error. So, while it doesn't always work cleanly, it will usually leave simple programs with a fairly reasonable program state. For example, any finally blocks are called. If you wanted to be a real jerk, you could catch ThreadDeath (or Error), and keep running, anyway!
If nothing else, this really makes me wish more code followed the IProgressMonitor approach - adding another parameter to methods that might take a while, and encouraging the implementor of the method to occasionally poll the Monitor object to see if the user wants the system to give up. I'll try to follow this pattern in the future, especially methods that might be interactive. Of course, you don't necessarily know in advance which methods will be used this way, but that is what Profilers are for, I guess.
As for the 'start another JVM entirely' method, that will take more work. I don't know if anyone has written a delegating class loader, or if one is included in the JVM, but that would be required for this approach.

Nobody answered it directly, so here's the closest thing i can give you in a short amount of psuedo code:
wrap the method in a runnable/callable. The method itself is going to have to check for interrupted status if you want it to stop (for example, if this method is a loop, inside the loop check for Thread.currentThread().isInterrupted and if so, stop the loop (don't check on every iteration though, or you'll just slow stuff down.
in the wrapping method, use thread.join(timeout) to wait the time you want to let the method run. or, inside a loop there, call join repeatedly with a smaller timeout if you need to do other things while waiting. if the method doesn't finish, after joining, use the above recommendations for aborting fast/clean.
so code wise, old code:
void myMethod()
{
methodTakingAllTheTime();
}
new code:
void myMethod()
{
Thread t = new Thread(new Runnable()
{
public void run()
{
methodTakingAllTheTime(); // modify the internals of this method to check for interruption
}
});
t.join(5000); // 5 seconds
t.interrupt();
}
but again, for this to work well, you'll still have to modify methodTakingAllTheTime or that thread will just continue to run after you've called interrupt.

The correct answer is, I believe, to create a Runnable to execute the sub-program, and run this in a separate Thread. THe Runnable may be a FutureTask, which you can run with a timeout ("get" method). If it times out, you'll get a TimeoutException, in which I suggest you
call thread.interrupt() to attempt to end it in a semi-cooperative manner (many library calls seem to be sensitive to this, so it will probably work)
wait a little (Thread.sleep(300))
and then, if the thread is still active (thread.isActive()), call thread.stop(). This is a deprecated method, but apparently the only game in town short of running a separate process with all that this entails.
In my application, where I run untrusted, uncooperative code written by my beginner students, I do the above, ensuring that the killed thread never has (write) access to any objects that survive its death. This includes the object that houses the called method, which is discarded if a timeout occurs. (I tell my students to avoid timeouts, because their agent will be disqualified.) I am unsure about memory leaks...
I distinguish between long runtimes (method terminates) and hard timeouts - the hard timeouts are longer and meant to catch the case when code does not terminate at all, as opposed to being slow.
From my research, Java does not seem to have a non-deprecated provision for running non-cooperative code, which, in a way, is a gaping hole in the security model. Either I can run foreign code and control the permissions it has (SecurityManager), or I cannot run foreign code, because it might end up taking up a whole CPU with no non-deprecated means to stop it.
double x = 2.0;
while(true) {x = x*x}; // do not terminate
System.out.print(x); // prevent optimization

I can think of a not so great way to do this. If you can detect when it is taking too much time, you can have the method check for a boolean in every step. Have the program change the value of the boolean tooMuchTime to true if it is taking too much time (I can't help with this). Then use something like this:
Method(){
//task1
if (tooMuchTime == true) return;
//task2
if (tooMuchTime == true) return;
//task3
if (tooMuchTime == true) return;
//task4
if (tooMuchTime == true) return;
//task5
if (tooMuchTime == true) return;
//final task
}