I'm reading J. Bloch's Effective Java and now I'm at the section which explains about Concurrency. The writer has provided the following example (Some modifications were applied to make it simpler):
Runnable action;
//...
executor.execute(new Runnable() {
public void run() {
ready.countDown();
try {
start.await();
action.run();
} catch (InterruptedException e) {
Thread.currentThread().interrupt(); // <------- Here
} finally {
done.countDown();
}
}
});
It's not clear that why we interrupt the Thread that already was interrupted? Couldn't you get a little explanation about what kind of troubles we may run into if we omit such interrupting?
Yes, it's right.
When an InterruptedException is thrown from a blocking method, the interrupt flag is cleared.
The right thing to do is to reset the interrupt flag (i.e. interrupt again) and stop running ASAP. Resetting the interrupt flag is necessary to let the executor (or any other calling code) know that the thread has been interrupted, and thus allow it to stop running.
I am currently running a Thread from a Service to do some background work.
Now there is the possibility that the Thread crashes or I want to
interrupt the thread from the Service. So how am I supposed to:
stop the Thread realiable, (hard)
catch exceptions and call the Service about the crash
handle InterruptedException if interrupted while sleep()
is Thread.isInterrupted a good way to detect if the Thread stopped?
What I have done so far is the following:
#Override
public void run() {
try {
while (!Thread.currentThread().isInterrupted()) {
doMyBackgroundWork();
sleep();
}
}catch(Exception e){
ExceptionHandler.logAndSendException(e);
Thread.currentThread().interrupt();
if(crashedListener != null){
crashedListener.onThreadCrashed();
}
}
LOG.i("Thread stops now.");
}
private void sleep() {
try {
sleep(frequency);
} catch (InterruptedException e) {
//what to do here? it can happen because I stopped it myself
}
}
So at first I am running my Thread until it gets interrupted.
If any exception occurs, I want to start a new Thread, therefore
my Service implements a listener interface and I call it, once an
Exception is thrown. I know that catching everything is discouraged,
but I need to know if the Thread stops, without polling Thread.isAlive()
all the time.
Additionally to my four questions above:
is my code reliable and does what I need?
is it ok to call interrupt on the Thread itself?
Thanks!
You are not actually interrupting your own thread because the catch block is outside of the while loop. Therefore, any exception would stop execution immediately.
Interruption is essentially just a request (usually from another thread) to stop doing what you are doing. The thread is free to ignore it and keep doing what it is doing. Normally you have to throw an exception in response to an interrupt, or stop execution some other way such as just breaking from the loop (you need this around the //what to do here? comment). It so happens that some library methods are "responsive to interruption" meaning they will throw an exception if the thread is ever interrupted, such as Thread.sleep(), which you will most likely have in your sleep call.
I recommend picking Java Concurrency In Practice. Among the excellent concurrency material, there is a chapter on interrupts which is very helpful.
EDIT:
I would remove the code where you interrupt your own thread. You will also need to rethrow the InterruptedException as a runtime exception to get out of the execution loop. Usually people will create a new Exception that extends RuntimeException that is something like MyInterruptedException. You can then add it to the catch block around your loop so that you know when the thread was interrupted vs execution failed.
As a general example you can do something like this:
public void run() {
try {
while (true) {
// check for interrupts in the loop, or somewhere in the work method
if (Thread.interrupted()) {
throw new MyInterruptedException("Important thread interrupted.");
}
doMyBackgroundWork();
sleep();
}
}
catch(Exception e){
ExceptionHandler.logAndSendException(e);
if(crashedListener != null){
crashedListener.onThreadCrashed();
}
}
catch(MyInterruptedException i) {
LOG.i("Execution stopping because of interrupt.");
}
}
private void sleep() {
try {
sleep(frequency);
} catch (InterruptedException e) {
throw new MyInterrptedException(e);
}
}
we have a nice and effective method called stop()(Thread.stop(void):void) which is deprecated, but it works and it's lovely.
Note that stop() throws ThreadDeath at the target thread which is not an exception(and it could any other throwable too), but an Error, so your code will not catch any signal about this.
public void run() {
try {
while (<<using_a_volatile_bool_type_is_better>>) {
...
}
}catch(Throwable t){/**/}/*use throwable instead of exception.*/}
}
Beside dear friend stop() we also have pause() method too, and it really pauses the target thread.
Not just one solution out there, but if it's really critical to keep thread run and run the emergency(or itself) just after any crash, you may run it as a separately app/process, plus get progress status(if any) that ensures you the target thread/app is not freezed(blocked,...)
I'm hoping someone can help me with this. I've been searching for about a week for an answer to this issue, with no avail.
I currently have a custom thread class that implements Runnable, which I'd like to pause upon a key press. Based on my research, I've learned that the best way to go about this is by using wait() and notify(), triggered by a key that's using a key binding.
My question is, how can I get this to work? I can't seem to set up a key binding without something going wrong, and how I might implement wait() and notify() without running into a deadlock is beyond me.
wait and notify are meant to be used for synchronization. It seems to me that you wanted to use methods like Thread.suspend(), Thread.stop() and Thread.resume(), but those have been deprecated for the risk of problems with lock that they cause.
The solution is to use a helper variable that the thread will check periodically to see if it should be running, otherwise, yield(or sleep)
Why not to use suspend, stop or resume: http://docs.oracle.com/javase/6/docs/technotes/guides/concurrency/threadPrimitiveDeprecation.html
Simple solutions:
How to Pause and Resume a Thread in Java from another Thread
http://www.tutorialspoint.com/java/java_thread_control.htm
Here is a simple snapshot that might get you started :
class PausableThread extends Thread {
private volatile boolean isPaused;
#Override
public void run() {
while (true /* or some other termination condition */) {
try {
waitUntilResumed();
doSomePeriodicAction();
} catch (InterruptedException e) {
// we've been interrupted. Stop
System.out.println("interrupted. Stop the work");
break;
}
}
}
public void pauseAction() {
System.out.println("paused");
isPaused = true;
}
public synchronized void resumeAction() {
System.out.println("resumed");
isPaused = false;
notifyAll();
}
// blocks current thread until it is resumed
private synchronized void waitUntilResumed() throws InterruptedException {
while (isPaused) {
wait();
}
}
private void doSomePeriodicAction() throws InterruptedException {
System.out.println("doing something");
thread.sleep(1000);
}
}
So, you start your thread somewhere new PausableThread().start();
And then in your button/keypress listeners on UI thread you call
in OnPauseKeyPress listener mPausableThread.pauseAction();,
and for OnResumeKeyPress you call mPausableThread.resumeAction();
To stop the tread altogether, just interrupt it : mPausableThread.interrupt();
Hope that helps.
I wrote a thread, it is taking too much time to execute and it seems it is not completely done. I want to stop the thread gracefully. Any help ?
The good way to do it is to have the run() of the Thread guarded by a boolean variable and set it to true from the outside when you want to stop it, something like:
class MyThread extends Thread
{
volatile boolean finished = false;
public void stopMe()
{
finished = true;
}
public void run()
{
while (!finished)
{
//do dirty work
}
}
}
Once upon a time a stop() method existed but as the documentation states
This method is inherently unsafe. Stopping a thread with Thread.stop causes it to unlock all of the monitors that it has locked (as a natural consequence of the unchecked ThreadDeath exception propagating up the stack). If any of the objects previously protected by these monitors were in an inconsistent state, the damaged objects become visible to other threads, potentially resulting in arbitrary behavior.
That's why you should have a guard..
The bad part about using a flag to stop your thread is that if the thread is waiting or sleeping then you have to wait for it to finish waiting/sleeping. If you call the interrupt method on the thread then that will cause the wait or sleep call to be exited with an InterruptedException.
(A second bad part about the flag approach is that most nontrivial code is going to be utilizing libraries like java.util.concurrent, where the classes are specifically designed to use interruption to cancel. Trying to use the hand rolled flag in a task passed into an Executor is going to be awkward.)
Calling interrupt() also sets an interrupted property that you can use as a flag to check whether to quit (in the event that the thread is not waiting or sleeping).
You can write the thread's run method so that the InterruptedException is caught outside whatever looping logic the thread is doing, or you can catch the exception within the loop and close to the call throwing the exception, setting the interrupt flag inside the catch block for the InterruptedException so that the thread doesn't lose track of the fact that it was interrupted. The interrupted thread can still keep control and finish processing on its own terms.
Say I want to write a worker thread that does work in increments, where there's a sleep in the middle for some reason, and I don't want quitting the sleep to make processing quit without doing the remaining work for that increment, I only want it to quit if it is in-between increments:
class MyThread extends Thread
{
public void run()
{
while (!Thread.currentThread().isInterrupted())
{
doFirstPartOfIncrement();
try {
Thread.sleep(10000L);
} catch (InterruptedException e) {
// restore interrupt flag
Thread.currentThread().interrupt();
}
doSecondPartOfIncrement();
}
}
}
Here is an answer to a similar question, including example code.
You should not kill Thread from other one. It's considered as fairly bad habit. However, there are many ways. You can use return statement from thread's run method.
Or you can check if thread has already been interrupted and then it will cancel it's work. F.e. :
while (!isInterrupted()) {
// doStuff
}
Make a volatile boolean stop somewhere. Then in the code that runs in the thread, regularly do
if (stop) // end gracefully by breaking out of loop or whatever
To stop the thread, set stop to true.
I think you must do it manually this way. After all, only the code running in the thread has any idea what is and isn't graceful.
You need to send a stop-message to the Thread and the Thread itself needs to take action if the message has been received. This is pretty easy, if the long-running action is inside loop:
public class StoppableThread extends Thread {
private volatile boolean stop = false;
public void stopGracefully() {
stop = true;
}
public void run() {
boolean finished = false;
while (!stop && !finished) {
// long running action - finished will be true once work is done
}
}
}
For a thread to stop itself, no one seems to have mentioned (mis)using exception:
abstract class SelfStoppingThread extends Thread {
#Override
public final void run() {
try {
doRun();
} catch (final Stop stop) {
//optional logging
}
}
abstract void doRun();
protected final void stopSelf() {
throw new Stop();
}
private static final class Stop extends RuntimeException {};
}
A subclass just need to override doRun() normally as you would with a Thread, and call stopSelf() whenever it feels like it wants to stop. IMO it feels cleaner than using a flag in a while loop.
How do you kill a java.lang.Thread in Java?
See this thread by Sun on why they deprecated Thread.stop(). It goes into detail about why this was a bad method and what should be done to safely stop threads in general.
The way they recommend is to use a shared variable as a flag which asks the background thread to stop. This variable can then be set by a different object requesting the thread terminate.
Generally you don't..
You ask it to interrupt whatever it is doing using Thread.interrupt() (javadoc link)
A good explanation of why is in the javadoc here (java technote link)
In Java threads are not killed, but the stopping of a thread is done in a cooperative way. The thread is asked to terminate and the thread can then shutdown gracefully.
Often a volatile boolean field is used which the thread periodically checks and terminates when it is set to the corresponding value.
I would not use a boolean to check whether the thread should terminate. If you use volatile as a field modifier, this will work reliable, but if your code becomes more complex, for instead uses other blocking methods inside the while loop, it might happen, that your code will not terminate at all or at least takes longer as you might want.
Certain blocking library methods support interruption.
Every thread has already a boolean flag interrupted status and you should make use of it. It can be implemented like this:
public void run() {
try {
while (!interrupted()) {
// ...
}
} catch (InterruptedException consumed)
/* Allow thread to exit */
}
}
public void cancel() { interrupt(); }
Source code adapted from Java Concurrency in Practice. Since the cancel() method is public you can let another thread invoke this method as you wanted.
One way is by setting a class variable and using it as a sentinel.
Class Outer {
public static volatile flag = true;
Outer() {
new Test().start();
}
class Test extends Thread {
public void run() {
while (Outer.flag) {
//do stuff here
}
}
}
}
Set an external class variable, i.e. flag = true in the above example. Set it to false to 'kill' the thread.
I want to add several observations, based on the comments that have accumulated.
Thread.stop() will stop a thread if the security manager allows it.
Thread.stop() is dangerous. Having said that, if you are working in a JEE environment and you have no control over the code being called, it may be necessary; see Why is Thread.stop deprecated?
You should never stop stop a container worker thread. If you want to run code that tends to hang, (carefully) start a new daemon thread and monitor it, killing if necessary.
stop() creates a new ThreadDeathError error on the calling thread and then throws that error on the target thread. Therefore, the stack trace is generally worthless.
In JRE 6, stop() checks with the security manager and then calls stop1() that calls stop0(). stop0() is native code.
As of Java 13 Thread.stop() has not been removed (yet), but Thread.stop(Throwable) was removed in Java 11. (mailing list, JDK-8204243)
There is a way how you can do it. But if you had to use it, either you are a bad programmer or you are using a code written by bad programmers. So, you should think about stopping being a bad programmer or stopping using this bad code.
This solution is only for situations when THERE IS NO OTHER WAY.
Thread f = <A thread to be stopped>
Method m = Thread.class.getDeclaredMethod( "stop0" , new Class[]{Object.class} );
m.setAccessible( true );
m.invoke( f , new ThreadDeath() );
I'd vote for Thread.stop().
As for instance you have a long lasting operation (like a network request).
Supposedly you are waiting for a response, but it can take time and the user navigated to other UI.
This waiting thread is now a) useless b) potential problem because when he will get result, it's completely useless and he will trigger callbacks that can lead to number of errors.
All of that and he can do response processing that could be CPU intense. And you, as a developer, cannot even stop it, because you can't throw if (Thread.currentThread().isInterrupted()) lines in all code.
So the inability to forcefully stop a thread it weird.
The question is rather vague. If you meant “how do I write a program so that a thread stops running when I want it to”, then various other responses should be helpful. But if you meant “I have an emergency with a server I cannot restart right now and I just need a particular thread to die, come what may”, then you need an intervention tool to match monitoring tools like jstack.
For this purpose I created jkillthread. See its instructions for usage.
There is of course the case where you are running some kind of not-completely-trusted code. (I personally have this by allowing uploaded scripts to execute in my Java environment. Yes, there are security alarm bell ringing everywhere, but it's part of the application.) In this unfortunate instance you first of all are merely being hopeful by asking script writers to respect some kind of boolean run/don't-run signal. Your only decent fail safe is to call the stop method on the thread if, say, it runs longer than some timeout.
But, this is just "decent", and not absolute, because the code could catch the ThreadDeath error (or whatever exception you explicitly throw), and not rethrow it like a gentlemanly thread is supposed to do. So, the bottom line is AFAIA there is no absolute fail safe.
'Killing a thread' is not the right phrase to use. Here is one way we can implement graceful completion/exit of the thread on will:
Runnable which I used:
class TaskThread implements Runnable {
boolean shouldStop;
public TaskThread(boolean shouldStop) {
this.shouldStop = shouldStop;
}
#Override
public void run() {
System.out.println("Thread has started");
while (!shouldStop) {
// do something
}
System.out.println("Thread has ended");
}
public void stop() {
shouldStop = true;
}
}
The triggering class:
public class ThreadStop {
public static void main(String[] args) {
System.out.println("Start");
// Start the thread
TaskThread task = new TaskThread(false);
Thread t = new Thread(task);
t.start();
// Stop the thread
task.stop();
System.out.println("End");
}
}
There is no way to gracefully kill a thread.
You can try to interrupt the thread, one commons strategy is to use a poison pill to message the thread to stop itself
public class CancelSupport {
public static class CommandExecutor implements Runnable {
private BlockingQueue<String> queue;
public static final String POISON_PILL = “stopnow”;
public CommandExecutor(BlockingQueue<String> queue) {
this.queue=queue;
}
#Override
public void run() {
boolean stop=false;
while(!stop) {
try {
String command=queue.take();
if(POISON_PILL.equals(command)) {
stop=true;
} else {
// do command
System.out.println(command);
}
} catch (InterruptedException e) {
stop=true;
}
}
System.out.println(“Stopping execution”);
}
}
}
BlockingQueue<String> queue=new LinkedBlockingQueue<String>();
Thread t=new Thread(new CommandExecutor(queue));
queue.put(“hello”);
queue.put(“world”);
t.start();
Thread.sleep(1000);
queue.put(“stopnow”);
http://anandsekar.github.io/cancel-support-for-threads/
Generally you don't kill, stop, or interrupt a thread (or check wheter it is interrupted()), but let it terminate naturally.
It is simple. You can use any loop together with (volatile) boolean variable inside run() method to control thread's activity. You can also return from active thread to the main thread to stop it.
This way you gracefully kill a thread :) .
Attempts of abrupt thread termination are well-known bad programming practice and evidence of poor application design. All threads in the multithreaded application explicitly and implicitly share the same process state and forced to cooperate with each other to keep it consistent, otherwise your application will be prone to the bugs which will be really hard to diagnose. So, it is a responsibility of developer to provide an assurance of such consistency via careful and clear application design.
There are two main right solutions for the controlled threads terminations:
Use of the shared volatile flag
Use of the pair of Thread.interrupt() and Thread.interrupted() methods.
Good and detailed explanation of the issues related to the abrupt threads termination as well as examples of wrong and right solutions for the controlled threads termination can be found here:
https://www.securecoding.cert.org/confluence/display/java/THI05-J.+Do+not+use+Thread.stop%28%29+to+terminate+threads
Here are a couple of good reads on the subject:
What Do You Do With InterruptedException?
Shutting down threads cleanly
I didn't get the interrupt to work in Android, so I used this method, works perfectly:
boolean shouldCheckUpdates = true;
private void startupCheckForUpdatesEveryFewSeconds() {
Thread t = new Thread(new CheckUpdates());
t.start();
}
private class CheckUpdates implements Runnable{
public void run() {
while (shouldCheckUpdates){
//Thread sleep 3 seconds
System.out.println("Do your thing here");
}
}
}
public void stop(){
shouldCheckUpdates = false;
}
Thread.stop is deprecated so how do we stop a thread in java ?
Always use interrupt method and future to request cancellation
When the task responds to interrupt signal, for example, blocking queue take method.
Callable < String > callable = new Callable < String > () {
#Override
public String call() throws Exception {
String result = "";
try {
//assume below take method is blocked as no work is produced.
result = queue.take();
} catch (InterruptedException e) {
Thread.currentThread().interrupt();
}
return result;
}
};
Future future = executor.submit(callable);
try {
String result = future.get(5, TimeUnit.SECONDS);
} catch (TimeoutException e) {
logger.error("Thread timedout!");
return "";
} finally {
//this will call interrupt on queue which will abort the operation.
//if it completes before time out, it has no side effects
future.cancel(true);
}
When the task does not respond to interrupt signal.Suppose the task performs socket I/O which does not respond to interrupt signal and thus using above approach will not abort the task, future would time out but the cancel in finally block will have no effect, thread will keep on listening to socket. We can close the socket or call close method on connection if implemented by pool.
public interface CustomCallable < T > extends Callable < T > {
void cancel();
RunnableFuture < T > newTask();
}
public class CustomExecutorPool extends ThreadPoolExecutor {
protected < T > RunnableFuture < T > newTaskFor(Callable < T > callable) {
if (callable instanceof CancellableTask)
return ((CancellableTask < T > ) callable).newTask();
else
return super.newTaskFor(callable);
}
}
public abstract class UnblockingIOTask < T > implements CustomCallable < T > {
public synchronized void cancel() {
try {
obj.close();
} catch (IOException e) {
logger.error("io exception", e);
}
}
public RunnableFuture < T > newTask() {
return new FutureTask < T > (this) {
public boolean cancel(boolean mayInterruptIfRunning) {
try {
this.cancel();
} finally {
return super.cancel(mayInterruptIfRunning);
}
}
};
}
}
After 15+ years of developing in Java there is one thing I want to say to the world.
Deprecating Thread.stop() and all the holy battle against its use is just another bad habit or design flaw unfortunately became a reality... (eg. want to talk about the Serializable interface?)
The battle is focusing on the fact that killing a thread can leave an object into an inconsistent state. And so? Welcome to multithread programming. You are a programmer, and you need to know what you are doing, and yes.. killing a thread can leave an object in inconsistent state. If you are worried about it use a flag and let the thread quit gracefully; but there are TONS of times where there is no reason to be worried.
But no.. if you type thread.stop() you're likely to be killed by all the people who looks/comments/uses your code. So you have to use a flag, call interrupt(), place if(!flag) all around your code because you're not looping at all, and finally pray that the 3rd-party library you're using to do your external call is written correctly and doesn't handle the InterruptException improperly.