Pausing a Thread on Java

Pausing a Thread on Java - java

given a Runnable:
public class MyRunnable implements Runnable {
public void run() {
doA();
for (i=0; i<18123 ; i++) {
doB();
}
doC();
}
}
where doA,B,C are defined with like 100 lines of code each.
what is the best way to make the thread FREEZE - in whatever line of code its at - and then continue from the next line of code where it last stopped.
I was searching around the net, and I saw here http://docs.oracle.com/javase/1.5.0/docs/guide/misc/threadPrimitiveDeprecation.html
that they suggest on using a boolean, so, does that mean that i need to check that boolean after every few lines of code? there's gotta be a nicer way...

That cannot work in general for the cause Thread.suspend() and Thread.resume() have been deprecated:
Thread.suspend is inherently deadlock-prone. If the target thread holds a lock on the monitor protecting a critical system resource when it is suspended, no thread can access this resource until the target thread is resumed. If the thread that would resume the target thread attempts to lock this monitor prior to calling resume, deadlock results. Such deadlocks typically manifest themselves as "frozen" processes.
If you absolutely need this behavior, you have to explicitly implemented it yourself.
The implementations could look like this (code not tested, nor compiled):
public abstract class SafeStoppableRunnable implements Runnable {
private boolean stopped = false;
public synchronized void stopSafe() {
this.stopped = true;
}
public synchronized void resumeSafe() {
this.stopped = false;
synchronized(this) {
this.notifyAll();
}
}
protected synchronized void waitWhenStopped() {
while(this.stopped) {
this.wait();
}
}
}
The stoppable Runnables should then extend SafeStoppableRunnable and call the method waitWhenStopped() at all the points in your program you want it to be stoppable. Stoppable points are probably points where the program does not hold global ressources other threads need to make progress.

Multithreaded systems are really complex. The golden rule of multithreaded programming is to avoid any code that can potentially raise the level of entropy of the system. Vague system states are impossible to predict and trace.
A thread paused at some unpredictable line is multithreaded programmer's nightmare. At development time it is simply impossible to make sure that the system will be working fine after the thread is woken up at some unknown line. You just can't keep track of all the alternatives. This will 100% result in all kind of weird defects in production.
Adding a couple of definite pause points in a thread is the only way to go.

Related

Simple thread crashes Android app

I found a very peculiar issue when creating an launching a thread inside an Android app.
If I have the following class thread:
public class TroubleThread extends Thread{
boolean running;
public boolean isRunning() {
return running;
}
public void setRunning(boolean running) {
this.running = running;
}
#Override
public void run() {
while (isRunning()){
}//end while
}//end run
}
and add it somewhere in the Activity's onCreate(...) method like:
public class MyActivity extends Activity {
TroubleThread myThread;
#Override
public void onCreate(Bundle savedInstanceState) {
//....
myThread = new TroubleThread();
myThread.setRunning(true);
myThread.start();
}
}
the app will crash.
But if I change the run() method to:
#Override
public void run() {
while (running){ //NOTE THE USE OF DIRECT FIELD ACCESS INSTEAD OF METHOD
}//end while
}//end run
it stops from crashing.
Even if I solved my problem by using locks, notify() and wait() the question still remains:
Why when using direct access to the field the app continues to work while when using the method it crashes?

First of all, since you did not provide a MCVE, other people cannot reproduce your problem. That is unfortunate because it means we cannot be sure what the real cause of the problem is. We can only propose hypotheses.
Some people have hypothesized that your problem is due to something else; e.g. an infinite loop on the GUI thread. That is plausible, but there is no clear evidence for this. (And we can't see the code ....)
My hypothesis is that this is a "memory visibility" issue. The Java Language Specification has a chapter that defines the circumstances under which one thread is guaranteed to see a value written to memory by another thread. The rules are rather complicated and technical, but the essence is that you need to analyzing whether there is a happens before relationship between a memory write by one thread and a subsequent memory read by another thread. A number of things will give you that relationship:
the threads write / reading a shared volatile variable
the threads synchronizing using the same lock
a thread start
a thread join
completion of a constructor (for final variables)
However, none of these things exist in your program with respect to the running variable. This means that there is no guarantee that the thread that is looping on the running variable will see the results of the setRunning call made by the other thread:
It might see the changes immediately
It might see the changes later
It might never see them
All three behaviors are possible .... in the absence of a happens before relationship.
So why does one version of the code behave differently to the other?
We can't be sure. To actually be sure, someone would need to do a deep analysis of the native (machine) code for your example. It could be something to do with (legal) reordering that the optimizer has done in one case and not the other. It could be a subtle timing effect.
But either way, the JLS says the the compiler is not obliged to make sure that the write is visible. Why? Because this code breaks the rules of the memory model.
Solutions:
In this case, the simplest solution is to declare running as volatile.
Another solution is to declare isRunning() and setRunning as synchronized methods.
Either of these are sufficient to provide the happens before relationship ... and guarantee that isRunning() sees the update made by setRunning().

How to stop a thread if one thread has completed a task first?

You have to find a file from your two drives of computer. If one thread found it first then the second thread will stop.

Have all threads periodically check a flag variable to see if it's been set and then stop searching if it has. You could do this check after each file, after each directory, or if it's been more than a second since the last time you checked, the mechanics are up to you, depending on your needs.
Then just have a thread set that flag if it finds the file. The other threads will soon pick it up and stop as well.
I prefer having each thread responsible for its own resource, including its lifespan, that's why I prefer this sort of solution to trying to kill a thread from outside it.
I don't know whether Java suffers from the same issues as pthreads (in terms of destroying threads that hold critical resources) but I'd rather be safe than sorry.

The first thing is, you cannot force a thread to stop in java. To stop a thrad safely it has to terminate "naturally". (Means, all the code inside has to be executed or an exception occurs which is not caugth).
In Java you can use the "interrupt" flag. Have both you threads check Thread.currentThread().isInterrupted() like this:
try {
while(!Thread.currentThread().isInterrupted()) {
[...]
Thread.sleep(); //May be you need to sleep here for a short period to not produce to much load on your system, ?
}
} catch (InterruptedException consumed) {
}
So the easiet implementation would be to have your two threads have a reference to one another and if one thread found the file it calls interrupt on the other thread which in turn terminates because of the above while loop.
For more information look Java Concurrency in practice.

You can try to use a flag whether the file was found. The thread will exit if the flag changes its state.
Example with an implemented Runnable
public class Finder implements Runnable {
private static boolean found = false;
#Override
public void run() {
for (ITERATE_THROUG_FILES) {
if (found) {
break;
}
if (FILE == SEARCHED_FILE) {
found = true;
}
}
}
}

Possible locking/performance issue

I am working on an application, that processes incoming messages. I am not proficient in java multithreading and I am asking your help, folks. Is there anything wrong with the following app structure.
There is main application class with stopRequested boolean field. And there is internal runnable class that listens for incoming messages and process them. Also there is another thread that sets stopRequested to true.
Is this approach working and reliable, or I am wrong?
Below there is a part of my code:
class ApplicationClass {
// we set this var in another thread
// when it is necessary to stop
private stopRequested = false;
public ApplicationClass() {
// starting message processing thread
(new Thread(new MessageProcessing())).start();
}
private class MessageProcessing implements Runnable {
public void run() {
while (!stopRequested) {
if (getNewMessagesCount() > 0) {
processNewMessages();
}
}
}
}
}
Thank you.

There are a few things to think about.
As sbridges noted stopRequested needs to be volatile to resolve visibility problems (a thread on another core may not see the change otherwise).
If getNewMessagesCount() doesn't block then your while loop will spin and consume the core; this will give you the lowest latency but ties up the entire core.
The code you've listed appears to be a simple processing queue; you're likely going to be better off going with an ArrayBlockingQueue.
It's dangerous to start a new thread from a constructor. The thing to worry about is what happens if getMessageCount() and processNewMessages() are invoked before ApplicationClass is finished being created. Since the instance of ApplicationClass could be in an incomplete state you could find a rather nasty bug. (For the same reason you never want to have your code subscribe as a listener to events from a constructor, by the way.) Check out Effective Java for more background on this topic.
Your while loop should check if the current thread has been interrupted so that it places nice; it should be while (!stopRequested && !Thread.currentThread().isInterrupted())
Writing correct concurrent programs is hard. I highly recommend reading Java Concurrency in Practice; it will save you a lot of pain.

It is hard to comment on performance without knowing more details, you will probably want to benchmark.
The code looks correct, except you will want to make stopRequested volatile. If it is not volatile, the processing thread may not see it being set to false. Rather than use methods like getNewMessageCount(), you might want to use a LinkedBlockingQueue, and use the poll() method on that.

Why can't a Java Thread object be restarted?

I know that it is not possible to restart a used Java Thread object, but I don't find an explanation why this is not allowed; even if it is guaranteed that the thread has finished (see example code below).
I don't see why start() (or at least a restart()) method should not be able to somehow reset the internal states - whatever they are - of a Thread object to the same values they have when the Thread object is freshly created.
Example code:
class ThreadExample {
public static void main(String[] args){
Thread myThread = new Thread(){
public void run() {
for(int i=0; i<3; i++) {
try{ sleep(100); }catch(InterruptedException ie){}
System.out.print(i+", ");
}
System.out.println("done.");
}
};
myThread.start();
try{ Thread.sleep(500); }catch(InterruptedException ie){}
System.out.println("Now myThread.run() should be done.");
myThread.start(); // <-- causes java.lang.IllegalThreadStateException
} // main
} // class

I know that it is not possible to
restart a used Java Thread object, but
I don't find an explanation why this
is not allowed; even if it is
guaranteed that the thread has
finished (see example code below).
My guestimation is that Threads might be directly tied (for efficiency or other constrains) to actual native resources that might be re-startable in some operating systems, but not in others. If the designers of the Java language had allowed Threads to be re-started, they might limit the number of operating systems on which the JVM can run.
Come to think of it, I cannot think of a OS that allows a thread or process to be restarted once it is finished or terminated. When a process completes, it dies. You want another one, you restart it. You never resurrect it.
Beyond the issues of efficiency and limitations imposed by the underlying OS, there is the issue of analysis and reasoning. You can reason about concurrency when things are either immutable or have a discrete, finite life-time. Just like state machines, they have to have a terminal state. Is it started, waiting, finished? Things like that cannot be easily reasoned about if you allow Threads to resurrect.
You also have to consider the implications of resurrecting a thread. Recreate its stack, its state, is is safe to resurrect? Can you resurrect a thread that ended abnormally? Etc.
Too hairy, too complex. All that for insignificant gains. Better to keep Threads as non-resurrectable resources.

I'd pose the question the other way round - why should a Thread object be restartable?
It's arguably much easier to reason about (and probably implement) a Thread that simply executes its given task exactly once and is then permanently finished. To restart threads would require a more complex view on what state a program was in at a given time.
So unless you can come up with a specific reason why restarting a given Thread is a better option than just creating a new one with the same Runnable, I'd posit that the design decision is for the better.
(This is broadly similar to an argument about mutable vs final variables - I find the final "variables" much easier to reason about and would much rather create multiple new constant variables rather than reuse existing ones.)

Because they didn't design it that way. From a clarity standpoint, that makes sense to me. A Thread represents a thread of execution, not a task. When that thread of execution has completed, it has done its work and it just muddies things were it to start at the top again.
A Runnable on the other hand represents a task, and can be submitted to many Threads as many times as you like.

Why don't you want to create a new Thread? If you're concerned about the overhead of creating your MyThread object, make it a Runnable and run it with a new Thread(myThread).start();

Java Threads follow a lifecycle based on the State Diagram below. Once the thread is in a final state, it is over. That is simply the design.

You can kind of get around this, either by using a java.util.concurrent.ThreadPoolExecutor, or manually by having a thread that calls Runnable.run() on each Runnable that it is given, not actually exiting when it is finished.
It's not exactly what you were asking about, but if you are worried about thread construction time then it can help solve that problem. Here's some example code for the manual method:
public class ReusableThread extends Thread {
private Queue<Runnable> runnables = new LinkedList<Runnable>();
private boolean running;
public void run() {
running = true;
while (running) {
Runnable r;
try {
synchronized (runnables) {
while (runnables.isEmpty()) runnables.wait();
r = runnables.poll();
}
}
catch (InterruptedException ie) {
// Ignore it
}
if (r != null) {
r.run();
}
}
}
public void stopProcessing() {
running = false;
synchronized (runnables) {
runnables.notify();
}
}
public void addTask(Runnable r) {
synchronized (runnables) {
runnables.add(r);
runnables.notify();
}
}
}
Obviously, this is just an example. It would need to have better error-handling code, and perhaps more tuning available.

If you are concerned with the overhead of creating a new Thread object then you can use executors.
import java.util.concurrent.Executor;
import java.util.concurrent.Executors;
public class Testes {
public static void main(String[] args) {
Executor executor = Executors.newSingleThreadExecutor();
executor.execute(new Testes.A());
executor.execute(new Testes.A());
executor.execute(new Testes.A());
}
public static class A implements Runnable{
public void run(){
System.out.println(Thread.currentThread().getId());
}
}
}
Running this you will see that the same thread is used for all Runnable objects.

A Thread is not a thread. A thread is an execution of your code. A Thread is an object that your program uses to create and, manage the life-cycle of, a thread.
Suppose you like playing tennis. Suppose you and your friend play a really awesome set. How would your friend react if you said, "That was incredible, let's play it again." Your friend might think you were nuts. It doesn't make sense even to talk about playing the same set again. If you play again you're playing a different set.
A thread is an execution of your code. It doesn't make sense to even talk about "re-using" a thread of execution for same reason that it makes no sense to talk about re-playing the same set in tennis. Even if another execution of your code executes all the same statements in the same order, it's still a different execution.
Andrzej Doyle's asked, "Why would you want to re-use a Thread?" Why indeed? If a Thread object represents a thread of execution---an ephemeral thing that you can't even talk about re-using---then why would you want or expect the Thread object to be re-useable?

i've been searching the same solution which you seem to be looking for, and i resolved it in this way. if you occur mousePressed Event you can terminate it also reuse it, but it need to be initialized, as you can see below.
class MouseHandler extends MouseAdapter{
public void mousePressed(MouseEvent e) {
if(th.isAlive()){
th.interrupt();
th = new Thread();
}
else{
th.start();
}
}
}

How to abort a thread in a fast and clean way in java?

Here is my problem: I've got a dialog with some parameters that the user can change (via a spinner for example). Each time one of these parameters is changed, I launch a thread to update a 3D view according to the new parameter value.
If the user changes another value (or the same value again by clicking many times on the spinner arrow) while the first thread is working, I would like to abort the first thread (and the update of the 3D view) and launch a new one with the latest parameter value.
How can I do something like that?
PS: There is no loop in the run() method of my thread, so checking for a flag is not an option: the thread updating the 3D view basically only calls a single method that is very long to execute. I can't add any flag in this method asking to abort either as I do not have access to its code.

Try interrupt() as some have said to see if it makes any difference to your thread. If not, try destroying or closing a resource that will make the thread stop. That has a chance of being a little better than trying to throw Thread.stop() at it.
If performance is tolerable, you might view each 3D update as a discrete non-interruptible event and just let it run through to conclusion, checking afterward if there's a new latest update to perform. This might make the GUI a little choppy to users, as they would be able to make five changes, then see the graphical results from how things were five changes ago, then see the result of their latest change. But depending on how long this process is, it might be tolerable, and it would avoid having to kill the thread. Design might look like this:
boolean stopFlag = false;
Object[] latestArgs = null;
public void run() {
while (!stopFlag) {
if (latestArgs != null) {
Object[] args = latestArgs;
latestArgs = null;
perform3dUpdate(args);
} else {
Thread.sleep(500);
}
}
}
public void endThread() {
stopFlag = true;
}
public void updateSettings(Object[] args) {
latestArgs = args;
}

The thread that is updating the 3D view should periodically check some flag (use a volatile boolean) to see if it should terminate. When you want to abort the thread, just set the flag. When the thread next checks the flag, it should simply break out of whatever loop it is using to update the view and return from its run method.
If you truly cannot access the code the Thread is running to have it check a flag, then there is no safe way to stop the Thread. Does this Thread ever terminate normally before your application completes? If so, what causes it to stop?
If it runs for some long period of time, and you simply must end it, you can consider using the deprecated Thread.stop() method. However, it was deprecated for a good reason. If that Thread is stopped while in the middle of some operation that leaves something in an inconsistent state or some resource not cleaned up properly, then you could be in trouble. Here's a note from the documentation:
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. Many uses of
stop should be replaced by code that
simply modifies some variable to
indicate that the target thread should
stop running. The target thread should
check this variable regularly, and
return from its run method in an
orderly fashion if the variable
indicates that it is to stop running.
If the target thread waits for long
periods (on a condition variable, for
example), the interrupt method should
be used to interrupt the wait. For
more information, see Why are
Thread.stop, Thread.suspend and
Thread.resume Deprecated?

Instead of rolling your own boolean flag, why not just use the thread interrupt mechanism already in Java threads? Depending on how the internals were implemented in the code you can't change, you may be able to abort part of its execution too.
Outer Thread:
if(oldThread.isRunning())
{
oldThread.interrupt();
// Be careful if you're doing this in response to a user
// action on the Event Thread
// Blocking the Event Dispatch Thread in Java is BAD BAD BAD
oldThread.join();
}
oldThread = new Thread(someRunnable);
oldThread.start();
Inner Runnable/Thread:
public void run()
{
// If this is all you're doing, interrupts and boolean flags may not work
callExternalMethod(args);
}
public void run()
{
while(!Thread.currentThread().isInterrupted)
{
// If you have multiple steps in here, check interrupted peridically and
// abort the while loop cleanly
}
}

Isn't this a little like asking "How can I abort a thread when no method other than Thread.stop() is available?"
Obviously, the only valid answer is Thread.stop(). Its ugly, could break things in some circumstances, can lead to memory/resource leaks, and is frowned upon by TLEJD (The League of Extraordinary Java Developers), however it can still be useful in a few cases like this. There really isn't any other method if the third party code doesn't have some close method available to it.
OTOH, sometimes there are backdoor close methods. Ie, closing an underlying stream that its working with, or some other resource that it needs to do its job. This is seldom better than just calling Thread.stop() and letting it experience a ThreadDeathException, however.

The accepted answer to this question allows you to submit batch work into a background thread. This might be a better pattern for that:
public abstract class dispatcher<T> extends Thread {
protected abstract void processItem(T work);
private List<T> workItems = new ArrayList<T>();
private boolean stopping = false;
public void submit(T work) {
synchronized(workItems) {
workItems.add(work);
workItems.notify();
}
}
public void exit() {
stopping = true;
synchronized(workItems) {
workItems.notifyAll();
}
this.join();
}
public void run() {
while(!stopping) {
T work;
synchronized(workItems) {
if (workItems.empty()) {
workItems.wait();
continue;
}
work = workItems.remove(0);
}
this.processItem(work);
}
}
}
To use this class, extend it, providing a type for T and an implementation of processItem(). Then just construct one and call start() on it.
You might consider adding an abortPending method:
public void abortPending() {
synchronized(workItems) {
workItems.clear();
}
}
for those cases where the user has skipped ahead of the rendering engine and you want to throw away the work that has been scheduled so far.

A thread will exit once it's run() method is complete, so you need some check which will make it finish the method.
You can interrupt the thread, and then have some check which would periodically check isInterrupted() and return out of the run() method.
You could also use a boolean which gets periodically checked within the thread, and makes it return if so, or put the thread inside a loop if it's doing some repetative task and it will then exit the run() method when you set the boolean. For example,
static boolean shouldExit = false;
Thread t = new Thread(new Runnable() {
public void run() {
while (!shouldExit) {
// do stuff
}
}
}).start();

Unfortunately killing a thread is inherently unsafe due to the possibilities of using resources that can be synchronized by locks and if the thread you kill currently has a lock could result in the program going into deadlock (constant attempt to grab a resource that cannot be obtained). You will have to manually check if it needs to be killed from the thread that you want to stop. Volatile will ensure checking the variable's true value rather than something that may have been stored previously. On a side note Thread.join on the exiting thread to ensure you wait until the dying thread is actually gone before you do anything rather than checking all the time.

You appear to not have any control over the thread that is rendering the screen but you do appear to have control of the spinner component. I would disable the spinner while the thread is rendering the screen. This way the user at least has some feedback relating to their actions.

I suggest that you just prevent multiple Threads by using wait and notify so that if the user changes the value many times it will only run the Thread once. If the users changes the value 10 times it will fire off the Thread at the first change and then any changes made before the Thread is done all get "rolled up" into one notification. That won't stop a Thread but there are no good ways to do that based on your description.

The solutions that purpose the usage of a boolean field are the right direction. But the field must be volatile.
The Java Language Spec says:
"For example, in the following (broken) code fragment, assume that this.done is a non-
volatile boolean field:
while (!this.done)
Thread.sleep(1000);
The compiler is free to read the field this.done just once, and reuse the cached value in each execution of the loop. This would mean that the loop would never terminate, even if another thread changed the value of this.done."
As far as I remember "Java Concurrency in Pratice" purposes to use the interrupt() and interrupted() methods of java.lang.Thread.

The way I have implemented something like this in the past is to implement a shutdown() method in my Runnable subclass which sets an instance variable called should_shutdown to true. The run() method normally does something in a loop, and will periodically check should_shutdown and when it is true, returns, or calls do_shutdown() and then returns.
You should keep a reference to the current worker thread handy, and when the user changes a value, call shutdown() on the current thread, and wait for it to shutdown. Then you can launch a new thread.
I would not recommend using Thread.stop as it was deprecated last time I checked.
Edit:
Read your comment about how your worker thread just calls another method which takes a while to run, so the above does not apply. In this case, your only real options are to try calling interrupt() and see if has any effect. If not, consider somehow manually causing the function your worker thread is calling to break. For example, it sounds like it is doing some complex rendering, so maybe destroy the canvas and cause it to throw an exception. This is not a nice solution, but as far as I can tell, this is the only way to stop a thread in suituations like this.

Since you're dealing with code you don't have access to you're probably out of luck. The standard procedure (as outlined in the other answers) is to have a flag that is checked periodically by the running thread. If the flag is set, do cleanup and exit.
Since that option is not available to you, the only other option is to force quit the running process. This used to be possible by calling Thread.stop(), but that method has been permanently deprecated for the following reason (copied from the javadocs):
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.
More info on this topic can be found here.
One absolute sure way you could accomplish your request (although this is not a very efficient way to do this) is to start a new java process via Runtime.exec() and then stopping that process as necessary via Process.destroy(). Sharing state between processes like this is not exactly trivial, however.

Instead of playing with thread starting and stopping, have you considered having the thread observe the properties that you're changing through your interface? You will at some point still want a stop condition for your thread, but this can be done this was as well. If you're a fan of MVC, this fits nicely into that sort of design
Sorry, after re-reading your question, neither this nor any of the other 'check variable' suggestions will solve your problem.

The correct answer is to not use a thread.
You should be using Executors, see the package: java.util.concurrent

Maybe this can help you: How can we kill a running thread in Java?
You can kill a particular thread by setting an external class variable.
Class Outer
{
public static flag=true;
Outer()
{
new Test().start();
}
class Test extends Thread
{
public void run()
{
while(Outer.flag)
{
//do your work here
}
}
}
}
if you want to stop the above thread, set flag variable to false. The other way to kill a thread is just registering it in ThreadGroup, then call destroy(). This way can also be used to kill similar threads by creating them as group or register with group.

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Java is a programming language and computing platform first released by Sun Microsystems in 1995.