Heisenbug: Thread doesn't run without a sysout [duplicate] - java

This question already has an answer here:
Loop doesn't see value changed by other thread without a print statement
(1 answer)
Closed 7 years ago.
I have experienced weird behaviour while coding a simple game. Basically, I create a thread with an infinite loop that fires an event in my game every couple of seconds. With the Sysout(runAnimation) in place everything works fine. However as soon as I remove it the even stops occurring.
It appears that the system.out calll is affecting the behavior of the program, does anyone have an idea why this could be happening
Here is the loop:
private void run(){
long lastTime = -1;
while(true){
int timePerStep = (int) Math.ceil(ANIM_TIME/frequency);
System.out.println(runAnimation);
if(runAnimation && System.currentTimeMillis() - lastTime > timePerStep){
lastTime = System.currentTimeMillis();
controller.step();
try {
Thread.sleep(timePerStep);
} catch (InterruptedException e) {
Thread.currentThread().interrupt();
}
}
}
}
It's started during the construction of my class as follows:
animThread = new Thread(new Runnable() {
#Override
public void run() {
GUIView.this.run();
}
});
animThread.start();

The System.out.println() call is synchronized (most PrintWriter method calls are) so I suspect that there is something that you are not synchronizing on inside of your code. I wonder about the runAnimation field. Could it be that it is set in another thread? Maybe it needs to be volatile? Any field that is modified in one thread and read in another needs to be synchronized or volatile. Reading the Java thread tutorial around synchronization may help.
Without seeing more of the code, it's hard to put a finger on it but I suspect this answer will help anyway.
Lastly, do you really want your thread to spin until runAnimation is true? That's a very bad practice. Maybe you should sleep for some time in the loop if runAnimation is false as well. Another idea is to use a CountDownLatch or other signaling mechanism to pause your thread until it needs to do the animation.

If your field is not volatile, the JIT can assume it doesn't change and place it in a register or even inline it in code. Placing a system call in this tight loop can
prevent the JIT from optimising the code this way as it cannot make assumptions about whether the thread modifies the field.
slow down the loop so it is not run 10,000 times (which is the point at which the JIT kicks in)
For more details, here is an article I wrote on the subject Java Memory Model and optimisation.

Related

is it necessary to extend thread class to use the sleep method..? [duplicate]

This question already has answers here:
What is the difference between a static method and a non-static method?
(13 answers)
Closed 4 years ago.
So I'm learning java and I kinda got confused here...
So while learning threads I learned that it is necessary to extend Thread class or implement Runnable class.
And while going through this program Thread.sleep() is used without doing any of the above process.
link:
http://www.abihitechsolutions.com/applets-mini-project-in-java-free-source-code/
Can someone explain me what is going on?
By using Thread.sleep(X) actually you are pausing execution for time X.
This is suitable for sending requests to servers or database. Nobody wants to send a huge request to DB or server in one time.
Making small portions is always reasonable. You can divide your requests and send them by waiting specified time duration.
This is one of the important usages of Thread.sleep()
There is a much proper way to handle thread pausing which is wait/notify. I suggest using it.
You can check from there;
https://docs.oracle.com/javase/tutorial/essential/concurrency/guardmeth.html
It is much better to implement Runnable than to extends Thread. Thread.sleep() is a static method so you can call it from anywhere.
class RunMe implements Runnable {
#Override
public void run() {
for (int i = 0; i < 100; i++) {
System.out.println("Hello");
try {
Thread.sleep(1000);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
}
Bear in mind that all Java code runs in a thread so calling Thread.sleep() can be done anywhere.
The reason why you can use Thread.sleep without actually creating a thread is because the main program is also running on a thread. You're simply calling sleep on the main thread. When you call Thread.sleep, java will figure out for you which thread it is actually running on.

Changing the while loops condition in another thread, nothing happens? [duplicate]

This question already has an answer here:
Loop doesn't see value changed by other thread without a print statement
(1 answer)
Closed 7 years ago.
So, the other day I made a fractal drawing program, and when it got too slow, I thought I would make it a bit faster by multithreading it. So I introduced a couple of threads to each draw a portion of the image. To know when I was done with the threads, I introduced two variables, AMOUNT_OF_THREADS and threadsDone. I created AMOUNT_OF_THREADS threads, that each drew their own partition and increased threadsDone when done. In the main method, I put in
while (threadsDone < AMOUNT_OF_THREADS)
;
It surprised me that that loop never finished looping. I knew it because each thread printed out "Thread done" and when the main method loop was done it should have printed out "Main done" but it didn't. I tried to print out threadsDone < AMOUNT_OF_THREADS and now the loop stopped. Weird.
I did some further testing and this problem seems only to occur if the thread takes >2ms to run or the loop takes >2ms each time it loops. Sometimes this bug occurs when the thread takes 2ms to run.
I suspect that the print function made the loop take >2ms to run and that made it work when I printed out the value, but I still wonder why this happens.
So my question is, why does this happen? Is it a bug in the compiler/JVM, or anything else? I asked my dad, and he replied after thinking for a few minutes that it has to be the while loop running too fast, but he wasn't sure.
Here's a little example of some code that has this problem:
public class WhileThread {
public static boolean threadDone = false;
public static void main(String[] args) {
new Thread(new Runnable() {
#Override
public void run() {
try {
Thread.sleep(10);
} catch (InterruptedException e) {
e.printStackTrace();
}
threadDone = true;
System.out.println("Thread done.");
}
}).start();
while (!threadDone) {
}
System.out.println("Main program done.");
}
}
When you change the 0 in the Thread.sleep into a 10, this bug never happens.
If it matters, I use a Macbook Air with Java SE 8.
So my question is, why does this happen? Is it a bug in the compiler/JVM, or anything else?
It's not a bug, it's a deliberate design decision. To make code like this work reliably, JVMs would have to assume that any thread could change any variable at any time unless they could prove that this couldn't happen. This would inhibit all kinds of extremely valuable optimizations and make Java code run much more slowly.
So instead, these optimizations were explicitly allowed and several methods (volatile, synchronized, and so on) were added to allow coders to show where variables could have their values changed by other threads.

Java Suitable Multithreading Implementation?

I'm very new to multithreading - I've done a few hours of research and tested a few different ways. I've recently implemented this code:
public class resetDataThread implements Runnable {
VertexHashMap allVertices;
public resetDataThread(VertexHashMap allVertices){
this.allVertices = allVertices;
}
#Override
public void run() {
for (int i = 0; i < allVertices.data.length; i++)
{
if (allVertices.data[i] != null){
allVertices.data[i].inClosed = false;
allVertices.data[i].inOpen = false;
}
}
System.out.println("Thread Finished!");
}
}
Please note, VertexHashMap is a self implemented hash map that stores Vertices.
Runnable r = new resetDataThread(allVertices);
Thread t1 = new Thread(r);
I call the method like so:
t1.run();
This line of code gets called many times without creating new instances of r or t1.
My reason for implementing this is so my program can fetch new data and output results while it resets the storage's boolean variables (allVertices). My main aim is execution speed of my program, so I figured I could multi thread this section of code.
So my question is, is this safe, suitable or a good way to do this?
Example
...code that changes allVertices variables
t1.run(); //this will run while otherSectionOfProgram and yetAnotherSectionOfProgram is executed
otherSectionOfProgram();
yetAnotherSectionOfProgram();
The example above can be called many times however the contents of allVertices will always be the same (except for the boolean settings), so I'm just wanting to make sure it's okay to use the run() method the way I have - as I want to ensure I practise good java/programming.
If what you're after is to have a single background thread running then call t1.start() to start a background thread.
If you want to get more advanced with creating threads look into the Executor Service for java Java Executor Service
It's also worth noting that unless allVertices is pretty large in size it shouldn't take much time to run the run() method anyway, but it's easy enough to time it to see how much difference it makes.
EDIT:
Also make sure otherSectionOfProgram(); and yetAnotherSectionOfProgram(); do not modify allVertices if you aren't creating a new instance of that because your background thread will be operating on it as well.
t1.run();
This line of code gets called many times
Call start() on your thread, not run().
What's the difference between Thread start() and Runnable run()
You start created threads with t.start() method, t.run() won't start a new thread.
Also, calling t.start() multiple times still won't create a new thread each time it's called, your thread t is unusable(dead) after it's done once. You need to create new Thread for every ... well... thread.
For starting new thread you should call start() method.
For program safety, you should take care about mutual exclusion, I should know logic of program to advise you more about the correct solution, but you may want synchronize allVertices if another sections of program may change it simultaneously or if you want to create more than one instance of your thread:
#Override
public void run() {
synchronized(allVertices){
for (int i = 0; i < allVertices.data.length; i++)
{
if (allVertices.data[i] != null){
allVertices.data[i].inClosed = false;
allVertices.data[i].inOpen = false;
}
}
}
System.out.println("Thread Finished!");
}
OK, first of all, calling Thread.run() will not start a new Thread. It will run the method on the calling thread. To actually start a new thread you have to use Thread.start(). In the code you posted there is no multithreading happening.
Secondly, the line VertexHashMap is a self implemented hash map that stores Vertices. screams non-thread-safe to me. You know there is a special ConcurrentHashMap version of HashMap to deal with concurrent (i.e. multithreaded) use? Which structure did you base your class on? Did you make your own objects thread-safe? Inter-thread visibility problems may arise.
Thirdly, reusing threads is tricky business. Just calling run multiple times won't even make a single Thread, but calling start more than once will throw an exception. As some of the other answers pointed out - using an ExecutorService is a great alternative - you can set it up to use whatever number of threads you want (and have flexibility in how it behaves under load), and have it reuse its threads (instead of re-inventing the complex machinery behind it).
Finally - you may want to think about visibility of your objects. In your case you have the VertexHashMap as default visibility. This means that other classes can see and use it directly. This may be a problem if, for example, you are in the process of clearing it, or re-populating. Then the user gets a half-ready object, whose methods may easily fail. Java has a Future interface just for that - it's an interface to be used with multithreading that "promises" to return a ready object. It's the basis for Swings SwingWorker and Androids AsyncTask - objects that do tasks in the background thread, and then return ready results to the foreground for further use.

Do I have to use thread.interrupted()?

I am writing a GUI for a program that takes some inputs and runs an algorithm on them. The code for the algorithm is fairly long and complex so I have just been launching a new thread from the GUI in order to perform the computations on the inputs.
//algorithmThread previously initialized
if(e.getSource() == startButton) {
if(update.updateStrings(textFields)) {
algorithmThread.start();
}
}
We want to add functionality that will allow the user to stop the computation (it runs for about half an hour on my laptop before producing a result) in the case that they have provided the wrong input files. This is how I am handling that.
else if(e.getSource() == stopButton) {
//if the user presses the stop button then intterupt the
//thread running the algorithm
algorithmThread.interrupt();
System.out.println("algorithm stopped"); //debugging code
//recreate the thread in case the user hits the start button again
algorithmThread = new Thread() {
public void run() {
runNOC();
}
};
}
The program does successfully stop the algorithm(although I think I should do some exception handling), allow the user to enter new input, and restart. My question is, under what conditions would I have to check Thread.interrupted() in the code for the algorithm? Is it necessary/best practice? Or is it acceptable to stop a thread in the manner illustrated above?
All the Thread.interrupt() method does is set an "interrupted" flag, so stopping a thread in this manner requires its cooperation. For example, the algorithm should poll the interrupt status every once in a while, for example once per iteration. You can see an example of this in the Java concurrency tutorial.
Since you are working with a GUI, you may find it easier to run the background thread using a SwingWorker. This class has many features convenient for GUI programming, like updating the GUI when the computation has finished using the done method and canceling the computation without using Thread.interrupt(). Canceling in this manner still requires cooperation from the thread, but is safer because interrupting a thread causes an InterruptedException to be thrown in some situations, such as when the thread is sleeping in Thread.sleep or waiting on a lock in Object.wait.
interrupt is not always evil following this thread:
Is Thread.interrupt() evil?
Around here, we use this method in one specific place: handling
InterruptedExceptions. That may seem a little strange but here's what
it looks like in code:
try {
// Some code that might throw an InterruptedException.
// Using sleep as an example
Thread.sleep(10000);
}
catch (InterruptedException ie) {
System.err.println("Interrupted in our long run. Stopping.");
Thread.currentThread().interrupt();
}
This does two things for us:
It avoids eating the interrupt exception. IDE auto-exception handlers
always provide you with something like ie.printStackTrace(); and a
jaunty "TODO: Something useful needs to go here!" comment.
It restores
the interrupt status without forcing a checked exception on this
method. If the method signature that you're implementing does not have
a throws InterruptedException clause, this is your other option for
propagating that interrupted status.

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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