From the book "Effective java" i have following famous code of stopping one thread from another
public class StopThread {
private static boolean stopRequested;
private static synchronized void requestStop() {
stopRequested = true;
}
private static synchronized boolean stopRequested() {
return stopRequested;
}
public static void main(String[] args)
throws InterruptedException {
Thread backgroundThread = new Thread(new Runnable() {
public void run() {
int i = 0;
while (!stopRequested()) {
i++;
}
}
});
backgroundThread.start();
TimeUnit.SECONDS.sleep(1);
requestStop();
}
}
A line is written there that is "synchronization has no effect unless both read and write
operations are synchronized."But it is clear that if we don't use synchronized keyword with method requestStop the code will work fine,i.e,it terminates nearly after 1 second which is desired.One thing more here is that if we don't synchronize both the method we will(most probably) go into infinite loop because of code optimization.So my questions are:-
1.How and in what scenario things can go wrong if we don't synchronize 'stopRequested' method?Although here if we don't synchronize it,the program runs as desired,i.e,it terminates nearly in 1sec.
2.Does synchronized keyword enforces the VM to stop optimization each time?
1.How and in what scenario things can go wrong if we don't synchronize 'stopRequested' method?Although here if we don't synchronize it,the program runs as desired,i.e,it terminates nearly in 1sec.
Things can go wrong if JVM decides to optimize the code within the run method of your backgroundThread. The read of stopRequested() can be reordered for optimization by JVM because of which it may never call the stopRequested() method again. But these days almost all JVM implementations take care of this and hence without making stopRequested as synchronized your code will still run fine. Also point to be noted here is that if you donot make stopRequested synchronized then the change done to stopRequested boolean variable may not be seen immediately by other non synchronized threads. Only if you used synchronization can the other threads immediately detect any change as an entry into synchronized method clears the cache and loads the data from the memory fresh. This immediate detection of memory changes is important in a highly concurrent system
2) Does synchronized keyword enforces the VM to stop optimization each time?
Synchronized keyword doesnot enforce VM to stop optimization but it makes it to adhere to the things listed below. VM can still do an optimization but it has to take care of the below things.
Synchronization effectively does the following things:-
It guarantees happens before relationship. If one action happens-before another, then the first is visible to and ordered before the second.
It guarantees memory visibility that is all the modifications done within the block which may be cached are immediately flushed before the exit of synchronization block which results in any other synchronized thread to see the memory updates immediately. This will be important in case of highly concurrent systems.
Changes by a thread to a variable are not necessarily seen right away by other threads. Using synchronized here makes sure that the update by one thread is visible to the other thread.
1) The change would possibly not become visible to the other thread. In the absence of synchronization or volatile or atomic fields there's no assurance when the other thread will see the change.
2) The synchronized keyword helps the VM decide on limits on instruction reordering and on what the VM can optimize.
Testing this on your machine will not necessarily display the same results as using a server with more processors. Different platforms may do more optimizing. So just because it works on your machine doesn't necessarily mean it's ok.
1.How and in what scenario things can go wrong if we don't synchronize 'stopRequested' method?
Assume if one thread is writing (updating) the field stopRequested, now before the first thread updates the value of stopRequested from requestStop(), another thread can read the value of stopRequested by calling stopRequested() (if stopRequested() was not synchronized. Thus it would not get the updated value.
2.Does synchronized keyword enforces the VM to stop optimization each time?
Not always, Escape Analysis implemented from JDK6U23 also plays a part in this.
Synchronization creates a memory barrier which ensures a happens-before relationship. i.e, any block of code executed after a synchronized block is sure to have the updated value (changes made earlier are reflected).
Statements can be executed out-of-order within a synchronized block to improve efficiency provided the happens-before holds good. On the other hand a synchronized block can be removed by the JVM if it determines that the block can be accessed only by a single thread.
Just make stopRequested volatile. Then method stopRequest does not have to be synchronized, because it does not change anything.
I am using the Bluetooth on Android; the short of it is that I only want to open a new Thread for receiving if the socket is currently not connected - I am testing this using a boolean.
So:
class Main {
protected boolean mConnected;
public void startClientConnection() {
ClientRunnable thread = new ClientRunnable() {
#Override public void manageSocket(BluetoothSocket pSocket) {
synchronized (this) {
if (!mConnected) openReadingThread(pSocket);
} // end synchronized()
} // end manageSocket()
}; // end ClientRunnable
} // end startClientConnection()
} // End CLASS
Edited:
Essentially, what I need to know is, because the Runnable will be run on a separate Thread, but the mConnected variable will only changed in the main Thread, does it need to be synchronized.
in this case, each thread you create has its own lock (the object itself)
you should use something static (like a class) for the lock.
The simple answer: mConnected must be accessed only within a synchronized block. The code you show is good, but the code where you change the value must also be in a synch block. The reason for this is that otherwize Java is under no obligation to let one thread see changes made by the other. In this case, you don't really need the synch blocks to synchronize anything, just to force each thread to see the other's changes.
Less simple: This is almost a good case for making mConnected volatile. You could then skip the synchronized block. But then it would be possible to get two threads in openReadingThread at the same time.
So keep the synch block in your example (which is more for openReadingThread than for mConnected), and either use another when you set mConnected or make mConnected volatile. Volatile fields are expensive, but then so are synch blocks, particularly when you don't need to synchronize but only make a field visible across threads. I'd say if changes are few and you go through the code above a lot, skip the volatile and use a second synch block. But if you change the value a lot, go with a volatile mConnected and only one synch block (the one in your example, which is now needed only for the method, not for mConnected).
Addtional: I was about to tell Philipp Sander that he was wrong about the "this" but, on second glance, he's not; it needs fixing.
I have two blocks of code, one waits for the other to notify it.
synchronized(this) {
wait();
}
and
while(condition) {
//do stuff
synchronized(this) {
notify();
}
}
Weirdly enough that didn't wait for the notify while this did:
synchronized(objectLock) {
objectLock.wait();
}
and
while(condition) {
//do stuff
synchronized(objectLock) {
objectLock.notify();
}
}
I'm very curious about the difference of both sets, and why the first one worked while the other didn't. Note that the two blocks reside in two different threads on two different methods (if that helps).
I hope someone could explain why this is so. I edited my question so it would be more detailed.
It didn't work because you synchronized on this which in two different threads pointed to two different Thread objects.
Synchronization with wait() and notify() would only work properly when you synchronize on the same object for locking like the objectLock that you used later on.
EDIT:
If the two thread instances belonged to the same MyThread class then to achieve the effect that you thought you're code was having, you would have to acquire a lock on their class object itself:
synchronized(MyThread.class)
You can use any object you like. However, it is generally clearer to other programmers to see an explicit lock object.
My wild guess as to why this didn't work for you is you had a different this in scope. (ie, in an anonymous function/callback). You can be explicit about which this to use by appending the class name, eg, WonderClass.this - again a reason why this is not as clear. (edit: actually WhateverClass.this won't help you if this really is a different instance)
Also do read this: http://docs.oracle.com/javase/tutorial/essential/concurrency/locksync.html - I generally find it easier to put all the thread-unsafe code into small synchronized methods (which do an implict lock on this)
When you say the two blocks reside in two different threads that makes me think they're not locking on the same object because this is not the same thing. When you name an explicit lock you're using the same thing to lock on.
By the way you should call wait in a loop, like this:
synchronized(someLock) {
while (!someCondition) {
wait();
}
// now the thread has the lock and it can do things
// knowing for sure that someCondition is true
}
Without this you will be vulnerable to spurious wakeups (not all notifications come from your application code) and the order in which wait and notify are called becomes problematic (if you have two threads and one notifies before the other waits then that notification never gets seen).
I'd advise using the Monitor pattern (http://en.wikipedia.org/wiki/Monitor_(synchronization)) anyway, that could save you from errors later on, especially as your use case gets more complex:
class Monitor
{
/** Initialised to `false` by default in Java. */
boolean condition;
synchronized void waitForSomething()
{
while(!condition)
{
wait();
}
}
synchronized void signal()
{
condition = true;
notify();
}
}
That way everything is nicely encapsulated and protected (I don't usually use private modifiers in examples, but you might want to enforce additional "privacy" in your code, specifically making the condition private.)
As you can observe, in my condition loop there is wait() call, as opposed to your example where you have notify() in the loop instead. In most use cases doing what you did with notify is a mistake, although I can't speak for your particular case as you didn't provide us with enough details. I am willing to bet yours is the typical one though, for which the Monitor pattern applies beautifully.
The usage scenario is along the following: thread that wants to wait for something calls waitForSomething and another thread may cause it to continue by invoking signal method which will set the condition flag.
While reviewing this question I noticed this code:
class MyThread extends Thread {
private boolean stop = false;
public void run() {
while(!stop) {
doSomeWork();
}
}
public void setStop() {
this.stop = true;
}
}
However I don't understand why would this fail. Do other threads not get access to the "actual" stop variable?
The JIT compiler can re-order reads and writes in an application so long as
the actions are sequentially consistent and
the altered actions do not violate intra-thread semantics.
That is just a fancy way of saying, all actions should appear to happen the same way as if it were executed by only a single thread. So you can get the JIT to recompile your code to look like this
class MyThread extends Thread {
private boolean stop = false;
public void run() {
if(!stop){
while(true){
}
}
}
This is a legal optimization called hoisting. It still acts the same as if serial but offers surprising results when using multiple threads.
By declaring a field volatile you are telling Java not to execute any re orderings. Along with the memory consistency as mentioned by Nathan Hughes
The instance variable stop needs to be volatile, otherwise there's no guarantee the other threads will see changes to it. There are a lot of conflicting interests at work: threads want a consistent view of the program state, CPUs want to be able to cache data, the JVM wants to be able to reorder instructions. Making the instance variable volatile means that it can't be cached and that happens-before relationships are established that limit instruction reordering.
See this other answer (+1) for a good example of what reordering may happen without marking the variable volatile.
(By the way using interruption for thread cancellation is preferable to using an instance variable.)
The variable stop must be declared as volatile.
Although i prefer using interrupt to stop a thread.
Other threads are not guaranteed to see updated values of stop - you need to establish a "happens before" relationship. The simplest way would be to make stop volatile.
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.