ScheduledThreadPoolExecutor (which implements ScheduledExecutorService) seems to be only running the SwingWorker class once when using the ScheduleAtFixedRate method. The original code is kinda long, so I made a new code that produces the same results below.
import java.util.concurrent.ScheduledThreadPoolExecutor;
import java.util.concurrent.TimeUnit;
import javax.swing.SwingUtilities;
import javax.swing.SwingWorker;
public class ScheduledThreadPoolExecutorTest extends SwingWorker<Void, Void>{
#Override
protected Void doInBackground() {
System.out.println("Yay!");
return null;
}
#Override
protected void done() {
try {
get();
} catch(Exception e) {
e.printStackTrace();
}
System.out.println("Woohoo!");
}
public static void main(String[] args) {
SwingUtilities.invokeLater(new Runnable() {
#Override
public void run() {
ScheduledThreadPoolExecutor executor = new ScheduledThreadPoolExecutor(1);
executor.scheduleAtFixedRate(new ScheduledThreadPoolExecutorTest(), 0, 30, TimeUnit.MILLISECONDS);
}
});
}
}
This yields the results:
Yay!
Woohoo!
Why is ScheduledThreadPoolExecutor running SwingWorker only once? And what can I do to make the SwingWorker run every 30 milliseconds as indicated in the code?
While SwingWorker does implement the Runnable interface, per its API section on the doInBackground() method:
Note that this method is executed only once.
So while its inner run() method may repeatedly run, the doInBackground() will only run once. Not only that, but the run() method is marked final within SwingWorker, and so you can't override it to call doInBackground multiple times.
A better solution is not use a SwingWorker at all but rather a simpler Runnable-derived class.
SwingWorker extends Runnable, however, it uses FutureTask to run its computation.
From the javadoc:
A cancellable asynchronous computation. This class provides a base
implementation of {#link Future}, with methods to start and cancel
a computation, query to see if the computation is complete, and
retrieve the result of the computation. The result can only be
retrieved when the computation has completed; the {#code get}
methods will block if the computation has not yet completed. Once
the computation has completed, the computation cannot be restarted
or cancelled (unless the computation is invoked using
{#link #runAndReset}).
That is, the FutureTask will run only once, if you try to run it again, it will simply return.
public void run() {
if (state != NEW ||
!UNSAFE.compareAndSwapObject(this, runnerOffset,
null, Thread.currentThread()))
return;
try {
Callable<V> c = callable;
if (c != null && state == NEW) {
V result;
boolean ran;
try {
result = c.call();
ran = true;
} catch (Throwable ex) {
result = null;
ran = false;
setException(ex);
}
if (ran)
set(result);
}
} finally {
// runner must be non-null until state is settled to
// prevent concurrent calls to run()
runner = null;
// state must be re-read after nulling runner to prevent
// leaked interrupts
int s = state;
if (s >= INTERRUPTING)
handlePossibleCancellationInterrupt(s);
}
}
Related
I create a Callable which should make a syncExec call. I delegate the Callable to a subclass of RecursiveTask (ForkJoinPool) which executes the call method of the Callable. The problem is that the code inside the run method is never reached. Do you know why and how to fix that?
public class someClass{
public static void main (String[] args){
Callable<Object> c = new Callable<Object>() {
#Override
public Object call() throws Exception {
PlatformUI.getWorkbench().getDisplay().syncExec(new Runnable() {
#Override
public void run() {
System.out.println("hi");
}
});
return null;
}
});
ATLockTask task = new ATLockTask();
task.setCallable(c);
ForkJoinPool pool = new ForkJoinPool();
pool.invoke(task);
}
}
public class ATLockTask extends RecursiveTask<Object[]>{
Callable callable;
#Override
protected Object[] compute() {
try {
callable.call();
} catch (Exception e) {
e.printStackTrace();
}
return null;
}
}
ForkJoinPool.invoke blocks the current thread until the given task has completed. Display.syncExec waits until the SWT UI thread executes Display.readAndDispatch so it will wait forever because ForkJoinPool.invoke is blocking the UI thread.
Use ForkJoinPool.execute to start the task without blocking and the code works.
I have a long running Runnable object and I wanted to provide a more graceful interrupt mechanism than having to call interrupt on the thread the object is running on.
The before code:
public class MyRunnable implements Runnable {
public void run() {
while(!Thread.currentThread().isInterrupted()) {
//do stuff
}
}
}
public class MyClass {
public static void main(String[] args) {
Runnable myRunnable = new MyRunnable();
Thread t = new Thread(myRunnable, "myRunnableThread");
t.start();
//do stuff
t.interrupt();
//do stuff
}
}
And the new code:
public class MyRunnable implements Runnable {
private Thread myThread = null;
public void run() {
myThread = Thread.currentThread();
while(!myThread.isInterrupted()) {
//do stuff
}
}
public void shutdown() {
if (myThread != null) {
myThread.interrupt();
//do other shutdown stuff
}
}
}
public class MyClass {
public static void main(String[] args) {
Runnable myRunnable = new MyRunnable();
Thread t = new Thread(myRunnable, "myRunnableThread");
t.start();
//do stuff
myRunnable.shutdown();
//do stuff
}
}
My question is, are there possible side effects or unknowns that holding a reference to your own thread, and providing limited access to that thread through public methods (as above) could cause? This is assuming that no-one ever calls the run() method directly, that it is always started from a new thread.
And I'm aware that I could use a volatile or atomic Boolean in the run() and shutdown() methods for communicating intent to shutdown, I'm more interested in learning than a solution. But solutions are still welcome!
For me your first approach is much better as less error prone and more "standard". But actually what you try to implement already exists (which proves that it makes sense and that it is not a bad practice but it is not easy to make it properly), it is called FutureTask, instead of shutdown you have cancel(boolean mayInterruptIfRunning) with true as value of mayInterruptIfRunning if you want to interrupt the thread running the task, I quote the javadoc:
Attempts to cancel execution of this task. This attempt will fail if
the task has already completed, has already been cancelled, or could
not be cancelled for some other reason. If successful, and this task
has not started when cancel is called, this task should never run. If
the task has already started, then the mayInterruptIfRunning
parameter determines whether the thread executing this task should be
interrupted in an attempt to stop the task.
For example:
// Task that will only sleep for 1 sec and print a message on interrupted
FutureTask<Void> myRunnable = new FutureTask<>(
new Callable<Void>() {
#Override
public Void call() throws Exception {
try {
System.out.println("Sleep");
Thread.sleep(1_000L);
} catch (InterruptedException e) {
System.out.println("Interrupted !!!");
throw e;
}
return null;
}
}
);
new Thread(myRunnable, "myRunnableThread").start();
// Wait long enough to make sure that myRunnableThread is sleeping
Thread.sleep(500L);
// Cancel the task and interrupt myRunnableThread
myRunnable.cancel(true);
Output:
Sleep
Interrupted !!!
It already has a reference:
Thread.currentThread()
From the javadoc:
Returns a reference to the currently executing thread object.
I'm trying to do multiple heavy calculations using threads.
Then I need to do something with the results after making sure all threads have finished its job.
Here's the basic code:
private class Runner implements Runnable {
private String _result = "fail";
public String get_results() {
return _result;
}
public void run() {
_result = "do complex calculation";
}
}
public void test() {
List<Thread> threads = new ArrayList<Thread>();
List<Runner> threadObjects = new ArrayList<Runner>();
for (int i = 0; i < 10; i++) {
Runner runner = new Runner();
Thread t = new Thread(runner);
t.start();
threads.add(t);
threadObjects.add(runner);
}
for (Thread thread : threads) {
try {
thread.join();
} catch (InterruptedException ex) {
}
}
for (Runner threadObject : threadObjects) {
System.out.println(threadObject.get_results());
}
}
My question is, is above snippet a common or a good approach to get calculation value?
If not please suggest a better ones.
Also sometimes I got runner.get_results() reponse = "fail", it seems calculation does not processed at all.
Thanks
You can use an ExecutorService such as the ScheduledThreadPoolExecutor;
ExecutorService executor = new ScheduledThreadPoolExecutor(numOfThreads);
With numOfThreads being the number of threads you want sitting in the thread pool.
You can then use the submit(Callable<T> task) method provided by the ScheduledThreadPoolExecutor class to execute the calculation.
You would then have a Callable implementation of your calculation and pass it to the submit() method in ExecutorService to execute the calculation;
class Calculation implements Callable {
#Override
public Object call() throws Exception { // The signature can be changed to return a different type (explained at the end)
return "do complex calculation";
}
}
As we can see from the method signature of the submit(Callable<T> task) method it returns a Future.
public <T> Future<T> submit(Callable<T> task)
The get() method of the Future class will return the result upon successful completion. This would ensure that your calculation completed before reading it.
A further note on the return type of the call() method;
Although this returns Object there is nothing stopping you changing the type of object it returns (this is known as co-variant returns)
For example the following is perfectly legal:
#Override
public String call() throws Exception {
return "do complex calculation";
}
I have a class which processes something. I'm trying to run a number of instances of this class in parallel.
However, I'm not sure if in TaskManager.startAll(), when I call r.go(), whether this would cause r to start running in its own thread, or within the main thread?
The total execution time that I'm getting seems to be very high, and despite my attempts at optimizing, nothing seems to be having any effect. Also, if I run a profiler on my project in Netbeans, it shows all the threads as sleeping. So I'd like to know if I'm doing something wrong?
This is the structure of the class:
public class TaskRunner implements Runnable {
private boolean isRunning = false;
public void run() {
while(true) {
while (! running) {
try {
Thread.sleep(1);
} catch (Exception e) {
e.printStackTrace();
}
}
process();
}
}
public void go() {
isRunning = true;
}
public void stop() {
isRunning = false;
}
private void process() {
//Do some number crunching and processing here
}
}
Here's how these are being run / managed:
public class TaskManager {
private ArrayList<TaskRunner> runners = new ArrayList<>();
public TaskManager() {
for (int i = 0; i < 10; i++) {
TaskRunner r = new TaskRunner();
new Thread(r).start();
runners.add(r);
}
}
public void startAll() {
for (TaskRunner r : runners) {
r.go();
}
}
}
Indeed, you are not "doing it right." If you want to create a multi-threaded Java application, the place to start is with the java.util.concurrent package.
It appears from your code that you want to run ten tasks in parallel. I assume that after "number crunching and processing," you'll want to aggregate the results and do something with them in the main thread. For this, the invokeAll() method of ExecutorService works well.
First, implement Callable to do the work you show in your process() method.
final class YourTask implements Callable<YourResults> {
private final YourInput input;
YourTask(YourInput input) {
this.input = input;
}
#Override
public YourResults call()
throws Exception
{
/* Do some number crunching and processing here. */
return new YourResults(...);
}
}
Then create your tasks and run them. This would take the place of your main() method:
Collection<Callable<YourResults>> tasks = new List<>(inputs.size());
for (YourInput i : inputs)
tasks.add(new YourTask(i));
ExecutorService workers = Executors.newFixedThreadPool(10);
/* The next call blocks while the worker threads complete all tasks. */
List<Future<YourResult>> results = workers.invokeAll(tasks);
workers.shutdown();
for (Future<YourResult> f : results) {
YourResult r = f.get();
/* Do whatever it is you do with the results. */
...
}
However, I'm not sure if in TaskManager.startAll(), when I call r.go(), whether this would cause r to start running in its own thread, or within the main thread?
So my first comment is that you should make isRunning be volatile since it is being shared between threads. If the threads are not starting when it goes to true (or seem to be delayed in starting) then I suspect that's your problem. volatile provides memory synchronization between the threads so the thread that calls go() and makes a change to isRunning will be seen immediately by the thread waiting for the change.
Instead of spinning like this, I would use wait/notify:
// this synchronizes on the instance of `TaskRunner`
synchronized (this) {
// always do your wait in a while loop to protect against spurious wakeups
while (!isRunning && !Thread.currentThread().isInterrupted()) {
try {
// wait until the notify is called on this object
this.wait();
} catch (InterruptedException e) {
Thread.currentThread().interrupt();
e.printStackTrace();
}
}
Then in the go() method you should do the following. stop() would be similar.
public void go() {
synchronized (this) {
isRunning = true;
this.notifyAll();
}
}
Notice that you should handle thread interrupts carefully. Test for isInterrupted() in the while running loop and re-interrupt a thread when InterruptedException is thrown is always a good pattern.
The total execution time that I'm getting seems to be very high, and despite my attempts at optimizing, nothing seems to be having any effect. Also, if I run a profiler on my project in Netbeans, it shows all the threads as sleeping.
So although the threads are mostly sleeping, they are still each looping 1000 times a second because of your Thread.sleep(1). If you increased the time sleeping (after making isRunning be volatile) they would loop less but the right mechanism is to use the wait/notify to signal the thread.
Awful solution, terrible. first I highly recommend you start reading some tutorial like [this]
Second, if threads should wait for a signal to go for some job, so why just don't you wait them!!!!!, something like this
import java.util.ArrayList;
public class TaskManager
{
//////////////////////
public volatile static Signal wait=new Signal();
//////////////////////
private ArrayList<TaskRunner> runners = new ArrayList<>();
public TaskManager()
{
for (int i = 0; i < 10; i++)
{
TaskRunner r = new TaskRunner();
new Thread(r).start();
runners.add(r);
}
try {
Thread.sleep(1000);
startAll();
Thread.sleep(1000);
pauseAll();
Thread.sleep(1000);
startAll();
Thread.sleep(1000);
haltAll();System.out.println("DONE!");
}catch(Exception ex){}
}
public void startAll()
{
synchronized(wait){
wait.setRun(true);;
wait.notifyAll();
}
}
public void pauseAll(){
wait.setRun(false);
}
public void haltAll(){
for(TaskRunner tx:runners){tx.halt();}
}
public static void main(String[] args) {
new TaskManager();
}
}
class TaskRunner implements Runnable
{
private Thread thisThread;
private volatile boolean run=true;
public void run()
{
thisThread=Thread.currentThread();
while(run){
if(!TaskManager.wait.isRun()){
synchronized(TaskManager.wait)
{
if(!TaskManager.wait.isRun()){
System.out.println("Wait!...");
try
{
TaskManager.wait.wait();
}
catch (Exception e)
{
e.printStackTrace();
break;
}
}
}}
process();
}
}
private double r=Math.random();
private void process(){System.out.println(r);try {
Thread.sleep(10);
} catch (Exception e) {
// TODO: handle exception
}}
public void halt(){run=false;thisThread.interrupt();}
}
class Signal{
private boolean run=false;
public boolean isRun() {
return run;
}
public void setRun(boolean run) {
this.run = run;
}
}
in above sample, all runners works till the Signal run boolean is true, and simple TaskManager class set tit as false for every time it needs to pause the threads. and about the halt, it just set the shutdown(run) flag to false, and also interrupt the thread because of if thread is in wait state.
I hope I could prove your solution is like dream-on story, and also could explained enough about my solution.
have a good parallel application :)
Is there a way to wait for all Runnables submitted to the SWT UI Thread via asyncExec(...) to finish?
Background:
I have a long-running operation, which among other things is triggering events that in turn submit Runnables to the SWT UI thread via the asyncExec(...) instance method of Display.
The progress of the long-running operation is shown in a ProgressMonitorDialog, and I would like to close the dialog only after the UI thread has finished executing the Runnables.
Changing the calls from asyncExec(...) to syncExec(...) is not an option, as the latter is not desired when the events are triggered from other contexts.
org.eclipse.swt.widgets.Display.readAndDispatch() will process an event from the event queue and return false if there are no more events to process. But you probably don't want to use this as it processes an event.
asyncExec(*) is a FIFO queue (although OS graphics events supersede the asyncExecs), so you could do most of your long-running op processing and then place a final asyncExec in the queue:
final boolean[] done = new boolean[1];
Runnable r = new Runnable() {
public void run() {
done[0] = true;
}
};
// now wait for the event somehow. The brute force method:
while (!done[0]) {
Thread.sleep(200);
}
In theory, all of the other asyncExecs spawned from your long running op will be finished by the time you get to the last one.
EDIT: potential other option
Create your own org.eclipse.core.runtime.jobs.Job and then join() it at the end:
public static class RefCountJob extends Job {
public RefCountJob() {
super("REF_COUNT");
}
int count = 0;
public void increment() {
count++;
}
public void decrement() {
count--;
}
#Override
protected IStatus run(IProgressMonitor monitor) {
monitor.beginTask("WAITING", IProgressMonitor.UNKNOWN);
while (count > 0) {
Thread.sleep(200);
monitor.worked(1);
}
monitor.done();
return Status.OK_STATUS;
}
}
To use it, increment() it every time you are going to fire off events, and have them decrement it when they're done (You have to make sure they decrement it no matter what exception is thrown :-)
RefCountJob ref = new RefCountJob();
// ... do stuff, everybody increments and decrements ref
ref.increment();
// ... do more stuff
ref.increment();
// at the end of your long-running job
ref.schedule();
ref.join();
Thanks, I ended up with the following. I think it is a pretty clean solution. By the way I would upvote your answer if I had enough reputation for that :)
public class SWTThreadingUtils
{
public static void waitForAsyncExecsToFinish(Display display)
{
Object waitObj = new Object();
display.asyncExec(new DummyRunnable(waitObj));
synchronized (waitObj)
{
try {
waitObj.wait();
} catch (InterruptedException e) {
throw new RuntimeException(e);
}
}
}
private static class DummyRunnable implements Runnable
{
private Object waitObj;
public DummyRunnable(Object waitObj)
{
this.waitObj = waitObj;
}
#Override
public void run()
{
synchronized (waitObj)
{
waitObj.notify();
}
}
}
}