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• How to detect that the internet connection has got disconnected through a java desktop application?

  - by Yatendra Goel

  I am developing a Java Desktop Application that access internet. It is a multi-threaded application, each thread do the same work (means each thread is an instance of same Thread class). Now, as all the threads need internet connection to be active, there should be some mechanism that detects whether an internet connection is active or not. Q1. How to detect whether the internet connection is active or not? Q2. Where to implement this internet-status-check-mechanism code? Should I start a separate thread for checking internet status regularly and notifies all the threads when the status changes from one state to another? Or should I let each thread check for the internet-status itself? Q3. This issue should be a very common issue as every application accessing an internet should deal with this problem. So how other developers usually deal with this problem? Q4. If you could give me a reference to a good demo application that addresses this issue then it would greatly help me.

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• C# parameter count mismatch when trying to add AsyncCallback into BeginInvoke()

  - by PunX

  I have main form (PrenosForm) and I am trying to run Form2 asynchronously. It works without callback delegate: this.BeginInvoke(cp, new object[] { datoteke, this.treeView1.SelectedNode.FullPath.ToString(), this, efekt }, null); //works 1. Doesn't work with callback delegate (parameter count mismatch): this.BeginInvoke(cp, new object[] { datoteke, this.treeView1.SelectedNode.FullPath.ToString(), this, efekt }, new AsyncCallback(callBackDelegate), null); //doesn't work parameter count mismatch 2. Works with callback delegate if I do it like this: cp.BeginInvoke(datoteke, this.treeView1.SelectedNode.FullPath.ToString(), this, efekt, new AsyncCallback(callBackDelegate), null); //works 3. My question is why does one way work and the other doesn't? I'm new at this. Would anyone be so kind as to answer my question and point out my mistakes? private delegate void copyDelegat(List<ListViewItem> datoteke, string path, PrenosForm forma, DragDropEffects efekt); private delegate void callBackDelegat(IAsyncResult a); public void doCopy(List<ListViewItem> datoteke, string path, PrenosForm forma, DragDropEffects efekt) { new Form2(datoteke, path, forma, efekt); } public void callBackFunc(IAsyncResult a) { AsyncResult res = a.AsyncState as AsyncResult; copyDelegat delegat = res.AsyncDelegate as copyDelegat; delegat.EndInvoke(a); } public void kopiraj(List<ListViewItem> datoteke, DragDropEffects efekt) { copyDelegat cp = new copyDelegat(doCopy); callBackDelegat callBackDelegate = new callBackDelegat(callBackFunc); this.BeginInvoke(cp, new object[] { datoteke, this.treeView1.SelectedNode.FullPath.ToString(), this, efekt }, new AsyncCallback(callBackDelegate), null); //doesn't work parameter count missmatch 2. this.BeginInvoke(cp, new object[] { datoteke, this.treeView1.SelectedNode.FullPath.ToString(), this, efekt }, null); //works 1. cp.BeginInvoke(datoteke, this.treeView1.SelectedNode.FullPath.ToString(), this, efekt, new AsyncCallback(callBackDelegate), null); //works 3. }

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• uh-oh windows mobile threading issues!

  - by violet313

  specifically WM6x, winCE5x Now my current understanding from trawling the msdn etal is that the IMAPIAdviseSink::OnNotify callback can be made from any old thread; from (ce)mapi or perhaps even from a third-party service provider. Under WM6x, i cannot seem to coax an in-thread response by invoking HrThisThreadAdviseSink, since while this function is declared in mapiutil.h, a definition appears not to exist (in cemapi.lib or wherever??) ~But i notice that all the OnNotify callbacks i get, derive from windows messages that i am receiving on my thread (=looks to me like an in-thread implementation in any case under cemapi)... So, can anyone confirm that this is infact always the case -or am i just getting lucky right now? ah, i should add that my advise source is IMAPISession::Advise (ActiveSync) erm i should also say that i might have cross-posted this on the msdn forum -but they're mostly numptys over there,,

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• Does UIActivityIndicator require manual threading on iPhone

  - by Akusete

  I am running creating an iPhone application which performs a costly operation and I wanted to create an activityIndicator to let the user know the application has not frozen. The operation is performed entirely in one event call... so there is no chance for the UI framework to receive control to actually display and animate this indicator. The sample apps which use the UIActivityIndicator (or any other similar animation) start and stop the animation in different events, triggered separately at different stages of the program. Do I need to manually create a separate thread to run my operation in, or is there already default support for this kind of behavior?

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• If I allocate memory in one thread in C++ can I de-allocate it in another

  - by Shane MacLaughlin

  If I allocate memory in one thread in C++ (either new or malloc) can I de-allocate it in another, or must both occur in the same thread? Ideally, I'd like to avoid this in the first place, but I'm curious to know is it legal, illegal or implementation dependent. Edit: The compilers I'm currently using include VS2003, VS2008 and Embedded C++ 4.0, targetting XP, Vista, Windows 7 and various flavours of Windows CE / PocketPC & Mobile. So basically all Microsoft but across an array of esoteric platforms.

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• Thread runs only once

  - by folone

  When a Thread is finished, you cannot run it once more, using start() method: it throws an Exception. Could anyone explain, why? What stands behind such an architectural decision?

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• Swing: what to do when a GUI update takes too long and freezes other GUI elements?

  - by java.is.for.desktop

  Hello, everyone! I know that GUI code in Java Swing must be put inside SwingUtilities.invokeAndWait or SwingUtilities.invokeLater. This way threading works fine. Sadly, in my situation, the GUI update it that thing which takes much longer than background thread(s). More specific: I update a JTree with about just 400 entries, nesting depth is maximum 4, so should be nothing scary, right? But it takes sometimes one second! I need to ensure that the user is able to type in a JTextPane without delays. Well, guess what, the slow JTree updates do cause delays for JTextPane during input. It refreshes only as soon as the tree gets updated. I am using Netbeans and know empirically that a Java app can update lots of information without freezing the rest of the UI. How can it be done? NOTE 1: All those DefaultMutableTreeNodes are prepared outside the invokeAndWait. NOTE 2: When I replace invokeAndWait with invokeLater the tree doesn't get updated.

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• Creating WPF components in background thread

  - by mizipzor

  Im working on a reporting system, a series of DocumentPage are to be created through a DocumentPaginator. These documents include a number of WPF components that are to be instantiated so the paginator includes the correct things when later sent to the XpsDocumentWriter (which in turn is sent to the actual printer). My problem now is that the DocumentPage instances take quite a while to create (enough for Windows to mark the application as frozen) so I tried to create them in a background thread, which is problematic since WPF expects the attributes on them to be set from the GUI thread. I would also like to have a progress bar showing up, indicating how many pages have been created so far. Thus, it looks like Im trying to get two things to happen in parallell on the GUI. The problem is hard to explain and Im really not sure how to tackle it. In short: Create a series of DocumentPage's. These include WPF components These are to be created on a background thread, or use some other trick so the application isnt frozen. After each page is created, a WPF ProgressBar should be updated. If there is no decent way to do this, alternate solutions and approaches are more than welcome.

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• How to synchronize threads in python?

  - by Eric

  I have two threads in python (2.7). I start them at the beginning of my program. While they execute, my program reaches the end and exits, killing both of my threads before waiting for resolution. I'm trying to figure out how to wait for both threads to finish before exiting. def connect_cam(ip, execute_lock): try: conn = TelnetConnection.TelnetClient(ip) execute_lock.acquire() ExecuteUpdate(conn, ip) execute_lock.release() except ValueError: pass execute_lock = thread.allocate_lock() thread.start_new_thread(connect_cam, ( headset_ip, execute_lock ) ) thread.start_new_thread(connect_cam, ( handcam_ip, execute_lock ) ) In .NET I would use something like WaitAll() but I haven't found the equivalent in python. In my scenario, TelnetClient is a long operation which may result in a failure after a timeout.

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• Asynchronous Delegates Vs Thread/ThreadPool?

  - by claws

  Hello, I need to execute 3 parallel tasks and after completion of each task they should call the same function which prints out the results. I don't understand in .net why we have Asychronous calling (delegate.BeginInvoke() & delegate.EndInvoke()) as well as Thread class? I'm little confused which one to use when? Now in this particular case, what should I use Asychronous calling or Thread class? I'm using C#.

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• Different standard streams per POSIX thread

  - by Roman Nikitchenko

  Is there any possibility to achieve different redirections for standard output like printf(3) for different POSIX thread? What about standard input? I have lot of code based on standard input/output and I only can separate this code into different POSIX thread, not process. Linux operation system, C standard library. I know I can refactor code to replace printf() to fprintf() and further in this style. But in this case I need to provide some kind of context which old code doesn't have. So doesn't anybody have better idea (look into code below)? #include <pthread.h> #include <stdio.h> void* different_thread(void*) { // Something to redirect standard output which doesn't affect main thread. // ... // printf() shall go to different stream. printf("subthread test\n"); return NULL; } int main() { pthread_t id; pthread_create(&id, NULL, different_thread, NULL); // In main thread things should be printed normally... printf("main thread test\n"); pthread_join(id, NULL); return 0; }

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• remote function with pthread

  - by user311130

  Hi all, I wrote some code in c, using pthread (I configured the linker and compiler in eclipse IDE first). #include <pthread.h> #include "starter.h" #include "UI.h" Page* MM; Page* Disk; PCB* all_pcb_array; void* display_prompt(void *id){ printf("Hello111\n"); return NULL; } int main(int argc, char** argv) { printf("Hello\n"); pthread_t *thread = (pthread_t*) malloc (sizeof(pthread_t)); pthread_create(thread, NULL, display_prompt, NULL); printf("Hello\n"); return 1; } that works fine. However, when I move display_prompt to UI.h no "Hello111 " output is printed. anyone know how to solve that? Elad

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• java methods and race condition in a jsp/servlets application.

  - by A.S al-shammari

  Hi. Suppose that I have a method called doSomething() and I want to use this method in a multithreaded application (each servlet inherits from HttpServlet).I'm wondering if it is possible that a race condition will occur in the following cases: doSomething() is not staic method and it writes values to a database. doSomething() is static method but it does not write values to a database. what I have noticed that many methods in my application may lead to a race condition or dirty read/write. for example , I have a Poll System , and for each voting operation, a certain method will change a single cell value for that poll as the following: [poll_id | poll_data ] [1 | {choice_1 : 10, choice_2 : 20}] will the JSP/Servlets app solve these issues by itself, or I have to solve all that by myself? Thanks..

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• ruby thread programming , ruby equivalent of java wait/notify/notifyAll

  - by Geo

  I would like to know what are ruby's alternatives to the Java methods : wait notify notifyAll Could you please post a small snippet or some links ?

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• passing pipe to threads

  - by alaamh

  I see it's easy to open pipe between two process using fork, but how we can passing open pipe to threads. Assume we need to pass out of PROGRAM A to PROGRAM B "may by more than one thread", PROGRAM B send his output to PROGRAM C #include <stdio.h> #include <stdlib.h> #include <pthread.h> struct targ_s { int fd_reader; }; void *thread1(void *arg) { struct targ_s *targ = (struct targ_s*) arg; int status, fd[2]; pid_t pid; pipe(fd); pid = fork(); if (pid == 0) { dup2(STDIN_FILENO, targ->fd_reader); close(fd[0]); dup2(fd[1], STDOUT_FILENO); close(fd[1]); execvp ("PROGRAM B", NULL); exit(1); } else { close(fd[1]); dup2(fd[0], STDIN_FILENO); close(fd[0]); execl("PROGRAM C", NULL); wait(&status); return NULL; } } int main(void) { FILE *fpipe; char *command = "PROGRAM A"; char buffer[1024]; if (!(fpipe = (FILE*) popen(command, "r"))) { perror("Problems with pipe"); exit(1); } char* outfile = "out.dat"; FILE* f = fopen (outfile, "wb"); int fd = fileno( f ); struct targ_s targ; targ.fd_reader = fd; pthread_t thid; if (pthread_create(&thid, NULL, thread1, &targ) != 0) { perror("pthread_create() error"); exit(1); } int len; while (read(fpipe, buffer, sizeof (buffer)) != 0) { len = strlen(buffer); write(fd, buffer, len); } pclose(fpipe); return (0); }

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• Diffrernce between BackgroundWorker.ReportProgress() and Control.Invoke()

  - by ohadsc

  What is the difference between options 1 and 2 in the following? private void BGW_DoWork(object sender, DoWorkEventArgs e) { for (int i=1; i<=100; i++) { string txt = i.ToString(); if (Test_Check.Checked) //OPTION 1 Test_BackgroundWorker.ReportProgress(i, txt); else //OPTION 2 this.Invoke((Action<int, string>)UpdateGUI, new object[] {i, txt}); } } private void BGW_ProgressChanged(object sender, ProgressChangedEventArgs e) { UpdateGUI(e.ProgressPercentage, (string)e.UserState); } private void UpdateGUI(int percent, string txt) { Test_ProgressBar.Value = percent; Test_RichTextBox.AppendText(txt + Environment.NewLine); } Looking at reflector, the Control.Invoke() appears to use: this.FindMarshalingControl().MarshaledInvoke(this, method, args, 1); whereas BackgroundWorker.Invoke() appears to use: this.asyncOperation.Post(this.progressReporter, args); (I'm just guessing these are the relevant function calls.) If I understand correctly, BGW Posts to the WinForms window its progress report request, whereas Control.Invoke uses a CLR mechanism to invoke on the right thread. Am I close? And if so, what are the repercussions of using either ? Thanks

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• Dynamically refresh JTextArea as processing occurs?

  - by digiarnie

  I am trying to create a very simple Swing UI that logs information onto the screen via a JTextArea as processing occurs in the background. When the user clicks a button, I want each call to: textArea.append(someString + "\n"); to immediately show up in the UI. At the moment, the JTextArea does not show all log information until the processing has completed after clicking the button. How can I get it to refresh dynamically?

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• How do I make my ArrayList Thread-Safe? Another approach to problem in Java?

  - by thechiman

  I have an ArrayList that I want to use to hold RaceCar objects that extend the Thread class as soon as they are finished executing. A class, called Race, handles this ArrayList using a callback method that the RaceCar object calls when it is finished executing. The callback method, addFinisher(RaceCar finisher), adds the RaceCar object to the ArrayList. This is supposed to give the order in which the Threads finish executing. I know that ArrayList isn't synchronized and thus isn't thread-safe. I tried using the Collections.synchronizedCollection(c Collection) method by passing in a new ArrayList and assigning the returned Collection to an ArrayList. However, this gives me a compiler error: Race.java:41: incompatible types found : java.util.Collection required: java.util.ArrayList finishingOrder = Collections.synchronizedCollection(new ArrayList(numberOfRaceCars)); Here is the relevant code: public class Race implements RaceListener { private Thread[] racers; private ArrayList finishingOrder; //Make an ArrayList to hold RaceCar objects to determine winners finishingOrder = Collections.synchronizedCollection(new ArrayList(numberOfRaceCars)); //Fill array with RaceCar objects for(int i=0; i<numberOfRaceCars; i++) { racers[i] = new RaceCar(laps, inputs[i]); //Add this as a RaceListener to each RaceCar ((RaceCar) racers[i]).addRaceListener(this); } //Implement the one method in the RaceListener interface public void addFinisher(RaceCar finisher) { finishingOrder.add(finisher); } What I need to know is, am I using a correct approach and if not, what should I use to make my code thread-safe? Thanks for the help!

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• Testing approach for multi-threaded software

  - by Shane MacLaughlin

  I have a piece of mature geospatial software that has recently had areas rewritten to take better advantage of the multiple processors available in modern PCs. Specifically, display, GUI, spatial searching, and main processing have all been hived off to seperate threads. The software has a pretty sizeable GUI automation suite for functional regression, and another smaller one for performance regression. While all automated tests are passing, I'm not convinced that they provide nearly enough coverage in terms of finding bugs relating race conditions, deadlocks, and other nasties associated with multi-threading. What techniques would you use to see if such bugs exist? What techniques would you advocate for rooting them out, assuming there are some in there to root out? What I'm doing so far is running the GUI functional automation on the app running under a debugger, such that I can break out of deadlocks and catch crashes, and plan to make a bounds checker build and repeat the tests against that version. I've also carried out a static analysis of the source via PC-Lint with the hope of locating potential dead locks, but not had any worthwhile results. The application is C++, MFC, mulitple document/view, with a number of threads per doc. The locking mechanism I'm using is based on an object that includes a pointer to a CMutex, which is locked in the ctor and freed in the dtor. I use local variables of this object to lock various bits of code as required, and my mutex has a time out that fires my a warning if the timeout is reached. I avoid locking where possible, using resource copies where possible instead. What other tests would you carry out?

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• Is Work Stealing always the most appropriate user-level thread scheduling algorithm?

  - by Il-Bhima

  I've been investigating different scheduling algorithms for a thread pool I am implementing. Due to the nature of the problem I am solving I can assume that the tasks being run in parallel are independent and do not spawn any new tasks. The tasks can be of varying sizes. I went immediately for the most popular scheduling algorithm "work stealing" using lock-free deques for the local job queues, and I am relatively happy with this approach. However I'm wondering whether there are any common cases where work-stealing is not the best approach. For this particular problem I have a good estimate of the size of each individual task. Work-stealing does not make use of this information and I'm wondering if there is any scheduler which will give better load-balancing than work-stealing with this information (obviously with the same efficiency). NB. This question ties up with a previous question.

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• Is System.nanoTime() consistent across threads?

  - by obvio171

  I want to count the time elapsed between two events in nanoseconds. To do that, I can use System.nanoTime() as mentioned here. The problem is that the two events are happening in different threads. Since nanoTime() doesn't return an absolute timestamp but instead can only be used to calculate time differences, I'd like to know if the values I get on the two different threads are consistent with the physical time elapsed between the two events.

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• Which is more robust and scalable method?

  - by Dhruv Arya

  I am implementing a distributed chat system, in this system we have the following options : Make the client and server running at each node run as separate threads. The server acting as the receiver will be running as the daemon thread and the client taking the user input as a normal thread. Fork two processes one for the client and one for the server. I am not able to reason out with which one to proceed. Any insight would be great !

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• Issue with Java join() method.

  - by gmunk

  First of all here are some code snippets: http://pastebin.com/9ZCwekXs http://pastebin.com/TtLLXPYP I'm trying to animate some images. The thing is that I want the main thread to wait for the animation thread to finish and then to continue. I searched around, read a little bit and decided to use the join() method. It perfectly waits for the thread to finish but I doesn't animate correctly. The repaint() method gets called 2 times instead of nine. I think maybe the problem is because I used singletons. Here is the singleton implementation. http://pastebin.com/bA3qLZJE

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• How to manage db connections on server?

  - by simpatico

  I have a severe problem with my database connection in my web application. Since I use a single database connection for the whole application from singleton Database class, if i try concurrent db operations (two users) the database rollsback the transactions. This is my static method used: All threads/servlets call static Database.doSomething(...) methods, which in turn call the the below method. private static /* synchronized*/ Connection getConnection(final boolean autoCommit) throws SQLException { if (con == null) { con = new MyRegistrationBean().getConnection(); } con.setAutoCommit(true); //TODO return con; } What's the recommended way to manage this db connection/s I have, so that I don't incurr in the same problem.

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• help me reason about F# threads

  - by Kevin Cantu

  In goofing around with some F# (via MonoDevelop), I have written a routine which lists files in a directory with one thread: let rec loop (path:string) = Array.append ( path |> Directory.GetFiles ) ( path |> Directory.GetDirectories |> Array.map loop |> Array.concat ) And then an asynchronous version of it: let rec loopPar (path:string) = Array.append ( path |> Directory.GetFiles ) ( let paths = path |> Directory.GetDirectories if paths <> [||] then [| for p in paths -> async { return (loopPar p) } |] |> Async.Parallel |> Async.RunSynchronously |> Array.concat else [||] ) On small directories, the asynchronous version works fine. On bigger directories (e.g. many thousands of directories and files), the asynchronous version seems to hang. What am I missing? I know that creating thousands of threads is never going to be the most efficient solution -- I only have 8 CPUs -- but I am baffled that for larger directories the asynchronous function just doesn't respond (even after a half hour). It doesn't visibly fail, though, which baffles me. Is there a thread pool which is exhausted? How do these threads actually work?

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