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• Do the ‘up to date’ guarantees for values of Java's final fields extend to indirect references?

  - by mattbh

  The Java language spec defines semantics of final fields in section 17.5: The usage model for final fields is a simple one. Set the final fields for an object in that object's constructor. Do not write a reference to the object being constructed in a place where another thread can see it before the object's constructor is finished. If this is followed, then when the object is seen by another thread, that thread will always see the correctly constructed version of that object's final fields. It will also see versions of any object or array referenced by those final fields that are at least as up-to-date as the final fields are. My question is - does the 'up-to-date' guarantee extend to the contents of nested arrays, and nested objects? An example scenario: Thread A constructs a HashMap of ArrayLists, then assigns the HashMap to final field 'myFinal' in an instance of class 'MyClass' Thread B sees a (non-synchronized) reference to the MyClass instance and reads 'myFinal', and accesses and reads the contents of one of the ArrayLists In this scenario, are the members of the ArrayList as seen by Thread B guaranteed to be at least as up to date as they were when MyClass's constructor completed? I'm looking for clarification of the semantics of the Java Memory Model and language spec, rather than alternative solutions like synchronization. My dream answer would be a yes or no, with a reference to the relevant text.

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• Multithreading/Parallel Processing in PHP

  - by manyxcxi

  I have a PHP script that will generate a report using PHPExcel from data queried from a MySQL DB. Currently, it is linear in processing in that it gets the data back from MySQL, reads in the Excel template, writes the data to the template, then outputs it. I have optimized the code to the point that the data is only iterated over once, and there is very little processing done on the PHP side. The query returns hundreds of lines in less than .001 seconds, so it is running fast enough. After some timing I have found my bottlenecks to be (surprise, surprise) reading the template and writing the output. I would like to do this: Spawn a thread/process to read the template Spawn a thread/process to fetch the data Return back to parent thread - Parent thread will wait until both are complete Proceed on as normal My main questions are is this possible, is it worth it? If yes to both, how would you tackle it? Also, it is PHP 5 on CentOS

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• Java LockSupport Memory Consistency

  - by Lachlan

  Java 6 API question. Does calling LockSupport.unpark(thread) have a happens-before relationship to the return from LockSupport.park in the just-unparked thread? I strongly suspect the answer is yes, but the Javadoc doesn't seem to mention it explicitly.

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• atomic writes to ehcache

  - by Jacques René Mesrine

  Context I am storing a java.util.List inside ehcache. Key(String) --> List<UserDetail> The ordered List contains a Top 10 ranking of my most active users. Problem Concurrent 3rd party clients might be requesting for this list. I have a requirement to be as current as possible with regards to the ranking. Thus if the ranking is changed due the activities of users, the ordered List in the cache must not be left stale for very long. Once I've recalculated a new List, I want to replace the one in cache immediately. Consider a busy scenario whereby multiple concurrent clients are requesting for the ranking; how can I replace the cache item in an fashion such that: Clients can continue to pull a possibly stale snapshot. They should never get a null value. There will only be 1 server thread that writes to the cache.

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• Peterson algorithm in Java?

  - by Gabriel Šcerbák

  Is there example implementation of Peterson algorithm for mutual exclusion in Java?

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• Java AtomicInteger: what are the differences between compareAndSet and weakCompareAndSet?

  - by WizardOfOdds

  (note that this question is not about CAS, it's about the "May fail spuriously" Javadoc). The only difference in the Javadoc between these two methods from the AtomicInteger class is that the weakCompareAndSet contains the comment: "May fail spuriously". Now unless my eyes are cheated by some spell, both method do look to be doing exactly the same: public final boolean compareAndSet(int expect, int update) { return unsafe.compareAndSwapInt(this, valueOffset, expect, update); } /* ... * May fail spuriously. */ public final boolean weakCompareAndSet(int expect, int update) { return unsafe.compareAndSwapInt(this, valueOffset, expect, update); } So I realize that "May" doesn't mean "Must" but then why don't we all start adding this to our codebase: public void doIt() { a(); } /** * May fail spuriously */ public void weakDoIt() { a(); } I'm really confused with that weakCompareAndSet() that appears to do the same as the compareAndSet() yet that "may fail spuriously" while the other can't. Apparently the "weak" and the "spurious fail" are in a way related to "happens-before" ordering but I'm still very confused by these two AtomicInteger (and AtomicLong etc.) methods: because apparently they call exactly the same unsafe.compareAndSwapInt method. I'm particularly confused in that AtomicInteger got introduced in Java 1.5, so after the Java Memory Model change (so it is obviously not something that could "fail spuriously in 1.4" but whose behavior changed to "shall not fail spuriously in 1.5").

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• Java Memory Model: reordering and concurrent locks

  - by Steffen Heil

  Hi The java meomry model mandates that synchronize blocks that synchronize on the same monitor enforce a before-after-realtion on the variables modified within those blocks. Example: // in thread A synchronized( lock ) { x = true; } // in thread B synchronized( lock ) { System.out.println( x ); } In this case it is garanteed that thread B will see x==true as long as thread A already passed that synchronized-block. Now I am in the process to rewrite lots of code to use the more flexible (and said to be faster) locks in java.util.concurrent, especially the ReentrantReadWriteLock. So the example looks like this: // in thread A synchronized( lock ) { lock.writeLock().lock(); x = true; lock.writeLock().unlock(); } // in thread B synchronized( lock ) { lock.readLock().lock(); System.out.println( x ); lock.readLock().unlock(); } However, I have not seen any hints within the memory model specification that such locks also imply the nessessary ordering. Looking into the implementation it seems to rely on the access to volatile variables inside AbstractQueuedSynchronizer (for the sun implementation at least). However this is not part of any specification and moreover access to non-volatile variables is not really condsidered covered by the memory barrier given by these variables, is it? So, here are my questions: Is it safe to assume the same ordering as with the "old" synchronized blocks? Is this documented somewhere? Is accessing any volatile variable a memory barrier for any other variable? Regards, Steffen

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• How is spin lock implemented under the hood?

  - by httpinterpret

  This is a lock that can be held by only one thread of execution at a time. An attempt to acquire the lock by another thread of execution makes the latter loop until the lock is released. How does it handle the case when two threads try to acquire the lock exactly the same time? I think this question also applies to various of other mutex implementation.

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• Lost Update Anomaly in Sql Server Update Command

  - by Javed

  Hi, I am very much confused. I have a transaction in ReadCommitted Isolation level. Among other things I am also updating a counter value in it, something similar to below: Update tblCount set counter = counter + 1 My application is a desktop application and this transaction happens to occur quite frequently and concurrently. We recently noticed an error that sometimes the counter value doesn't get updated or is missed. We also insert one record on each counter update so we are sure that records have been inserted but somehow counter fails to update. This happens once in 2000 simulaneous transactions. I seriously doubt it is a lost update anomaly I am facing but if you look at the command above, it's just update the counter from its own value: if I have started a transaction and the transaction has reached this statement, it should have locked the row. This should not cause lost update, but it's happening somehow. Is the thing that this update command works in two parts? Like first it reads the counter value (during which it doesn't get the exclusive lock) and then writes the new calculated value (when it does get an exclusive lock)? Please help, I have got really confused.

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• Java 1.4 singleton containing a mutable field

  - by Philippe

  Hi, I'm working on a legacy Java 1.4 project, and I have a factory that instantiates a csv file parser as a singleton. In my csv file parser, however, I have a HashSet that will store objects created from each line of my CSV file. All that will be used by a web application, and users will be uploading CSV files, possibly concurrently. Now my question is : what is the best way to prevent my list of objects to be modified by 2 users ? So far, I'm doing the following : final class MyParser { private File csvFile = null; private static Set myObjects = Collections.synchronizedSet(new HashSet); public synchronized void setFile(File file) { this.csvFile = file; } public void parse() FileReader fr = null; try { fr = new FileReader(csvFile); synchronized(myObjects) { myObjects.clear(); while(...) { // foreach line of my CSV, create a "MyObject" myObjects.add(new MyObject(...)); } } } catch (Exception e) { //... } } } Should I leave the lock only on the myObjects Set, or should I declare the whole parse() method as synchronized ? Also, how should I synchronize - both - the setting of the csvFile and the parsing ? I feel like my actual design is broken because threads could modify the csv file several times while a possibly long parse process is running. I hope I'm being clear enough, because myself am a bit confused on those multi-synchronization issues. Thanks ;-)

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• testing SpringMVC controllers

  - by Don

  Hi, I'm unit-testing my SpringMVC (v. 2.5) controllers using code like the following: public void testParamValidation() { MyController controller = new MyController(); MockHttpServletRequest request = new MockHttpServletRequest(); request.addParameter("foo", "bar"); request.addParameter("bar", baz"); ModelAndView mav = controller .handleRequest(request, new MockHttpServletResponse()); // Do some assertions on mav } This controller is a subclass of AbstractCommandController, so the parameters are bound to a command bean, and any binding or validation errors are stored in an object that implements the Errors interface. I can't seem to find any way to access this Errors from within the test, is this possible? Thanks, Don

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• Django: testing get query

  - by Brant

  Okay, so I am sick of writing this... res = Something.objects.filter(asdf=something) if res: single = res[0] else: single = None if single: # do some stuff I would much rather be able to do something like this: single = Something.objects.filter(asdf=something) if single: #do some stuff I want to be able to grab a single object without testing the filtered results. In other words, when i know there is either going to be 1 or 0 matching entries, I would like to jump right to that entry, otherwise just get a 'None'. The DoesNotExist error that goes along with .get does not always work so well when trying to compress these queries into a single line. Is there any way to do what I have described?

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• How to download images in playframework jobs?

  - by MrROY

  I have a playframework Job class like this: public class ImageDownloader extends Job { private String[] urls; private String dir; public ImageDownloader(){} public ImageDownloader(String[] urls,String dir){ this.urls = urls; this.dir = dir; } @Override public void doJob() throws Exception { if(urls!=null && urls.length > 0){ for (int i = 0; i < urls.length; i++) { String url = urls[i]; //Dowloading } } } } Play(1.2.4) has lots of amazing tools to make things easy. So i wonder whether there's a way to make the downloading easy and beautiful in play ?

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• Testing MPMoviePlayerViewController in iPad simulator

  - by hgpc

  I have a view that shows a MPMoviePlayerViewController modally. When testing it in the iPad simulator it works well on the first try. If I dismiss the video and then show the view again, the player only plays the audio, but not the video. Is this a simulator quirk or am I doing something wrong? Here's my code: - (void)viewWillAppear:(BOOL)animated { [super viewWillAppear:animated]; MPMoviePlayerViewController* v = [[MPMoviePlayerViewController alloc] initWithContentURL:url]; [[NSNotificationCenter defaultCenter] addObserver:self selector: @selector(playbackDidFinish:) name:MPMoviePlayerPlaybackDidFinishNotification object:v.moviePlayer]; [self presentMoviePlayerViewControllerAnimated:v]; [v release]; } -(void) playbackDidFinish:(NSNotification*)aNotification { MPMoviePlayerController *player = [aNotification object]; [[NSNotificationCenter defaultCenter] removeObserver:self name:MPMoviePlayerPlaybackDidFinishNotification object:player]; [player stop]; [self dismissMoviePlayerViewControllerAnimated]; }

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• Is it safe to spin on a volatile variable in user-mode threads?

  - by yongsun

  I'm not quite sure if it's safe to spin on a volatile variable in user-mode threads, to implement a light-weight spin_lock, I looked at the tbb source code, tbb_machine.h:170, //! Spin WHILE the value of the variable is equal to a given value /** T and U should be comparable types. */ template<typename T, typename U> void spin_wait_while_eq( const volatile T& location, U value ) { atomic_backoff backoff; while( location==value ) backoff.pause(); } And there is no fences in atomic_backoff class as I can see. While from other user-mode spin_lock implementation, most of them use CAS (Compare and Swap).

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• Automation of testing using JUnit

  - by Vijay Selvaraj

  Hi, i am trying to automate manual testing of modules in my project. We are dealing with IBM Websphere Message queue software. We have a trigger component written in core java which when executed polls for availability of message in the configured queue. Its an indefinite while loop that keeps the trigger component running. I have written test cases in JUnit to put message in the queue and now will i be able to start/stop the trigger component on demand? Invoking the trigger component keeps it running and i am not getting the control back to check the expected output. If i start it in thread then the log files to which the trigger component is supposed to update when processing the message is not getting updated. How can i resolve this situation. Your suggestion and directions is highly appreciated. Thanks, -Vijay

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• Simplest possible voting/synchronization algorithm

  - by Domchi

  What would be a simplest algorithm one or more people could use to decide who of them should perform some task? There is one task, which needs to be done only once, and one or more people. People can speak, that is, send messages one to another. Communication must be minimal, and all people use the exact same algorithm. One person saying "I'm doing it" is not good enough since two persons may say it at a same time. Simplest that comes to my mind is that each person says a number and waits a bit. If somebody responds in that time, the person with lower number "wins" and does the task. If nobody responds, person says that she's doing it and does it. When she says that she does it, everybody else backs off. This should be enough to avoid two persons doing the task in the same time (since there is wait/handhake period), but might need a "second round" if both persons say the same number. Is there something simpler? For those curious, I'm trying to synchronize several copies of SecondLife LSL script to do something only once.

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• Handling Exceptions for ThreadPoolExecutor

  - by HonorGod

  I have the following code snippet that basically scans through the list of task that needs to be executed and each task is then given to the executor for execution. The JobExecutor intern creates another executor (for doing db stuff...reading and writing data to queue) and completes the task. JobExecutor returns a Future for the tasks submitted. When one of the task fails, I want to gracefully interrupt all the threads and shutdown the executor by catching all the exceptions. What changes do I need to do? public class DataMovingClass { private static final AtomicInteger uniqueId = new AtomicInteger(0); private static final ThreadLocal<Integer> uniqueNumber = new IDGenerator(); ThreadPoolExecutor threadPoolExecutor = null ; private List<Source> sources = new ArrayList<Source>(); private static class IDGenerator extends ThreadLocal<Integer> { @Override public Integer get() { return uniqueId.incrementAndGet(); } } public void init(){ // load sources list } public boolean execute() { boolean succcess = true ; threadPoolExecutor = new ThreadPoolExecutor(10,10, 10, TimeUnit.SECONDS, new ArrayBlockingQueue<Runnable>(1024), new ThreadFactory() { public Thread newThread(Runnable r) { Thread t = new Thread(r); t.setName("DataMigration-" + uniqueNumber.get()); return t; }// End method }, new ThreadPoolExecutor.CallerRunsPolicy()); List<Future<Boolean>> result = new ArrayList<Future<Boolean>>(); for (Source source : sources) { result.add(threadPoolExecutor.submit(new JobExecutor(source))); } for (Future<Boolean> jobDone : result) { try { if (!jobDone.get(100000, TimeUnit.SECONDS) && success) { // in case of successful DbWriterClass, we don't need to change // it. success = false; } } catch (Exception ex) { // handle exceptions } } } public class JobExecutor implements Callable<Boolean> { private ThreadPoolExecutor threadPoolExecutor ; Source jobSource ; public SourceJobExecutor(Source source) { this.jobSource = source; threadPoolExecutor = new ThreadPoolExecutor(10,10,10, TimeUnit.SECONDS, new ArrayBlockingQueue<Runnable>(1024), new ThreadFactory() { public Thread newThread(Runnable r) { Thread t = new Thread(r); t.setName("Job Executor-" + uniqueNumber.get()); return t; }// End method }, new ThreadPoolExecutor.CallerRunsPolicy()); } public Boolean call() throws Exception { boolean status = true ; System.out.println("Starting Job = " + jobSource.getName()); try { // do the specified task ; }catch (InterruptedException intrEx) { logger.warn("InterruptedException", intrEx); status = false ; } catch(Exception e) { logger.fatal("Exception occurred while executing task "+jobSource.getName(),e); status = false ; } System.out.println("Ending Job = " + jobSource.getName()); return status ; } } }

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• Clojure agents consuming from a queue

  - by erikcw

  I'm trying to figure out the best way to use agents to consume items from a Message Queue (Amazon SQS). Right now I have a function (process-queue-item) that grabs an items from the queue, and processes it. I want to process these items concurrently, but I can't wrap my head around how to control the agents. Basically I want to keep all of the agents busy as much as possible without pulling to many items from the Queue and developing a backlog (I'll have this running on a couple of machines, so items need to be left in the queue until they are really needed). Can anyone give me some pointers on improving my implementation? (def active-agents (ref 0)) (defn process-queue-item [_] (dosync (alter active-agents inc)) ;retrieve item from Message Queue (Amazon SQS) and process (dosync (alter active-agents dec))) (defn -main [] (def agents (for [x (range 20)] (agent x))) (loop [loop-count 0] (if (< @active-agents 20) (doseq [agent agents] (if (agent-errors agent) (clear-agent-errors agent)) ;should skip this agent until later if it is still busy processing (not sure how) (send-off agent process-queue-item))) ;(apply await-for (* 10 1000) agents) (Thread/sleep 10000) (logging/info (str "ACTIVE AGENTS " @active-agents)) (if (> 10 loop-count) (do (logging/info (str "done, let's cleanup " count)) (doseq [agent agents] (if (agent-errors agent) (clear-agent-errors agent))) (apply await agents) (shutdown-agents)) (recur (inc count)))))

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• iPhone: NSOperationQueue running operations serially

  - by Greg Maletic

  I have a singleton NSOperationQueue that handles all of my network requests. I'm noticing, however, that when I have one particularly long operation running (this particular operation takes at least 25 seconds), my other operations don't run until it completes. maxConcurrentOperationCount is set to NSOperationQueueDefaultMaxConcurrentOperationCount, so I don't believe that's the issue. Any reason why this would be happening? Besides spawning multiple NSOperationQueues (a solution that I'm not sure would work, nor am I sure it's a good idea), what's the best way to fix this problem? Thanks.

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• Ways to polling server status

  - by Yijinsei

  Hi guys, I am try to create a JSP page that will show all the status in a group of local servers. Currently I create a schedule class that will constantly poll to check the status of the server with 30 second interval, with 5 second delay to wait for each server reply, and provide the JSP page with the information. However I find this way to be not accurate as it will take some time before the information of the schedule class to be updated. Do you guys have a better way to check the status of several server within a local network?

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• How do I prove whether or not a dining philosophers suffers from deadlock or starvation possibilitie

  - by Marvin Tenegard

  Hi, I have a homework task to prove whether or not a particular variation on the dining philosophers problem suffers from deadlock or starvation. I suspect that the situation does not suffer at all from either but I'm finding this tricky to prove and I don't really know where to start. Is there a general strategy for attacking this sort of problem?

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• Why is my multithreaded Java program not maxing out all my cores on my machine?

  - by James B

  Hi, I have a program that starts up and creates an in-memory data model and then creates a (command-line-specified) number of threads to run several string checking algorithms against an input set and that data model. The work is divided amongst the threads along the input set of strings, and then each thread iterates the same in-memory data model instance (which is never updated again, so there are no synchronization issues). I'm running this on a Windows 2003 64-bit server with 2 quadcore processors, and from looking at Windows task Manager they aren't being maxed-out, (nor are they looking like they are being particularly taxed) when I run with 10 threads. Is this normal behaviour? It appears that 7 threads all complete a similar amount of work in a similar amount of time, so would you recommend running with 7 threads instead? Should I run it with more threads?...Although I assume this could be detrimental as the JVM will do more context switching between the threads. Alternatively, should I run it with fewer threads? Alternatively, what would be the best tool I could use to measure this?...Would a profiling tool help me out here - indeed, is one of the several profilers better at detecting bottlenecks (assuming I have one here) than the rest? Note, the server is also running SQL Server 2005 (this may or may not be relevant), but nothing much is happening on that database when I am running my program. Note also, the threads are only doing string matching, they aren't doing any I/O or database work or anything else they may need to wait on. Thanks in advance, -James

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• Understanding this Pascal-FC threaded code

  - by dmindreader

  **Program Parcial2; type buffer = channel of integer; var buffers : array [1..2] of buffer; val:integer; process sleeper (id:integer); var i : integer; begin for i:=1 to 10 do begin sleep (random(10*id)); **buffers (id):any;** end; end; process troll; begin **buffers[1]: random(10);** end;** What are buffers(id):any and buffers[1]:random(10) doing?

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• Erlang-style light-weight processes in .NET

  - by alexey

  Is there any way to implement Erlang-style light-weight processes in .NET? I found some projects that implement Erlang messaging model (actors model). For example, Axum. But I found nothing about light-weight processes implementation. I mean multiple processes that run in a context of a single OS-thread or OS-process.

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