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• Browser application & local file system access

  - by Beyond HTML

  I want to enhance my browser-based web application with functionality that enables management of local files and folders. E.g. folder tree structures should be synchronized between local workstation and server via HTTP(S). I am aware of security-related limitations in browser-based applications. However, there are some techniques that "work around" these issues: Signed Java applets (full trust) .NET Windows Forms browser controls (no joke, that works. Just the configuration is horrible) ActiveX My question is: What do you use/suggest, both technology and implementation practice? Key requirement is that the installation process is as simple as possible. Thanks for your opinions!

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• StringBuilder/StringBuffer vs. "+" Operator

  - by matt.seil

  I'm reading "Better, Faster, Lighter Java" (by Bruce Tate and Justin Gehtland) and am familiar with the readability requirements in agile type teams, such as what Robert Martin discusses in his clean coding books. On the team I'm on now, I've been told explicitly not to use the "+" operator because it creates extra (and unnecessary) string objects during runtime. But this article: http://www.ibm.com/developerworks/java/library/j-jtp01274.html Written back in '04 talks about how object allocation is about 10 machine instructions. (essentially free) It also talks about how the GC also helps to reduce costs in this environment. What is the actual performance tradeoffs between using "+," "StringBuilder," or "StringBuffer?" (In my case it is StringBuffer only as we are limited to Java 1.4.2.) StringBuffer to me results in ugly, less readable code, as a couple of examples in Tate's book demonstrates. And StringBuffer is thread-synchronized which seems to have its own costs that outweigh the "danger" in using the "+" operator. Thoughts/Opinions?

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• Stopping looping thread in Java

  - by halfwarp

  I'm using a thread that is continuously reading from a queue. Something like: public void run() { Object obj; while(true) { synchronized(objectsQueue) { if(objectesQueue.isEmpty()) { try { objectesQueue.wait(); } catch (InterruptedException e) { e.printStackTrace(); } obj = objectesQueue.poll(); } } // Do something with the Object obj } } What is the best way to stop this thread? I see two options: 1 - Since Thread.stop() is deprecated, I can implement a stopThisThread() method that uses a n atomic check-condition variable. 2 - Send a Death Event object or something like that to the queue. When the thread fetches a death event it exists. I prefer the 1st way, however, I don't know when to call the stopThisThread() method, as something might be on it's way to the queue and the stop signal can arrive first (not desirable). Any suggestions?

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• SQL Server database synchronization issues

  - by George2

  Hello everyone, I have two SQL Server 2008 Enterprise databases (on two machines), and one of the databases is master database and another database is slave database (master database is read/write, slave database is readonly). I want to have daily update from master database to slave database (i.e. new data inserted/updated/deleted in master database could be synchronized to slave database daily or manually controlled). I only need to sync several tables of the databases, not all of the database. Any solutions or documents? thanks in advance, George

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• Are C++ Reads and Writes of an int atomic

  - by theschmitzer

  I have two threads, one updating an int and one reading it. This value is a statistic where the order of the read and write is irrelevant. My question is, do I need to synchronize access to this multi-byte value anyway? Or, put another way, can part of the write be complete and get interrupted, and then the read happen. For example, think of value = ox0000FFFF increment value to 0x00010000 Is there a time where the value looks like 0x0001FFFF that I should be worried about? Certainly the larger the type, the more possible something like this is I've always synchronized these types of accesses, but was curious what the community thought.

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• Generating Ant build file for a project in eclipse

  - by Negai

  Hi everyone. I have a project written in Java using eclipse and I would like to generate an Ant build file for it. How can it be done? I constantly need to keep it synchronized with the source code, so that I could promote it along with the code to the common repository when with the project structure is modified. I'm sorry for a stupid question like this. I've browsed across the forums, but still no luck. Thanks in advance.

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• Benefits of arrays

  - by Vitalii Fedorenko

  As I see it, the advantages of List over array are pretty obvious: Generics provide more precise typing: List<Integer>, List<? extends Number>, List<? super Integer>. List interface has a bunch useful methods: addAll, remove etc. While for arrays all standard operations except get/set must be performed in a procedure manner by passing it to a static method. Collections offer different implementations like ArrayList, LinkedList, unmodifieable and synchronized lists, which can be hidden under common List interface. OOB length control. As disadvantages I can only mention absence of syntactic sugar and runtime type check. At the same time supporting of both structures requires frequent using of asList and toArray methods, which makes code less readable. So I am curious if there are any important benefits of using arrays that I miss.

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• JPA in distributed Java EE configuration

  - by sof

  Hello, I'm developing a JEE application to run on Glassfish: Database (javaDB, MS SQL, MySQL or Oracle) EJB layer with JPA (Toplink essentials - from Glassfish) for database access JSF/Icefaces based web UI accessing the EJB layer The application will have a lot of concurrent web client, so I want to run it on different physical servers and use a load-balancer. My problem is now how to keep the applications synchronized. I intend to set up multiple servers, each running Glassfish with my EAR app installed. Whenever on one of the servers data is added to or removed from the database (via JPA, no direct SQL queries), this change should be reflected in the JPA layer on the other servers. I've been looking around for solutions to this, but couldn't find anything I really like (the full Toplink from Oracle claims to have a solution, but don't know). Doing a refresh before every access to a JPA entity could work, but is far from efficient. Are there any patterns, libraries, ... that could help here? Thanks a lot!

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• why Observable snapshot observer vector

  - by han14466

  In Observable's notifyObservers method, why does the coder use arrLocal = obs.toArray();? Why does not coder iterate vector directly? Thanks public void notifyObservers(Object arg) { Object[] arrLocal; synchronized (this) { /* We don't want the Observer doing callbacks into * arbitrary code while holding its own Monitor. * The code where we extract each Observable from * the Vector and store the state of the Observer * needs synchronization, but notifying observers * does not (should not). The worst result of any * potential race-condition here is that: * 1) a newly-added Observer will miss a * notification in progress * 2) a recently unregistered Observer will be * wrongly notified when it doesn't care */ if (!changed) return; arrLocal = obs.toArray(); clearChanged(); } for (int i = arrLocal.length-1; i>=0; i--) ((Observer)arrLocal[i]).update(this, arg); }

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• MySQL Locking table from Stored FUNCTION

  - by Brandon

  I have a function in a MySQL Database that determines some sync parameters for a mobile device. The function determines the last date/time the user synchronized with the database. During my sync operation I call this server side function twice. As soon as I call it the second time - the entire Sync_Records table is locked. I cannot write to it from any other connection anywhere (note, after first call, the table is not locked). I changed the function to a Procedure - and all is fine - no locking after the second call. The entire sync operation (including both calls to the function/procedure) is within a transaction. This is an InnoDb table. The function/procedure simply does two select statements. They are storing results in local variables and then returning the date time variable. I don't understand why the tables are locked. Does anyone have any ideas?

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• Using locks inside a loop

  - by Xaqron

  // Member Variable private readonly object _syncLock = new object(); // Now inside a static method foreach (var lazyObject in plugins) { if ((string)lazyObject.Metadata["key"] = "something") { lock (_syncLock) { if (!lazyObject.IsValueCreated) lazyObject.value.DoSomething(); } return lazyObject.value; } } Here I need synchronized access per loop. There are many threads iterating this loop and based on the key they are looking for, a lazy instance is created and returned. lazyObject should not be created more that one time. Although Lazy class is for doing so and despite of the used lock, under high threading I have more than one instance created (I track this with a Interlocked.Increment on a volatile shared int and log it somewhere). The problem is I don't have access to definition of Lazy and MEF defines how the Lazy class create objects. My questions: 1) Why the lock doesn't work ? 2) Should I use an array of locks instead of one lock for performance improvement ?

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• What would be my best MySQL Synchronization method?

  - by Kerry

  We're moving a social media service to be on separate data centers with global load balancing, as our other hosting provider's entire data center went down. Twice. This means that both websites need to be synchronized in some sense -- I'm less worried about the code of the pages, that's easy enough to sync, but they need to have the same database data. From my research on SO, it seems MySQL Replication is a good option, but the MySQL manual, for scaling out, says that its best when there are far more reads then there are writes/updates: http://dev.mysql.com/doc/refman/5.0/en/replication-solutions-scaleout.html In our case, it's about equal. We're getting around 200-300 thousand requests a day right now, and we can grow rapidly. Every request is both a read and write request. What would be the best method or tool to handle this?

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• Thread help with Android game

  - by Ciph3rzer0

  I need some help dealing with three Threads in Android One thread is the main thread, the other is the GLThread, and the other is a WorkerThread I created to update the game state. The problem I have is they all need to access the same LinkedList of game objects. Both the GLThread and my WorkerThread only read from the LinkedList, so no problem there, but occasionally I have the main thread adding in another game object to the list. How can I manage this? I tried using synchronized in front of the functions involved but it really slows down the application. For some reason, just catching the errors and not rendering or updating the game state that frame, causes it to start lagging permanently. Anyone have any great ideas?

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• How to debug ConcurrentModificationException?

  - by Dani

  I encountered ConcurrentModificationException and by looking at it I can't see the reason why it's happening; the area throwing the exception and all the places modifying the collection are surrounded by synchronized (this.locks.get(id)) { ... } // locks is a HashMap<String, Object>; I tried to catch the the pesky thread but all I could nail (by setting a breakpoint in the exception) is that the throwing thread owns the monitor while the other thread (there are two threads in the program) sleeps. How should I proceed? What do you usually do when you encounter similar threading issues?

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• In web project can we write core services layer without knowledge of UI ?

  - by Silent Warrior

  I am working on web project. We are using flex as UI layer. My question is often we are writing core service layer separately from web/UI layer so we can reuse same services for different UI layer/technology. So practically is it possible to reuse same core layer services without any changes/addition in API with different kind of UI technologies/layers. For e.g. same core service layer with UI technology which supports synchronized request response (e.g. jsp etc.) and non synchronize or event driven UI technology (e.g Ajax, Flex, GWT etc.) or with multiple devices like (computers, mobiles, pdas etc.). Personally I feel its very tough to write core service layer without any knowledge of UI. Looking for thoughts from other people.

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• Java LinkedList iterator being exhausted prematurely

  - by Sujeet

  I am using LinkedList and retrieving an Iterator object by using list.iterator(). After that, I am checking it.hasNext(), real issue is while checking it.hasNext(), sometimes it returns false. I need help why this is happening, though I have elements in the list. Some code: public synchronized void check(Object obj) throws Exception { Iterator itr = list.iterator(); while(itr.hasNext()) { //This Line I get false.. though i have list size is 1 Item p = (Item)itr.next(); if(p.getId() == null) {continue;} if(p.getId().getElemntId() == obj.getId() || obj.getId() == 0 ) { p.setResponse(obj); notifyAll(); return; } } Log.Error("validate failed obj.getId="+obj.getId()+" **list.size="+list.size()*This shows 1*); throw new Exception("InvalidData"); }

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• WeakHashMap iteration and garbage collection

  - by Shamik

  I am using a WeaekHashMap to implement a Cache. I am wondering if I am iterating over the keys of this map, and at the same time garbage collector is actively removing keys from this map, would I receive a ConcurrentModificationException ? I do not think so, because as far as I understand, concurrentmodificationexception happens because of bugs in the application code where the developer forgot to understand that the same map is shared/used by other threads and in this case, it should not happen. But wondering how would JVM handle this when WeakHashMap is not synchronized ?

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• Is this plain stupid: GIT Sharing Via DropBox?

  - by yar

  I realize that there are similar questions, but my question is slightly different. I'm wondering whether sharing a bare repository via a synchronized DropBox folder on multiple computers would work for sharing code via GIT. Really what I want to know is: is sharing a GIT repo via DropBox (the repo gets updated on each person's local drive) the same as sharing it from one centralized location, e.g., via SSH, git or HTTP? Is this the same or different from sharing a GIT repo via a shared network drive? Note: This is not an empirical question: it seems to work fine. I'm asking whether the way a GIT repo is structured is compatible with this way of sharing. EDIT To clarify/repeat, I'm talking about keeping the GIT repository on DropBox as a bare repository. I'm not talking about keeping the actual files that are under source control in DropBox.

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• Python: UTF-8 problems (again...)

  - by blahblah

  I have a database which is synchronized against an external web source twice a day. This web source contains a bunch of entries, which have names and some extra information about these names. Some of these names are silly and I want to rename them when inserting them into my own database. To rename these silly names, I have a standard dictionary as such: RENAME_TABLE = { "Wsird" : "Weird", ... } As you can see, this is where UTF-8 comes into play. This is the function which performs renaming of all the problematic entries: def rename_all_entries(): all_keys = RENAME_TABLE.keys() entries = Entry.objects.filter(name__in=all_keys) for entry in entries: entry.name = RENAME_TABLE[entry.name] entry.save() So it tries to find the old name in RENAME_TABLE and renames the entry if found. However, I get a KeyError exception when using RENAME_TABLE[entry.name]. Now I'm lost, what do I do? I have... # -*- coding: utf-8 -*- ...in the top of the Python file.

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• Singleton design potential leak

  - by iBrad Apps

  I have downloaded a library off of github and have noticed that in the main singleton of the library there is a possible leak in this bit of code: +(DDGameKitHelper*) sharedGameKitHelper { @synchronized(self) { if (instanceOfGameKitHelper == nil) { [[DDGameKitHelper alloc] init]; } return instanceOfGameKitHelper; } return nil; } Now obviously there is no release or autorelease anywhere so I must do it but how and in what way properly? I have looked at various Singleton design patterns on the Internet and they just assign, in this case, instanceOfGameKitHelper to the alloc and init line. Anyway how would I properly fix this? Thanks!

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• Java Thread wait() => blocked?

  - by Chris

  According to http://java.sun.com/j2se/1.5.0/docs/api/java/lang/Thread.State.html calling wait() will result a thread to go in BLOCKED state. However this piece of code will result (after being called) in a Thread in WAITING State. class bThread extends Thread { public synchronized void run() { try { wait(); } catch (InterruptedException e) { e.printStackTrace(); } } } Have I got something wrong? Can anybody explain this behaviour to me? Any help would be appreciated!

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• C# design question (Connections)

  - by David

  Hello, I would like to hear your suggestions on kind of design problem which I have in c#. So, I am making a program where people can meet and draw in the same window over the internet or LAN. I am drawing into a bitmap and than I set it to a pictureBox component. I have a hard time to decide how to send updates to each user, what is the best way to do it. Should I send coordinates of mouse and than do the drawing on each users screen or stream the image to each. Maybe you know better solution to keep it synchronized and efficient. Thank you.

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• Efficient implementation of exclusive execution

  - by n0weak

  I have an ObjectManager class that is used to process payments. It is wrapped over the Order entities, so new instance has to be created when processing is required. I need to prevent the situation when several ObjectManager instances are dealing with the same order simultaneously (it happend once because of some errors on the remote payment processing center, somehow they called our callback urls twice). I'd love to get an advice how to implement it more efficiently. For now, I am thinking about something like that: public class OrderManager{ private static final CopyOnWriteArrayList<Integer> LOCKER = new CopyOnWriteArrayList<Integer>(); private static synchronized boolean tryLock(Integer key) { return LOCKER.addIfAbsent(key); } private static void releaseLock(Integer key) { LOCKER.remove(key); } public void processPayment(Integer orderId) throws Exception{ if (!tryLock(orderId)) { return; } try { //operate } finally { releaseLock(orderId); } } //remainder omitted }

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• Is it possible (and if so how) to delete a zookeeper node with apache camel?

  - by b_habegger

  I am looking into using Zookeeper to synchronized distributed Camel instances. In doing so it seems that there is not possibility to delete a znode from Zookeeper from Camel (I imagined some sort of producer operation). A look at the components source seems to confirm this. (I didn't find any code explicitely requesting a node removal). Am I missing something ? When would a node created via Camel (in my case EPHEMERAL) be deleted ? Maybe I am miss understanding something with zookeeper ?

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• Multi-Threaded Application vs. Single Threaded Application

  Why would we use a multi threaded application vs. a single threaded application? First we must define multithreading. Multithreading is a feature of an operating system that allows programs to run subcomponents or threads in parallel. Typically most applications only need to use one thread because they do not perform time consuming tasks. The use of multiple threads allows an application to distribute long running tasks so that they can be executed in parallel. This gives the user the perceived appearance that the application is working faster due to the fact that while one thread is waiting on an IO process the remaining tasks can make use of the available CPU. The allows working threads to execute in tandem so that they can be competed sooner. Multithreading Benefits Improved responsiveness — Users usually report improved responsiveness compared to single thread applications. Faster applications — Multiple threads can lead to improved application performance. Prioritization — Threads can be assigned a priority which would allow higher priority tasks to take precedence over lower priority tasks. Single Threading Benefits Programming and debugging —These activities are easier compared to multithreaded applications due to the reduced complexity Less Overhead — Threads add overhead to an application When developing multi-threaded applications, the following must be considered. Deadlocks occur when two threads hold a monitor that the other one requires. In essence each task is blocking the other and both tasks are waiting for the other monitor to be released. This forces an application to hang or deadlock. Resource allocation is used to prevent deadlocks because the system determines if approving the resource request will render the system in an unsafe state. An unsafe state could result in a deadlock. The system only approves requests that will lead to safe states. Thread Synchronization is used when multiple threads use the same instance of an object. The threads accessing the object can then be locked and then synchronized so that each task can interact with the static object on at a time.

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