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• Java Hibernate/C3P0 error: "Could not obtain connection metadata. An attempt by a client to checkout

  - by Allan

  I'm trying to get some code I was passed up and running. It appears to use the Hibernate framework. I've gotten past most of the errors tweaking the configuration, but this one has me dead stumped. It's trying to connect to two databases: gameapp and gamelog. Both exist. It seems to have issues connecting to gamelog, but none connecting to gameapp (later in the init, it connects to and loads other DBs just fine). Below, I've pasted the error and exception stack dump. I imaging there's something else in the configs, so I've also included the configuration file for that db. I know this is very vague, but I'm hoping some pro can see the stupid mistake I'm missing. <?xml version="1.0" encoding="GBK"?> <!DOCTYPE hibernate-configuration PUBLIC "-//Hibernate/Hibernate Configuration DTD 3.0//EN" "http://hibernate.sourceforge.net/hibernate-configuration-3.0.dtd"> <hibernate-configuration> <session-factory> <property name="dialect">org.hibernate.dialect.MySQLDialect</property> <property name="connection.driver_class">com.mysql.jdbc.Driver</property> <property name="connection.url">jdbc:mysql://127.0.0.1:3306/gamelog</property> <property name="connection.username">root</property> <property name="connection.password"></property> <property name="connection.useUnicode">true</property> <property name="connection.characterEncoding">UTF-8</property> <property name="hibernate.jdbc.batch_size">100</property> <property name="jdbc.fetch_size">1</property> <property name="hbm2ddl.auto">none</property><!-- update --> <property name="connection.useUnicode">true</property> <property name="show_sql">true</property> <!-- c3p0-configuration --> <property name="hibernate.connection.provider_class">org.hibernate.connection.C3P0ConnectionProvider</property> <property name="hibernate.c3p0.min_size">5</property> <property name="hibernate.c3p0.max_size">10</property> <property name="hibernate.c3p0.timeout">30</property> <property name="hibernate.c3p0.idle_test_period">30</property> <property name="hibernate.c3p0.max_statements">0</property> <property name="hibernate.c3p0.acquire_increment">5</property> </session-factory> </hibernate-configuration> Exception and stack trace: 2010-04-30 17:50:00,411 WARN [org.hibernate.cfg.SettingsFactory] - Could not obtain connection metadata java.sql.SQLException: An attempt by a client to checkout a Connection has timed out. at com.mchange.v2.sql.SqlUtils.toSQLException(SqlUtils.java:106) at com.mchange.v2.sql.SqlUtils.toSQLException(SqlUtils.java:65) at com.mchange.v2.c3p0.impl.C3P0PooledConnectionPool.checkoutPooledConnection(C3P0PooledConnectionPool.java:527) at com.mchange.v2.c3p0.impl.AbstractPoolBackedDataSource.getConnection(AbstractPoolBackedDataSource.java:128) at org.hibernate.connection.C3P0ConnectionProvider.getConnection(C3P0ConnectionProvider.java:35) at org.hibernate.cfg.SettingsFactory.buildSettings(SettingsFactory.java:76) at org.hibernate.cfg.Configuration.buildSettings(Configuration.java:1933) at org.hibernate.cfg.Configuration.buildSessionFactory(Configuration.java:1216) at com.database.hibernate.util.HibernateFactory.<init>(Unknown Source) at com.database.hibernate.util.HibernateUtil.<clinit>(Unknown Source) at com.server.databaseop.goodOp.GoodOpImpl.initBreedGoods(Unknown Source) at com.server.databaseop.goodOp.GoodOpImpl.access$000(Unknown Source) at com.server.databaseop.goodOp.GoodOpImpl$1.run(Unknown Source) at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:471) at java.util.concurrent.FutureTask$Sync.innerRunAndReset(FutureTask.java:351) at java.util.concurrent.FutureTask.runAndReset(FutureTask.java:178) at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.access$201(ScheduledThreadPoolExecutor.java:165) at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.run(ScheduledThreadPoolExecutor.java:267) at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1110) at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:603) at java.lang.Thread.run(Thread.java:636) Caused by: com.mchange.v2.resourcepool.TimeoutException: A client timed out while waiting to acquire a resource from com.mchange.v2.resourcepool.BasicResourcePool@ca470 -- timeout at awaitAvailable() at com.mchange.v2.resourcepool.BasicResourcePool.awaitAvailable(BasicResourcePool.java:1317) at com.mchange.v2.resourcepool.BasicResourcePool.prelimCheckoutResource(BasicResourcePool.java:557) at com.mchange.v2.resourcepool.BasicResourcePool.checkoutResource(BasicResourcePool.java:477) at com.mchange.v2.c3p0.impl.C3P0PooledConnectionPool.checkoutPooledConnection(C3P0PooledConnectionPool.java:525) ... 18 more

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• Handling file upload in a non-blocking manner

  - by Kaliyug Antagonist

  The background thread is here Just to make objective clear - the user will upload a large file and must be redirected immediately to another page for proceeding different operations. But the file being large, will take time to be read from the controller's InputStream. So I unwillingly decided to fork a new Thread to handle this I/O. The code is as follows : The controller servlet /** * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse * response) */ protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { // TODO Auto-generated method stub System.out.println("In Controller.doPost(...)"); TempModel tempModel = new TempModel(); tempModel.uploadSegYFile(request, response); System.out.println("Forwarding to Accepted.jsp"); /*try { Thread.sleep(1000 * 60); } catch (InterruptedException e) { // TODO Auto-generated catch block e.printStackTrace(); }*/ request.getRequestDispatcher("/jsp/Accepted.jsp").forward(request, response); } The model class package com.model; import java.io.IOException; import java.util.concurrent.ExecutionException; import java.util.concurrent.Future; import javax.servlet.http.HttpServletRequest; import javax.servlet.http.HttpServletResponse; import com.utils.ProcessUtils; public class TempModel { public void uploadSegYFile(HttpServletRequest request, HttpServletResponse response) { // TODO Auto-generated method stub System.out.println("In TempModel.uploadSegYFile(...)"); /* * Trigger the upload/processing code in a thread, return immediately * and notify when the thread completes */ try { FileUploaderRunnable fileUploadRunnable = new FileUploaderRunnable( request.getInputStream()); /* * Future<FileUploaderRunnable> future = ProcessUtils.submitTask( * fileUploadRunnable, fileUploadRunnable); * * FileUploaderRunnable processed = future.get(); * * System.out.println("Is file uploaded : " + * processed.isFileUploaded()); */ Thread uploadThread = new Thread(fileUploadRunnable); uploadThread.start(); } catch (IOException e) { // TODO Auto-generated catch block e.printStackTrace(); } /* * catch (InterruptedException e) { // TODO Auto-generated catch block * e.printStackTrace(); } catch (ExecutionException e) { // TODO * Auto-generated catch block e.printStackTrace(); } */ System.out.println("Returning from TempModel.uploadSegYFile(...)"); } } The Runnable package com.model; import java.io.File; import java.io.FileInputStream; import java.io.FileNotFoundException; import java.io.FileOutputStream; import java.io.IOException; import java.io.InputStream; import java.nio.ByteBuffer; import java.nio.channels.Channels; import java.nio.channels.ReadableByteChannel; public class FileUploaderRunnable implements Runnable { private boolean isFileUploaded = false; private InputStream inputStream = null; public FileUploaderRunnable(InputStream inputStream) { // TODO Auto-generated constructor stub this.inputStream = inputStream; } public void run() { // TODO Auto-generated method stub /* Read from InputStream. If success, set isFileUploaded = true */ System.out.println("Starting upload in a thread"); File outputFile = new File("D:/06c01_output.seg");/* * This will be changed * later */ FileOutputStream fos; ReadableByteChannel readable = Channels.newChannel(inputStream); ByteBuffer buffer = ByteBuffer.allocate(1000000); try { fos = new FileOutputStream(outputFile); while (readable.read(buffer) != -1) { fos.write(buffer.array()); buffer.clear(); } fos.flush(); fos.close(); readable.close(); } catch (FileNotFoundException e) { // TODO Auto-generated catch block e.printStackTrace(); } catch (IOException e) { // TODO Auto-generated catch block e.printStackTrace(); } System.out.println("File upload thread completed"); } public boolean isFileUploaded() { return isFileUploaded; } } My queries/doubts : Spawning threads manually from the Servlet makes sense to me logically but scares me coding wise - the container isn't aware of these threads after all(I think so!) The current code is giving an Exception which is quite obvious - the stream is inaccessible as the doPost(...) method returns before the run() method completes : In Controller.doPost(...) In TempModel.uploadSegYFile(...) Returning from TempModel.uploadSegYFile(...) Forwarding to Accepted.jsp Starting upload in a thread Exception in thread "Thread-4" java.lang.NullPointerException at org.apache.coyote.http11.InternalInputBuffer.fill(InternalInputBuffer.java:512) at org.apache.coyote.http11.InternalInputBuffer.fill(InternalInputBuffer.java:497) at org.apache.coyote.http11.InternalInputBuffer$InputStreamInputBuffer.doRead(InternalInputBuffer.java:559) at org.apache.coyote.http11.AbstractInputBuffer.doRead(AbstractInputBuffer.java:324) at org.apache.coyote.Request.doRead(Request.java:422) at org.apache.catalina.connector.InputBuffer.realReadBytes(InputBuffer.java:287) at org.apache.tomcat.util.buf.ByteChunk.substract(ByteChunk.java:407) at org.apache.catalina.connector.InputBuffer.read(InputBuffer.java:310) at org.apache.catalina.connector.CoyoteInputStream.read(CoyoteInputStream.java:202) at java.nio.channels.Channels$ReadableByteChannelImpl.read(Unknown Source) at com.model.FileUploaderRunnable.run(FileUploaderRunnable.java:39) at java.lang.Thread.run(Unknown Source) Keeping in mind the point 1., does the use of Executor framework help me in anyway ? package com.utils; import java.util.concurrent.Future; import java.util.concurrent.ScheduledThreadPoolExecutor; public final class ProcessUtils { /* Ensure that no more than 2 uploads,processing req. are allowed */ private static final ScheduledThreadPoolExecutor threadPoolExec = new ScheduledThreadPoolExecutor( 2); public static <T> Future<T> submitTask(Runnable task, T result) { return threadPoolExec.submit(task, result); } } So how should I ensure that the user doesn't block and the stream remains accessible so that the (uploaded)file can be read from it?

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• Simple Java networking game engine [on hold]

  - by Florian Peschka

  I want to create a simple java networking game and search a networking engine that eases use of sockets etc. I have already read some questions on here and the internet about java networking for games, but many of them were over 10 years old or not really answered. I have no idea whatsoever about what exactly I need to send in terms of messages, but I figured simple strings or integers will be enough for my purposes. It's basically a peer to peer game, so I don't need a centralized server structure. Messaging doesn't have to have an extremely low ping, yet of course all players need to have a synchronized view. What are the possibilities I have here?

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• Open source Java CMS for Google App Engine?

  - by markvgti

  I am looking for an open source Java CMS (Web CMS, actually) to run on Google App Engine. I have looked at related older questions on this topic (What CMS runs on Google AppEngine?, CMS over Google App Engine, with SEO etc.) but the problem is that they all largely list Python-based CMSes. Plus these questions are pretty old, and since GAE is a fast-moving target, I thought it might be worthwhile to ask again. I want a CMS for creating some websites (for myself and for others), but would rather not start writing one from scratch. A "good" (very subjective, I know) open source WCMS allows me to start using a product, while still being able to add to/extend the product/project. On the one hand I am looking for a somewhat mature product/project, on the other hand it's easier to start contributing to the development cycle of a young product/project (conflicting, I know :-). Here are some features that would be preferable: [X]HTML/XML/CSS based templating Ability to create multiple blogs Galleries Ability to create a "Downloads" section (is this pretty much standard?) Separate management for digital assets (images, PDFs, binary files etc.) Roles like "Administrator", "Editor", "Contributor" etc. (or their equivalents) Ability to move/reorganize pages Export to PDF Reformat content for printing Is the CMS you are about to suggest especially well-suited to publishing an online book? My idea is that while the book may be offered as a downloadable eBook, the latest, most current version will be the one available on the website.

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• Java Spotlight Episode 84: Anil Gaur on JavaEE 7

  - by Roger Brinkley

  Tweet Interview with Anil Gaur, VP of Java Platform for Enterprise Edition and GlassFish Server, on JavaEE 7. Joining us this week on the Java All Star Developer Panel are Dalibor Topic, Java Free and Open Source Software Ambassador and Arun Gupta, Java EE Guy. Right-click or Control-click to download this MP3 file. You can also subscribe to the Java Spotlight Podcast Feed to get the latest podcast automatically. If you use iTunes you can open iTunes and subscribe with this link:  Java Spotlight Podcast in iTunes. Show Notes News Tori Wieldt - Judges Selected for Duke's Choice Awards Donald Smith - #OpenJDK interview in Java Magazine Henrik Ståhl - Java 7 adoption at 23% JavaOne Kicks Off with Sunday Keynotes at Masonic Auditorium Jersey 2.0 M4 JSF 2.2 Latest Snapshot NetBeans IDE 7.2 - Deploy to Cloud Events May 30, OTN Java Developer Day, Redwood Shores June 11-14, Cloud Computing Expo, New York City June 12, Boulder JUG June 13, Denver JUG June 13, Eclipse Juno DemoCamp, Redwoood Shore June 13, JUG Münster June 14, Java Klassentreffen, Vienna, Austria June 18-20, QCon, New York City June 26-28, Jazoon, Zurich, Switzerland July 5, Java Forum, Stuttgart, Germany July 30-August 1, JVM Language Summit, Santa Clara Feature InterviewAnil Gaur is the Vice President of Java Platform, Enterprise Edition, and GlassFish Server at Oracle in the Fusion Middleware Group. Is responsible for creation of Java EE Specifications, Reference Implementation, and Compatibility Test Suites. Leading the evolution on Java EE into Cloud and PaaS environment through the Java EE 7 standard. Prior to that, managed the delivery of Java EE 6 Platform and SDK which quickly gained momentum in enterprise application development and deployments. In this episode we talk about GlassFish 3.1 release. Mail Bag What’s Cool RFR (L): Adding core file parsing on Mac OS X to SA Sergio Del Valle @swdelvalle is the 1,000 @JavaSpotlight twitter follower

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• Dell XPS 15 (L502x) sound problem

  - by lauramolenaar

  I have a problem with my sound on my Dell XPS 15. First, when I had Windows 7, my sound was pretty good (I have a JBL 2.1 speaker system with Waves Maxx audio), but since I installed Ubuntu 12.04 it sounded very cheap and as if I put my laptop in a tin can. I've already tried installing alsa-hda-dkms from the alsa-daily ppa (http://ppa.launchpad.net/ubuntu-audio-dev/alsa-daily) This is my audio controller: laura@laura-XPS-L502X:~$ lspci -v | grep -A7 -i audio 00:1b.0 Audio device: Intel Corporation 6 Series/C200 Series Chipset Family High Definition Audio Controller (rev 05) Subsystem: Dell Device 04b6 Flags: bus master, fast devsel, latency 0, IRQ 51 Memory at f1c00000 (64-bit, non-prefetchable) [size=16K] Capabilities: <access denied> Kernel driver in use: snd_hda_intel Kernel modules: snd-hda-intel I hope you can help me, and you can always ask me for more information. Result of aplay -l: **** List of PLAYBACK Hardware Devices **** card 0: PCH [HDA Intel PCH], device 0: ALC665 Analog [ALC665 Analog] Subdevices: 0/1 Subdevice #0: subdevice #0 card 0: PCH [HDA Intel PCH], device 1: ALC665 Digital [ALC665 Digital] Subdevices: 1/1 Subdevice #0: subdevice #0 card 0: PCH [HDA Intel PCH], device 3: HDMI 0 [HDMI 0] Subdevices: 1/1 Subdevice #0: subdevice #0 Result of aplay -L: default Playback/recording through the PulseAudio sound server sysdefault:CARD=PCH HDA Intel PCH, ALC665 Analog Default Audio Device front:CARD=PCH,DEV=0 HDA Intel PCH, ALC665 Analog Front speakers surround40:CARD=PCH,DEV=0 HDA Intel PCH, ALC665 Analog 4.0 Surround output to Front and Rear speakers surround41:CARD=PCH,DEV=0 HDA Intel PCH, ALC665 Analog 4.1 Surround output to Front, Rear and Subwoofer speakers surround50:CARD=PCH,DEV=0 HDA Intel PCH, ALC665 Analog 5.0 Surround output to Front, Center and Rear speakers surround51:CARD=PCH,DEV=0 HDA Intel PCH, ALC665 Analog 5.1 Surround output to Front, Center, Rear and Subwoofer speakers surround71:CARD=PCH,DEV=0 HDA Intel PCH, ALC665 Analog 7.1 Surround output to Front, Center, Side, Rear and Woofer speakers iec958:CARD=PCH,DEV=0 HDA Intel PCH, ALC665 Digital IEC958 (S/PDIF) Digital Audio Output hdmi:CARD=PCH,DEV=0 HDA Intel PCH, HDMI 0 HDMI Audio Output dmix:CARD=PCH,DEV=0 HDA Intel PCH, ALC665 Analog Direct sample mixing device dmix:CARD=PCH,DEV=1 HDA Intel PCH, ALC665 Digital Direct sample mixing device dmix:CARD=PCH,DEV=3 HDA Intel PCH, HDMI 0 Direct sample mixing device dsnoop:CARD=PCH,DEV=0 HDA Intel PCH, ALC665 Analog Direct sample snooping device dsnoop:CARD=PCH,DEV=1 HDA Intel PCH, ALC665 Digital Direct sample snooping device dsnoop:CARD=PCH,DEV=3 HDA Intel PCH, HDMI 0 Direct sample snooping device hw:CARD=PCH,DEV=0 HDA Intel PCH, ALC665 Analog Direct hardware device without any conversions hw:CARD=PCH,DEV=1 HDA Intel PCH, ALC665 Digital Direct hardware device without any conversions hw:CARD=PCH,DEV=3 HDA Intel PCH, HDMI 0 Direct hardware device without any conversions plughw:CARD=PCH,DEV=0 HDA Intel PCH, ALC665 Analog Hardware device with all software conversions plughw:CARD=PCH,DEV=1 HDA Intel PCH, ALC665 Digital Hardware device with all software conversions plughw:CARD=PCH,DEV=3 HDA Intel PCH, HDMI 0 Hardware device with all software conversions

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• Tech Article: Powerful Logging in Java ME

  - by OracleTechnologyNetwork

  Learn how to add logging to your Java ME MIDlets with Microlog, an open-source logging library.

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• java.lang.ClassCastException: $Proxy99 cannot be cast

  - by svaret

  Hi, I am using JBoss4.2.2 and java6. The deployed ear's name is apa.ear In a servlet I have the following code line: placeBid = (PlaceBid) context.lookup("apa/" + PlaceBid.class.getSimpleName() + "/remote"); I have a generated jboss-app.xml like this: <jboss-app> <loader-repository>apa:app=ejb3</loader-repository> </jboss-app> When trying to get the PlaceBid via the context I get this exception java.lang.ClassCastException: $Proxy99 cannot be cast to se.nextit.actionbazaar.buslogic.PlaceBid The PlaceBid interface looks like this: @Remote public interface PlaceBid { Long addBid(String userId, Long itemId, Double bidPrice); } When I run the example coming with EJB3 in action it works. EJB3 in action sample code comes with ant building. I want to use Maven so I have rearranged the code some. However, I don't understan what I am doing wrong here. I have some thoughts about the jboss-app.xml file. I am not sure of how its content should look like. Grateful for any help. Best wishes Lasse

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• Java Transaction Service without the application server

  - by johnny

  Is it possible to have a Java standalone application (no application server attached) that exposes some operations that a client can call and be the one to manage the transactions? I was thinking this application to expose JNDI resources and get a hold of a java:comp/UserTransaction from there, get also a bean from there and call methods A, B and C on it and coordinate the transaction from the client? The application I'm writing isn't complex enough so that I need a big application server around it so I'm thinking to have a standalone JTS inside it that the client could interact with from a transactions point of view. I don't have much experience with distributed transactions and don't really know how to tackle the issue. Is it even possible? Am I getting myself into something beyond what a mere mortal (programmer) can handle? How can I approach this?

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• ca-certificates-java fails to install

  - by Jonas

  I use a VPS with Ubuntu Server 10.10. I want to use Jetty and run the command sudo apt-get install jetty but it fails because the installation encounted errors while processing ca-certificates-java. I have tried to install the failed package with: sudo apt-get install ca-certificates-java How can I solve this? I have run sudo apt-get update and sudo apt-get upgrade but I get the same errors after that. I have also installed Ubuntu Server x64 on a VirtualBox, but the two Ubuntu Server 10.10 has different kernel versions (2.6.35 on VirtualBox and 2.6.18 on my VPS). And on VirtualBox I can install Jetty without any problems.

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• Feasibility to take over a JavaMe Project by Coders who have no experience in JavaMe

  - by Stephenmjm

  As the original JavaMe team will leave to do other items. The JavaMe project will be taken over by some guys knowing nothing about JavaMe. Transition period: One month About this JavaMe project: about 3.5 million lines of code (more than 180 java file, SourceCode is 8.5KB in total) using the Polish, Proguard document: The JavaMe project itself have no document. No UML map. Difficulties I guess: familiar with the JavaMe, this should be okay In order to do the further development. We need to Read the sourceCode ---- It's not easy to read 3.5 million lines of code having not enough comment Adaptation work for more than 100 phone These are the questions, thank you! In the case of our guys have no experience in JavaMe, Is one month too hasty? In order to take the job in time . What we should ask the original JavaMe team to do . Considering we hava no experience in JavaMe. The complication we taking the Adaptation work without the original JavaMe team? Any other suggestions?

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• Eloqua API Full Code Example in JAVA

  - by Shawn Spencer

  Is there anyone out there who has mastered to retrieve some data programmatically from Eloqua? First of all, I'm more or less a newbie, as far as JAVA. I can follow tutorials, take directions and will Google till my fingers bleed. I understand the basics and am slightly familiar with OOP. My main problem is that I have a Friday deadline (and tomorrow is Thanksgiving). At any rate, all the Eloqua code snippets (that I've been able to find) illustrate one aspect of a specific issue, and that's it. In my case, I would greatly appreciate a JAVA project of some sort, with all the necessary files to do web services (WSDL, SOAP and perhaps WSIT) and the main class and all that included. No, I don't want you to do my work for me! Just give me enough to find my way around, enter the information I need to retrieve and all that. I'll take it from there. Any pointers, links or suggestions?

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• Java ExecutorService java heap space ptoblems

  - by Sergey Aganezov jr

  I have a little bit of a problem in a multitasking java department. I have a class, called public class ThreadWorker implements Runnable { //some code in here public void run(){ // invokes some recursion method in the ThreadWorker itself, // which will stop eventually { } all in all, pretty simple "worker" that can work on it's on. To work with threads I'm using public static int THREAD_NUMBER = 4; public static ExecutorServide es = Executors.newFixedThreadPool(THREAD_NUMBER); adding instances of ThreadWroker class happens here: public void recursiveMethod(Arraylist<Integers> elements, MyClass data){ if (elements.size() == 0 && data.qualifies()){ ThreadWorker tw = new ThreadWorker(data); es.execute(tw); return; } for (int i=0; i< elements.size(); i++){ // some code to prevent my problem MyClass data1 = new MyClass(data); MyClass data2 = new MyClass(data); ArrayList<Integer> newElements = (ArrayList<Integer>)elements.clone(); data1.update(elements.get(i)); data2.update(-1 * elements.get(i)); newElements.remove(i); recursiveMethod(newElements, data1); recursiveMethod(newElements, data2); { } and the problem is that the depth of the recursion tree is quite big, so as it's width, so a lot of ThreadWorkers are added to the ExecutorService, so after some time on the big input a get Exception in thread "pool-1-thread-2" java.lang.OutOfMemoryError: Java heap space which is caused, as I think because of a ginormous number of ThreadWorkers i'm adding to ExecutorSirvice to be executed, so it runs out of memory. Every ThreadWorker takes about 40 Mb of RAM for all it needs. Is there a method to get how many threads (instances of classes implementing runnable interface) have been added to ExecutorService? So I can add it in the shown above code (int the " // some code to prevent my problem"), as while ("number of threads in the ExecutorService" > 10){ Thread.sleep(10000); } so I won't go to deep or to broad with my recursion and prevent those exception-throwing situations. Sincerely, Sergey Aganezov jr.

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• Question about casting a class in Java with generics

  - by Florian F

  In Java 6 Class<? extends ArrayList<?>> a = ArrayList.class; gives and error, but Class<? extends ArrayList<?>> b = (Class<? extends ArrayList<?>>)ArrayList.class; gives a warning. Why is (a) an error? What is it, that Java needs to do in the assignment, if not the cast shown in (b)? And why isn't ArrayList compatible with ArrayList? I know one is "raw" and the other is "generic", but what is it you can do with an ArrayList and not with an ArrayList, or the other way around?

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• Java: volatile guarantees and out-of-order execution

  - by WizardOfOdds

  Note that this question is solely about the volatile keyword and the volatile guarantees: it is not about the synchronized keyword (so please don't answer "you must use synchronize" for I don't have any issue to solve: I simply want to understand the volatile guarantees (or lack of guarantees) regarding out-of-order execution). Say we have an object containing two volatile String references that are initialized to null by the constructor and that we have only one way to modify the two String: by calling setBoth(...) and that we can only set their references afterwards to non-null reference (only the constructor is allowed to set them to null). For example (it's just an example, there's no question yet): public class SO { private volatile String a; private volatile String b; public SO() { a = null; b = null; } public void setBoth( @NotNull final String one, @NotNull final String two ) { a = one; b = two; } public String getA() { return a; } public String getB() { return b; } } In setBoth(...), the line assigning the non-null parameter "a" appears before the line assigning the non-null parameter "b". Then if I do this (once again, there's no question, the question is coming next): if ( so.getB() != null ) { System.out.println( so.getA().length ); } Am I correct in my understanding that due to out-of-order execution I can get a NullPointerException? In other words: there's no guarantee that because I read a non-null "b" I'll read a non-null "a"? Because due to out-of-order (multi)processor and the way volatile works "b" could be assigned before "a"? volatile guarantees that reads subsequent to a write shall always see the last written value, but here there's an out-of-order "issue" right? (once again, the "issue" is made on purpose to try to understand the semantics of the volatile keyword and the Java Memory Model, not to solve a problem).

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• Java Cryptography Extension

  - by Adam Tannon

  I was told that in order to support AES256 encryption inside my Java app that I would need the JCE with Unlimited Strength Jurisdiction Policy Files. I downloaded this from Oracle and unzipped it and I'm only seeing 2 JARs: local_policy.jar; and US_export_polic.jar I just want to confirm I'm not missing anything here! My understanding (after reading the README.txt) is that I just drop these two into my <JAVA_HOME>/lib/security/ directory and they should be installed. By the names of these JARs I have to assume that its not the Java Crypto API that cannot handle AES256, but it's in fact a legal issue, perhaps? And that these two JARs basically tell the JRE "yes, it's legally-acceptable to run this level of crypto (AES256)." Am I correct or off-base?

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• Resources for game networking in Java

  - by pudelhund

  I am currently working on a Java multiplayer game. The game itself (single player) already works perfectly fine and so does the chat. The only thing that is really missing is the multiplayer part. Sadly I am absolutely clueless on where to start with that. I roughly know that I will have to work with packages, and I also know many things about streaming etc (chat is already working). Oh and it should - according to this article - be a UDP server. My problem is that I can't find any resources on how to do this. A tutorial (book or website) would be perfect, alternatively a good example of an open source client/server (in Java of course) would be fine as well. If you feel like doing something helpful I'd also really appreciate someone "privately" teaching me via email or some chat program :) Thank you!

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• Can't decide between Java or Python for college [on hold]

  - by Will Harrison

  I'm returning to college in about a month for Computer Science. My problem is, I have been programming on the web since I left (4 years ago), using PHP, ASP.NET, and JavaScript. I want to bone up on a more general purpose language that is cross platform before I begin. I would also like to be using a language that is common at my school and I know that they teach the students C++, Java, and Python. I would like to choose between Java or Python but I'm not sure which one. What do you think would be better based on job prospects in the next 2 years and community?

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• java 2D and swing

  - by user384706

  Hi, I have trouble understanding a fundamental concept in Java 2D. To give a specific example: One can customize a swing component via implementing it's own version of the method paintComponent(Graphics g) Graphics is available to the body of the method. Question: What is exactly this Graphics object, I mean how it is related to the object that has the method paintComponent? Ok, I understand that you can do something like: g.setColor(Color.GRAY); g.fillOval(0, 0, getWidth(), getHeight()); To get a gray oval painted. What I can not understand is how is the Graphics object related to the component and the canvas. How is this drawing actually done? Another example: public class MyComponent extends JComponent { protected void paintComponent(Graphics g) { System.out.println("Width:"+getWidth()+", Height:"+getHeight()); } public static void main(String args[]) { JFrame f = new JFrame("Some frame"); f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); f.setSize(200, 90); MyComponent component = new MyComponent (); f.add(component); f.setVisible(true); } } This prints Width:184, Height:52 What does this size mean? I have not added anything to the frame of size(200,90). Any help on this is highly welcome Thanks

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• Using Java classes in JRuby

  - by kerry

  I needed to do some reflection today so I fired up interactive JRuby (jirb) to inspect a jar.  Surprisingly, I couldn’t remember exactly how to use Java classes in JRuby.  So I did some searching on the internet and found this to be a common question with many answers.  So I figure I will document it here in case I forget how in the future. Add it’s folder to the load path, require it, then use it! $: << '/path/to/my/jars/' require 'myjar' # so we don't have to reference it absolutely every time (optional) include Java::com.goodercode my_object = SomeClass.new

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• Approaching Java/JVM internals

  - by spinning_plate

  I've programmed in Java for about 8 years and I know the language quite well as a developer, but my goal is to deepen my knowledge of the internals. I've taken undergraduate courses in PL design, but they were very broad academic overviews (in Scheme, IIRC). Can someone suggest a route to start delving into the details? Specifically, are there particular topics (say, garbage collection) that might be more approachable or be a good starting point? Is there a decent high-level book on the internals of the JVM and the design of the Java programming language? My current approach is going to be to start with the JVM spec and research as needed.

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• Programming in academic environment vs industry environment [closed]

  - by user200340

  Possible Duplicate: Differences between programming in school vs programming in industry? This is a general discussion about programming in the industry environment. The background story is that my colleague sent me a very interesting article called "10 Things Entrepreneurs Don’t Learn in College." The first point in that post is about the author's experience of programming in the academic environment vs industry environment. After finishing a 4 year Computer Science degree course, I am currently working in the academic environment as a developer, mainly writing Java, J2EE, Javascript code. I know there are differences between academic programming and industry programming, but I was shocked after reading that post. Trying to avoid this happening on me in the future, or the others. Can anyone from industry give some general advice about how to program in industry. For example, What exactly happens when a task is received? What is the flow from the beginning to the end? What are the main differences between the programming in industry and academia? Is it more structured? Are more frameworks used? It would be great if some code examples could be given. Thanks.

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• Java Application for handling records(CRUD)

  - by LivingThing

  I am new to JavaEE and am faced with a tight situation here. I have to develop a Java application for (CRUD) handling records and saving and loading an XML concerning that record. Obviously, I won't be asking you to do this for me. What I would be asking you is to give me some hints/pointer. Initially I thought JAXB would be enough for this but after putting a lot of time learning it and implementing the program I realized that it just can create the XML and read it but for update, delete I would have to do something else. Even if it wasn't for update and delete features requirement for my project I would still think that by just using JAXB is not a good implementation. I was wondering if "REST with Java (JAX-RS) using Jersey" should do the trick for me. ?

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• Immutable Method in Java

  - by Chris Okyen

  In Java, there is the final keyword in lieu of the const keyword in C and C++. In the latter languages there are mutable and immutable methods such as stated in the answer by Johannes Schaub - litb to the question How many and which are the uses of “const” in C++? Use const to tell others methods won't change the logical state of this object. struct SmartPtr { int getCopies() const { return mCopiesMade; } }ptr1; ... int var = ptr.getCopies(); // returns mCopiesMade and is specified that to not modify objects state. How is this performed in Java?

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• DI/IoC in Java for a .NET'er used to Castle.Windsor

  - by Ciddan

  Is there a Java DI container that works in a similar way to the most excellent Castle.Windsor container on the .NET side? The Java containers I've had a look at all seem to rely on annotations (Guice) within my services, which I don't dig all that much - I'd like to go POJO all the way if possible. Spring on the other hand can do without the annotations, but it requires a lot of XML. XML configuration != maintainability. One of the really nice things about Castle.Windsor is the wiring you're able to set up in code with Installers, auto wiring based on naming conventions and whatnot. Ideally the container should also support lifecycle management and configuration; i.e. registering components as transient, singleton, pooled etc. Another bonus would be support for interceptors. Any tips would be greatly appreciated.

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