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• MySQL Connection error in spring MVC

  - by nidhi

  i am working on spring MVC facing the following exception while connecting to my sql. Blocorg.springframework.jdbc.CannotGetJdbcConnectionException: Could not get JDBC Connection; nested exception is org.apache.commons.dbcp.SQLNestedException: Cannot create PoolableConnectionFactory (Communications link failure The last packet sent successfully to the server was 0 milliseconds ago. The driver has not received any packets from the server.) at org.springframework.jdbc.datasource.DataSourceUtils.getConnection(DataSourceUtils.java:80) at org.springframework.jdbc.core.JdbcTemplate.execute(JdbcTemplate.java:572) at org.springframework.jdbc.core.JdbcTemplate.query(JdbcTemplate.java:636) at org.springframework.jdbc.core.JdbcTemplate.query(JdbcTemplate.java:665) at org.springframework.jdbc.core.JdbcTemplate.query(JdbcTemplate.java:673) at org.springframework.jdbc.core.JdbcTemplate.queryForObject(JdbcTemplate.java:728) at org.springframework.jdbc.core.JdbcTemplate.queryForObject(JdbcTemplate.java:744) at org.springframework.jdbc.core.JdbcTemplate.queryForInt(JdbcTemplate.java:775) at com.trackmeetings.dao.InboxDAOImpl.getNotificationsCount(InboxDAOImpl.java:51) at com.trackmeetings.service.InboxService.getNotificationsCount(InboxService.java:56) at com.trackmeetings.controller.HomePageController.getLogInUserDetails(HomePageController.java:80) at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39) at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25) at java.lang.reflect.Method.invoke(Method.java:597) at org.springframework.web.bind.annotation.support.HandlerMethodInvoker.invokeHandlerMethod(HandlerMethodInvoker.java:175) at org.springframework.web.servlet.mvc.annotation.AnnotationMethodHandlerAdapter.invokeHandlerMethod(AnnotationMethodHandlerAdapter.java:421) at org.springframework.web.servlet.mvc.annotation.AnnotationMethodHandlerAdapter.handle(AnnotationMethodHandlerAdapter.java:409) at org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:774) at org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:719) at org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:644) at org.springframework.web.servlet.FrameworkServlet.doPost(FrameworkServlet.java:560) at javax.servlet.http.HttpServlet.service(HttpServlet.java:710) at javax.servlet.http.HttpServlet.service(HttpServlet.java:803) at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:290) at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206) at com.trackmeetings.util.NoCacheFilter.doFilter(NoCacheFilter.java:56) at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:235) at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206) at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:233) at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:175) at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:128) at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:102) at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:109) at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:286) at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:844) at org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.process(Http11Protocol.java:583) at org.apache.tomcat.util.net.JIoEndpoint$Worker.run(JIoEndpoint.java:447) at java.lang.Thread.run(Thread.java:680) Caused by: org.apache.commons.dbcp.SQLNestedException: Cannot create PoolableConnectionFactory (Communications link failure The last packet sent successfully to the server was 0 milliseconds ago. The driver has not received any packets from the server.) at org.apache.commons.dbcp.BasicDataSource.createDataSource(BasicDataSource.java:1225) at org.apache.commons.dbcp.BasicDataSource.getConnection(BasicDataSource.java:880) at org.springframework.jdbc.datasource.DataSourceUtils.doGetConnection(DataSourceUtils.java:111) at org.springframework.jdbc.datasource.DataSourceUtils.getConnection(DataSourceUtils.java:77) ... 38 more Caused by: com.mysql.jdbc.exceptions.jdbc4.CommunicationsException: Communications link failurekquote how can i resolve this. please help,

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• How does Java handle multithreading?

  - by badcodenotreat

  How does Java decide which core to assign a thread or a process? Is there any way to control that? to prevent two large threads from executing on the same core? Basically what I am asking is for further information on either how multi-threading works in Java, or how to control it within Java.

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• Recommendations for easy jQuery Autocomplete using AJAX that will set form value based on selected t

  - by Ben Dauphinee

  I am looking for suggestions on the easiest to set up and use AJAX Autocomplete plugin that will use JSON key/value pairs. I want a plugin that supports setting the form value to the key of the name selected. I have been working with jQuery UI Autocomplete, but it is a total pain in the rear to get this working. Does anyone know of anything that actually supports this out of the box, or have a thread or page where this is already detailed?

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• mysql result set joining existing table

  - by Yang

  is there any way to avoid using tmp table? I am using a query with aggregate function (sum) to generate the sum of each product: the result looks like this: product_name | sum(qty) product_1 | 100 product_2 | 200 product_5 | 300 now i want to join the above result to another table called products. so that i will have a summary like this: product_name | sum(qty) product_1 | 100 product_2 | 200 product_3 | 0 product_4 | 0 product_5 | 300 i know 1 way of doing this is the dump the 1st query result to a temp table then join it with products table. is there a better way?

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• preverification error in netbeans and eclipse while including j2ssh maverick

  - by user568651

  problem-- Preverification errors: Error preverifying class com.maverick.ssh.components.ComponentFactory java/lang/NoClassDefFoundError: java/lang/Cloneable j2ssh Unknown Java Problem this error comes while j2ssh is added as a library in my application. console-- Uncaught exception: java.lang.NoClassDefFoundError: com/maverick/ssh/LicenseManager - mobilessh.MobileSSH.run(), bci=2 - java.lang.Thread.run(), bci=11 this error shows both in eclipse and netbeans ide 6.9.1 jdk 1.6 installed.. wtk 3.0 and 2.5.2.

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• Help Required on track active directory changes

  - by Faiasl Iqbal

  I am going to write a small software to track active directory changes. I need an expert opinion from you. I want to display information about What ( with before and after values), When, Where and Who of any change in AD. I am going to implement this by one of the following way 1. Change Notification Control 2. DirSync Control Both of these method give us the attribute that are changed and we can compare new values with some dump in SQL Server to get old values. My problem is how can I find “Who” has made this change. I have tried but there is no attribute “LastModifiedBy” in active directory. Please give your opinion how can I track who has made this change. --Faisal Iqbal

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• COMException Problem

  - by Jack Harvin

  Wondering if anyone could help with my problem. Below is the code, and after the code an explination of where the exception is thrown. using System; using System.Collections.Generic; using System.ComponentModel; using System.Data; using System.Drawing; using System.Linq; using System.Text; using System.Windows.Forms; using System.Web; using WatiN.Core; using System.Threading; using System.IO; namespace WindowsFormsApplication1 { public partial class Form1 : System.Windows.Forms.Form { public Form1() { InitializeComponent(); } private void Form1_Load(object sender, EventArgs e) { } private void button1_Click(object sender, EventArgs e) { Thread t = new Thread(createApplications); Settings.AutoStartDialogWatcher = false; t.SetApartmentState(System.Threading.ApartmentState.STA); t.Start(); } private void createApplications() { createApp("username", "password", "Test App", "This is just a test description", "http:/mysite.com"); } private void createApp(String username, String password, String appName, String description, String appUrl) { var currentBrowser = new IE("http://mysite.com/login/php"); currentBrowser.TextField(Find.ById("username")).TypeText(username); currentBrowser.TextField(Find.ById("password")).TypeText(password); currentBrowser.Button(Find.ById("submit")).Click(); currentBrowser.GoTo("http://mysite.com/createmusicapp.php"); currentBrowser.TextField(Find.ById("application_name")).TypeText(appName); currentBrowser.TextField(Find.ById("application_description")).TypeText(description); currentBrowser.TextField(Find.ById("application_url")).TypeText(appUrl); currentBrowser.RadioButton(Find.ById("client_application_desktop_1")).Click(); currentBrowser.RadioButton(Find.ById("client_application_is_writable_1")).Click(); WatiN.Core.Image captchaImage = currentBrowser.Div(Find.ById("recaptcha_image")).Image(Find.ByStyle("display", "block")); Form2 captcha = new Form2(captchaImage.Src); captcha.ShowDialog(); } } } The exception is thrown on this line: currentBrowser.TextField(Find.ById("username")).TypeText(username); BUT, it's thrown when it gets to this line: captcha.ShowDialog(); It logs in, and fills in the app details and Form2 loads fine, but once loaded, after around 2-3 seconds the exception happens. I am wondering if it's anything to do with the threads? But I wouldn't know how to solve it if it was. The complete exception thrown is: The object invoked has disconnected from its clients. (Exception from HRESULT: 0x80010108 (RPC_E_DISCONNECTED))

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• How can Perl's XML::Simple ignore HTML embedded in XML?

  - by Miriam Raphael Roberts

  I have an XML file that I am pulling from the web and parsing. One of the items in the XML is a 'content' value that has HTML. I am using XML::Simple::XMLin to parse the file like so: $xml= eval { $data->XMLin($xmldata, forcearray => 1, suppressempty=> +'') }; When I use Data::Dumper to dump the hash, I discovered that SimpleXML is parsing the HTML into the hash tree: 'content' = { 'div' = [ { 'xmlns' = 'http://www.w3.org/1999/xhtml', 'p' = [ { 'a' = [ { 'href' = 'http://miamiherald.typepad.com/.a/6a00d83451b26169e20133ec6f4491970b-pi', 'style' = 'FLOAT: left', 'img' = [ etc..... This is not what I want. I want to just grab content inside of this entry. How do I do this?

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• Mysql-how to update the "domain.com" in "[email protected]"

  - by w00t

  Hi there, In my database I have a lot of users who've misspelled their e-mail address. This in turn causes my postfix to bounce a lot of mails when sending the newsletter. Forms include (but are not limited to) "yaho.com", "yahho .com" etc. Very annoying! So i have been trying to update those record to the correct value. After executing select email from users where email like '%@yaho%' and email not like '%yahoo%'; and getting the list, I'm stuck because I do not know how to update only the yaho part. I need the username to be left intact. So I thought I would just dump the database and use vim to replace, but I cannot escape the @ symbol.. BTW, how do I select all email addresses written in CAPS? select upper(email) from users; would just transform everything into CAPS, whereas I just needed to find out the already-written-in-CAPS mails.

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• Can't find method in the activity

  - by Synesso

  I'm starting with Scala + Android. I'm trying to wire a button action to a button without the activity implementing View.OnClickListener. The button click fails at runtime because the method cannot be found. The document I'm working through says that I need only declare a public void method taking a View on the action, and use that method name in the layout. What have I done wrong? MainActivity.scala package net.badgerhunt.hwa import android.app.Activity import android.os.Bundle import android.widget.Button import android.view.View import java.util.Date class MainActivity extends Activity { override def onCreate(savedInstanceState: Bundle) = { super.onCreate(savedInstanceState) setContentView(R.layout.main) } def calculate(button: View): Unit = println("calculating with %s ...".format(button)) } res/layout/main.xml <?xml version="1.0" encoding="utf-8"?> <Button xmlns:android="http://schemas.android.com/apk/res/android" android:id="@+id/button" android:text="" android:onClick="calculate" android:layout_width="fill_parent" android:layout_height="fill_parent"/> the failure onclick D/AndroidRuntime( 362): Shutting down VM W/dalvikvm( 362): threadid=3: thread exiting with uncaught exception (group=0x4001b188) E/AndroidRuntime( 362): Uncaught handler: thread main exiting due to uncaught exception E/AndroidRuntime( 362): java.lang.IllegalStateException: Could not find a method calculate(View) in the activity E/AndroidRuntime( 362): at android.view.View$1.onClick(View.java:2020) E/AndroidRuntime( 362): at android.view.View.performClick(View.java:2364) E/AndroidRuntime( 362): at android.view.View.onTouchEvent(View.java:4179) E/AndroidRuntime( 362): at android.widget.TextView.onTouchEvent(TextView.java:6540) E/AndroidRuntime( 362): at android.view.View.dispatchTouchEvent(View.java:3709) E/AndroidRuntime( 362): at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:884) E/AndroidRuntime( 362): at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:884) E/AndroidRuntime( 362): at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:884) E/AndroidRuntime( 362): at com.android.internal.policy.impl.PhoneWindow$DecorView.superDispatchTouchEvent(PhoneWindow.java:1659) E/AndroidRuntime( 362): at com.android.internal.policy.impl.PhoneWindow.superDispatchTouchEvent(PhoneWindow.java:1107) E/AndroidRuntime( 362): at android.app.Activity.dispatchTouchEvent(Activity.java:2061) E/AndroidRuntime( 362): at com.android.internal.policy.impl.PhoneWindow$DecorView.dispatchTouchEvent(PhoneWindow.java:1643) E/AndroidRuntime( 362): at android.view.ViewRoot.handleMessage(ViewRoot.java:1691) E/AndroidRuntime( 362): at android.os.Handler.dispatchMessage(Handler.java:99) E/AndroidRuntime( 362): at android.os.Looper.loop(Looper.java:123) E/AndroidRuntime( 362): at android.app.ActivityThread.main(ActivityThread.java:4363) E/AndroidRuntime( 362): at java.lang.reflect.Method.invokeNative(Native Method) E/AndroidRuntime( 362): at java.lang.reflect.Method.invoke(Method.java:521) E/AndroidRuntime( 362): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:860) E/AndroidRuntime( 362): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:618) E/AndroidRuntime( 362): at dalvik.system.NativeStart.main(Native Method) E/AndroidRuntime( 362): Caused by: java.lang.NoSuchMethodException: calculate E/AndroidRuntime( 362): at java.lang.ClassCache.findMethodByName(ClassCache.java:308) E/AndroidRuntime( 362): at java.lang.Class.getMethod(Class.java:1014) E/AndroidRuntime( 362): at android.view.View$1.onClick(View.java:2017) E/AndroidRuntime( 362): ... 20 more

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• Spawning worker threads

  - by LB

  In C#, How would one go about spawning multiple threads and then sequentially adding results to a list before returning the entire result set? What are some best practices? I'm so far using an ManualResetEvent to signal when the last element has been processed by a thread. But when it returns, I need to have them consolidate the result sets in sequential order so that we don't get into contention issues with the return value list (total results).

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• Rails - asynchronous tasks, forked processes, best practices

  - by LisaPatton

  I'm using a Observer on my classes. When one of the records is created/updated I need to notfify another service (via a URL call). What is the best way to do this to avoid slowing down my class? Would using a gem liked delayed_job be overkill? In my Observer's after_update() / after_create() I just want to launch a thread that calls the URL...

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• How can you prevent an individual window being captured by Print-Screen?

  - by John

  I noticed a few times than when I print-screen while Media Player is running, the content is replaced with a grey rectangle. I've no idea if it's deliberate due to DRM, or some technical issue, but it struck me that being able to deliberately block screen-capture might be a useful feature in some scenarios. I am not looking to fix the problem, but replicate it! Is there some specific message each HWND gets for print-screen - does Windows do a special repaint or simply dump buffers to the clipboard? I guess I'd prefer to focus this on Win32 specifically, but as long as it's Windows-related then fine.

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• Multiple calendars on wordpress highlighting only specific categories

  - by zac

  Does anyone know of how, in wordpress, I could implement many calendars on a site, each only highlighting days that are within a specific category? I am hoping to setup multiple calendars, each tracking all of the events from their own cat. After researching for a while the best I could find was this thread: http://wordpress.org/support/topic/266627?replies=11 But it seems to peter out before there is ever a working solution

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• What's a good 2nd level cache for JEE applications?

  - by Hank

  Can anyone recommend a good 2nd level object caching solution for JEE 6 applications, and give background to your recommendation? I'm using JPA 2.0 as persistence provider. I am particularly worried about having to run the cache client as a single-thread / singleton bean. Is that the case? If so, is that an issue? I've good experience using memcached from a PHP webapp, but PHP is of course single-threaded, so that was never an issue...

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• Reading chunked data from HttpEntity

  - by Gagan

  I have the following code: HttpClient FETCHER HttpResponse response = FETCHER.execute(host, httpMethod); Im trying to read its contents to a string like this: HttpEntity entity = response.getEntity(); InputStream st = entity.getContent(); StringWriter writer = new StringWriter(); IOUtils.copy(st, writer); String content = writer.toString(); The problem is, when i fetch http://www.google.co.in/ page, the transfer encoding is chunked, and i get only the first chunk. It fetches till first "". How do i get all the chunks at once so i can dump the complete output and do some processing on it ?

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• Can managed and unmanaged C++/MFC be mixed in one dll?

  - by Walter Williams

  Previously we had software in MFC (VC6), VB6 and C# applications that needed to call the same engine written in C++ (and MFC). The engine required C++ for speed. At the time we decided to use COM as the interface because all three could use it with the least issues in marshalling, etc. Our MFC application is now deprecated and we have recently decided to dump VB6, so what we've got left is C#. We can just leave the COM engine as-is, but it would be nice to get away from COM registration, etc., and have a managed interface to work with. COM registration occasionally causes support issues if there is something wrong with the person's machine. Is it possible to have a dll with the existing unmanaged C++/MFC, and a .NET front end interface?

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• Rails CSV import, adding to a related table

  - by Jack

  Hi, I have a csv importing system on my app (used locally only) which parses the csv file line by line and adds the data to the database table. This is based on a tutorial here. require 'csv' def csv_import @parsed_file=CSV::Reader.parse(params[:dump][:file]) n = 0 @parsed_file.each_with_index do |row, i| next if i == 0 #ignore the first row course = Course.new course.title = row[0] course.unit_code = row[1] course.course_type = row[2] course.value = row[3] course.pass_mark = row[4] if course.save n = n+1 GC.start if n%50==0 end flash.now[:message] = "CSV Import Successful, #{n} new courses added to the database." end redirect_to(courses_url) end This is all in the courses controller and works fine. There is a relationship that courses HABTM years and years HABTM courses. In the csv file (effectively in row[5] to row[8]) are the year_id s. Is there a way that I can add this within the method above. I am confused as to how to loop over the 4 items and add them to the courses_years table. Thank you Jack

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• Synchronising local and remote DB

  - by nico

  Hi everyone, I have a general question about DB synchronisation. So, I'm developing a website locally (PHP + MySQL) and I would like to be able to synchronise at least the structure (and maybe the contents) of the two DB when one of the two is changed (normally I would change the local copy). Right now what I'm doing is to use mysqldump to dump the modified tables and then import them in the remote DB or do it by hand if the changes are minimal. However I find this tedious and error-prone. For the PHP I'm currently using Quanta+ which has the handy feature of finding files that have changed and just upload those. Is there something similar for MySQL? Otherwise how do you keep your DBs synchronised? Thanks nico PS: I'm sorry if this was already asked, I saw other questions that deal with similar topics, but couldn't really find an answer.

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• Is the first persistance of an Entity Data Model in EF 4.0 slower due to the connection cost ?

  - by Scott Davies

  Hi, I've got a console app written that persists an object graph via Entity Framework 4.0. I loop through this to dump the execution times for each persistance. The first persistance is always the largest. Is this due to EF making the initial connection to the database and/or JIT'ing ? Here's a sample of the output: Persisted graph in **3318** millseconds. Persisted graph in 25 millseconds. Persisted graph in 26 millseconds. Persisted graph in 22 millseconds. Thanks, Scott

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• wamp server not working? or bad php code

  - by lclaud

  I have this PHP code: <?php $username="root"; $password="******";// censored out $database="bazadedate"; mysql_connect("127.0.0.1",$username,$password); // i get unknown constant localhost if used instead of the loopback ip @mysql_select_db($database) or die( "Unable to select database"); $query="SELECT * FROM backup"; $result=mysql_query($query); $num=mysql_numrows($result); $i=0; $raspuns=""; while ($i < $num) { $data=mysql_result($result,$i,"data"); $suma=mysql_result($result,$i,"suma"); $cv=mysql_result($result,$i,"cv"); $det=mysql_result($result,$i,"detaliu"); $raspuns = $raspuns."#".$data."#".$suma."#".$cv."#".$det."@"; $i++; } echo "<b> $raspuns </b>"; mysql_close(); ?> And it should return a single string containing all data from the table. But it says "connection reset when loading page". the log is : [Tue Jun 15 16:20:31 2010] [notice] Parent: child process exited with status 255 -- Restarting. [Tue Jun 15 16:20:31 2010] [notice] Apache/2.2.11 (Win32) PHP/5.3.0 configured -- resuming normal operations [Tue Jun 15 16:20:31 2010] [notice] Server built: Dec 10 2008 00:10:06 [Tue Jun 15 16:20:31 2010] [notice] Parent: Created child process 2336 [Tue Jun 15 16:20:31 2010] [notice] Child 2336: Child process is running [Tue Jun 15 16:20:31 2010] [notice] Child 2336: Acquired the start mutex. [Tue Jun 15 16:20:31 2010] [notice] Child 2336: Starting 64 worker threads. [Tue Jun 15 16:20:31 2010] [notice] Child 2336: Starting thread to listen on port 80. [Tue Jun 15 16:20:35 2010] [notice] Parent: child process exited with status 255 -- Restarting. [Tue Jun 15 16:20:35 2010] [notice] Apache/2.2.11 (Win32) PHP/5.3.0 configured -- resuming normal operations [Tue Jun 15 16:20:35 2010] [notice] Server built: Dec 10 2008 00:10:06 [Tue Jun 15 16:20:35 2010] [notice] Parent: Created child process 1928 [Tue Jun 15 16:20:35 2010] [notice] Child 1928: Child process is running [Tue Jun 15 16:20:35 2010] [notice] Child 1928: Acquired the start mutex. [Tue Jun 15 16:20:35 2010] [notice] Child 1928: Starting 64 worker threads. [Tue Jun 15 16:20:35 2010] [notice] Child 1928: Starting thread to listen on port 80. Any idea why it outputs nothing?

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• How to set priority to an operation contract in WCF

  - by Kamarey

  Is it possible to set priority to a thread that currently run some specific contract operation? Say I have WCF service with Func1, Func2 and Func3. I want that Func1 will have more priority over Func2 and Func3.

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• Java socketserver: How to handle many incoming connections?

  - by SlappyTheFish

  I am writing a simple multithreaded socketserver and I am wondering how best to handle incoming connections: create a new thread for each new connection. The number of concurrent threads would be limited and waiting connections limited by specifying a backlog add all incoming connections into a queue and have a pool of worker threads that process the queue I am inclined to go for option 2 because I really don't want to refuse any connections, even under high loads, but I am wondering if there are any considerations I should be aware of with accepting effectively unlimited connections?

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• Huge file in Clojure and Java heap space error

  - by trzewiczek

  I posted before on a huge XML file - it's a 287GB XML with Wikipedia dump I want ot put into CSV file (revisions authors and timestamps). I managed to do that till some point. Before I got the StackOverflow Error, but now after solving the first problem I get: java.lang.OutOfMemoryError: Java heap space error. My code (partly taken from Justin Kramer answer) looks like that: (defn process-pages [page] (let [title (article-title page) revisions (filter #(= :revision (:tag %)) (:content page))] (for [revision revisions] (let [user (revision-user revision) time (revision-timestamp revision)] (spit "files/data.csv" (str "\"" time "\";\"" user "\";\"" title "\"\n" ) :append true))))) (defn open-file [file-name] (let [rdr (BufferedReader. (FileReader. file-name))] (->> (:content (data.xml/parse rdr :coalescing false)) (filter #(= :page (:tag %))) (map process-pages)))) I don't show article-title, revision-user and revision-title functions, because they just simply take data from a specific place in the page or revision hash. Anyone could help me with this - I'm really new in Clojure and don't get the problem.

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• Implementing a robust async stream reader for a console

  - by Jon

  I recently provided an answer to this question: C# - Realtime console output redirection. As often happens, explaining stuff (here "stuff" was how I tackled a similar problem) leads you to greater understanding and/or, as is the case here, "oops" moments. I realized that my solution, as implemented, has a bug. The bug has little practical importance, but it has an extremely large importance to me as a developer: I can't rest easy knowing that my code has the potential to blow up. Squashing the bug is the purpose of this question. I apologize for the long intro, so let's get dirty. I wanted to build a class that allows me to receive input from a Stream in an event-based manner. The stream, in my scenario, is guaranteed to be a FileStream and there is also an associated StreamReader already present to leverage. The public interface of the class is this: public class MyStreamManager { public event EventHandler<ConsoleOutputReadEventArgs> StandardOutputRead; public void StartSendingEvents(); public void StopSendingEvents(); } Obviously this specific scenario has to do with a console's standard output. StartSendingEvents and StopSendingEvents do what they advertise; for the purposes of this discussion, we can assume that events are always being sent without loss of generality. The class uses these two fields internally: protected readonly StringBuilder inputAccumulator = new StringBuilder(); protected readonly byte[] buffer = new byte[256]; The functionality of the class is implemented in the methods below. To get the ball rolling: public void StartSendingEvents(); { this.stopAutomation = false; this.BeginReadAsync(); } To read data out of the Stream without blocking, and also without requiring a carriage return char, BeginRead is called: protected void BeginReadAsync() { if (!this.stopAutomation) { this.StandardOutput.BaseStream.BeginRead( this.buffer, 0, this.buffer.Length, this.ReadHappened, null); } } The challenging part: BeginRead requires using a buffer. This means that when reading from the stream, it is possible that the bytes available to read ("incoming chunk") are larger than the buffer. Since we are only handing off data from the stream to a consumer, and that consumer may well have inside knowledge about the size and/or format of these chunks, I want to call event subscribers exactly once for each chunk. Otherwise the abstraction breaks down and the subscribers have to buffer the incoming data and reconstruct the chunks themselves using said knowledge. This is much less convenient to the calling code, and detracts from the usefulness of my class. Edit: There are comments below correctly stating that since the data is coming from a stream, there is absolutely nothing that the receiver can infer about the structure of the data unless it is fully prepared to parse it. What I am trying to do here is leverage the "flush the output" "structure" that the owner of the console imparts while writing on it. I am prepared to assume (better: allow my caller to have the option to assume) that the OS will pass me the data written between two flushes of the stream in exactly one piece. To this end, if the buffer is full after EndRead, we don't send its contents to subscribers immediately but instead append them to a StringBuilder. The contents of the StringBuilder are only sent back whenever there is no more to read from the stream (thus preserving the chunks). private void ReadHappened(IAsyncResult asyncResult) { var bytesRead = this.StandardOutput.BaseStream.EndRead(asyncResult); if (bytesRead == 0) { this.OnAutomationStopped(); return; } var input = this.StandardOutput.CurrentEncoding.GetString( this.buffer, 0, bytesRead); this.inputAccumulator.Append(input); if (bytesRead < this.buffer.Length) { this.OnInputRead(); // only send back if we 're sure we got it all } this.BeginReadAsync(); // continue "looping" with BeginRead } After any read which is not enough to fill the buffer, all accumulated data is sent to the subscribers: private void OnInputRead() { var handler = this.StandardOutputRead; if (handler == null) { return; } handler(this, new ConsoleOutputReadEventArgs(this.inputAccumulator.ToString())); this.inputAccumulator.Clear(); } (I know that as long as there are no subscribers the data gets accumulated forever. This is a deliberate decision). The good This scheme works almost perfectly: Async functionality without spawning any threads Very convenient to the calling code (just subscribe to an event) Maintains the "chunkiness" of the data; this allows the calling code to use inside knowledge of the data without doing any extra work Is almost agnostic to the buffer size (it will work correctly with any size buffer irrespective of the data being read) The bad That last almost is a very big one. Consider what happens when there is an incoming chunk with length exactly equal to the size of the buffer. The chunk will be read and buffered, but the event will not be triggered. This will be followed up by a BeginRead that expects to find more data belonging to the current chunk in order to send it back all in one piece, but... there will be no more data in the stream. In fact, as long as data is put into the stream in chunks with length exactly equal to the buffer size, the data will be buffered and the event will never be triggered. This scenario may be highly unlikely to occur in practice, especially since we can pick any number for the buffer size, but the problem is there. Solution? Unfortunately, after checking the available methods on FileStream and StreamReader, I can't find anything which lets me peek into the stream while also allowing async methods to be used on it. One "solution" would be to have a thread wait on a ManualResetEvent after the "buffer filled" condition is detected. If the event is not signaled (by the async callback) in a small amount of time, then more data from the stream will not be forthcoming and the data accumulated so far should be sent to subscribers. However, this introduces the need for another thread, requires thread synchronization, and is plain inelegant. Specifying a timeout for BeginRead would also suffice (call back into my code every now and then so I can check if there's data to be sent back; most of the time there will not be anything to do, so I expect the performance hit to be negligible). But it looks like timeouts are not supported in FileStream. Since I imagine that async calls with timeouts are an option in bare Win32, another approach might be to PInvoke the hell out of the problem. But this is also undesirable as it will introduce complexity and simply be a pain to code. Is there an elegant way to get around the problem? Thanks for being patient enough to read all of this.

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