Turning off the system beeper

This is the magic command that turns off the system beeper:

sudo rmmod pcspkr

And just for completence, this is how to turn it on again:

sudo modprobe pcspkr

Update (Thanks to the ArsGeek site.) This only works for the current session - the beeper is only muted till the next reboot. In order to make the beeper silent for ever, you need to add the following line to the file /etc/modprobe.d/blacklist:

blacklist pcspkr

Ubuntu: Connecting to a Windows shared network drive

I got a new PC at work and I made a clean installation of Ubuntu Gutsy Gibbon 7.10, so I had to make some configuration from scratch. This is a quick note about how I connected to our file server, using samba

From the Places menu, select Connect to server. From the window that appears, select Windows share in the Type drop-down. Then provide the server's IP in the Server field and the folder's name in the Folder field and you are done.

Datasources, Tomcat and Netbeans

I am currently developing a JSF application using NetBeans 6.0 and Tomcat 6.0.14. When I tried to deploy the application on our production server (by just dumping the .war file in Tomcat's webapps directory, it didn't work properly. I could access the app's start page, but it failed to render the following pages.

A look in tomcat's log files revealed that there was a problem with the mysql driver.

Caused by: java.lang.ClassNotFoundException: com.mysql.jdbc.Driver

I tried to make sure that the mysql-connector jar is included in the project's imported libraries and that it can be found in META-INF/lib, but this didn't help. I later found out that in Netbeans, under Tools→Servers→Deployment there is a "Enable JDBC driver deployment option". Disabling this resulted in a replication of the problem on my development machine, even when using NetBeans. So I had to find out what it was that NetBeans was doing when this option was enabled and repeat it on the production server.

I started making a research on the net, based on the error message I was getting. The oslution proposed by most people was to place the mysql driver jar in tomcat's common/lib directory and nowhere else, but this just wouldn't do the trick for me. I also checked that the datasource's declarations in the web.xml and context.xml files were correct.

A final attempt to go through the steps to create and declare a datasource as described in tomcat's website revealed a small detail in step 1: the driver jar file needs to be placed in the $CATALINA_HOME/lib directory rather than $CATALINA_HOME/common/lib.

Still remains to check whether there is a difference in the directory structure in versions 5.5 and 6.0 of tomcat, since no common directory appears to exist in installations of tomcat 6.0.

Setting the time on linux.

For some reason I find the manpage for the date command a bit confusing. Here is how you can set the time in a linux box through the command line:

date MMDDhhmmYYYY

where MM is the month, DD the day, hh the hour, mm the minutes and YYYY the year.

The system's hardware clock can be synchronized with this time by executing the following command:

sudo hwclock --utc --systohc

Allowing ssh access with iptables.

This post describes how you can allow incoming ssh connections from a certain IP address, using iptables.

First of all, the current list of iptables rules can be viewed with the command

iptables --list

A new rule to allow access from the IP 100.100.100.100 can be appended to this list with the command

sudo iptables -A INPUT -p tcp -s 100.100.100.100 --dport 22 -j ACCEPT

As it can be seen from the command above, root access is required to alter the iptables.

However, in my case this didn't work, because the rule was appended to the end of the list, which means that it was preceded by a DROP rule that blocked ssh access to all IP addresses that did not explicitly have an ACCEPT rule. So, I had to add the rule further up in the list. This can be achieved with the -I switch.

sudo iptables -I INPUT 11 -p tcp -s 100.100.100.100 --dport 22 -j ACCEPT

In the example above, the rule will be added to the 11th position in the list.

How to create a Datasource and a Connection Pool on Tomcat.

This post describes how you can setup a Connection Pool for a web application under Tomcat. I have been using NetBeans 5.5.1, with bundled Tomcat (version 5.5.17). NetBeans does offer some visual tools to create datasources and connection pools, but as far as I understand, these only work with the Sun Application Server.

Before doing anything else, the following jars are required:

• mysql-connector-java-5.0.7-bin.jar
• commons-dbcp-1.2.2.jar
• commons-collections-3.2.jar
• commons-pool-1.3.jar

The first one is the official mysql driver which can be downloaded from the mysql site. The other three are part of the Apache Commons collection.
They need to be placed in the WEB-INF/lib subdirectory of the webapp. (Of course this can also be done through the NetBeans IDE.) Actually, although I read on various websites about the last 3 jars, I am not absolutely sure if they are definitely required. Even after I removed them, my application continued to work fine.
Important: The MySQL Connector/J jar needs to be placed under the Tomcat common/lib/ subdirectory. (For the bundled with the NetBeans IDE version of Tomcat, this directory is $NETBEANS_HOME/enterprise3/apache-tomcat-5.5.17/common/lib.

1. Add this to the context.xml file (which is located in the META-INF directory of the web-app.

<Context path="/ePmashup">
  <Resource 
      name="jdbc/dataSource"
      auth="Container"
      type="javax.sql.DataSource"
      factory="org.apache.tomcat.dbcp.dbcp.BasicDataSourceFactory"
      username="username"
      password="password"
      driverClassName="com.mysql.jdbc.Driver"
      url="jdbc:mysql://localhost:3306/databaseName"
      maxWait="1000"
      removeAbandoned="true"
      maxActive="30"
      maxIdle="10"
      removeAbandonedTimeout="60"
      logAbandoned="true"/>    
</Context>

The options in italics have to be substituted with the appropriate values.

2. Create a DataSource object in the application code.

Context initContext = new InitialContext();
Context envContext  = (Context)initContext.lookup("java:/comp/env");
datasource = (DataSource)envContext.lookup("jdbc/dataSource");

This snippet has to be wrapped in a try-catch block, because it might throw a NamingEsception. In my code, I placed this piece of code in the init() method of the Servlet that contacts the database and returns the results to the client.

 public void init() throws ServletException {
   super.init();
   try {
       Context initContext = new InitialContext();
       Context envContext  = (Context)initContext.lookup("java:/comp/env");
       datasource = (DataSource)envContext.lookup("jdbc/dataSource");
          
   } catch( NamingException ne ) {
     throw new RuntimeException( "Unable to aquire data source", ne );
  }
 }

Obviously, the string passed in the lookup method, needs to be the Resource name (in context.xml).

3. Now, a connection from the pool can be retrieved and used like this:

Connection conn = dataSource.getConnection();

Popup windows in JSF

Is is possible to raise a popup window in Java Server Faces (JSF) using Javascript, more or less the same way as in HTML.

This example shows how this can be done with a commandButton.

<h:commandButton value="Press me to bring up a popup window" onclick="window.open('popup.jsp', 'popup_window')" />

This exact method won't work with a commandLink, because this does not offer an onclick option. However the mouseup event can be used instead:

<h:commandLink onclick="window.open('popup.jsp', 'popup_window')">
    Press me to bring up a popup window
</h:commanndLink>

Of course, both commandButton and commandLink items need to be enclosed in a form element. This means that the form will be submitted when the commandButton or commandLink is pressed/clicked. In order to avoid it, we have to return false from within the onclick or mouseup attribute.

<h:commandButton value="Press me to bring up a popup window" onclick="window.open('popup.jsp', 'popup_window'); return false;" />