5 November 2023

Leafpad with access to system files

As I have explained earlier, I am using Leafpad instead of gedit in my Ubuntu system. However, recent versions of Ubuntu does not have Leafpad available through APT any longer, you have to install it with Snap instead. This is quite annoying, since the Snap packaging of Leafpad is too restrictive, it cannot access hidden files in your home directory, nor anything in ~/bin/. This is not good, since I often want to use Leafpad to edit such files.

28 October 2023

From WordPress to Hugo

I have used self-hosted WordPress for this blog since its start in 2006, but I finally got fed up with having to frequently update WordPress itself and a bunch of plugins, or having to constantly worry about security issues. Since I am the only one publishing on this blog, I decided to switch to a static site generator instead, and I chose Hugo, mainly because it is easy to install and does not require any heavy runtime (most of its competitors require Python, Ruby or Node.

14 October 2023

From screen to tmux

I have written about how to configure screen in XTerm to support Ctrl-Tab and convenient scrolling back in history. This has worked well for over a decade now, but it seems like screen is not developed very much any longer, and tmux has gained popularity as a more modern and actively developed alternative. So I decided to try it, and was able to replicate my setup with tmux instead of screen.

1 May 2023

In-memory database for testing

I have written about using an embedded database like HSQLDB or H2 for testing Java applications. This can still be useful (although JavaEE is not so popular any longer). But even though H2 claims to emulate PostgreSQL and several other popular production SQL databases, sometimes your application uses very specific features of a particular database and you need to test against the real thing to be really sure your application works correctly.

20 April 2023

Alpine rather than distroless

I have been using the distroless Docker base images to package my applications, mainly since I want slim and simple image without unnecessary cruft. However, they are based on Debian, and Debian is unfortunately not so diligent to fix serious security issues as other distributions like Ubuntu or Alpine. If you scan a distroless image with the grype tool, you get this result: $ grype gcr.io/distroless/java17-debian11 NAME INSTALLED FIXED-IN TYPE VULNERABILITY SEVERITY libharfbuzz0b 2.

10 February 2023

How to enable multi-platform Docker builds on Ubuntu 22.04

Docker’s official documentation on how to enable multi-platform Docker builds is a bit intimidating, suggesting you to run the Docker image tonistiigi/binfmt in privileged mode on your machine. I searched for alternatives on Ubuntu, and found this very detailed description. However, with recent versions of Ubuntu and Docker, it is now much easier than that: Install QEMU’s static user mode emulation binaries: sudo apt install qemu-user-static Install Docker Engine with Buildx support according instructions here (do not install docker-ce or docker.

2 November 2022

How to capture log events in tests with Log4j 2

Sometimes you want to verify that your program actually logs what it is supposed to be logging. When using Apache Log4j 2 as logging framework, it can be done like this: Include this dependency (for Maven, adjust appropriately for other build systems): <dependency> <groupId>org.apache.logging.log4j</groupId> <artifactId>log4j-core</artifactId> <type>test-jar</type> <scope>test</scope> </dependency> Import some stuff: import org.apache.logging.log4j.core.Logger; import org.apache.logging.log4j.core.LoggerContext; import org.apache.logging.log4j.test.appender.ListAppender; Add a ListAppender to the logger you are interested in: var loggerContext = LoggerContext.

30 July 2019

How to run exec-maven-plugin only if a file is not up-to-date

In my Maven build, I use exec-maven-plugin to run an external tool for Java code generation. In this case the flatbuffers compiler (flatc). By default, flatc always runs, so you always get a new .java file. This will always trigger Java compiler to run, which will in turn always trigger jar packaging to run, etc. I would like flatc to only run if it’s source file is updated, so I can get a properly incremental build and avoid needlessly recompiling.

14 June 2016

Support fake Android TV devices

Android TV is included in some modern Smart TVs, and there are also external set-top-boxes to get Android TV for any TV with HDMI input. However, there are also some set-top-boxes which almost, but not quite, support Android TV. With not so much extra effort, it is possible to make an Android app supporting those fake Android TV devices as well as the real ones. In addition to the guidelines from Google for building Android TV apps, do this to support the fake TV Android TV devices as well:

10 February 2016

Data structures and Domain Driven Design

After listening to a presentation about Domain Driven Security, I got some additional insights about what a data structure (as opposed to OOP object) should be able to do. Data structures (I call them struct) should have a subset of OOP features which I think is useful and harmless. A struct should allow constructors. Such constructors can have two purposes, to convert one type of data into another and to validate the input.

8 February 2016

Objects vs. data structures

Several popular statically typed programming languages, including C++, Java and C#, have a serious design flaw. They make no useful distinction between objects in the OOP sense and plain data structures. A plain data structure should only be a dumb container for data. It should not have any behaviour, only simple accessors for data items in it. It should not be possible to override data accessors. It should be clear and transparent what it does.

3 August 2015

Typesafe’s Reactive Straw man

In their quest to promote Reactive, Typesafe is beating up a straw man by portraying blocking I/O in a particularly stupid way which is rarely (if ever) done in practice. In a recent webinar, I found this slide which suggests that a blocking I/O operation will waste CPU time while waiting for the I/O to complete. If I understand it correctly, it does not actually work like this in any reasonable runtime environment / operating system.

21 July 2015

Leafpad > gedit

I want a simple, fast and lightweight text editor for my Linux desktop. I don’t want to learn a lot of new arcane key bindings, so not Vim or Emacs. I want a real GUI desktop application, not a console based one, so not nano (even though nano is nice when you only have a text console available). I don’t want a full-blown IDE for programming (I already have that) so I don’t need syntax highlighting and similar features.

11 June 2015

How to fix keyboard layout in Ubuntu 14.04

I regularly use Swedish keyboard layout, but I keep the English layout around in case I would like to temporary switch to it. Ubuntu 14.04 sometimes mess this up and I suddenly get English layout when I log in. I fix this by installing dconf-editor, go to desktop/ibus/general, and make sure that the values engines-order and preload-engines are the same and in the desired order with the default layout first.

16 May 2015

Custom Preference for Android

I need a setting with a numeric value in my Android app. There is no obvious fit among the standard Preferences in Android, so I decided to implement my own Preference subclass. This was a bit more involved than what I hoped for. I ended up with this. An improvement would be to be able to configure min and max values through XML attributes. public class NumericPreference extends DialogPreference { private static final int DEFAULT_VALUE = 0; private NumberPicker mWidget; private int value = DEFAULT_VALUE; public NumericPreference(Context context, AttributeSet attrs) { super(context, attrs); setDialogLayoutResource(R.

26 December 2014

Running Adobe Lightroom 4.4 in Ubuntu 14.04

I use Adobe Lightroom 4.4 for photo editing. There is one very annoying aspect of this program, it is not available for Linux (only for Windows and Max OS X). In order to run Lightroom on my computer, I had to use VirtualBox and install Windows 7 in it. This works, but is quite clumsy and annoying. And since Lightroom is the only reason for me to run Windows, I would like to get rid of it.

26 October 2014

Create a self-contained .jar file with Maven

If you want to create a self-contained .jar file, with all library dependencies included, you can do it with Maven. Include this in your pom.xml: <build> <plugins> <plugin> <groupId>org.apache.maven.plugins</groupId> <artifactId>maven-assembly-plugin</artifactId> <version>2.4</version> <configuration> <archive> <manifest> <mainClass>com.yourcompany.youapp.Main</mainClass> </manifest> </archive> <descriptorRefs> <descriptorRef>jar-with-dependencies</descriptorRef> </descriptorRefs> </configuration> <executions> <execution> <id>make-jar-with-dependencies</id> <phase>package</phase> <goals> <goal>single</goal> </goals> </execution> </executions> </plugin> </plugins> </build> Then when you run mvn package, you will get a target/*-jar-with-dependencies.jar file which you can run with java -jar.

2 October 2014

Running Jetty as a service in Ubuntu Linux

Jetty is a popular open source Java application server. In order to run it as a service on a Linux server, they recommend using a horribly overcomplicated and quite fragile script. In Ubuntu server, there is a better way. Leverage Upstart and its declarative job definitions. First install Jetty into /opt/jetty and create a jetty user: useradd --user-group --shell /bin/false --home-dir /opt/jetty/temp jetty Then create a file /etc/init/jetty.conf with content like this:

5 June 2014

Don’t use large BLOBs in MySQL from Java

The MySQL database can store large chunks of binary data (up to 1 GB) using the BLOB data types. However, this does not work well if you access the MySQL database from Java (or any other JVM based language) using the MySQL JDBC driver. The JDBC API supports an efficient stream based way to handle BLOBs, but the MySQL JDBC driver does not implement this properly. It works, but it will buffer all data in memory in a way which is very inefficient and can make your JVM run out of memory if the BLOBs are large.

4 March 2014

Don’t use PipedOutputStream on Android

I was using java.io.PipedOutputStream in an Android app. The app performed horribly bad. After doing some profiling, it turned out that the call to PipedOutputStream.write(byte[]) that was really slow. After digging into the Android source code, I discovered that PipedOutputStream.write(byte[]) was not implemented, it just delegated to the default implementation in OutputStream which iterate through the array and call PipedOutputStream.write(byte) for each byte. Since PipedOutputStream.write(byte) does some synchronization and Object.notifyAll() each time, it is really slow to do this 1000 times when you write a block of 1 KB data.

17 December 2013

Running Adobe Lightroom 4.4 in Linux

I use Adobe Lightroom 4.4 for photo editing. There is one very annoying aspect of this program, it is not available for Linux (only for Windows and Max OS X). In order to run Lightroom on my computer, I had to use VirtualBox and install Windows 7 in it. This works, but is quite clumsy and annoying. And since Lightroom is the only reason for me to run Windows, I would like to get rid of it.

13 October 2013

Using an embedded SQL database in Java web application

You have a Java web application needing a relational SQL database. According to JavaEE conventions, you declare the DataSource in web.xml: <resource-ref> <res-ref-name>jdbc/DS</res-ref-name> <res-type>javax.sql.DataSource</res-type> <res-auth>Container</res-auth> </resource-ref> and then fetch it in code with (DataSource)new InitialContext().lookup("java:comp/env/jdbc/DS"). Using Maven, Jetty and HSQLDB, you can very easily run the web application for local testing without having to setup neither a web container, nor a database. Add this to pom.xml: <build> <plugins> <plugin> <groupId>org.eclipse.jetty</groupId> <artifactId>jetty-maven-plugin</artifactId> <version>9.

29 August 2013

GIMP 2.8 in Ubuntu 12.04

This is no longer relevant, since the PPA have updated to GIMP 2.8.10, which has fixed the bug. If you use Ubuntu 12.04 LTS, you only have access to the old GIMP 2.6 in the standard repositories. This is unfortunate since GIMP 2.8 have a lot of new useful features. The standard way to get GIMP 2.8 is to use the a PPA, as described here. This used to work fine.

1 January 2013

Using AAC music files in Android

If you have a file with .aac extension, it is AAC encoded audio in an ADTS container. If you want to play such file on an Android device, you have problems. Android 2.x does not support this file format at all, and not even the latest version of Android supports reading metadata tags from it. The solution is to repackage the audio in an MPEG-4 container to a file with .

24 August 2012

How to design a RESTful protocol

What is REST? In most cases, it is sufficient say that REST is a way to design a network protocol based on HTTP. I perfer to call it a RESTful protocol, but it can also be called RESTful API or RESTful Web Service. It is important to keep in mind that it technically is a network protocol, and not an API like an interface with some methods in a regular programming language.

2 July 2012

How to add password protection to GRUB2

These instructions are tested with Ubuntu desktop 12.04, but will probably be useful in other Linux distros with GRUB2 as well. The goal is to block everything except booting the default system. In paricular, it should not be possible for anyone to boot into recovery mode, since that will bypass normal login and give root access directly. Run grub-mkpasswd-pbkdf2 from a terminal and enter the desired password, copy the output. Edit /etc/grub.

14 January 2012

Bachata Linux

Based on what I have desscribed in the two previous posts, I have now made a Linux distribution with an install CD. It is called Bachata Linux, check it out here.

4 January 2012

How to roll your own bootable Linux CD-ROM

When booting a regular Linux system, it just mounts some partition with a nice file system (such as ext4) on your HDD on / read/write and there you go. When booting from a CD-ROM, it’s not that simple. The CD-ROM file system, ISO 9660, does not support the file names and attributes that a Linux system normally needs. And the CD-ROM is read only. There are ways to work around these issues.

11 December 2011

How to roll your own Debian based Linux distro

Goal To build a minimal Debian based Linux system with a fully functional bash shell, TCP/IP networking with DHCP client and apt setup to be able to install any package from the Debian repositories. The resulting system will use about 157 MB disk space and consume less than 10 MB RAM. This is now implemented in Bachata Linux. Prerequisites A Debian based Linux system to work from (e.g. Ubuntu desktop) with the debootstrap and extlinux packages installed.

9 December 2011

How to move MySQL data directory in Ubuntu Server

By default, the MySQL data is placed in /var/lib/mysql, which is a reasonable default. However, sometimes you want to place it somewhere else, such as on an other file system. Using a symlink doesn’t seem to work, so you have follow this procedure. To move the MySQL data directory from /var/lib to /mnt/mydata, run these commands as root: apt-get install mysql-server service mysql stop mv /var/lib/mysql /mnt/mydata/ replace /var/lib/mysql with /mnt/mydata/mysql in /etc/passwd – mysql /etc/mysql/my.

24 November 2011

Using ZTE Blade Android phone with Ubuntu 11.10

Using a ZTE Blade Android phone connected with USB is a bit tricky with Ubuntu 11.10. First you need to apply the patch in this bug, after doing that you should get USB storage to work. After doing that, it is also possible to get development and debugging to work over USB. Create a group androiddev (addgroup --system androiddev), and add yourself to it (gpasswd -a yourUsername androiddev). Then create a file /etc/udev/rules.

27 October 2011

How to disable activity logging in Ubuntu 11.10 Oneiric Ozelot

Ubuntu has mechanism to log user activity such as used documents. This is used to facilitate searching, but can also be intrusive to your privacy. Here is a way to disable this logging without breaking Unity or any other part of the system, execute these commands in a terminal: sudo mv /etc/xdg/autostart/zeitgeist-datahub.desktop /etc/xdg/autostart/zeitgeist-datahub.desktop-inactive rm ~/.local/share/recently-used.xbel mkdir ~/.local/share/recently-used.xbel rm -rf ~/.local/share/zeitgeist then log out and log in again.

31 July 2011

Backup your mobile phone in Linux

To backup data from a non-smart SonyEricsson mobile phone (such as W890i) in Linux, use the gammu utility. Install gammu, it is available as a package in the standard repositories for Debian and Ubuntu, just install the gammu package. Create a ~/.gammurc file with the following content:> > [gammu] > port = /dev/ttyACM0 > connection = at Connect your mobile phone to the computer with the USB cable and select Phone mode

20 July 2011

PHP session timeout

The developers of PHP has, in their infinite wisdom, decided that the default session timeout should be 24 minutes (1440 seconds). This means that if you have a MediaWiki wiki and are editing a single page for half an hour and then click the save button, you are logged out and all your changes are lost. I just learned this the hard way. Fortunately, you can change this with the session.

25 May 2011

Upgrade to Ubuntu 11.04 without Unity

The new Ubuntu release 11.04 Natty contains the new Unity desktop environment which is quite controversal. I have tried it for a while and I think it is neat but too buggy and immature. However, it is easy to revert to the old Gnome 2 desktop environment and have things working almost as in 10.10. First do a normal distribution upgrade to 11.04, then go to System Settings -> Login Screen and select Ubuntu Classic as default session and reboot.

28 April 2011

Using Sveon SNT1020 WiFi USB adapter in Ubuntu

I just brought an Sveon SNT1020 WiFi USB adapter. It works very well with Ubuntu Linux 10.10, just connect it to an USB 2 port and you can start using it with NetworkManager right away, no drivers or setup necessary. You should possibly disable any built-in WiFi adapters first though. It cost €35.

25 November 2010

Web application frameworks in Java

When you know which type of web application you are to develop, it’s time to have a look at some possible choices. I have tried to categorize some modern and popular web application frameworks in Java. Simple server driven MVC page based This category contains the traditional frameworks used for developing web applications with purely server driven application logic. They are based on complete HTML pages and uses the Model-View-Controller design pattern.

25 November 2010

Web applications and web frameworks

If you are to develop a web application, there are a lot if frameworks to choose between. I assume that the web application by its nature needs to have bi-directional communication between the web browser and the server during the execution, initial downloading of resources is not enough. I also assume that the available technologies are HTML, CSS and JavaScript/AJAX; no Flash, Java applets, ActiveX, Silverlight or other browser plug-ins are used.

21 August 2010

Mount your home directory with SSHFS in Linux

If you have your home directory on another machine, it’s common to use NFS to access it. NFS is easy to set up and works more or less out of the box on Linux. However, NFS is not secure, and you need to have a carefully setup firewall in order to use it safely. SSHFS is a more secure alternative, but it’s quite tricky to set up on the client side.

9 June 2010

Using Vaadin with Maven

Vaadin is a comprehensive framework for developing web applications in Java. The Vaadin web site presents a number of ways to use Vaadin with Maven, but I am not completely satisfied with any of those. Here is how I do it. Use a pom.xml like this: <?xml version="1.0" encoding="UTF-8"?> <project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/maven-v4_0_0.xsd"> <modelVersion>4.0.0</modelVersion> <groupId>com.thecompany</groupId> <artifactId>theapp</artifactId> <version>1.0</version> <packaging>war</packaging> <name>The App</name> <dependencies> <dependency> <groupId>com.vaadin</groupId> <artifactId>vaadin</artifactId> <version>6.3.3</version> </dependency> </dependencies> <build> <plugins> <plugin> <groupId>org.

29 April 2010

Implementing POX Web Services with Spring WS and JAXB

Spring Web Services together with JAXB 2.0 provides a convenient way to implement POX Web Services in Java. POX means Plain Old XML, and a POX Web Service is a protocol based on sending XML over HTTP without using any well-known protocol framework like SOAP or XML-RPC. First you have to define annotated JAXB classes for all XML request and response messages. If you have a schema, you can generate them with the xjc tool.

16 April 2010

How to implement RESTful JSON Web Services in Java

You can implement RESTful Web Services in Java using the JAX-RS framework. JAX-RS is part of the JavaEE 6 platform. But if you are not using a JavaEE 6 application server, you can use the reference implementation Jersey and embed it in any web application server. However, it’s quite awkward to produce JSON output from Jersey. Jersey has some support for producing JSON via JAXB, but to get the NATURAL encoding (which you probably want) you need JAXB 2.

15 November 2009

Linux with / mounted read-only 2.0

(This is a new version of a previous post updated to work with Ubuntu 9.10 (karmic).) I wondered why you usually mount / (the root file system) read-write in Linux and decided to do some experiments to find out if it is possible to have it mounted read-only. So why do you want to do that? Perhaps you have the root file system on a read-only media, such as CD-ROM. Or on a writable media which can only handle a limited number of writes, such as a CD-RW or flash disk.

18 September 2009

How to get microphone to work un Ubuntu 9.04

I finally got the microphone input on my Acer Veriton X270 to work un Ubuntu 9.04. The trick is to uninstall PulseAudio and use ALSA only. It even works in Skype.

13 August 2009

java.util.Map is broken in Java 5

Java 5 added generics. The collection classes was modified to make use generics to provide compile-time type-safe collections. Sadly, this was not done properly. The worst problem is in the interface java.util.Map: public interface Map<K,V> { // more methods... V get(Object key); V remove(Object key); boolean containsKey(Object key); boolean containsValue(Object key); } The key parameters to these methods ought to be declared to be K and not Object. Now we don’t get any type-safety for those methods.

17 July 2009

Misuse of HTTP GET is a cardinal sin

According to the RESTful style, you should make use of the four HTTP methods GET, POST, PUT and DELETE. However, in many cases only GET and POST is used, and POST is used when you really should use PUT or DELETE. I consider this as a quite minor issue. However, using GET instead of POST (or PUT or DELETE) is much worse. The current HTTP 1.1 specfication (RFC-2616) clearly states that a GET request must be safe, i.

3 July 2009

Running Ubuntu Linux on Acer Veriton X270

I recently brought an Acer Veriton X270. Ubuntu Linux 8.04 works well except for sound. Basic stereo sound output works, but sound input (microphone) and some advanced 3D and surround sound does not work. The front headphone jack does not work correctly either. (All this works in Windows, so it’s not a hardware problem.) Apart from the poor sound support in Linux, I am quite happy with this computer. It’s small (though not super small like Mac Mini or Fujitsu ESPRIMO Q), quiet, has all features you need and works out of the box.

23 April 2009

Type safe JSP and JSTL

When using JavaServer Pages, you want to use JSTL to be able to to flow-control (iterations and conditionals) in a reasonable way. And the recommended way to use JSTL is to use the Expression Language (EL). However, using EL is not a good idea at all. Contrary to Java and plain JSP, EL lacks static typing. This means that many errors which the compiler can catch is not detected until runtime when using EL.

23 April 2009

java -classpath *.jar

It’s quite annoying that you cannot use wildcards in the -classpath command line option to java and javac. Quite often you want to include all .jar files in one directory. Here is a way to get that effect: java -classpath `echo lib/*.jar | sed -e “s/ /:/g”` org.foo.MyApp You can even include all .jar files in a whole hierarchy of directories: java -classpath `find repository -name *.jar -printf %p:` org.foo.MyApp

22 January 2009

Things you might want to change in Ubuntu 8.04 (hardy) desktop, part 2

After installing Ubuntu 8.04 (hardy) desktop, there are some things you might want to change. This article focus on user configuration (mostly editing dot files in your home directory) and do not require superuser access. Some of this changes requires that you log out to take effect. Customize the bash shell By default, bash save all commands in a history file. This can be quite annoying when you run several instances of the shell in parallel, and may also be a security concern.

19 January 2009

How to setup PPTP VPN in Linux

Create a file /etc/ppp/peers/name: pty "pptp host --nolaunchpppd" name username remotename PPTP require-mppe-128 file /etc/ppp/options.pptp ipparam name Add this line to the file /etc/ppp/chap-secrets: username PPTP password * Create a file /etc/ppp/ip-up.d/tunnel #!/bin/sh if [ "${PPP_IPPARAM}" = "name" ]; then route add -net RemoteNetworkWithNetmask dev ${PPP_IFACE} fi RemoteNetworkWithNetmask is the network on the remote side you want to access via the VPN tunnel, e.g. 172.16.0.0/12. Connect by running sudo pon name

16 January 2009

Things you might want to change in Ubuntu 8.04 (hardy) desktop, part 1

After installing Ubuntu 8.04 (hardy) desktop, there are some things you might want to change. This article focus on system configuration (mostly editing in /etc) and requires superuser access (using sudo). Some of this changes requires reboot to take effect. Fix terminal font rendering bug There is a bug which gives bad font rendering in the terminal window. Fix it by doing this in a terminal: cd /etc/fonts/cond.d sudo unlink 10-hinting-medium.

11 January 2009

How to use mousewheel in GNU Screen

GNU Screen has support for scrollback, but by default you have to use awkward keys to use it. I would like to be able to use Shift-PageUp, Shift-PageDown and the mousewheel to scroll, just like you can do in xterm. It was not easy to configure Screen for this, and it involves cooperation with the terminal emulator. But I finally managed to achieve a solution which works pretty well. Add this to your ~/.

10 January 2009

How to use Ctrl-Tab in GNU Screen

GNU Screen allows you to open several sub-windows within one terminal window. By default, you switch between them using Ctrl-A followed by n or p. I think this is a bit clumsy, I would like to switch with Ctrl-Tab and Ctrl-Shift-Tab just like you switch tabs in Firefox and many other applications. The sub-windows in Screen is conceptually just like tabs in Firefox, so it’s logical to use the same keys to switch between them.

16 December 2008

Configure web applications in JBoss

When you deploy a web application in a JavaEE application server, it usually consist of a .war archive. Sometimes, the web application needs some configuration parameters. The most common way to do this is to have <context-param> in web.xml. That is simple and works fine except that the web.xml file needs to be inside the .war archive. This makes it inconvenient to change a configuration parameter since you have to repack the .

8 December 2008

Packages you might want to remove from Ubuntu 8.04 (hardy) desktop

When you install Ubuntu 8.04 (hardy) desktop, you get a lot of packages installed by default. Most users will never use many of these packages, so you end up with a lot of unnecessary packages. Most of these packages are harmless and only waste disk space when not used. However, some packages can actually affect the system in a negative way, such as draining resources (other then disk space), and you should consider removing them if you not actively use them.

26 July 2008

Linux with / mounted read-only

(This post has been edited since it was first published.) I wondered why you usually mount / (the root file system) read-write in Linux and decided to do some experiments to find out if it is possible to have it mounted read-only. So why do you want to do that? Perhaps you have the root file system on a read-only media, such as CD-ROM. Or on a writable media which can only handle a limited number of writes, such as a CD-RW or flash disk.

31 October 2007

Why can only root listen to ports below 1024?

(This article has been edited since it’s first publication.) In Linux, and other UNIX-like systems, you have to be root (have superuser privileges) in order to listen to TCP or UDP ports below 1024 (the well-known ports). This port 1024 limit is a security measure. But it is based on an obsolete security model and today it only gives a false sense of security and contributes to security holes. The port 1024 limit forces you to run all network daemons with

21 May 2007

Common mistakes with exceptions in Java

Unintentional catching of runtime exceptions Unfortunately, checked exceptions in Java are defined as all Exceptions which are not RuntimeExceptions. This makes it a bit tricky to catch all checked exceptions (but not any runtime exceptions). It would have been better if checked exceptions were defined by a specific class, CheckedException, but that’s too late to change now. Too often Java programmers catch all exceptions when they really only should catch checked exceptions.

28 March 2007

Linux on diskless workstation

When the built-in HDD controller in my computer broke down, I decided to try running my computer diskless with LAN booting. I use Ubuntu Linux 6.10 and I have another computer with a large HDD acting as server. I followed the instructions in this HOWTO. I changed MODULES to “netboot” in initramfs.conf and I had to add a driver for my network adapter to the modules file in the initramfs configuration.

8 September 2006

Poor Javascript performance is an obstacle in AJAX development

I’m currently developing my first AJAX based web application. The goal is to have a pure AJAX application, i.e. never reload the entire page, use only background XMLHTTPRequest to contact the server. My observations so far is that it is possible to do most of the application logic I need. But the performance of Javascript execution in the browser is a big obstacle. The application is centered around a list of data items in a scrollable area.

11 May 2006

Who needs rmiregistry?

Java Remote Method Invocation (RMI) contains a tool rmiregistry to run a stand-alone remote object registry. According to the RMI tutorial, rmiregistry is normally used in an RMI application. I wonder why. It’s actually almost as simple to run the remote object registry within the RMI server. You only have to use java.rmi.registry.LocateRegistry#createRegistry(int port) method like this: Registry registry = LocateRegistry.createRegistry(port); registry.bind("Foo", stub); Here is a complete example. This is also more efficient since you don’t have to start a separate JVM for the registry.