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 2.1. And that can be problematic since JAXB 2.0 is bundled with JavaSE 6 and with some application servers (such as WebLogic). Overriding that with a later version of JAXB can be really difficult. Using JAXB is also a bit clumsy if you only want to produce JSON.

(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. It would also increase security since it will be more difficult (though not impossible) for some malware to infect your system.

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.

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. If you create a HashMap<String,String> and then by mistake try to use an int as key when looking up a value, you will not get any compile-time error. Worse yet, you will not even get any run-time error, it will just appear as if there is no value with that key (which in some sense is correct).

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.e. not have any significant side-effect on the server. So in order to change anything on the server, you must use POST (or PUT or DELETE). The older HTTP 1.0 specification (RFC-1945) from 1996 said the same.

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. And it has a compelling price.

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. And even worse, EL doesn’t even do proper type checking at runtime. In many cases you just end up with an empty string when it in fact is a type error.

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

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. Add this line at the beginning of ~/.bashrc to only keep command in memory until you exit the shell:

1. 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

2. Add this line to the file /etc/ppp/chap-secrets:

username PPTP password *

3. 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.