Getting Started with Java on Linux

Write once, run everywhere: that's the slogan the Java community uses to propagate their language-of-choice. It's probably true, but only if you first manage to set up the beast on your box. This article gets you started with Java on Linux by showing you how to get the Java Compiler and Virtual Machine installed so you can run core Java programs. It also shows you how to set up a Java web application server. If you follow each step in this article carefully, you will end up with a full-featured Java application server.

For the core Java, we will use Sun's JDK (Java Development Kit), although the IBM equivalent would work equally well. IBM's JDK is even known to be a little faster than the Sun JDK.

Resin will be used for the application server. Resin is both fast and easy to set up. It's also robust and includes all the features that Java developers are looking for, including Servlet/JSP, XML and EJB. For now, we will bother only with the Servlet/JSP portion.

We start by downloading the JDK. For most people, the standard edition of the Java Development Kit (J2SE) will work fine. However, if you need support for EJB (Enterprise Java Beans), you should get the J2EE (Enterprise Edition) instead. In either case, point your browser at java.sun.com/download.html, where you can get both J2SE and J2EE. At the time of this writing, the latest version of the J2SE SDK is 1.4.0 (about 26 Mb). My recommendation is to get the BIN-version of the file, although the RPM-package works if you're using an RPM-based distribution. BIN gives you more control over how and where you want to have your Java-environment installed. And that can be nice when you're dealing with large packages like this one.

The file should be on your local machine, so it's time to start the installation (make sure the file has permissions 755):

root@localhost:~# ./j2sdk-1_4_0_01-linux-i586.bin

As you can see, I choose to execute the file as root. You could do this as a regular user as well, but you would not be able to install it in a system-wide directory.

After you've read the license agreement, you will be asked

Do you agree to the above license terms? [yes or no]

Type yes and press Enter. The file-extraction process will begin now, placing all the files in a sub-directory below your current working directory (cwd). The most common location for Java installations is /usr/local, so we'll go with that:

root@localhost:~# mv j2sdk-1_4_0_01-linux-i586.bin /usr/local

Personally, I think it's nice to always have JDK installed in /usr/local/jdk but still be able to easily see what version is used. To accomplish this, you can simply do:

root@localhost:~# ln -s /usr/local/<REPLACE> /usr/local/jdk

This creates a symbolic link, /usr/local/jdk, pointing to the /usr/local/jdk1.4.0_01 -directory. If you ever upgrade your JDK, all you need to do is update the symbolic link, and the Java compiler (for example) will still reside in /usr/local/jdk/bin, saving you from the work of changing your PATH variable and/or updating scripts that use files in /usr/local/jdk. Executing a ls -als in /usr/local should output something like this:

root@localhost:/usr/local# ls -als
   4 drwxrwsr-x   17 root     staff        4096 Aug 13 18:31 ./
   4 drwxr-xr-x   14 root     root         4096 Aug  1 14:29 ../
   4 drwxr-xr-x    2 root     staff        4096 Jun 13 21:15 bin/
   4 drwxr-xr-x    8 root     staff        4096 Aug 13 18:31 j2sdk1.4.0_01/
   0 lrwxrwxrwx    1 root     staff          24 Aug 13 18:31 jdk ->/usr/local/j2sdk1.4.0_01/
   4 drwxrwsr-x    6 root     staff        4096 Aug  1 14:28 lib/
   4 drwxrwsr-x    6 root     staff        4096 Oct 22  2001 man/
   4 drwxrwsr-x    2 root     staff        4096 Jun 13 21:17 sbin/
   4 drwxrwsr-x    9 root     staff        4096 Mar  5 13:31 share/
   4 drwxrwsr-x    2 root     staff        4096 Apr 15  2001 src/

Speaking of the PATH variable, that is also something we need to take care of--if you want to be able to access the executables in /usr/local/jdk/bin, that is. As a Java developer, you will need to access these files quite often, so it's generally a good idea to set it up.

Fortunately, editing the PATH variable is a simple task. All you have to do is to add the line below to your /etc/profile (or whatever file holds your global shell variables).

export PATH=$PATH:/usr/local/jdk/bin

This means set the variable PATH to the previous value of PATH, and add /usr/local/bin/jdk to it. Then, reload your /etc/profile by issuing:

root@localhost:~# source /etc/profile

You can then test to see if your shell can find the Java compiler simply by typing javac. If everything is okay, you should see something like:

root@localhost:~# javac
Usage: javac <options> <source files>
where possible options include:
  -g                        Generate all debugging info
  -g:none                   Generate no debugging info
  -g:{lines,vars,source}    Generate only some debugging info
  -O                        Optimize; may hinder debugging or enlarge class file
  -nowarn                   Generate no warnings
  -verbose                  Output messages about what the compiler is doing
  -deprecation              Output source locations where deprecated APIs are used
  -classpath <path>         Specify where to find user class files
  -sourcepath <path>        Specify where to find input source files
  -bootclasspath <path>     Override location of bootstrap class files
  -extdirs <dirs>           Override location of installed extensions
  -d <directory>            Specify where to place generated class files
  -encoding <encoding>      Specify character encoding used by source files
  -source <release>         Provide source compatibility with specified release
  -target <release>         Generate class files for specific VM version
  -help                     Print a synopsis of standard options

If you get a command not found message instead, make sure your PATH variable is set up correctly, and then try again.

In addition to PATH, another variable needs to be defined. JAVA_HOME defines where on your filesystem the JDK base directory can be found. In our case, this is /usr/local/jdk. Other applications that use Java (such as Resin, which we will look at a little later) use this variable to find the JDK. Add this line to your /ets/profile:

export JAVA_HOME=/usr/local/jdk

And reload /etc/profile:

root@localhost:~# source /etc/profile

Time for testing.