Are you an absolute beginner? Here’s a brief introduction to using the JDK.
Java has taken the Internet and programming communities by storm during the past year with its promise to enable the creation of software that can run on any platform from a single binary file and be used securely in a distributed network environment.
The Java concept is simple: a single source code file is compiled to a single pseudo-binary file containing Java byte codes. This binary file can be run on any platform for which a Java interpreter or other runtime engine exists.
In order to develop and test these Java applications, it is necessary to have access to several critical development tools, including a compiler, a debugger and a Java interpreter for testing applications. Numerous Java development environments already exist—primarily for MS-Windows systems. These full professional development tools are produced by the likes of Borland and Symantec. For Linux, and Unix in general, this type of commercial development tool hasn’t become available.
Nonetheless, this doesn’t mean that Linux users are unable to develop Java applets and applications. Sun Microsystems has developed a free Java Developer’s Kit (JDK) which includes a compiler, a debugger, a runtime environment and an applet viewer for testing Java applets embedded in web pages.
Obtaining the JDK
The Java Developer’s Kit was originally developed by Sun for SPARC, Solaris and Windows NT/95. These versions of the kit, along with more recent versions for x86 Solaris and MacOS, are available from the JavaSoft web site at http://www.javasoft.com/
Sun has also allowed other organizations to port the JDK to other platforms. The Blackdown Organization (Randy Chapman) has ported version 1.0.1 of the JDK to Linux and makes binaries available for x86 versions of Linux. The JDK distribution for Linux is available from ftp://ftp.blackdown.org/pub/Java/linux/. Blackdown also has a web page at www.blackdown.org/. On the web site you will find three files:
linux.jdk-1.0.1-try3.common.tar.gz linux.jdk-1.0.1-try3.static-motif-bin.tar.gz linux.jdk-1.0.1-try3.shared-motif-bin.tar.gz
It is necessary to download two of these three files:
The last file requires an ELF binary of version 2.0 of the Motif libraries (libXm.so.2). If you don’t have Motif, download linux.jdk-1.0.1-try3.static-motif-bin.tar.gz instead—it is a larger file but will work regardless of whether you have the Motif libraries or not. Throughout this article we will be using linux.jdk-1.0.1-try3.static-motif-bin.tar.gz.
All versions of the Java Developer’s Kit also require the following libraries, many of which may already be on your system:
If you are missing any of these libraries, the JDK will not work. These libraries are all freely available on the Internet.
Installing the JDK
Once the “common” tar file and one of the Motif tars have been downloaded, they need to be uncompressed. The documentation with the Linux JDK recommends installing the JDK in the /usr/local directory (although it can be installed elsewhere). To do this, copy the two compressed tar files to /usr/local with:
cp linux.jdk-1.0.1-try3.common.tar.gz /usr/local cp linux.jdk-1.0.1-try3.static-motif-bin.taR.GZ /usr/local
Then, the files can be uncompressed and untarred with:
tar xzvf linux.jdk-1.0.1-try3.common.tar.gz linux.jdk-1.0.1-try3.static-motif-bin.tar.gz
This will create a directory java/ under /usr/local which contains four subdirectories: bin, demo, include and lib. /usr/local/java will also contain a zip file with the source and various README and HOWTO files. The bin/ directory contains the scripts used to execute all the components of the JDK. The components are:
- appletviewer: A viewer to test applets embedded in HTML documents.
- javac: The Java compiler: compiles Java source code to Java byte code binary files (known as class files). The class files produced by javac can be run by a Java interpreter on any platform.
- java: The Java interpreter: used to execute Java class files under Linux.
- jdb: The Java Debugger: a command-line debugger which is in alpha development.
Each of these scripts in java/bin actually call executable files in java/bin/i586. These scripts expect certain tools to exist in fixed locations in your system. Specifically, the appletviewer script expects mkdir to be in /usr/bin and pwd to be in /bin. On some systems, this may not be true (for instance, in RedHat-derived systems, you may find mkdir in /bin). There are two solutions to this problem. One is to edit java/bin/appletviewer and replace the incorrect occurrences of /usr/bin/mkdir or /bin/pwd with the correct full paths of these programs.
The second solution is to create symbolic links in the directories expected by appletviewer. For instance, on RedHat systems where mkdir is in /bin, a symbolic link could be created in /usr/bin with the command:
ln -s /bin/mkdir /usr/bin/mkdir
If you expect to be using the JDK components frequently, you will probably want to add the java/bin directory to your path. Assuming you installed the JDK under /usr/local and you are running the bash shell (the default shell for many Linux distributions), you could add the following lines to the .bashrc file in your home directory:
PATH=/usr/local/java/bin:$PATH export PATH
If you are running C shell, the following line at the end of your .cshrc file in your home directory will do the job:
setenv PATH /usr/local/java/bin:$PATH
Using the JDK
Although the details of programming and developing Java applications and applets are beyond the scope of this article, we will briefly cover how to go about compiling and running Java applications and applets. Java source code is saved in files with the .java extension. Once compiled, a class file (with a .class extension) will be created. Assuming the java/bin directory is in your path as outlined above, a Java source file can be compiled with:
Class files for applications can be executed using:
Applets are a little more complicated. Applets are run as embedded pieces of web pages and are included in web pages with a special <APPLET> tag. You can test an embedded applet with appletviewer or Netscape Navigator 2.0 or 3.0. With Navigator, simply choose Open from the File menu and then open the HTML file with the embedded applet.
With appletviewer, simply type:
Running applets as well as some Java applications requires that you are working in an X-Windows environment. If you don’t have X-Windows installed on your system you won’t be able to test applets or any applications which make use of Java’s GUI development capabilities.
Troubleshooting Your Installation
On most Linux systems the steps outlined above should be all that is required to get the JDK up and running. However, on some systems you may experience some difficulty; some of the common errors and their solutions are outlined below:
- You get an error message referring to /dev/zero. The device /dev/zero needs to have world read and write permissions. Set these permissions using:
chmod 666 /dev/zero
- You get “dirname: too many arguments” or “cannot find class” errors. The component you are trying to run cannot find the native Java class files. The JDK uses the environment variable CLASSPATH to find these files. This variable is set in java/bin/.java_wrapper and java/bin/appletviewer. However, with your shell, these scripts are having trouble determining the correct directory. You can edit these files so that the CLASSPATH gets set correctly.
In .java_wrapper, change the line which reads:
(or wherever you installed the JDK) A similar change needs to be made in appletviewer.
Getting More Information
Once you have installed the Java Developer’s Kit, you will probably want more information about developing Java applications and applets. Aside from Sun’s official Java home page at http://www.javasoft.com/, the Gamelan directory (http://www.gamelan.com/) provides an extensive collection of applets and applications—many with source code—as well as pointers to other reference material. The comp.lang.java newsgroup is a high-volume newsgroup which is actively used by many Java experts and novice programmers.