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Setting Up A Java Development Enviroment For Linux

By Chris Gibbs

Contents

Obtaining the JDK

Kernel Support For Java

JDK

Java Beans

Swing

Java Accessibility Utilities

JSDK

Documentation

Java Tutorial

Sun's HotJava Browser

Next Month

Introduction

This article is intended to help those new to either Linux or Java set their machines up to run Java applications, as well as providing an effective environment for developing new Java applications.

Specifically I shall explain how to set up Sun's 1.1.x JDK and other related packages. This is not the only way to run Java on Linux.

Blackdown.org now offers the new 1.2.x JDK for Linux (at the time of writting this is still a pre-release version). I only recommend using this if you are also using the new 2.2.x Linux kernel. If you choose to install the 1.2.x JDK then please note that you do not need to obtain the Swing and JSDK packages, since they are already included. Also you may find some compatibility problems if you use older Java applications with the 1.2.x JDK. In particular if you intend to implement the javapache extensions to Apache you will need to use JDK 1.1.x.

In addition there exist other third party Java environments and Java compilers such as IBM's jikes. Use these at your own peril! IMHO Sun have defined and developed the Java language so I expect their Java environments to be the most standard. Other environments may be of interest, but I cannot cover them here.

Unfortunately the development of the Java language for Linux lags the development for other operating systems. Having said that there are still many advantages to using Java on Linux, not least of which is the easy availability of sophisticated development tools. If you have not seen a recent version of DDD (The Dynamic Display De-bugger), you may not realise that it now supports Java. If you write any programs under Linux DDD is a must have utility.

Why use Java at all?

Java's greatest advantage is also its greatest disadvantage. For reasons given below, the fact that the same Java code can be run on any platform without re-compiling is responsible for the fact that Java applications can appear slow. It is this slowness that has lead some to question the need to use Java at all.

It is true that if speed in loading and running an application is the only criteria for choosing a programming language then I would never consider using Java at all. Although applications such as jedit and the Java cd player described in earlier editions of this e-zine are functionaly equivalent to applications written in C they are very slow to start up and I am loath to use them when I have much faster alternatives.

But Java is not meant for such applications. Java excells in what it was originally designed for, which is to provide a means of making applications available over the Internet. Over the Internet the speed of loading an application is not really important when compared to the time it takes to download the application.

Now consider your friendly ISP, and the servers that provide you with Internet access. Your ISP (even your local Intranet) needs the fastest machines possible given certain criteria (cost etc). The cost of the actual server may not be important when compared to the software investment of server-side applications. Using Java to implement server-side applications leaves your ISP free to choose the best machine architecture and operating system without having to worry about the cost and feasibility of re-implementing such applications. They can simply be copied from one machine to another. Perhaps one day Microsoft will develop a reliable operating system that is superior to a *NIX operating system. If that day ever comes I will want to change from Linux, using Java will allow me to do that and keep exactly the same server-side applications I am running now.

If you only want to use a stand-alone computer, with no network access, either locally or over the Internet, then you probably do not need or want to consider using Java. But if your machine is going to be part of a local network or the Internet then the use of Java should be taken very seriously. Java is not the only way to develop network applications. For instance Perl can be used in a very similar way. But Java is a very attractive option.

Obtaining the JDK

Please do not obtain your JDK directly from Sun. They provide support only for Microsoft Windows, Solaris etc. Instead go to Blackdown.org and find the version you require.

All other packages mentioned here should be obtained from Sun.

The Linux port of the JDK can either be found on a Linux distribution disk, or can be found at http://blackdown.org. You will find you have a bewildering choice of files to install. There is the JRE as well as the JDK. I recommend you get the full JDK package since it contains everything in the others.

Kernel Support For Java

Linux was possibly the first operating system to provide direct kernel support for Java. This allows Linux to run both Java applications and Java applets directly, without the need for a browser such as Netscape. But if you think direct kernel support means that the kernel has anything to do with actually running Java code you are sadly mistaken.

To understand what is happening you need to realize that Java is really only one step removed from running interpreted programs rather than compiled programs. In the early days of computing, it was common to use an interpreted language, such as GW-BASIC, to write simple applications. The code that was written was never compiled into a program, rather the code would be parsed by an interpreter which would perform the actual execution of instructions. This is a very inefficient way of doing things and the results will always be slow.

Java differs from an interpreted language, such as early versions of BASIC, in that the source code is compiled. But the source is not compiled into something a computer can directly execute as would be the case if you compiled a C program with gcc. Instead the source code is byte-compiled into a standard format. The reason that the same Java application can be run on any platform is precisely because this byte-code is in a standard format. The programs java and appletviewer, provided by Sun or Blackdown.org perform the function of parsing this byte-code and executing the instructions. This is the reason Java applications can appear slow, the byte-code must be parsed by an interpreter before it can be executed, though the situation is not nearly as bad as it was with early BASIC programs, since the byte-code has already been optimized by the java compiler.

When you compile Java support into the Linux kernel what you are really doing is telling the operating system to invoke either java or appletviewer when a request is made to execute a java application. Linux is simply giving you a shorthand way of saying 'appletviewer myapplication'.

For this reason you need to tell the kernel where your Java executables are kept. If you obtain the JDK from Blackdown.org then the chances are that java and appletviewer will be placed in /usr/local/java/bin. If you install the JDK from a S.u.S.E. distribution disk then they will be in /usr/lib/java/bin. Of course you might have installed the JDK to some non-standard location.

Before you re-compile your Linux kernel, edit the file /usr/src/linux/fs/binfmt_java.c to reflect the situation you actually have. 

     #include <linux/malloc.h>
     #include <linux/binfmts.h>

     #define _PATH_JAVA      "/usr/local/java/bin/java"          // Replace with correct location for your system.
     #define _PATH_APPLET    "/usr/local/java/bin/appletviewer"  // Replace with correct location for your system.
     #define _PATH_SH        "/bin/bash" 

     char binfmt_java_interpreter[65] = _PATH_JAVA; 
     char binfmt_java_appletviewer[65] = _PATH_APPLET;

Please note that the references to java and appletviewer should be to the actual location of the binaries and not to any symbolic link you might have created. In the example above the directory /usr/local/java might be a symbolic link to /usr/lib/java-1.1.7, but the actual file refereed to is not a symbolic link, it actually exists in /usr/lib/java-1.1.7/bin.

Now you can decide if you want to compile Java support directly into the kernel, or have it as a module. I choose to use a module. The only comment I feel necessary is that when I try to execute a html file kernld will not automatically load the binfmt_java module, so I must "insmod binfmt_java" first.

Execute a HTML file?

Yes, Linux will allow you to execute certain HTML files as though they are applications. To understand why you would want to do this you need to understand the difference between a Java applet and a full blown Java application. Whilst a Java application provides all the resources it needs itself, an applet does not. Normally an applet needs to be included in a web page which is viewed by a Java enabled browser such as Netscape or HotJava because it has no top level window of its own. An alternative is to use the appletviewer program to view the applet.

Linux gives you a further option. if you add the line 

        <!--applet-->

to a HTML file (and '<' must be the first character in the file), and chmod the file to be executable, then you will be able to launch the applet(s) embedded in the file directly.

In a similar manner a Java application is made up of one or more class files. To make the application executable by linux simply chmod the top level class file so it is executable.

JDK

Reading the documentation that comes with the JDK, we find that there is no need to set the CLASSPATH or JDK_HOME environments. Whilst this is true for the JDK itself, I have found that there are some applications that expect these environments to be set. In any case setting them will not cause you any harm and I recommend you take the following steps to install the JDK correctly:

  1. export PATH=$PATH:{JDK_HOME}/bin
  2. export JDK_HOME={JDK_HOME}
  3. export JAVA_HOME={JDK_HOME}
  4. export CLASSPATH=./:{JDK_HOME}/lib/classes.zip

Where {JDK_HOME} is the top-level directory to which you installed the JDK.

Java Beans

The JavaBeans API allows you to create component software in the Java programming language. Components are complete, self-contained, reusable software items that can be visually linked into any Java program, applet or servlet. Such software items are refereed to as Beans. Many Java IDE's are available, such as NetBeans or Visarj, that use this technology to provide application builder tools. Included in the package is BeanBox, a simple JavaBean IDE to get you started.

When you startup BeanBox, by running bin.sh in the BDK/beanbox sub-directory, three windows will open for you:

The ToolBox initially contains a number of example beans, you can add your own to create your own visual programming environment. Select a bean by clicking your mouse on it. Then click somewhere in the BeanBox. The javaBean will appear and a list of its properties will appear in the Properties window. You may then use the Properties window to customize the JavaBean for your application. When you have finished adding Beans, and are happy with the appearance of your application in the BeanBox you can select File/Make applet from the BeanBox windows menu and an applet complete with example html file will be generated for you.

As should now be clear, JavaBeans provide a means to develop applications without writing a single line of code. Of course you need to write the Beans themselves but many third party vendors make Beans available and the Internet is a fine place to look for new components.

A feature of JavaBeans is that they are packed into the JAR file format. This format has many advantages over java class files including:

Additional features are included in JDK 1.2.x including package versioning, package sealing and packaging for extensions.

INSTALLATION

  1. Obtain the BDK from http://java.sun.com/beans
  2. Unpack the archive to a subdirectory, e.g. /usr/lib/BDK
  3. Optionally add the new classes to your CLASSPATH. e.g. export CLASSPATH=$CLASSPATH:/usr/lib/BDK/beanbox/classes
  4. Enter the demo sub-directory and run "make -f GNUMakefile"
  5. Enter the demo/html directory and make the files executable in the manner described above and try them out.
  6. Enter the beanbox sub-directory and run "run.sh".

Swing

Not all Java applications are equal! Some look better than others. Swing provides several enhancements to the Java language. Whilst you do not need Swing to develop and run Java applications and applets, you may well need it to compile or run certain Java applications (such as jedit) that are freely available on the Internet.

The Java Foundation Classes (JFC, this is included in the Swing package), together with Swing, provide a set of "lightweight" components that work the same on all platforms. Effectively they provide GUI extensions to the Java language that can provide many enhancements to the look and feel of your applications.

The screen-shot above shows the initial screen when you start the swingset application. This can be used to demonstrate the enhancements Swing provides. by selecting a category such as ListBox you will be able to display and alter the various options that are available and so become familiar with the effects that Swing can provide.

An application I can recommend that makes full use of Swing is jedit, a very nice editor. This is available in full source code, and is suitable for editing plain text files such as HTML, Java source, Perl scripts, LaTeX documents, etc. The editor supports the inclusion of plug-ins and a couple of trivial examples are included to get you started.

INSTALLATION

  1. Unpack the Swing package (I recommend version 1.1) to the directory of your choice. e.g. /usr/local/Swing
  2. export SWING_HOME={SWING_HOME}

    optionally,
  3. Add the .jar files and src.zip under {SWING_HOME} to your CLASSPATH.

Where {SWING_HOME} is the directory under which you unpacked Swing.

Java Accessibility Utilities

Actually, there are two packages here: The Java Accessibility API and the Java Accessibility Utilities themselves.

The Java Accessibility API defines a contract between individual user-interface components that make up a Java application and an assistive technology that is providing access to that Java application. If a Java application fully supports the Java Accessibility API, then it should be compatible with, and friendly toward, assistive technologies such as screen readers, screen magnifiers, etc. With a Java application that fully supports the Java Accessibility API, no off screen model would be necessary because the API provides all of the information normally contained in an off screen model.

In order to provide access to a Java application, an assistive technology requires more than the Java Accessibility API: it also requires support in locating the objects that implement the API as well as support for being loaded into the Java virtual machine, tracking events, etc. The Java Accessibility utility classes provide this assistance.

As you may have guessed from the screen-shot above, the Java Accessibility Utilities require that you have installed the Swing package.

Several example utilities are provided in the package. The screen-shot above shows the Java Monitor which allows you to obtain information about objects being displayed on the screen by a Java Virtual Machine. Also included are:

INSTALLATION

This is a little different to the other packages mentioned here, since the different utilities included have to be explicitly enabled by you.

When you start any Java application you will also be able to automatically stat one of the utilities mentioned above by following the steps below:

  1. Unpack the packace to the directory of your choice. e.g. /usr/local/jaccess
  2. Install the Swing package as described above.
  3. Add the new jar files to your CLASSPATH e.g. export CLASSPATH=$CLASSPATH:{jaccess_home}/jaccess.jar:{jaccess_home}/jaccess-examples.jar
  4. Edit the JDK_HOME/lib/awt.properties file to enable jaccess by adding the following line: 
           AWT.EventQueueClass=com.sun.java.accessibility.util.EventQueueMonitor
  5. Decide which utitility(s) you want to enable and add a line for each one to the awt.properties file. For example to enable the JavaMonitor add the line: 
           AWT.assistive_technologies=JavaMonitor

JSDK

The Java Servlet API is a standard extension to the Java Development Kit. Servlet's are bodies of code that run inside servers to extend their functionality. For example, servlet's offer an efficient platform-independent replacement for CGI scripts. Servers that can host servlet's are java-enabled servers that respond to client requests.

The diagram above shows a typical use of a Java servlet. A HTTP server is running a servlet that is responsible for taking data from a HTML order-entry form and applies a companies rules for updating their order database. Simply put, a java servlet is a servers version of a java applet. As one might expect, since the server will be running remotely and unattended (from a users viewpoint at least) servlet's have no graphical user interface.

Servlet's provide a way to generate dynamic documents that is both easier to write and faster to run. Servlet's also address the problem of doing server-side programming with platform-specific APIs: they are developed with the Java Servlet API, a standard Java extension.

So use servlet's to handle HTTP client requests. For example, have servlet's process data POSTed over HTTP using an HTML form, including purchase order or credit card data. A servlet like this could be part of an order-entry and processing system, working with product and inventory databases, and perhaps an on-line payment system.

Amongst the many other possible uses for servlet's are the following:

If you use Apache then you will be interested in the javapache project, available from http://java.apache.org. To use this you will need to use the 1.1x JDK, and must use version 2.0 of the JSDK. Next month I hope to make an introduction to this project available.

The JSDK is available for download from http://java.sun.com/products/java-server/servlets

If you are using the newer 1.2.x JDK please remember you do not need to download this package, since the JSDK is already included.

INSTALLATION

  1. Unpack the JSDK to your preferred location (e.g. /usr/lib/JSDK)
  2. Add the JSDK /bin directory to your path. The best way to do this is to place a line 
            export PATH=$PATH:/usr/lib/JSDK/bin
    to your /etc/profile file.

  3. Add the JSDK Classes to your CLASSPATH, e.g. 

            export CLASSPATH=$CLASSPATH:/usr/lib/JSDK/lib/jsdk.jar

    Again you can add this line to your /etc/profile file.

    Documentation

    Each package includes its own documentation in HTML format. However, since you need to get your JDK from Blackdown.org, rather than from Sun, the documentation for the JDK will be missing from the package.

    Sun provides full documentation for both 1.1.x and 1.2.x versions of the JDK. Documentation for the 1.1.x version exists for version 1.1.3. I do not think it has been updated since then.

    In the demo sub-directory you will find many examples. Find the HTML files that launch java applets and make them executable in the manner I have described above so you can run them as conventional programs.

    The docs subdirectory contains a complete programmers guide to the Java language. This is further enhanced by the Java Tutorial.

    Java Tutorial

    Sun have a comprehensive Java tutorial which can get you up to speed programming in Java very quickly. Both the entire tutorial and specific parts of it are available for download:

    Bundle

    Format

    Compressed /
    Uncompressed

    Link to Download

    Online Tutorial
    (last updated December 21)

    Zip

    8.5 MB / 14.7 MB

    ftp://ftp.javasoft.com/docs/tutorial.zip

    tar/compress

    10.5 MB / 14.7 MB

    ftp://ftp.javasoft.com/docs/tutorial.tar.Z

    tar/GNUzip

    7.2 MB / 14.7 MB

    ftp://ftp.javasoft.com/docs/tutorial.tar.gz

    Tutorial Examples
    (last updated December 21)

    Zip

    2.9 MB / 4.4 MB

    ftp://ftp.javasoft.com/docs/tut-examples.zip

    tar/compress

    3.6 MB / 4.4 MB

    ftp://ftp.javasoft.com/docs/tut-examples.tar.Z

    tar/GNUzip

    2.5 MB / 4.4 MB

    ftp://ftp.javasoft.com/docs/tut-examples.tar.gz

    JFC/Swing Trail
    (last updated December 21)

    Zip

    2.8 MB / 4.6 MB

    ftp://ftp.javasoft.com/docs/tut-swing.zip

    Servlets Bookstore Example
    (last updated December 21)

    Zip

    18 KB / 51 KB

    ftp://ftp.javasoft.com/docs/tut-bookstore.zip

    BINGO Example
    (last updated December 21)

    Zip

    35 KB / 108 KB

    ftp://ftp.javasoft.com/docs/tut-bingo.zip

    Alternatively you can read the tutorial on-line at http://java.sun.com/docs/books/tutorial.

    Please note: Whilst you are free to mirror this tutorial on a local network or Intranet, Sun request that you do not make it available publicly on the Internet.

    Sun's HotJava Browser

    This is a web browser written entirely in Java. For that reason even on a 133Mhz pentium class processor it is very slow. Its advantage is that it is one of the very few browsers available for Linux that can run Java applets. It is also very interesting for other reasons.

    HotJava is highly modular, the user interface is easily customizable and has a small footprint (does not take up a large amount of system resources). Well that is what Sun claims. I find in practice that if I try loading java applets whilst running other applications the browser will die on me. I would have thought that if it does have a small footprint this would not happen - whatever.

    Sun further claim that the browser is ideal for a variety of devices - such as screen phones as well as desktop PC's. All I can say is that until the browser can perform faster (on my pentium class box with 32Meg ram it takes about three minuets to start up and load the default screen), and is more reliable, then it is unlikely to be of much use unless you have a very fast processor.

    The heart of HotJava is the HotJava HTML Component. This is a JavaBean that parses and renders HTML. This can be embedded into your own applications, anything from a newsreader to a microwave oven (yes there are microwaves and even fridges that give you Internet access now), can make use of it to display information.

    HotJava supports the following Internet standards:-

    The following packages are available depending on your needs:

    Next Month

    Last month I explained how to set up the Apache HTTP server. Next month I intend to tie these two articles together by showing you how to incorporate the javapache extensions to Apache so you can use Java to create dynamic HTML pages and server side applications to enhance your web site.

    In a future article I shall give an overview of the available Netscape Plug-ins and show how simple it is to develop your own plug-ins. In the meantime I welcome your comments and suggestions for future articles.

Editor's note: October's Linux Journal, in the Strictly On-Line section, contains two articles related to Java:

Copyright © 1999, Chris Gibbs
Published in Issue 45 of Linux Gazette, September 1999

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