Using and Writing Java Servlets

With the power of the Java API set, servlets can provide complex, dynamic web pages and form processing.

Creating dynamic web pages is necessary if a web site wants to display the current state of data accurately, for example showing temperatures around the world. There are several ways to accomplish this, such as using Perl or shell scripts. In this article, I discuss the viability of using Java programs (servlets) with a web server. A servlet is a Java application that performs a task that may generate a dynamic web page or process input from a web page form.

Advantages and Disadvantages of Using Servlets

The first advantage is that only one instance of the JVM (Java Virtual Machine) needs to be started. For each servlet, a new thread is created and managed by the same JVM. In contrast, Perl and shell CGI (common gateway interface) scripts require a new process to be created each time the script is run. This may become a problem if dynamic web pages are being created for many (possibly thousands) of web pages per minute. The JVM itself is more efficient because it only needs to keep one copy of the actual servlet code to create the dynamic portions, such as variable data and program context, for each instance of the servlet. Another plus is that if you have Java expertise, it is not necessary to relearn a new scripting language with a new set of APIs and quirks.

Since the JVM has a rapport with the web server, it is possible for servlets to communicate to the server directly. This, of course, poses a security risk but does allow servlets to be written that manipulate the web server on the fly.

The user does not need to have a Java-capable browser because there are still many small internet devices that do not have the capacity to run complex Java APIs. Also, the Java servlet code is portable across operating systems and machine architectures.

None of the graphical user interface components of the Java API can be used by a servlet, as its display is an HTML web page. However, it is possible to use the imaging APIs to create graphs and display them as a final rendered image. In addition, the servlet has access not only to a vast amount of standard APIs for accessing databases and other information but also to third party APIs for Java.

As Java progresses through various versions, APIs are deprecated and eventually discarded, and sometimes even the language itself is changed. This may not cause a problem as the Java-generated pseudo code (a type of machine-independent machine code) can still execute on a different version of the JVM. However, at some point (several years down the line) you will be required to recompile and port the servlet with the Java Development Kit (JDK) of the day.

Another disadvantage is that Java, unfortunately, is slow. Although great strides have been made in performance improvements, Java code will not be as fast as native executables but probably is faster than several other popular scripting languages.

Since the servlet manager (in this case Tomcat) is a Java program and is separate from the web server, Apache, the communication between the two is not instantaneous. This may produce a performance hitch on heavily active web servers.

Getting Started

The biggest hiccup with servlets is the installation and running of the various components that are required for servlet support. Here is a list of some requirements:

  • A Java installation. Although some Linux distributions provide Java, the version may be limited in its API set or may be an older version.

  • A web server that can support servlets. There are several available. In this tutorial, I use Apache but recommend Jetty as well. You need to have Apache installed and functioning for servlet support. Most Linux distributions install Apache by default and already have it running, so this may not be a problem.

  • You need a mechanism by which the web server can execute Java servlets. For example, a popular servlet support extension to Apache is Tomcat. However, Linux distributions most likely will not provide this mechanism, and you will need to download it from the Web.

Gotchas for Apache and Tomcat

You need to inform Tomcat of the location of the Java distribution; if the installation process does not detect it at install time, you will need to edit /etc/rc.d/init.d/tomcat. Be particularly careful if you have more than one version of Java installed (e.g., 1.1.18 and 1.2.2).

Also, whenever Tomcat is restarted, the Apache server must be restarted as well. This is to establish the communication path between Apache and Tomcat. The Apache server can be restarted without having to restart Tomcat.

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code to display charts

raghu's picture

i want to display more than one cahrts in a web page by using java by passing x-axis nad y-axis. and the data should fetch from oracle database. the query should execute in the backend.

Can u pls sort this out

rimi's picture

hi.thanks for the wonderful stuff.
i have a problem, however.
I developed a login form in html,got it validated for user authentication through java servlets,and if user is valid the servlet is redirected to another html page, but now i wish to edit certain tags of the html page before redirection, kindly find some time to answer this soon.

Posting from Http DoPost without html form

Anonymous's picture

Hi,

How can I call DoPost directly from the code, without html post method?

Thanks,

Re: Using and Writing Java Servlets

Anonymous's picture

Good review. Right on target on the configuration being troublesome. The beating of old drums such as 'recompiling for new API's' or 'Java is slow' is unfortunately pure bantor. Everybody in every language recompiles code from time to time. And saying 'java is slow' is akin to the windoz jokers preaching that 'linux has no desktop interface'. How very yesterday....

running servlet

alankar's picture

Hi,

I tried your example of form response servlet. Can you tell me where should I put the html form file and the class file of the sevlet? so that I can try it on Abyss Server/Tomcat/Servletrunner

Thanks,

Alankar

Re: Using and Writing Java Servlets

Anonymous's picture

I like the tutorial, it will help me learn. I wish some of the value judgements could have been left out. I am, however, sick of Java people bashing "scripting" langs that they clearly have no knowledge of.

Eric

Re: Using and Writing Java Servlets

Anonymous's picture

I really like this Tutorial too! I just start using Servlet yesterday and I have already create a WebApp information Table from what I have learned from this session. Thank you!

==Sandy

Re: Using and Writing Java Servlets

Anonymous's picture

No credit is given to mod_perl, which only loads one instance of the Perl interpreter. Within mod_perl, pages are compiled once and run many times during the life of the server process. Many major web sites use the power of mod_perl to leverage the speed of Apache and the power and flexibility of Perl to generate rich, dynamic, and lightning-fast Web sites.

Re: Using and Writing Java Servlets

Anonymous's picture

The article is not about mod_perl. However I do agree the article is a bit misleading as nobody in their right mind uses perl/php via CGI with Apache.

I like this article but can u

Amit's picture

I like this article but can u plz tell me the full way to run Servlet on tomcat.And how to modified or create web.xml for the servlet.

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