Introduction to Java Servlets
Why Use Servlets?
Servlets have a variety of enhancements over plain CGI.
The most fundamental advantage of Servlets is that they are tightly coupled to the Web Server.
If you recall from the diagram above, CGI scripts are typically launched as a separate process by the web server whose lifetime exists only during the particular user request.
It turns out that the CGI model is extremely inefficient and slow especially with interpreted languages such as Perl or Java.
First, operating systems typically incur some overhead in simply launching another process.
When you think about it, an operating system really has to do quite a bit.
It has to:
allocate memory for the process
copy information about the environment that the parent process was running in so that there is a security context
and the web server (parent process) must be constantly aware of the child process in order to send the CGI output back to the user's web browser.
Second, an interpreted language typically has to load an entire program to interpret the script before the program can even execute.
In the case of Perl, this is the Perl executable.
In the case of Java, it is the Java binary.
Even on a small program and a relatively fast machine, a Java program can take a second or more to start.
Enter Java Servlets to the save the day.
The Servlet engine runs persistently alongside the Web Server itself.
First, because the Servlet engine is running constantly, the only time a new process is ever necessary if at all is when the web server starts.
Second, the class files themselves can be cached inside the JVM (Java Virtual Machine), so subsequent calls to Servlets do not have to go through the process of loading them into memory and past result sets can be cached in the web server itself.
The mere placement of the Java Virtual Machine inside a Web Server was really just the first step for Java Servlets.
Subsequent enhancements have been made to the API that add many value-added features.
These include:
session persistence
JSPs (Java Server Pages)
and external utility libraries to take advantage of the caching nature of Servlets for resources such as handles to relational databases.
The following is a brief listing of the major features of the current
Servlet API (v2.2):
- Persistent Java Engine
- Cached Classes
- Cached Resources (eg Database handles)
- Session handling (w/cookies and/or URL rewriting)
- Java Server Pages
- Servlet Chaining
- Inter-Servlet Communication
- HTTP Header Integration
- Internationalization features for world-wide use
- Support for Distributed Servlets
- Support for Web Application Archives (WAR files)
- Response Buffering
NEXT
Gunther Birznieks contributes to JavaBoutique's Web/Networking column.
Gunther currently works for Barclays Capital in London, one of the leading global investment banks in Europe and has previously worked as a senior computer scientist in the Human Genome Project.
Gunther is also known for writing several books on Web Programming (Perl, CGI, Java) as well as for co-creating Extropia with Selena Sol.
Extropia is one of the best known public domain web programming archives
Email: gunther@extropia.com.
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