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New features added to Servlet 2.5

Servlets updated with annotations, web.xml conveniences, and more

By Jason Hunter, JavaWorld.com, 01/02/06

On September 26, 2005, Sun Microsystems and the Java Specification Request 154 Expert Group issued a maintenance release of the Servlet API. Under normal circumstances, a JSR maintenance releases includes just a handful of nominally interesting specification clarifications. However, in this case, the release added several new features and changes, and made enough of an impact on servlets to justify a bump in the version number to Servlet 2.5.

In this article, I explain what's new in this Servlet 2.5 release. I describe each change, provide background on why the change was necessary, and show how to use the change in your own servlet-based programs.

This is actually my sixth article documenting Servlet API updates for JavaWorld. I intend these articles to serve two purposes: in the short term, to introduce you to the new features and, in the long term, to act as a historical compendium of changes, so when you find yourself coding against a past Servlet API version, you can determine exactly what features and behavior you can and can't depend on. Check Resources for my previous servlet articles.

Please remember: When experimenting with these new features and capabilities, not all servlet containers or Java Enterprise Edition (JEE) application servers jump immediately to the latest Servlet API release. At the time of this writing, the Jetty 6 server and Sun's GlassFish server are the two best-known servlet containers that include 2.5 support. Apache Tomcat 5.5 and JBoss 4.0 still support Servlet 2.4.

Among the changes introduced in Servlet 2.5:

• A new dependency on J2SE 5.0
• Support for annotations
• Several web.xml conveniences
• A handful of removed restrictions
• Some edge case clarifications

Dependency on J2SE 5.0

To begin with, the Servlet 2.5 specification now lists J2SE 5.0 (JDK 1.5) as its minimum platform requirement. While this limits Servlet 2.5 to those platforms with J2SE 5.0 implementations, this change means that all the new language features from J2SE 5.0 (generics, autoboxing, an improved for loop, a new enum type, static importing, varargs, and metadata annotations) are guaranteed available to Servlet 2.5 programmers.

Traditionally, servlet and JEE releases have moved forward at the measured pace of one JDK level at a time, but this time, the servlet release skipped version 1.4. The expert groups considered the double jump to be justified because J2SE 5.0 offered one compelling feature that the servlet and JEE specifications wanted to use themselves: annotations.

Annotations

Annotations are a new language feature provided as part of JSR 175 (A Metadata Facility for the Java Programming Language). Annotations provide a mechanism for decorating Java code constructs (classes, methods, fields, etc.) with metadata information. Annotations aren't executed like code, but, rather, mark code in such a way that code processors may alter their behavior based on the metadata information.

When you think about it, we've been annotating classes and methods all along with different tricks like the Serializable marker interface (to alter serialization) or the @deprecated Javadoc comment (to alter compilation). The new metadata facility simply provides a standard mechanism for doing annotations and a vehicle for libraries to create custom annotation types.