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Tue, 01/06/2009 - 16:00
Servlets New Features
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.
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:
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
enumtype, 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
Serializablemarker interface (to alter serialization) or the@deprecatedJavadoc 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.Muhammad Sajid Khan
BITF03Alumni
03004360542
Cont...
Here's a simple Web service annotation example:
The
@WebServiceand@WebMethodannotation types, specified in JSR 181 (Web Services Metadata for the Java Platform) and imported just like classes, mark this class as a Web service and mark itshelloWorld()method as a Web service method. By themselves, the annotations don't do anything but sit there, kind of like Post-It notes; however, a container, upon loading this class and seeing those annotations in the bytecode, can wire up the class for Web services.Annotations may accept name/value parameters. The parameter information is kept with the annotation and can be used to alter the behavior requested by the annotation. For example, here's a more advanced annotation example:
Upon loading this class, a proper container will respect the annotations and their parameters, and wire up the class as a
PingServicewith aFoooperation using the remote-procedure-call/literal encoding style. In a sense, annotations define the contract a class wishes to have with its container.The Java language itself (through JSR 175) specifies only a tiny number of annotation types. The interesting annotation types come from other JSRs:
Annotations in Servlet 2.5
Coming back to Servlet 2.5, the new specification describes how several annotations work in a servlet environment. Simple servlet containers can ignore these rules, while servlets in a JEE container must abide by them.