Servlet 2.4: What's in store

On March 7, 2003, Sun Microsystems (working with the JSR (Java Specification Request) 154 expert group) published the "Proposed Final Draft 2" of the Servlet 2.4 specification (see Resources for a link to the formal specification). As it's still in the Proposed Final Draft stage, the specification is not quite finished, and technical details may change. A Proposed Final Draft 3 is even a possibility. However, changes should not prove significant before the specification's final release, and, in fact, server vendors have already begun to implement the new features. That makes now a good time to start learning about what's coming in Servlet 2.4 and check out its integration with J2EE (Java 2 Platform, Enterprise Edition) 1.4.

In this article, I describe what changed between 2.3 and 2.4. I also explain the decision-making process behind the changes and tell you about a few things that didn't make it. To keep the article focused, I assume you're familiar with the classes and methods of previous Servlet API versions. If that's not the case, you can peruse Resources for links to sites (and my book!) that will help get you up to speed.

Servlet 2.4 lacks some of the fireworks of past releases. Servlet 2.2 introduced the notion of self-contained Web applications. Servlet 2.3 added the power of filters and filter chains. Servlet 2.4, while adding several interesting features, has no superstars and spends more time polishing and clarifying the features that came before—a tying up of loose ends. This work's effect is that servers faithfully implementing 2.4 will be more interoperable than any past servers. But don't let me imply there's nothing new in Servlet 2.4! Here's a list of what's new:

• Servlets now require HTTP/1.1 and J2SE (Java 2 Platform, Standard Edition) 1.3, and can work with J2EE 1.4
• ServletRequest has new methods to observe the client connection
• New support for internationalization and charset choice
• RequestDispatcher has new features and clarifications
• New ServletRequest listener classes and methods
• A deprecated SingleThreadModel
• HttpSession details and interaction with logins has been clarified
• Classloading and welcome-file behavior has been clarified
• The web.xml file now uses XML Schema and has added a slew of new elements

Before we begin looking at these changes, let me point out that most servers don't yet have fully compliant Servlet 2.4 implementations. If you want to test those features, your best bet is to download the official reference implementation server, Apache Tomcat 5.0. It's open source, and you can download the server for free. Tomcat 5.0 will be the first Tomcat version to support Servlet 2.4, but, of course, its latest release is still an alpha. (See Resources for more information on Tomcat.)

Upgraded support for HTTP, J2SE, and J2EE

Servlet 2.4 depends on HTTP/1.1 and J2SE 1.3. Previously, servlets relied upon HTTP/1.0 and J2SE 1.2. Having 2.4 upgrade these minimum-level requirements means servlet authors can reliably depend on the new features of HTTP/1.1 and J2SE 1.3. At the same time, these requirements complicate the task of the servlet container developer because HTTP/1.1 has more special cases and complexities than HTTP/1.0. Some servers already support HTTP/1.1, but those that don't will have to spend some time upgrading. Note that, unlike what some people think, having J2SE 1.3 as a minimum requirement does not mean that if you implement Servlet 2.4 on J2SE 1.2, you've succeeded with a good hack. That breaks the contract with the servlet author, a contract that says an author may rely on J2SE 1.3 features when he writes against Servlet 2.4.

As another change, Servlet 2.4 will be part of the upcoming J2EE 1.4 (in fact, the two specs will be released concurrently, probably around JavaOne 2003). Of course, it's important to note that servlets can run standalone; you don't have to buy a full J2EE container to run servlets. Apache Tomcat, for example, doesn't implement all of J2EE. But when you run servlets as part of J2EE 1.4, you can take advantage of extra features and extra deployment descriptor elements exposing JNDI (Java Naming and Directory Interface) resources, EJB (Enterprise JavaBeans), message queues, and JAX-RPC (Java API for XML-based Remote Procedure Call) services. I'll talk about some of those elements later.

The upgrade to HTTP/1.1 caused one code change. Servlets have a new static constant HttpServletResponse.SC_FOUND to represent status code 302. Found is the HTTP/1.1 name for what HTTP/1.0 called Moved temporarily. HttpServletResponse.SC_MOVED_TEMPORARILY still exists and represents 302, but SC_FOUND is now preferred. SC_MOVED_TEMPORARILY can be considered deprecated, but deprecating variables, even public constants is technically impossible in Java.

New ServletRequest methods

The ServletRequest interface (and the ServletRequestWrapper class) adds four new methods in Servlet 2.4:

• getRemotePort(): Returns the IP source port of the client or last proxy that sent the request
• getLocalName(): Returns the host name of the IP interface on which the request was received
• getLocalAddr(): Returns the IP address of the interface on which the request was received
• getLocalPort(): Returns the IP port number of the interface on which the request was received

These methods provide a mechanism to query the low-level IP connection details and understand how the connection routed. The getRemotePort() method, combined with the preexisting getRemoteAddr() and getRemoteHost() methods, exposes the client side of the IP connections. The new getLocalPort(), getLocalAddr(), and getLocalName methods expose the server side of the IP connections. The preexisting getServerName() and getServerPort() methods have been newly defined to expose the HTTP-layer details by simply returning the "host:port" extracted from the HTTP Host header. On a virtual hosted or load-balanced system, these methods provide a way to learn what clients, proxies, or load-balance devices connect, where they physically connect, and where they virtually connect.

Internationalization

Also in Servlet 2.4, the ServletResponse interface (and the ServletResponseWrapper) adds two new methods:

• setCharacterEncoding(String encoding): Sets the response's character encoding. This method provides an alternative to passing a charset parameter to setContentType(String) or passing a Locale to setLocale(Locale). This method has no effect if called after getWriter() has been called or if the response has committed. For a list of acceptable Internet charsets, see Resources.
• getContentType(): Returns the response's content type. This may include a charset parameter set by either setContentType(), setLocale(), or setCharacterEncoding(). If no type has been specified, the method returns null.

The setCharacterEncoding() method pairs with the preexisting getCharacterEncoding() method to provide an easy way to manipulate and view the response's character encoding (charset). You can now avoid setting the charset via the awkward setContentType("text/html; charset=UTF-8") call.

The new getContentType() method pairs with the preexisting setContentType() method to expose the content type you've assigned. Formerly, this wouldn't have been too interesting, but now the type might be dynamically set with a combination of setContentType(), setLocale(), and setCharacterEncoding() calls, and this method provides a way to view the generated type string.

So which is better, setLocale() or setCharacterEncoding()? It depends. The former lets you specify a locale like ja for Japanese and lets the container handle the work of determining an appropriate charset. That's convenient, but, of course, many charsets might work for a given locale, and the developer has no choice in the matter. The latter method provides a new, easy way to choose a specific charset, letting you override the container's choice of Shift_JIS with EUC-JP, for example.

However, the story doesn't end there. Servlet 2.4 also introduces a new <locale-encoding-mapping-list> element in the web.xml deployment descriptor to let the deployer assign locale-to-charset mappings outside servlet code. It looks like this:

<locale-encoding-mapping-list>
  <locale-encoding-mapping>
    <locale>ja</locale>
    <encoding>Shift_JIS</encoding>
  </locale-encoding-mapping>
  <locale-encoding-mapping>
    <locale>zh_TW</locale>
    <encoding>Big5</encoding>
  </locale-encoding-mapping>
</locale-encoding-mapping-list>

Now within this Web application, any response assigned to the ja locale uses the Shift_JIS charset, and any assigned to the zh_TW Chinese/Taiwan locale uses the Big5 charset. These values could later be changed to UTF-8 when it grows more popular among clients. Any locales not mentioned in the list will use the container-specific defaults as before.

RequestDispatcher changes

Servlet 2.4 adds five new request attributes to provide extra information during a RequestDispatcher forward() call. In case you've forgotten, when you forward() to a servlet, the servlet container changes the target servlet's path environment as if it were the first servlet being invoked. The methods getRequestURI(), getContextPath(), getServletPath(), getPathInfo(), and getQueryString() all return information based on the URI (Uniform Resource Identifier) passed to the getRequestDispatcher() method. However, sometimes an advanced forward() target servlet might like to know the true original request URI. For this, Servlet 2.4 adds the following request attributes:

• javax.servlet.forward.request_uri
• javax.servlet.forward.context_path
• javax.servlet.forward.servlet_path
• javax.servlet.forward.path_info
• javax.servlet.forward.query_string

Inside a forwarded servlet you'll see getRequestURI() return the path to the target servlet as always, but now if you want the original path, you can call request.getAttribute("javax.servlet.forward.request_uri"). One special caveat: if forward() happens through a getNamedDispatcher() call, these attributes aren't set because, in that case, the original path elements aren't changed.

This set of attributes may remind you of these request attributes added with Servlet 2.2:

• javax.servlet.include.request_uri
• javax.servlet.include.context_path
• javax.servlet.include.servlet_path
• javax.servlet.include.path_info
• javax.servlet.include.query_string

However, these work just the opposite of the forward() attributes. In an include(), the path elements don't change, so the include attributes act as the backdoor to access the target servlet's path elements. Compare this with a forward() where the path elements change so the forward attributes represent the backdoor to the original path elements. Yes, it gets complicated. As soon as servlets began to use the URI space as an internal dispatch mechanism, the door to complexity opened.

Another area where we see this complexity is in the interaction between the RequestDispatcher and filters. Should filters invoke for forwarded requests? Included requests? What about for URIs invoked via the <error-page> mechanism? Before Servlet 2.4, these questions were left as open issues. Now Servlet 2.4 makes it a developer's choice. There's a new <dispatcher> element in the deployment descriptor with possible values REQUEST, FORWARD, INCLUDE, and ERROR. You can add any number of <dispatcher> entries to a <filter-mapping> like this:

<filter-mapping>
  <filter-name>Logging Filter</filter-name>
  <url-pattern>/products/*</url-pattern>
  <dispatcher>REQUEST</dispatcher>
  <dispatcher>FORWARD</dispatcher>
</filter-mapping>

This indicates the filter should be applied to requests directly from the client as well as forward requests. Adding the INCLUDE and ERROR values also indicates that the filter should additionally be applied for include requests and <error-page> requests. Mix and match for what you want. If you don't specify any <dispatcher> elements, the default is REQUEST.

The last RequestDispatcher change is to allow, for the first time, relative paths in request.getRequestDispatcher() calls. The path will be interpreted relative to the current request's path. It's a minor change, but comes in handy when dispatching to a sibling servlet.

Listeners

Servlet 2.3 introduced the idea of context and session listeners, classes that could observe when a context or session was initialized or about to be destroyed, and when attributes were added or removed to the context or session. Servlet 2.4 expands the model to add request listeners, allowing developers (or more likely tool vendors) to observe as requests are created and destroyed, and as attributes are added and removed from a request. Servlet 2.4 adds the following classes:

• ServletRequestListener
• ServletRequestEvent
• ServletRequestAttributeListener
• ServletRequestAttributeEvent

These classes have been modeled after the familiar ServletContextListener, ServletContextEvent, ServletContextAttributeListener, and ServletContextAttributeEvent design, and are assigned to execute using the same <listener> elements. The request variety of listeners were added primarily to help debugging tools hook into the request handling. The practical applications beyond that may be slim, so I don't dig into details here.