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Even as a mid-level API ensconced in modern UI component-based Web frameworks and Web services technologies, the incoming Servlet 3.0 specification (JSR 315) will have groundbreaking impact on Java Web application development. Author Xinyu Liu explains in detail why asynchronous processing is foundational for the collaborative, multi-user applications that define Web 2.0. He also summarizes Servlet 3.0's other enhancements such as ease of configuration and pluggability. Level: Intermediate
The Java Servlet specification is the common denominator for most server-side Java Web technologies, including JavaServer Pages (JSP), JavaServer Faces (JSF), numerous Web frameworks, SOAP and RESTful Web services APIs, and newsfeeds. The servlets running underneath these technologies make them portable across all Java Web servers (servlet containers). Any proposed changes to this widely accepted API for handling HTTP communications will potentially affect all affiliated server-side Web technologies.
The upcoming Servlet 3.0 specification, which passed public review in January 2009, is a major release with important new features that will change the lives of Java Web developers for the better. Here's a list of what you can expect in Servlet 3.0:
Asynchronous support is Servlet 3.0's most significant enhancement, intended to make the server-side processing of Ajax applications much more efficient. In this article I'll focus on asynchronous support in Servlet 3.0, starting by explaining the connection and thread-consumption issues that underlie the need for asynchronous support. I'll then explain how real-world applications today utilize asynchronous processing in server push implementations like Comet, or reverse Ajax. Finally, I'll touch on Servlet 3.0's other enhancements such as pluggability and ease of configuration, leaving you with a good impression of Servlet 3.0 and its impact on Java Web development.
Web 2.0 technologies drastically change the traffic profile between Web clients (such as browsers) and Web servers. Asynchronous support introduced in Servlet 3.0 is designed to respond to this new challenge. In order to understand the importance of asynchronous processing, let's first consider the evolution of HTTP communications.
A major improvement in the HTTP 1.1 standard is persistent connections. In HTTP 1.0, a connection between a Web client and server is closed after a single request/response cycle. In HTTP 1.1, a connection is kept alive and reused for multiple requests. Persistent connections reduce communication lag perceptibly, because the client doesn't need to renegotiate the TCP connection after each request.
Figuring out how to make Web servers more scalable is an ongoing challenge for vendors. Thread per HTTP connection, which is based on HTTP 1.1's persistent connections, is a common solution vendors have adopted. Under this strategy, each HTTP connection between client and server is associated with one thread on the server side. Threads are allocated from a server-managed thread pool. Once a connection is closed, the dedicated thread is recycled back to the pool and is ready to serve other tasks. Depending on the hardware configuration, this approach can scale to a high number of concurrent connections. Experiments with high-profile Web servers have yielded numerical results revealing that memory consumption increases almost in direct proportion with the number of HTTP connections. The reason is that threads are relatively expensive in terms of memory use. Servers configured with a fixed number of threads can suffer the thread starvation problem, whereby requests from new clients are rejected once all the threads in the pool are taken.