A Fast Introduction to Basic Servlet Programming
- The Advantages of Servlets Over "Traditional" CGI
- Basic Servlet Structure
- The Servlet Life Cycle
- The Client Request: Form Data
- The Client Request: HTTP Request Headers
- The Servlet Equivalent of the Standard CGI Variables
- The Server Response: HTTP Status Codes
- The Server Response: HTTP Response Headers
- Session Tracking
Topics in This Chapter
The advantages of servlets over competing technologies
The basic servlet structure and life cycle
Servlet initialization parameters
Access to form data
HTTP 1.1 request headers, response headers, and status codes
The servlet equivalent of the standard CGI variables
Cookies in servlets
Servlets are Java technology's answer to Common Gateway Interface (CGI) programming. They are programs that run on a Web server, acting as a middle layer between a request coming from a Web browser or other HTTP client and databases or applications on the HTTP server. Their job is to perform the following tasks, as illustrated in Figure 21.
Figure 21 The role of Web middleware.
Read the explicit data sent by the client. The end user normally enters this data in an HTML form on a Web page. However, the data could also come from an applet or a custom HTTP client program.
Read the implicit HTTP request data sent by the browser. Figure 21 shows a single arrow going from the client to the Web server (the layer where servlets and JSP execute), but there are really two varieties of data: the explicit data the end user enters in a form 2. and the behind-the-scenes HTTP information. Both varieties are critical to effective development. The HTTP information includes cookies, media types and compression schemes the browser understands, and so forth.
Generate the results. This process may require talking to a database, executing an RMI or CORBA call, invoking a legacy application, or computing the response directly. Your real data may be in a relational database. Fine. But your database probably doesn't speak HTTP or return results in HTML, so the Web browser can't talk directly to the database. The same argument applies to most other applications. You need the Web middle layer to extract the incoming data from the HTTP stream, talk to the application, and embed the results inside a document.
Send the explicit data (i.e., the document) to the client. This document can be sent in a variety of formats, including text (HTML), binary (GIF images), or even a compressed format like gzip that is layered on top of some other underlying format.
Send the implicit HTTP response data. Figure 21 shows a single arrow going from the Web middle layer (the servlet or JSP page) to the client. But, there are really two varieties of data sent: the document itself and the behind-the-scenes HTTP information. Both varieties are critical to effective development. Sending HTTP response data involves telling the browser or other client what type of document is being returned (e.g., HTML), setting cookies and caching parameters, and other such tasks.
Many client requests can be satisfied by prebuilt documents, and the server would handle these requests without invoking servlets. In many cases, however, a static result is not sufficient, and a page needs to be generated for each request. There are a number of reasons why Web pages need to be built on-the-fly like this:
The Web page is based on data sent by the client. For instance, the results page from search engines and order-confirmation pages at online stores are specific to particular user requests. Just remember that the user submits two kinds of data: explicit (i.e., HTML form data) and implicit (i.e., HTTP request headers). Either kind of input can be used to build the output page. In particular, it is quite common to build a user-specific page based on a cookie value.
The Web page is derived from data that changes frequently. For example, a weather report or news headlines site might build the pages dynamically, perhaps returning a previously built page if that page is still up to date.
The Web page uses information from corporate databases or other server-side sources. For example, an e-commerce site could use a servlet to build a Web page that lists the current price and availability of each sale item.
In principle, servlets are not restricted to Web or application servers that handle HTTP requests but can be used for other types of servers as well. For example, servlets could be embedded in FTP or mail servers to extend their functionality. In practice, however, this use of servlets has not caught on, and I'll only be discussing HTTP servlets.
2.1 The Advantages of Servlets Over "Traditional" CGI
Java servlets are more efficient, easier to use, more powerful, more portable, safer, and cheaper than traditional CGI and many alternative CGI-like technologies.
With traditional CGI, a new process is started for each HTTP request. If the CGI program itself is relatively short, the overhead of starting the process can dominate the execution time. With servlets, the Java virtual machine stays running and handles each request with a lightweight Java thread, not a heavyweight operating system process. Similarly, in traditional CGI, if there are N requests to the same CGI program, the code for the CGI program is loaded into memory N times. With servlets, however, there would be N threads, but only a single copy of the servlet class would be loaded. This approach reduces server memory requirements and saves time by instantiating fewer objects. Finally, when a CGI program finishes handling a request, the program terminates. This approach makes it difficult to cache computations, keep database connections open, and perform other optimizations that rely on persistent data. Servlets, however, remain in memory even after they complete a response, so it is straightforward to store arbitrarily complex data between client requests.
Servlets have an extensive infrastructure for automatically parsing and decoding HTML form data, reading and setting HTTP headers, handling cookies, tracking sessions, and many other such high-level utilities. Besides, you already know the Java programming language. Why learn Perl too? You're already convinced that Java technology makes for more reliable and reusable code than does Visual Basic, VBScript, or C++. Why go back to those languages for server-side programming?
Servlets support several capabilities that are difficult or impossible to accomplish with regular CGI. Servlets can talk directly to the Web server, whereas regular CGI programs cannot, at least not without using a server-specific API. Communicating with the Web server makes it easier to translate relative URLs into concrete path names, for instance. Multiple servlets can also share data, making it easy to implement database connection pooling and similar resource-sharing optimizations. Servlets can also maintain information from request to request, simplifying techniques like session tracking and caching of previous computations.
Servlets are written in the Java programming language and follow a standard API. Servlets are supported directly or by a plug-in on virtually every major Web server. Consequently, servlets written for, say, iPlanet Enterprise Server can run virtually unchanged on Apache, Microsoft Internet Information Server (IIS), IBM WebSphere, or StarNine WebStar. They are part of the Java 2 Platform, Enterprise Edition (J2EE; see http://java.sun.com/j2ee/), so industry support for servlets is becoming even more pervasive.
One of the main sources of vulnerabilities in traditional CGI stems from the fact that the programs are often executed by general-purpose operating system shells. So, the CGI programmer must be careful to filter out characters such as backquotes and semicolons that are treated specially by the shell. Implementing this precaution is harder than one might think, and weaknesses stemming from this problem are constantly being uncovered in widely used CGI libraries.
A second source of problems is the fact that some CGI programs are processed by languages that do not automatically check array or string bounds. For example, in C and C++ it is perfectly legal to allocate a 100-element array and then write into the 999th "element," which is really some random part of program memory. So, programmers who forget to perform this check open up their system to deliberate or accidental buffer overflow attacks.
Servlets suffer from neither of these problems. Even if a servlet executes a system call (e.g., with Runtime.exec or JNI) to invoke a program on the local operating system, it does not use a shell to do so. And, of course, array bounds checking and other memory protection features are a central part of the Java programming language.
There are a number of free or very inexpensive Web servers that are good for development use or deployment of low- or medium-volume Web sites. Thus, with servlets and JSP you can start with a free or inexpensive server and migrate to more expensive servers with high-performance capabilities or advanced administration utilities only after your project meets initial success. This is in contrast to many of the other CGI alternatives, which require a significant initial investment for the purchase of a proprietary package.