Programming Tutorials

A Servlet That Generates HTML

By: aathishankaran in JSP Tutorials on 2007-02-14  

Most servlets generate HTML, not plain text as in the previous example. To build HTML, you need two additional steps:

1. Tell the browser that you are sending back HTML, and
2. Modify the println statements to build a legal Web page.

You accomplish the first step by setting the HTTP Content-Type response header. In general, headers are set by the setHeader method of HttpServletResponse, but setting the content type is such a common task that there is also a special setContentType method just for this purpose. The way to designate HTML is with a type of text/html, so the code would look like this:

response.setContentType("text/html");

Although HTML is the most common type of document servlets create, it is not unusual to create other document types. For example, in the previous article (Using Servlets to Generate GIF Images) shows how servlets can build and return custom images, specifying a content type of image/gif. As a second example, in (The contentType Attribute) article shows how to generate and return Excel spreadsheets, using a content type of application/vnd.ms-excel. Don't be concerned if you are not yet familiar with HTTP response headers; they are discussed in next article. Note that if you need to set response headers before actually returning any of the content via the Print-Writer. That's because an HTTP response consists of the status line, one or more headers, a blank line, and the actual document, in that order.

The headers can appear in any order, and servlets buffer the headers and send them all at once, so it is legal to set the status code (part of the first line returned) even after setting headers. But servlets do not necessarily buffer the document itself, since users might want to see partial results for long pages. In version 2.1 of the servlet specification, the PrintWriter output is not buffered at all, so the first time you use the PrintWriter, it is too late to go back and set headers.

In version 2.2, servlet engines are permitted to partially buffer the output, but the size of the buffer is left unspecified. You can use the get-BufferSize method of HttpServletResponse to determine the size, or use setBufferSize to specify it. In version 2.2 with buffering enabled, you can set headers until the buffer fills up and is actually sent to the client. If you aren't sure if the buffer has been sent, you can use the isCommitted method to check.

The second step in writing a servlet that builds an HTML document is to have your println statements output HTML, not plain text. The structure of an HTML document is discussed more in the article (Simple HTML-Building Utilities), but it should be familiar to most readers. This program gives an example servlet, with the result.