By: aathishankaran Viewed: 153337 times Printer Friendly Format
Servlet That Generates HTML
Most servlets generate HTML, not plain text as in the previous
example. To build HTML, you need two additional steps:
Tell the browser that you’re sending back HTML, and
Modify the println
to build a legal Web page.
accomplish the first step by setting the HTTP Content-Type
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
just for this purpose. The way to designate HTML is with a type of text/html,
so the code would look like this:
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
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
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
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
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.
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