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26

Chapter 2 First Servlets

2.3 A Servlet That Generates HTML

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're 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, Section 7.5 (Using

Servlets to Generate GIF Images) shows how servlets can build and return cus 

tom images, specifying a content type of 

image/gif

. As a second example,

Section 11.2 (The contentType Attribute) 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 head 

ers; they are discussed in detail in Chapter 7. Note that 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 head 

ers 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.

Second edition of this book: www.coreservlets.com; Sequel: www.moreservlets.com.

Servlet and JSP training courses by book's author: courses.coreservlets.com.