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238

Chapter 10 JSP Scripting Elements

10.3 JSP Scriptlets

If you want to do something more complex than insert a simple expression,

JSP scriptlets let you insert arbitrary code into the servlet's 

_jspService

method (which is called by 

service

). Scriptlets have the following form: 

<% Java Code %>

Scriptlets have access to the same automatically defined variables as

expressions (

request

, 

response

, 

session

, 

out

, etc.; see Section 10.5). So,

for example, if you want output to appear in the resultant page, you would

use the 

out

 variable, as in the following example.

<% 

String queryData = request.getQueryString();

out.println("Attached GET data: " + queryData); 

%>

In this particular instance, you could have accomplished the same effect

more easily by using the following JSP expression:

Attached GET data: <%= request.getQueryString() %>

In general, however, scriptlets can perform a number of tasks that cannot

be accomplished with expressions alone. These tasks include setting response

headers and status codes, invoking side effects such as writing to the server

log or updating a database, or executing code that contains loops, condition 

als, or other complex constructs. For instance, the following snippet specifies

that the current page is sent to the client as plain text, not as HTML (which is

the default).

<% response.setContentType("text/plain"); %>

It is important to note that you can set response headers or status codes

at various places within a JSP page, even though this capability appears to

violate the rule that this type of response data needs to be specified before

any document content is sent to the client. Setting headers and status codes

is permitted because servlets that result from JSP pages use a special type

of 

PrintWriter

 (of the more specific class 

JspWriter

) that buffers the

document before sending it. This buffering behavior can be changed, how 

ever; see Section 11.6 for a discussion of the 

autoflush

 attribute of the

page

 directive.

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

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