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11.4 The session Attribute

259

tem guarantees that there will not be simultaneous access to the same servlet

instance. The system can satisfy this guarantee either by queuing up all

requests and passing them to the same servlet instance or by creating a pool

of instances, each of which handles a single request at a time.

You use 

<%@ page isThreadSafe="false" %>

 to indicate that your code

is not thread safe and thus that the resulting servlet should implement 

Sin 

gleThreadModel

. (See Section 2.6 (The Servlet Life Cycle.) The default

value is 

true

, which means that the system assumes you made your code

thread safe, and it can consequently use the higher performance approach of

multiple simultaneous threads accessing a single servlet instance. Be careful

about using 

isThreadSafe="false"

 when your servlet has instance vari 

ables (fields) that maintain persistent data. In particular, note that servlet

engines are permitted (but not required) to create multiple servlet instances

in such a case and thus instance variables are not necessarily unique. You

could still use 

static

 fields in such a case, however.

11.4 The session Attribute

The 

session

 attribute controls whether or not the page participates in

HTTP sessions. Use of this attribute takes one of the following two forms:

<%@ page session="true" %> <%   Default   %>

<%@ page session="false" %>

A value of 

true

 (the default) indicates that the predefined variable 

ses 

sion

 (of type 

HttpSession

) should be bound to the existing session if one

exists; otherwise, a new session should be created and bound to 

session

. A

value of 

false

 means that no sessions will be used automatically and

attempts to access the variable 

session

 will result in errors at the time the

JSP page is translated into a servlet. 

11.5 The buffer Attribute

The 

buffer

 attribute specifies the size of the buffer used by the 

out

 variable,

which is of type 

JspWriter

 (a subclass of 

PrintWriter

). Use of this

attribute takes one of two forms:

<%@ page buffer="sizekb" %>

<%@ page buffer="none" %>

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