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Chapter 11 The JSP page Directive: Structuring Generated Servlets

11.3 The isThreadSafe Attribute

The 

isThreadSafe

 attribute controls whether or not the servlet that results

from the JSP page will implement the 

SingleThreadModel

 interface. Use of

the 

isThreadSafe

 attribute takes one of the following two forms:

<%@ page isThreadSafe="true" %> <%!   Default   %>

<%@ page isThreadSafe="false" %>

With normal servlets, simultaneous user requests result in multiple

threads concurrently accessing the 

service

 method of the same servlet

instance. This behavior assumes that the servlet is thread safe; that is, that the

servlet synchronizes access to data in its fields so that inconsistent values will

not result from an unexpected ordering of thread execution. In some cases

(such as page access counts), you may not care if two visitors occasionally get

the same value, but in other cases (such as user IDs), identical values can

spell disaster. For example, the following snippet is not thread safe since a

thread could be preempted after reading 

idNum

 but before updating it, yield 

ing two users with the same user ID.

<%! private int idNum = 0; %>

<% 

String userID = "userID" + idNum;

out.println("Your ID is " + userID + ".");

idNum = idNum + 1; 

%>

The code should have used a 

synchronized

 block. This construct is written

synchronized(someObject) { ... }

and means that once a thread enters the block of code, no other thread can

enter the same block (or any other block marked with the same object refer 

ence) until the first thread exits. So, the previous snippet should have been

written in the following manner.

<%! private int idNum = 0; %>

<% 

synchronized(this) {

String userID = "userID" + idNum;

out.println("Your ID is " + userID + ".");

idNum = idNum + 1; 

}

%>

That's the normal servlet behavior: multiple simultaneous requests are dis 

patched to multiple threads concurrently accessing the same servlet instance.

However, if a servlet implements the 

SingleThreadModel

 interface, the sys 

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