This attribute support the facility of maintaining thread for sending multiple and concurrent requests from the JSP container to the JSP page if you specify the true value of the attribute otherwise if you specify the false value of the attribute then the JSP container can send only one request at one time. The default value of the attribute is true.
A thread-safe JSP/servlet is one that works correctly when more than one thread is running at the same time. To make your JSPs thread-safe, you can implement the SingleThreadModel interface that prevents two threads from accessing the service method at the same time.
By default, the servlet container considers the JSP page code safe for reuse in a single instance by multiple request threads. If the page's code risks unintentional sharing its state across simultaneous requests, the following directive will cause the servlet container use separate instances of the page in each request:
<%@ page isThreadSafe="false" %>
With this, instead of a single instance of the servlet generated for your JSP page loaded in memory, you will have N instances of the servlet loaded and initialized, with the service method of each instance effectively synchronized. You can typically control the number of instances (N) that are instantiated for all servlets implementing SingleThreadModel through the admin screen for your JSP engine.
Whether your JSP is thread-safe or not is a consequence of the way you implemented your JSP. When the code in the JSP avoids holding a state (such as member and session variables), the servlet container can rely on the default "true" value of the "isThreadSafe" attribute to reply faster and with a smaller memory footprint.
For example, if you happen to use <%! %>, this will place the code in the class level and not in the _jspService method. Introducing class members into the JSP opens a way to lose thread safety in the single instance usage.
If your JSP is not thread-safe as a single instance serving parallel requests you will have to add the isThreadSafe=false in order for things to work correctly. This will preserve the web application's thread safety by instructing the servlet container to work around the thread-unsafety of the JSP, at a cost:
If isThreadSafe=true then the JSP container may choose to dispatch multiple outstanding client requests to the page simultaneously. Page authors using true must ensure that they properly synchronize access to the shared state of the page.
If isThreadSafe=false then the JSP container shall dispatch multiple outstanding client requests, one at a time, in the order they were received, to the page implementation for processing.
Note that even if the isThreadSafe attribute is false the JSP page author must ensure that accesses to any shared objects are properly synchronized. The objects may be shared in either the ServletContext or the HttpSession.
