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JSP Interview Questions and Answers

Ques 41. How does JSP handle run-time exceptions?

Ans. You can use the errorPage attribute of the page directive to have uncaught run-time exceptions automatically forwarded to an error processing page. For example:

<%@ page errorPage=\"error.jsp\" %> redirects the browser to the JSP page error.jsp if an uncaught exception is encountered during request processing. Within error.jsp, if you indicate that it is an error-processing page, via the directive: <%@ page isErrorPage=\"true\" %> Throwable object describing the exception may be accessed within the error page via the exception implicit object. Note: You must always use a relative URL as the value for the errorPage attribute.

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Ques 42. How can I implement a thread-safe JSP page? What are the advantages and Disadvantages of using it?
Ans. You can make your JSPs thread-safe by having them implement the SingleThreadModel interface. This is done by adding the directive <%@ page isThreadSafe="false" %> within your JSP page. 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. More importantly, avoid using the tag for variables. If you do use this tag, then you should set isThreadSafe to true, as mentioned above. Otherwise, all requests to that page will access those variables, causing a nasty race condition. SingleThreadModel is not recommended for normal use. There are many pitfalls, including the example above of not being able to use <%! %>. You should try really hard to make them thread-safe the old fashioned way: by making them thread-safe .
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Ques 43. How do I use a scriptlet to initialize a newly instantiated bean?
Ans. A jsp:useBean action may optionally have a body. If the body is specified, its contents will be automatically invoked when the specified bean is instantiated. Typically, the body will contain scriptlets or jsp:setProperty tags to initialize the newly instantiated bean, although you are not restricted to using those alone.

The following example shows the “today” property of the Foo bean initialized to the current date when it is instantiated. Note that here, we make use of a JSP expression within the jsp:setProperty action.

<jsp:useBean id=”foo” class=”com.Bar.Foo” >

<jsp:setProperty name=”foo” property=”today”

value=”<%=java.text.DateFormat.getDateInstance().format(new java.util.Date()) %>” / >

<%– scriptlets calling bean setter methods go here –%>

</jsp:useBean >
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Ques 44. How can I prevent the word “null” from appearing in my HTML input text fields when I populate them with a resultset that has null values?
Ans. You could make a simple wrapper function, like

<%!

String blanknull(String s) {

return (s == null) ? \”\” : s;

}

%>

then use it inside your JSP form, like

<input type=”text” name=”lastName” value=”<%=blanknull(lastName)% >” >
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Ques 45. What's a better approach for enabling thread-safe servlets and JSPs? SingleThreadModel Interface or Synchronization?
Ans. Although the SingleThreadModel technique is easy to use, and works well for low volume sites, it does not scale well. If you anticipate your users to increase in the future, you may be better off implementing explicit synchronization for your shared data. The key however, is to effectively minimize the amount of code that is synchronized so that you take maximum advantage of multithreading.

Also, note that SingleThreadModel is pretty resource intensive from the server’s perspective. The most serious issue however is when the number of concurrent requests exhaust the servlet instance pool. In that case, all the unserviced requests are queued until something becomes free – which results in poor performance. Since the usage is non-deterministic, it may not help much even if you did add more memory and increased the size of the instance pool.
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