JSP Servlet J2EE Questions 3

11: How does JSP handle run-time exceptions?
A: 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.

12: How can I implement a thread-safe JSP page? What are the advantages and
Disadvantages of using it?
A: 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 .

13: How do I use a scriptlet to initialize a newly instantiated bean?
A: 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 >

14: 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?
A: 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)% >" >

15: What's a better approach for enabling thread-safe servlets and JSPs?
SingleThreadModel Interface or Synchronization?
A: 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
synchronzied 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.