Servlet Java Interview Questions – Set 09

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In the Servlet 2.4 specification SingleThreadModel has been deprecated, why

Because it is not practical to have such model. Whether you set isThreadSafe to true or false, you should take care of concurrent client requests to the JSP page by synchronizing access to any shared objects defined at the page level.

Can you give some examples of thread racing conditions you had experienced

  • Declaring variables in JSP pages are not thread-safe. The declared variables in JSP pages end-up as instance variables in the converted Servlets. <%! Calendar c = Calendar.getInstance(); %>
  • Decalring instance variables in Servlets is not thread safe, as Servlets are inherently multi-threaded and gets accessed by multiple-threads. Same is true for the Actionclasses in the struts framework.
  • Some of the Java standard library classes likeSimpleDateFormat is not thread-safe. Always check the API to see if a particular class is thread-safe. If a particular class or library is not therad-safe, you could do one of three things.

Provide your own wrapper class that decorates the third-party library with proper synchronization.

This is a typical use of the decorator design pattern.

Use an alternative library, which is thread-safe if available.

For example, Joda Time Library.

Use it in a thread-safe manner.

For example, you could use the SimpleDateFormat class as shown below within a ThreadLocal class.

Each thread will have its own instance of the SimpleDateFormat object.

public class DateFormatTest

{

//          anonymous inner class. Each thread will have its own copy

private final static ThreadLocal<SimpleDateFormat> shortDateFormat

=  new ThreadLocal<SimpleDateFormat>()

{

protected SimpleDateFormat initialValue()

{

return new SimpleDateFormat(“dd/MM/yyyy”);

}

};

public Date convert(String strDate)throws ParseException

{

//          get the SimpleDateFormat instance for this thread and parse the date string

Date d = shortDateFormat.get().parse(strDate);

return d;

}

}

  • The one that is very popular with the interviewers is writing the singleton classes that are not thread-safe.