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The Server-side Java™ Magazine - September 1998 Issue
Servlet Central Ask the Experts
Paul Bonfanti, New Atlanta Communications

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If you have a servlet problem and can't find a solution, then send it to me at with a subject of "Ask the Expert" and let me take a crack at it. When sending a question, please be brief and to the point and include a code snippet if it will shed some light on your problem. I'll send you a response and the response will appear in the next "Ask the Experts" column.


Q: I tried to unload dynamically loaded java class from a regular java server. It worked fine. ( I nuke that class assigning it to null) and then run Systemgc() When I tried EXACTLY the same thing in servlet - it did not work. I have to stop/restart JSM default every time I recompile dynamically loaded class.

A: Most servlet engines automatically reload servlets which are placed in the Servlets directory. If your servlets are in the Servlets directory and the main servlet class changes then the Servlet Engine should automatically reload the servlet. If you continue to have problems then you should contact your Servlet Engine vendor.

Q: I have the following lines in my servlet:
File profile = new File("profile.ini");
try { Input = new FileInputStream(profile);
. . } catch (FileNotFoundException e) { System.out.println( "can't found the file" );
} catch (IOException e) { System.out.println("I/O Error"); }
It always displays "can't found the file". But the profile.ini file is in my current working directoy. I don't know if I can load a file with its relative pathname.

A: The current working directory may be different from what you expect. Try displaying the user.dir system property to determine what is the current working directory.

Q: I have a servlet that uses Cookies, when I call the servlet from an HTML page, it works fine, it gets all the cookies and can also create cookies, but calling the SAME servlet from a java Applet, the servlet never receives any cookies and does not seem to create any cookies either. the Applet is using the standard javax.servlet.http.* classes to communicate with the servlet. Is there a limitation on passing of cookies to the servlet when it is called/accessed thru an Applet vs. a HTML page.

A: When you call a servlet from an HTML page it is the browser which handles the sending of the cookies in the HTTP request header and the reading and storing of the cookies in the HTTP response header. When you call a servlet from an applet you need to handle adding the cookies to the HTTP request header and the reading and storing of the cookies in the HTTP response header.

Q: I would like to know whether a servlet can instruct the browser to go to a specified url. For example, after the servlet successfully processes data submitted by a user, I would like to display a thank you page in the browser. This thank you page would be a .html page at some specific location ( with other frames such as those containing the top company logo, other links of the site etc. As a workaround, what I am doing right now is to read the html page line by line and write it to the output stream. this has a problem that when the html page has an img tag in it, the servletrunner displays an error message "servlet img not found".

A: This can be done by using the sendRedirect() method of the response object.

Q: I'm trying to write a form base authorization (username/password) servlet. I'm using redirection to send users to the login.html when they attempt to access a protected page. However, I'm running into a problem. I want the user only to have to login once per browser session. I have tried shared objects and normal session putValue() and getValue() methods. Unfortunately, I can only get two behaviors. (1) login every time you attempt to access a protected page; (2) login once and never have to again until the server is reset. Is there anyway to detect the browser closing and use this event to manually invalidate the user's session?

A: There's no way to detect the browser closing to invalidate the user's session. Instead there is an invalidation time out value which indicates how long the session must be inactive before the servlet engine invalidates it. The default value is 30 minutes.

Q: I would like to know whether it is possible to get following info whenever a html page is viewed: 1) the name and version of the browser where the request came from 2) the url from which the current html page was referred to

A: This information can be retrieved by calling
req.getHeader( "User-Agent" ) and req.getHeader( "Referer" );

Paul Bonfanti works for New Atlanta Communications as the lead developer of the ServletExec servlet engine ( 


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