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I recently got an email from a customer saying that they were getting errors on one of our web apps. We have a date/time picker applet that's used throughout this app and it's not displaying for this user. The tag in the HTML page is:

<APPLET CODE="com.enerwise.applet.datetime.DateTimeSelector" WIDTH=250 HEIGHT=30  codebase="/japps" name="app_date_start" align="middle" archive="DateTimeSelector.jar,VisualSoftJBCalendar.jar">
<PARAM NAME="locale" VALUE= "en_US" ><PARAM NAME="interval" VALUE= "-1" >
</APPLET>

The error he's reporting is ClassNotFoundException in com.enerwise.applet.datetime.DateTimeSelector. He's tried accessing the page from several different computers in his office and gets the same error. He has Java 1.7 installed (the applet is old; likely compiled with 1.4) and has tried from Win XP/IE8 and Win 7/IE9 machines. I've tried it from a Win XP/IE8 and a Win 7/IE9 machine with java 1.7 and had no problem.

He's the only customer (that I know of) having this problem. I'm stumped! I suggested that he try from a computer that's NOT on his work network; I haven't gotten a reply yet.

Can anyone suggest something to check on his system? I found another thread where the problem was the users proxy setting in the Java Console. I'll have him check that as well.

Is it possible that his company's IT dept. Made a change to their security and it's blocking applets/jar files? Would he get a ClassNotFound message in that case?

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    'only sometimes' Seems to rule out proxies or security (unless they also are changing on the same basis/time as the applet is failing). That proxies or security environments are often changing, would itself be unusual. Commented Jun 22, 2012 at 12:37
  • Do you have a page where we can visit the applet? Does it need to be embedded in the page, or can it be free-floating? BTW - Check the applet info. page also & chase the links on deployJava.js. Commented Jun 22, 2012 at 12:39
  • Is there a chance the IT admin in his company have disabled some functionality on the browsers such as Java support? If you know of a similar service can you ask him to check that to see if a problem with the technology or your app? Commented Jun 22, 2012 at 12:49
  • The 'sometimes' part meant "it's not working for this customer, but works for others". It used to work for this customer until just recently, then he started getting errors. He says no changes were made to his computer recently. He had java 1.6.22 previously; he installed 1.7 after the errors started displaying. Commented Jun 22, 2012 at 12:59
  • The app is user-account accessible only and I don't know of a 'guest' type of account. Maintenance of this has just recently dropped into my lap, and I'm not strong in java; I know next to nothing about applets. Commented Jun 22, 2012 at 13:01

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The JAR files of java applets are downloaded by the browser like any other files. You should be able to create a URL to those JARs, and ask the user to enter them in the browser and see if they are downloaded.

For example, if your applet is embedded in a page accessible by 

http://yoursite.com/some/page.html

then the JAR URLs (in your case) should be something like:

http://yoursite.com/japps/DateTimeSelector.jar
http://yoursite.com/japps/VisualSoftJBCalendar.jar

(you have the /japps codebase specified in the applet tag).

Make your user enter them in the browser, and if they won't download, it's the company firewall, or they are physically missing from the web server.

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Something on the user's end was blocking the jar files. He could access them fine from his home computer but not from his office. Thanks!

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