JApplet - works on windows not on Linux

[Rob Hunter]

On my windows machine I have a html page that includes a simple JApplet within it (using applet tag). The class is located in the same directory as the html page. This works fine on Windows. On linux I have the same class in the same directory as the html page again but when I try loading the page the applet's area just shows Java's logo and the applet trying to load (never finishes). What do I have to do to get the class file to load in Linux? Is there any sort of setting I need to do to enable it's use? Any help would be much appreciated. Thanks.

[Maneesh Godbole]

Usually the java error console is informative in case something goes wrong. Can you post the stack trace?

[Rob Hunter]

Stack trace I got from running it on the browser (client -> Windows using IE 8; server -> Linux)

load: class mDraw.class not found.
java.lang.ClassNotFoundException: mDraw.class
	at sun.plugin2.applet.Applet2ClassLoader.findClass(Unknown Source)
	at java.lang.ClassLoader.loadClass(Unknown Source)
	at java.lang.ClassLoader.loadClass(Unknown Source)
	at sun.plugin2.applet.Plugin2ClassLoader.loadCode(Unknown Source)
	at sun.plugin2.applet.Plugin2Manager.createApplet(Unknown Source)
	at sun.plugin2.applet.Plugin2Manager$AppletExecutionRunnable.run(Unknown Source)
	at java.lang.Thread.run(Unknown Source)
Caused by: java.io.IOException: open HTTP connection failed:http://mainserver/cgi-bin/mdir/mDraw/class.class
	at sun.plugin2.applet.Applet2ClassLoader.getBytes(Unknown Source)
	at sun.plugin2.applet.Applet2ClassLoader.access$000(Unknown Source)
	at sun.plugin2.applet.Applet2ClassLoader$1.run(Unknown Source)
	at java.security.AccessController.doPrivileged(Native Method)
	... 7 more
Exception: java.lang.ClassNotFoundException: mDraw.class

[Paul Clapham]

So that error message is saying that this URL:
http://mainserver/cgi-bin/mdir/mDraw/class.class
returns a 404 error code. You could paste the URL into your browser and confirm that. Next try this URL:
http://mainserver/cgi-bin/mdir/mDraw.class
Better? If so, then wherever you configured "mDraw.class" to be the name of the class, change it to just say "mDraw".

[Rob Hunter]

Hi Paul,
I tried it without .class and nothing came from changing it. I tried in the URI and got a server error regarding http headers (the class is a JApplet). I find the /mDraw/class.class bit kind of strange. I don't have a package statement in the class, should I? Right now I have the following line for the applet in the html page loading :

<applet width="240" height="220" code="mDraw.class" style="border: solid 4px #000000;">

The cgi app and the .class mentioned are both in the same directory as well (as you can probably tell from the code). Any additional help would again be much appreciated. Thanks.

Rob

[Paul Clapham]

It looks to me as if something is applying Java rules to the string "mDraw.class" and interpreting that as a class named "class" in a package named mDraw. That would produce the URL ending in "mDraw/class.class".

Your original post just used phrases like "on Windows" without saying whether the problem occurred when you moved the server from Windows to Linux or whether you started using a browser on Linux instead of Windows. That sort of gets in the way because it helps to know what changed, and I can't tell whether it was the server or the client which changed.

I checked in my configuration and it does use "applet.class" as the name of the applet, not just "applet".

[Rob Hunter]

Hi Paul,
I'm using my windows machine (using IE 8) to view the app on both a Windows server and also a Linux server. On the windows server the applet appears fine, on Linux it doens't appear. Both machines are running the exact code (I compiled the .java files on both separately as well). It appears it has something to do with the different setup Linux has since the code (java code) and the html page are the same. Is there anything else (debug or possible solution) you can think of that I could try? Thanks again.

Rob

[Paul Clapham]

I would start by monitoring the two servers to see exactly what request the browser is sending when you start the applet.

[Rob Hunter]

I compiled the code on a Windows machine running

java version "1.6.0_17"
Java(TM) SE Runtime Environment (build 1.6.0_17-b04)
Java HotSpot(TM) Client VM (build 14.3-b01, mixed mode, sharing)

On the server running the app (where it's not working) it's running

java version "1.6.0_17"
OpenJDK Runtime Environment (IcedTea6 1.7.3) (suse-7.3-i386)
OpenJDK Client VM (build 14.0-b16, mixed mode)

Would that (if so, how would it) make a difference in getting the behaviour I'm getting from the program? Thanks again.

[Paul Clapham]

Rob Hunter wrote:On the server running the app...

Now I'm confused again. I thought we were talking about an applet here. But now you're talking about an application running on a server?

[Rob Hunter]

The applet (.class file) is on both a windows server and a linux server. I open the webpage from both on my PC in Internet Explorer 8. The applet shows fine when opening the page from the windows server and doesn't show opening the webpage from the Linux server.

[Paul Clapham]

Ah, okay. So there's no application and it's not running on the server.

But you said you compiled the applet using different compilers. Why did you do that? When you're installing Java code on more than one server, you should try to keep things consistent between the servers.

[Rob Hunter]

Actually first I didn't. I compiled on the Windows machine and moved it to the Linux as is. I tried it on both (on my machine in IE) and it didn't work when connecting to the webpage on Linux. I figured I'd tried compiling it on Linux and see if it would work (anything was worth a try). When compiling it on Linux I got a few errors trying to compile anyways so that didn't work regardless. I'm not sure if it has something to do with the 2 servers having JRE or something like that but all I know if the applets aren't loading in the webpages coming from Linux. I'd try looking and seeing what the appletviewer would do but there is no appletviewer in the java installation for Linux on the Linux machine.

[Campbell Ritchie]

I think this is too difficult a question for "beginning", so I shall move it.

[Rob Hunter]

Alright I'll outline once again:

I have a class (i.e. mDraw) that extends JApplet, at the top of the page I package the class into a /mreps/mmods directory or package mreps.mmods (this directory is under cgi-bin where the webpage is situated). In the same file that mDraw class is in (i.e. mDraw.java file), I also declare another class called mDrawing (extends JPanel). In my mDraw class I create an instance of mDrawing and then I add the mDrawing instance to my applet. Here is the error message I'm getting from the Java console after attempting to load the webpage from Linux (this all works fine coming from the Windows server's webpage). Any help would be appreciated.

Exception: java.lang.ClassNotFoundException: mreps.mmods.mDraw
load: class mreps.mmods.mDraw not found.
java.lang.ClassNotFoundException: mreps.mmods.mDraw

[Paul Clapham]

It appears to me that the difference is in how the servers on the two machines are configured. One of the servers returns your applet, the other one doesn't. So looking at the servers to see why that is would be my approach. I believe I already suggested that.

[Rob Hunter]

Where/how can I check to see what paths are set up for Java (i.e. JAVA_HOME, etc..,)? Should I have the directory the applet is in listed within a particular environment variable? What directory could I move the classes to to check if this is the case. The original way I had the script set up wasn't going to work anyways because the classes weren't part of a package, I thought once I had that change in it would work. Thanks again.

[Paul Clapham]

I don't understand what you are getting at there. The only involvement for Java here is in the browser. The server doesn't use Java in any way, it just serves (or fails to serve) a Java class when the browser asks for it. Exactly in the same way as it serves an image or an HTML file when the browser asks for it.

[Rob Hunter]

I was thinking it was something to do with linux trying to find the class. Maybe it was something to do with a java related path. The classes are stored in the exact same way (relative to the webpage on each server). On Windows it works and Linux it doesn't. What is it you want me to check?

[Paul Clapham]

Check whether the servers are returning the same thing when the same response is sent. E.g. if the client (browser) sends a request for one of your Java classes, what does each of the two servers do with that request?

[Rob Hunter]

Paul,
Sorry Paul but I haven't done a heck of a lot with applets nor with checking stuff server side for requests. What would you suggest doing (specifically) or do you have a handy doc to point to that would explain what approach you wanted me to take? Thanks again.
Rob

[Aditya Jha]

Have you thoroughly checked the package (directory) names and the class file name, especially any differences in upper-case or lower case naming? Windows machine has case-ignored names while Linux doesn't.

I must admit that I haven't gone through the posts in minute details. But since its a ClassNotFoundException, I thought the above should probably be pointed out.

[Rob Hunter]

Thanks for the reply. Yes, I've checked spelling multiple times before posting. Here's the output from the java console :

Exception: java.lang.ClassNotFoundException: mods.md.mdDraw
load: class mods.md.mdDraw not found.
java.lang.ClassNotFoundException: mods.md.mdDraw
	at sun.plugin2.applet.Applet2ClassLoader.findClass(Unknown Source)
	at java.lang.ClassLoader.loadClass(Unknown Source)
	at java.lang.ClassLoader.loadClass(Unknown Source)
	at sun.plugin2.applet.Plugin2ClassLoader.loadCode(Unknown Source)
	at sun.plugin2.applet.Plugin2Manager.createApplet(Unknown Source)
	at sun.plugin2.applet.Plugin2Manager$AppletExecutionRunnable.run(Unknown Source)
	at java.lang.Thread.run(Unknown Source)
Caused by: java.io.IOException: open HTTP connection failed:http://servername/cgi-bin/md/mods/md/mdDraw.class
	at sun.plugin2.applet.Applet2ClassLoader.getBytes(Unknown Source)
	at sun.plugin2.applet.Applet2ClassLoader.access$000(Unknown Source)
	at sun.plugin2.applet.Applet2ClassLoader$1.run(Unknown Source)
	at java.security.AccessController.doPrivileged(Native Method)
	... 7 more
Exception: java.lang.ClassNotFoundException: mods.md.mdDraw

The server (Windows) is running Sun Java installation and is where I compiled the code. The machine I'm running the webpage from on my browser is running Sun's JRE. Now on the Linux server it's running OpenJDK Runtime Environment (IcedTea6 1.7.3) (suse-7.3-i386). Not sure how much (if anything) that matters but it is a difference between the 2 servers so I thought I'd add that bit on. Any other ideas / suggestions? Thanks again.

-NOTE: the difference between this post and the original is I packaged the classes into a package called mods.md because I was getting failed attempts to access mDraw/class/class when accessing Linux.

[Paul Clapham]

Rob Hunter wrote: I haven't done a heck of a lot with applets nor with checking stuff server side for requests. What would you suggest doing (specifically) or do you have a handy doc to point to that would explain what approach you wanted me to take?

The first step would be to paste the URL http://servername/cgi-bin/md/mods/md/mdDraw.class into your browser's address bar and see what response you get. You said you were using Internet Explorer? Don't be misled by its "smart" error reporting, find out whether you're getting a 404 or if not, what error code you are getting.

You also keep telling us about Java on your servers, which I don't think is relevant. I'm not familiar with OpenJDK Runtime Environment so I can't give you any help on how to figure out whether it's responding to an HTTP request in the correct way. (Actually when I google that name I get something which appears to be a JRE... is it really an HTTP server as well?)

[Rob Hunter]

Well here's an interesting one. I've put a plain html page in that directory as well and try entering in the URL for it in the browser and it can't find the html page. So used to using Windows, where do I/should I set this directory to be browsable/executable/etc..,? The directory and the files underneath (for now) has 777 permissions.

[Paul Clapham]

Just to clarify, this is a question about which Linux server?

[Rob Hunter]

Yes, most of the uncertainties I have are with Linux only.

[Paul Clapham]

Let me clarify my request for clarification: What is the name of your Linux web server?

[Rob Hunter]

openSUSE 11.3 (i586)
VERSION = 11.3

[Paul Clapham]

That's interesting... I read the Wikipedia article about openSUSE and it doesn't mention web servers anywhere. Perhaps you aren't running one. That would explain why you aren't getting the applet returned, and the HTML file as well.

[Rob Hunter]

My pages in cgi-bin return fine though. I even run perl scripts on that server that parse html pages and return them in calls as well. Yes, a web server was installed shortly after obtaining the machine.

[Paul Clapham]

Paul Clapham wrote:Let me clarify my request for clarification: What is the name of your Linux web server?

[Rob Hunter]

I thought I had responded to the last post here. The name of the server is bluefish1.

[Rob Hunter]

What error is returned when typing the URL of the .class in the browser....

Server error!
The server encountered an internal error and was unable to complete your request. 

Error message: 
Premature end of script headers: mDraw.class 

If you think this is a server error, please contact the webmaster. 

Error 500