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Run a java class file from a webpage on the visitor computer?

I coded a Java program to read and modify a file on the computer. The program is based only on 1 class.
At the moment who want to use it has to run it from terminal, I'm looking on how to insert it on a webpage and make it run on the visitor's computer. It would be fine to have a file chooser (the user will want this modification).
I searched on internet and found Java applets, but I read that they aren't downloaded and executed locally so the program won't work.
How to provide a Java class file from a webpage, for use on the computer of the end-user?

If you really want to download a Java program and run it locally, you should check out Java Web Start.
Briefly, it allows the user to download and run a Java program locally on their machine. It does clever stuff like identify if an updated version is available for download, and will run the cached version if that's the current version.
Here's a tutorial.

..it would be fine to have a file chooser ..
In that case, there are basically the two options as I've outlined in comments throughout this question & the answers. I'll collect them together here:
Digitally sign the applet, get the user to accept the digitally signed code when prompted (before the applet is loaded), then offer a JFileChooser to browse to the file.
If the user has a plugin 2 JRE (chase the links in the JWS info. page for more details), it is possible to deliver the applet to the user unprompted, then leverage the JNLP API to produce a file chooser. The user will be prompted before the dialog appears, this time with a more specific warning.
JWS
For an example, see my applet based GIF animation tool which uses the JNLP API when the user goes to load image frames or save the animated GIF.
That applet is not open source (mostly because of my laziness in not wanting to revisit & tidy the code) but there is a much better example of using the JNLP file services that comes complete with source.
Digital signatures
I don't have any great links about the process of digitally signing code, but note that the 'example of using the JNLP file services' listed above provides one set of signed Jars for 2 different security environments. It also (hopefully obviously) demonstrates how to digitally sign code using Ant (it all happens by invoking the default task in the build.xml).

Applets can modify files locally, if they are signed and the user allows them to.
Read up on signed applets.

I read that they aren't downloaded and excuted locally
Whereever you read that, it is 100% incorrect. Applets are downloaded into the browser and executed at the client host.

How to upload files with java applet?

I know similar questions have been asked but i have searched for hours and as of yet have not come up with a workable solution.
I have a Java applet which will be a "paint" like application. So, I need the user to be able to upload images from their file system. I first tried using a JFileChooser which works great in the eclipse environment. However, when put online i get a "java.security.AccessControlException: access denied" exception.
I was thinking that perhaps within the applet i could call a script (located on my server) which would prompt the user to select a file - but i have no idea how to do this. I am using zymic web hosting, so the only supported scripting language is PHP.
I also tried signing the applet. Since i don't want to spend money on certificates, i self signed the applet. When i tried running it, I got an error stating "The Publisher Cannot Be Verified By A Trusted Source".
Any help would be greatly appreciated. - Thanks:)

A trusted applet can most certainly load files from the local file-system. The "Publisher Cannot Be Verified By A Trusted Source" message that is produced by self-signed applets is onerous & scary (for good reason), but if the user OKs it, it works just fine.
Here is a small demo. of exactly that.
Since the 'Next Generation' Plug-In, even sand-boxed applets can access the local file system. See the last 2 links on the Applet tag info page for further details.
Here is an applet that uses the Next Generation file abilities.

Unsigned applets can not access the file system for security reasons.
There is a tutorial about uploading files using PHP here
Maybe you can work that into your page in order to allow the file to be uploaded and then load it from your web server with the applet.
Do keep in mind the warning at the end of that example under the section "php - file upload: safe practices"

Ways to open a folder/explorer client side from a web page.... Java applet?

I have a need to "pop up" operating system folders from my web app, mostly to locate files in them. My users don’t want to use a conventional web upload/download paradigm. I have 7 or 8 static folders that need to be opened in explorer on a PC or in Finder on a mac. These folders are all network available, but are buried, and for convenience need to be shown on a web page.
There are IE tricks to do this, and I've written a sample flash app that only allows the browser to open, but I know most browsers sandbox this, and keep me from calling these folders. I am aware that some Java libraries deal with the opening of folders, does anyone have any thoughts or samples for this?

The only way I can imagine is to create a Signed Java Applet.
Applets cannot access the filesystem (and a lot of other ressources) if they are not valid signed. Maybe you can also grant the permission by a policyfile.
There ist a tutorial at JavaRanch.
But I am not sure, if this solution will be very helpful, because the JavaPlugin will be removed from a lott of browsers in near future. (I think FireFox already removed it).
I think there is actually NO solution to this problem.
There may be some workarrounds:
Put a Batch-File onto the server, which opens the folder, if executed
Can you create a local service, which handles requests from you webpage and opens the folder.
Create a webapplication, which opens the folder at serverside, and create a webpage, which displays the content in you website.

How to edit files on the users file system from my web server?

I am really looking for implementation advice as I have entered a new realm that I am not familiar with.
At the simplest level, I would like to find a way that I can read/write to a users machine from my web server. For this to work, I think I will have to install some sort of "plugin" on the users machine which can receive (or poll?) the server for instructions.
The above is the line of thought that I currently have, maybe using JAVA to do this. This needs to work on Linux, Mac and Windows OS.
I am really looking for advice on the above, is it a good idea? Is there a better way of doing this? Is there something out there already that I can build on top of?
I really appreciate all input and advice as this is something I have not done before.
Thanks all

For Java, you could launch a client application via Java Web Start that will be able to perform a limited set of operations on the file system.
If this is too restrictive, then you would need to provide a link to a download of a client application that would be installed / executed on the user's desktop machine.

I'm assuming you want to read and write specific files on the users' machine that are not normally accessible (i.e. not temp files, or files in a sandbox). And you want to do this from your webserver.
As you looking for cross platform, I'd go with java. Given that your needs are simple (read/write files from remote commands) you could probably target JRE 1.4, which is now many years old, but is installed on ca. 98% of desktops (source).
Here's an overview of how you can approach this:
Create a java applet or Java Web Start application that fetches a list of commands from a URL. The URL can contain any specific identification that you need to identify the machine, such as the users ID (see below for alternatives.)
Your webserver generates the list of commands that the applet should execute - create file, read file, write file and sends these as the response.
Sign the applet/application, so that it can escape the restrictions of the sandbox. To do this, you need to obtain a certificate. More on this later.
Inform and educate your users about what the applet is doing. E.g. a page on your site about why they are being asked to trust your certificate and what the implications are.
You can implement this as an applet or an application, the bulk of the work is pretty much the same. I talk about applet, but remember it applies equally to application.
In more detail:
The applet requests a list of commands from a URL. To hamper attempts at using your applet on another malicious site, you should use HTTPS to fetch the list of commands so that the server is authenticated. The URL should be hard-coded into your applet, so that any attempts to change this will break the signing. How you communicate the commands to the applet via the URL is up to you, e.g. you can use XML or use RMI and simply send over the list of commands as an object.
I mentioned using the userid to identify the machine - using the machine's MAC address is also a possibility. See how to get the mac address of the host.
Once you have the list of commands, your applet executes these using java file I/O apis. See File, FileReader/FileWriter in the javadocs. You include appropriate logging so the applets actions can be audited later if necessary. Once the applet has executed the commands, it sends the result of the commands back to the server, either as a POST operation, or another RMI method call, if you settle on RMI.
If you want to continually send commands from the server, then the applet can poll the URL/invoke the RMI method regularly. A Timer can help with this.
With communciation errors, it will be necessary for the client to request the list of commands more than once. Thus each time you produce a list of commands from the server, it is given an id. The server gives out the same list with the same id until it receives acknowledgement from the applet that the commands have been executed.
To sign the applet, you should obtain an rsa certificate from a certficiate authority. You can self-sign, but then you are opening the door to others modifying your app and impersonating your certificate. Details on obtaining and using certificates are given here.
If you don't know java, then all of this may not make a huge amount of sense, and it's not a trivial project to get started on, particularly considering the implications for your users if your implementation has holes and defects.
It may be wise to exercise caution: although you can do this, doesn't necessarily mean you should. I imagine that most expert users would frown on the use of this app, and would not accept the certificate. And then there is culpability - if your app accidentally deletes a critical file through a bug, misconfiguration, human error etc, how will you be prepared for that?

For the user, a web browser plugin may be the easiest, since the user won't have to manage yet another running application, however it has its limitation such as having to develop a plugin for each browser. In addition, the user would need their browser running.
You could have the user install a desktop application, which they would have to keep running, so installing it as a service might be ideal.
You could also use a plugin as a wedge between the web browser and a desktop application, which is how Flash works.

To really know you will have to answer the "why" question, why do you wan tot do this.
If you are just trying to store some state on the user's machine that you need to have locally then for small things you can fall back on cookies, or for larger needs you can use the new Web Database features in HTML5 http://dev.w3.org/html5/webdatabase/
If you need to access specific files then you are actually going to have to circumvent the security sand boxing that Java does when run on a webpage. I will leave that for others to go into, I don't know how to do it off hand myself.

In my opinion, Applet will be a better solution. I have made one applet which downloads a zip file from the server on client machine into temp folder, extracts the zip file into a directory specified by client (browser user) and then deletes the zip file.
thanks.

The lowest hanging fruit might be to provide WebDAV access through your web server to a given set of files, as this allows the users to see the files as a part of their usual filesystem with all their usual tools. You then do not have to do all that functionality.
Looks like Slide can deploy on Tomcat, and JBoss has ModeShape.
Will that be useful enough?

Java applet doesn't run on my local machine

I'm writing my own Java applet that can write to /tmp on my local unix machine.
this applet is only going to run on a html page, inside my machine only. No server involved.
But when I try to open my page inside Firefox, the applet doesn't seem to work at all. I don't understand why, since all the .class, .html and .jar files are on the same directory.
Need help.

A normal applet can't access the filesystem, as they are by default not trusted.
To enable filesystem access and other things (like native librarys) for an applet it has to be signed.
When a signed applet is loaded into the browser the first time, a notification pops up requesting the user to confirm that the applets signature is trusted.
More information here.

I don't know very much about applets, but I remember that one basic constraint is that they can't access the local filesystem.
If you want to do that one good option is to use Javascript. EDIT: But it will not work directly!