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Open a file with a java application

I made a text editor in java. It has a FileExplorer class which allows me to read and write to a file. Now I would like to know how I could open said file (text.txt) with my editor application from outside my application. Basically when I double-click on the file (text.txt) it should start my application and pass some variables(like name and path). The application is a .jar file.
I am not going to bother anyone to go through the 2k lines of code, so I won't post it in here. But it is just a JTextPane in a JFrame and a PrintWriter/BufferedReader reading and writing to the file.
PS: should preferably work cross-platform.
Thanks

This question is not about java. It is about mapping files to specific application using you OS tools.
If you are on Windows you have to map extension *.txt to your application. Take a look on this article for details.
Please note that your application must accept file path in command line.
To make association easier I'd recommend you to writer batch file that runs your application and also accepts file name in command line. Then you just have to associate your batch file with *.txt extension.
If you are on Linux association technique depends on your flavor, but you can google it. Obviously you will have to create shell script instead of batch file.
EDIT
Actually your question is mostly about installation process. There are a lot of installation tools that can do this work for you. Some of the tools are even cross platform (I can remember "install anywhere"). There are both commercial and free tools that do this.

Is writing into class resources a good way to save files?

When we want to load a static file e.g. a picture, a sound file, a file containing information about a game map,... we can store them as resources in jar file and use getClass.getResource("images/splash.png") (also getResourceAsStream) to load and use them. But when we want to read and write into a file like settings file, I don't think using resources is a good way, because i think resources are designed to store read/only files that are not supposed to change, like splash screen image or a game's background music; These are my reasons to think this way:
That is why return value of getResourceAsStream is an instance of InputStream and we don't have a similar function which gives us an OutputStream, because we're not supposed to alter resource files.
Writing into resources changes program .jar file and i guess it's not a good thing at all; Because if we do so: we can't use check-sums to verify file, if we are a limited user and system administrator doesn't give us write permission we can't make changes into main .jar file, user-specific preferences are hard or impossible to implement,...
So, my questions are:
Which parts of my thoughts and assumptions are right or wrong?
If they're right what is the best(I mean short and portable between OSs and Computers) way to store files like that? (Application setting/preferences, A game save file, ...)
(#Some user who may wants to mark this as duplicate: I don't think my question is a duplicate, i searched in the site, I admit it has some common parts with some questions but it's not duplicate!)

Your three observations in #2 above are valid reasons not to store settings in a resource file, regardless of the APIs provided.
There are a variety of ways to save settings in Java, including:
The Java system property "user.home" provides the user's home directory, to which the user should have write access. You can create an application-specific subdirectory underneath it.
Java provides a Preferences API. This may store settings in a directory or (on Windows) in the registry.
OSGI provides a preferences API.
If you're using the Eclipse RCP, you can write to the configuration directory using a ConfigurationScope. See the Eclipse FAQ "What is a preference scope").

Open and edit file in .jar with Java?

How would you go about opening an .xml file that is within a .jar and edit it?
I know that you can do...
InputStream myStream = this.getClass().getResourceAsStream("xmlData.xml");
But how would you open the xmlData.xml, edit the file, and save it in the .jar? I would find this useful to know and don't want to edit a file outside of the .jar... and the application needs to stay running the entire time!
Thank you!

Jar files are just .zip files with different file suffix, and naming convention for contents. So use classes from under java.util.zip to read and/or write contents.
Modifying contents is not guaranteed (or even likely) to effect running system, as class loader may cache contents as it sees fit.
So it might be good to know more about what you are actually trying to achieve with this. Modifying contents of a jar on-the-fly sounds like complicated and error-prone approach...

If you app. has a GUI and you have access to a web site/server, JWS might be the answer. The JNLP API that is available to JWS apps. provides services such as the PersistenceService. Here is a small demo. of the PersistenceService.
The idea would be to check for the XML in the JWS persistence store. If it is not there, write it there, otherwise use the cached version. If it changes, write a new version to the store.
The demo. writes to the store at shut-down, and reads at start-up. But there is no reason it could not be called by a menu item, timer etc.

Removing file locks in Windows and Java

I have a Java program that opens a file using the RandomAccessFile class.
I'd like to be able to rename that file while it is opened by Java. In Unix, this isn't a problem. Does anyone know how I can do this in Windows?
Should I set Java to open it a certain way?
Thanks in advance.
Edit, clarification: I'd like to be able to set this file sharing access in my Java code.

You need to close any handles on the file at any way. E.g. a FileOutputStream on that File must be closed before you can rename it. That's just the way how the Windows file system works.

in_stream.close();
out_stream.getChannel().close();
out_stream.close();
Anyhow in Windows it stay locked :*(

Opening file from Java

What I need is to instruct OS to open the file with the default program used for that file type. Exactly as if, i.e., that file was double-clicked by user under Windows.
The purpose is, i.e., "your PDF file was generated. Click here to open it".
In platform-independent way, if possible...
I dont know exact terms for what I want, so if someone could update tags, I'd most appreciate that ;)

You need the Desktop class, and the open() method in particular.
Launches the associated application to
open the file. If the specified file
is a directory, the file manager of
the current platform is launched to
open it.

Since Java 6, we have Desktop.open() for exactly that purpose.