Read a file from same folder as JAR file but still read resources folder when loading from IDE

Question

I've been trying to make jar application that can read a csv file in the same directory as it. This is, however, proving difficult as my means for accessing the file currently is:

InputStream is = getClass().getClassLoader().getResourceAsStream(filename);

Which works for my program running in the IDE and for my tests but doesn't work when I run the program from the compiled jar file. I have no idea how to get it to work for both. I seriously can't understand this path stuff, it seems like there are a million ways to do it and only one of them work for only one specific scenario.

Answer 1

I've been trying to make jar application that can read a csv file in the same directory as it.

Ah, there's your problem. That just isn't a thing.

There are only 2 types of files:

Application Resources

These are read only, and are as much part of your app as your class files are. It is not in any way relevant to think about 'editing' them - that's not the kind of thing they are. It is reasonable to assume that if this resource is somehow missing, the app is as corrupt / misinstalled as it would be if class files are missing.

For this, you use .getResource and .getResourceAsStream. And note that getClass().getClassLoader() is wrong, you want MyClass.class.getResource and then add a slash if you want to go from root (because getClass() potentially breaks when you subclass, and going via classloader is [A] just typing for no reason, and [B] breaks in bootload scenarios. MyOwnClassName.class.getResource never breaks, so, always use that).

This asks java to look in the same place class files are and nowhere else. Your class files are inside the jar files, and not next to them, therefore, it won't find a text file that is sitting next to jar files.

it does not make sense that it does work during development: That means you shoved a file inside the resources folder, which is equivalent to having a CSV file inside the jar file. You must have gone out of your way to tell your build system to do weird things. Don't do that.

If that CSV file is not intended to be user editable it should be inside the jar file and not next to it: That makes it an application resource. Examples of application resources:

• You have a GUI, and you need to store the icon files and splash screen art and such someplace.
• You ship static data with your app, such as a table of all US states along with the zipcodes they use (could be a text or csv file for example).
• Templates of config files. Not config files themselves.
• DLLs and the like that you need to unpack (because windows/linux/mac isn't going to look inside jars for them).
• You're a webapp and you want to ship the HTML static files along with your webapp.

If this is what your CSV file is, the fix is to put it in the jar, not next to it, then load it with MyClass.class.getResource(name).

Config files and project files

For example:

• For a rich text editor (like, say, LibreOffice Writer), the .odt files representing your writings.
• Save games for a game.
• A config file, which can be edited by the user, or is edited by your own app in a 'preferences' dialog. This stores for example whether to open the app full screen or not, or authentication info for a third party API you're using.

These should not be in the jar, should not be loaded with .getResource at all, and should not be in src/main/resources in the first place.

They also should not be next to your jar! That's an outdated and insecure model (the idea that editable files sit in the same place the app itself sits): A proper OS configuration means that an app cannot write to itself which is most easily accomplished by having it be incapable of writing to its directory. Some OSes (notably, windows) did this wrong for a while.

For example on windows, your app lives in C:\Program Files\MakorisAwesomeApp\makori.jar, and the data files for it live somewhere in C:\Users\UserThatInstalledIt\Documents\MakorisAwesomeApp.

oh linux, your app might be /usr/bin/makori and the data lives somewhere in the home dir. Config data might live in /etc/.

You don't "ship" your config files, you instead make installers that create them. You can do this part in-app by detecting that the relevant config file does not exist, load in a template (that is a resource, shipped inside your jar, loaded with getResource), and write it out, and tell the user to go look at it and edit it.

I really want a CSV file next to my jars!

Well, that's wrong, so, there are no libraries that make this easy. When you want to do silly things its good that APIs don't make that easy, right?

There are really hacky ways to do this. You can use .getResource to get a URL and then 'parse' this. This breaks the classloader abstraction concept (because in java, you can write your own classloaders and they can load from anywhere, not just files or entries in jars), but you can ask for 'yourself' (MyClass.class.getResource("MyClass.class")), pull the URL apart and figure out what's happening - does it start with file://? Then it is a file, so turn it into a j.i.File object, and go from there. Does it start with jar://? find the !, substring out the jar part, and now you know the jar. Make that a java.io.File, ask for the parent dir, and look there for the CSV.

You have to write all this. It's complicated code that is hard to test. You should not do this.