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cannot create inputstream for ico file

I tried to create an InputStream pointing at an .ico file, which is in a directory in the src directory. I also create an InputStream for a different jar file in my src.
InputStream inIco = Installer.class.getResourceAsStream("/res/" + iconName + ".ico");
InputStream inApp = Installer.class.getResourceAsStream("/res/" + applicationName + ".jar");
that is how I tried to load it. The inputstream for the jar file works, but the other one is null.
Edit: Sorry for the confusion guys. I didn't build the jar, I just ran it from my editor, which obviously gives me different results, now it is working. Thanks for your answers.

getResourceAsStream (gRAS) loads from the same place java loads class files. That's great - it means you can ship your app as a jar and put these resources inside. If it's not working for you, you've misconfigured your build. Specifically, java is first going to determine the classpath root of your Installer.class file and looks there. If you're not sure what that is, run this code:
System.out.println(Installer.class.getResource("Installer.class"));
which will print something like jar:file:/Users/carlos/projects/FooBar/dist/foobar.jar!com/foo/Installer.class
and this tells you that gRAS is going to look in that foobar.jar file.
From there, the resource is loaded relatively to the root (because of that leading slash): Within that jar, it will look for /res/app.ico. Without it, it loads relative to the same dir/package of Installer class (in this example above, gRAS("hello.txt") is the same as gRAS("/com/foo/hello.txt").
To make this work out, your build system is responsible. For maven and gradle, have src/main/java/com/foo/Installer.java along with src/main/resources/res/icon.ico and all should be well. If this is not working out, explain how you've set up your environment because something is misconfigured if this isn't working. If you're using another build tool (such as perhaps ant, sbt, or relying on your IDE to take care of it), name the tool and perhaps we can help address the misconfiguration.

Unable to reach resources folder when compiling from .jar to .exe using Launch4j

I made a small Java program for academic purposes, its main focus is to read some .txt files and present the information to the user. These files are present in the resources folder, under the src folder.
The program runs as intended when launched from Eclipse.
Using the Launch4j app I was able to successfully create an exe which runs fine and does what's intended, up until I try to read the .txt files I have in the resources folder, which appears not to be able to reach.
I'm guessing that when I launch the exe the run time path would change to where the exe was created, so I created the program in a desktop folder and specified this path in the program, but that doesn't seem to solve the situation.
As an alternative, I moved the .txt files out of the program and once again created the exe in a desktop folder with said .txt files, linked the program to this path and once again it didn't work.
The command used to get the .txt files is:
Files.readAllLines(Paths.get(doc)).get(line)
And doc is simply the path to the intended .txt file.
It's worth noting that I have no previous experience in Java and throughout the development of the program I tried my best to use commands I'd fully understand and to keep it as simple as possible. I hope the solution can be along these lines! I'm very confident this must be a rookie mistake, but I can't seem to find the solution to this specific problem anywhere.

The paths to files in Eclipse are different than the paths to files in an .exe or JAR file.
I will let this other user explain it because I am lazy :p
Rather than trying to address the resource as a File just ask the
ClassLoader to return an InputStream for the resource instead via
getResourceAsStream:
InputStream in = getClass().getResourceAsStream("/file.txt");
BufferedReader reader = new BufferedReader(new InputStreamReader(in));
As long as the file.txt resource is available on the classpath then
this approach will work the same way regardless of whether the
file.txt resource is in a classes/ directory or inside a jar.
The URI is not hierarchical occurs because the URI for a resource
within a jar file is going to look something like this:
file:/example.jar!/file.txt. You cannot read the entries within a jar
(a zip file) like it was a plain old File.
This is explained well by the answers to:
How do I read a resource file from a Java jar file?
Java Jar file: use resource errors: URI is not hierarchical
The original post is here, all credit to its author.
Fixing your URL should let you read from that file when you are using the .exe.
EDITED FOR CORRECTION. Thanks #VGR (see comments) for correcting my mistake.

how to find a local resource from dependent jar?

I have written a code that is packed to 1.jar
with this code:
return isProd? "/etc/waze/automation/devices.json":
DeviceRepositoryFromJsonFile.class.getClassLoader().getResource("devices.json").getPath().toString();
devices.json is here:
I have another project that depends on 1.jar
however the classLoader doesn't find the local devices.json file but rather one packed in the jar
anyhow it shows the file doesn't exist.
How can I fix this? just use a absolute path anyhow?

If as in your screenshot the devices.json locate in the src/main/resources and the package have successfully treat that as the package path and put in the jar file root directory, then you can just find the file via:
DeviceRepositoryFromJsonFile.class.getResource("/devices.json");
Note the "/" slash is important to indicate that to search from the root of the classpath.

It does not answer your question directly, but it may solve your problems faster.
As far as I can see you try to detect the absolute path to json file and pass it to another method so this file could be processed. Instead, it could be done simpler:
public byte[] getDevicesJsonBytes() {
return isProd
? IOUtils.toByteArray(ABSOLUTE_PATH_TO_PROD_FILE)
: IOUtils.toByteArray(DeviceRepositoryFromJsonFile.class.getResourceAsStream(RESOURCE_CLASSPATH);
}
The common way to read classpath resources it to use getResourceAsStream on class or classLoader instance. Also, many frameworks have their own resources abstractions, but I guess you don't need them now.

Getting location of file as string in resource folder

I have completed a program in eclipse and now I would like to export it as a single runnable jar file. The program contains a resource folder with images and text files in it. This is located beneath the source folder.
The res file is not added to the build path however when I run the program in Eclipse it still works.
The thing that is confusing me is that the res file is being saved into the runnable jar file when I export it as I can open the Jar file with WinRar and I see the folder is there with all the objects in it. But when I run the problem it stops at the point that the resource folder is referenced. To add to my confusion when I manually copy and paste the res folder next to where the runnable jar file is saved and run the program it works exactly as it should do.
Now I know this is something to do with how I reference the files in my code. At the moment I have it like this
reader = new LineNumberReader(new FileReader("res/usernames.txt"));
This works exactly how I want and accesses the res folder without any exceptions - in Eclipse and when I move the resource folder next to the Jar file.
I would like it to work normally but without having a folder outside of the Jar file I would like it all encapsulated in one Jar file.
I did a lot of research and what seems to be a common fix - may I add I don't really know how it works but everyone seems to mention it - is to somewhere use:
myClass().getResource()
When I create a new FileReader it needs a String input however when I use myClass().getResource() it returns a resource and not a string. I also don't have a clue how it is meant to reference the resource folder. Should I move the resource folder into the source folder?
Does anyone know how I can reference the resource folder from within the runnable jar file?
Sorry for rambling question I know what I want for my final product but I'm getting confused by the build paths and referencing from within classes and I have searched online for a long time trying to figure it out.

Resources, when you deploy your software, are not deployed as files in a folder. They are packaged as part of the jar of your application. And you access them by retrieving them from inside the jar. The class loader knows how to retrieve stuff from the jar, and therefore you use your class's class loader to get the information.
This means you cannot use things like FileReader on them, or anything else that expects a file. The resources are not files anymore. They are bundles of bytes sitting inside the jar which only the class loader knows how to retrieve.
If the resources are things like images etc., that can be used by java classes that know how to access resource URLs (that is, get the data from the jar when they are given its location in the jar), you can use the ClassLoader.getResource(String) method to get the URL and pass it to the class that handles them.
If you have anything you want to do directly with the data in the resource, which you would usually do by reading it from a file, you can instead use the method ClassLoader.getResourceAsStream(String).
This method returns an InputStream, which you can use like any other InputStream - apply a Reader to it or something like that.
So you can change your code to something like:
InputStream is = myClass().getResourceAsStream("res/usernames.txt");
reader = new LineNumberReader( new InputStreamReader(is) );
Note that I used the getResourceAsStream() method from Class rather than ClassLoader. There is a little difference in the way the two versions look for the resource inside the jar. Please read the relevant documentation for Class and ClassLoader.

Find a resource both in .jar and eclipse with the same String computation

I want to get the path to a resource for ImageIO to read out a BufferedImage from some .png s.
While developing the project I use a relative path to "/bin/stuff/icons/image.png" , but this will definetly not work when I put everything together into a .jar file, so I need a way to get the path to these resources both while testing in eclipse and when later running it within a .jar .

After a lot of trying out both finding the file and getting the input stream to the file I came to the conclusion that this approach works every time:
InputStream in = ClassLoader.getSystemClassLoader().getResourceAsStream(path)
BufferedImage image = ImageIO.read(in)
Where path is
"projectName/resourceFolder/" + nameOfResource.stuff
as found in the src directory of the eclipse project.
E.g.
"myProject/images/icon.png"
When getting only the resource and then getting the path of the resource to link to a file, you will get FileNotFoundExceptions when using a .jar (but not while testing with eclipse, so one should be warned to think that his code works).
And - no - I don't save images in the bin/ - but they are copied to this directory and thus I find them there while testing. Now everything seems to be working.

Don't put anything under the bin directory in Eclipse: if you run a clean on the project it will be erased.
What you can do is to define a new source folder like resources, and put the image there. This way it will be automatically copied to the bin folder.
If you include the resources folder into the Jar, it will be available in both environments by using something like:
ImageIO.read( getClass().getResource("/image.png") )
PS: You can evade using a different resources folder but mixing the sources and images will quickly pollute your source folder.