Why does the getResource() method return null in JDK 11?

I've tried your application with both openjdk 8 and 11 on MacOS and it does not work with both. I think you need to look at [1] and [2] in order to understand how getResourceAsStream works.

TLDR:

1. If the path is absolute (i.e. starts with a slash - /), then class.getResourceAsStream() searches in the provided path

2. If the path is NOT absolute (i.e. does not start with a slash) , then class.getResourceAsStream() searches in a constructed path that corresponds to the package name, where the dots are replaced with slashes

So whether it works or not depends on 2 things:

1. Is your path absolute or not ?
2. Is the file located in the same package as the class or not ?

Basically in your exaple as is provided, it can never work if the path is not absolute, because Class.class.getResourceAsStream() will always resolve the path to java/lang/<file>, so your file must be in a system package. So instead you must use <MyClass>.class.getResourceAsStream() or alternatively use an absolute path

[1] https://docs.oracle.com/javase/7/docs/api/java/lang/ClassLoader.html#getResource(java.lang.String)

[2] https://docs.oracle.com/javase/7/docs/api/java/lang/Class.html#getResource%28java.lang.String%29

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Update

Since Java SE 9, invoking getResourceXXX on a class in a named module will only locate the resource in that module, it will not search the class path as it did in previous release. So when you use Class.class.getResourceAsStream() it will attempt to locate the resource in module containing java.lang.Class, which is the java.base module. Obviously your resource is not in that module, so it returns null.

You have to make java 9+ search for the file in your module, which most probably is an "unnamed module". You can do that by changing Class to any class defined in your module in order to make java use the proper class loader.

There is another solution using ClassLoader

ClassLoader.getSystemResourceAsStream(file)

loads from all locations. I'm using it to load resources from JAR file which holds just resources and no executable code so the previous trick can't work because there is no SomeClass in the JAR to do SomeClass.class.getResourceAsStream()

I just don't understand the internals and I must change also the path of the file.

I have JAR file with file "my-file.txt" in the root of the JAR archive.

For java 8, this worked

Class.class.getResourceAsStream("/my-file.txt");

For java 11, I must remove the leading / even when the file is in the root path

ClassLoader.getSystemResourceAsStream("my-file.txt");