Why Does getResourceAsStream Return Null When Loading Resources from a JAR, and How Can I Fix It?

Resolving "getResourceAsStream Returns Null" for Loading Resources from JAR

When attempting to load a text file from a package within a compiled JAR, the getResourceAsStream method can return null, leaving developers stumped. This article delves into the issue and provides a solution.

The initial attempts to load the file using Class::getResourceAsStream and various arguments all resulted in null being printed. This unexpected behavior is due to the fact that the system class loader, which is used by Class::getResourceAsStream(...) method, does not recognize JARs and therefore cannot access the resources within the JAR file.

To resolve this issue, it is necessary to use the appropriate class loader that has access to the resources in the JAR. In Java, each class is loaded by a class loader, and the class loader that loaded Lifepaths class will have access to the resources in the JAR. Therefore, the correct way to load the file is to use Class::getResourceAsStream() method directly, as shown below:

public static InputStream loadResource() {
    return Lifepaths.class.getResourceAsStream("/initialization/Lifepaths.txt");
}

Note that when invoking getResourceAsStream(name), the name must start with "/". This ensures that the resource is loaded from the root of the classpath and is essential for the class loader to correctly locate the file.

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