Load resource from the classpath of application in dependency

Question

Essentially I have a client .jar that requires dynamic behaviour. It will need to load resources from the classpath of it's parent application. The main application is a Spring Boot application and the client .jar is a maven dependency of that project.

I find that when storing test.xml in a subfolder of src/main/resources of the Spring Boot parent project then using this code in the dependency:

InputStream fis = SSLSocketFactoryGenerator.class.getResourceAsStream("/subFolder/etc/test.xml");

will cause a null pointer as it can't find the file. Anyone know why this is and how to resolve this?

Answer 1

There can be 2 possible cases why you got the NPE:

1 (most likely). You incorrectly assembled your jar and it may well happen that /subFolder/etc/test.xml simply not in classpath. Take a look at ClassLoader::getResource(String name)

public URL getResource(String name) {
    URL url;
    if (parent != null) {
        url = parent.getResource(name);
    } else {
        url = getBootstrapResource(name);
    }
    if (url == null) { //not found in ClassPath, falling back to findResource.
        url = findResource(name);
    }
    return url;
}

But the "default" implementation of findResource is this:

protected URL findResource(String name) {
    return null;
}

So I would advice you to check your assembled jar-file.

2 (very unlikely). The class SSLSocketFactoryGenerator was loaded by a ClassLoader with the overriden methods for finding resources listed above. Actually I cannot imagine the case where we would override the behavoir of loading resources that are in classpath so we cannot load them. So 1. is most likely your case