How to find specific file from the classpath in java

Question

I have a project structure like below:

Now, I want to pass a specific keyword-based (which will help to find properties file in the project) on which I should directly look for that file in classpath and get it. Below is my code:

public class WithoutSubfolders {
    public static void main(String[] ar) throws Exception {
        WithoutSubfolders t = new WithoutSubfolders();
        String inputString = "Encounter";
        File path = t.getFileFromURL(inputString);
        HashMap< String, String> hMap = new HashMap< String, String>();
        File[] listOfFiles = path.listFiles();
        for (int i = 0; i < listOfFiles.length; i++) {
            System.out.println(listOfFiles[i]);
        }
    }

    private File getFileFromURL(String keyword) {

        URL url = this.getClass().getClassLoader().getResource(keyword+".*");
        File file = null;
        try {
            file = new File(url.toURI());
        } catch (Exception e) {
            file = new File(url.getPath());
        } finally {
            return file;
        }
    }  

When I run this code, I get Null Pointer Exception for below line:

File[] listOfFiles = path.listFiles();

Also, I don't want to pass any extensions for the file. Something like .* should work. So, where am I going wrong?

Answer 1

Your problem will be a lot more obvious if you add some temporary debug output for path and check value of listFiles() which can be null.

    System.out.println("path="+path);
    System.out.println("exists ? "+path.exists());
    System.out.println("isDirectory ? "+path.isDirectory());
    File[] listOfFiles = path.listFiles();
    for (int i = 0; listOfFiles != null && i < listOfFiles.length; i++) {
        System.out.println(listOfFiles[i]);
    }

But the real cause of the error is probably because you've appended ".*" to the keyword and missed that url == null in getFileFromURL. Move return outside finally :

private File getFileFromURL(String keyword) {

    URL url = this.getClass().getClassLoader().getResource(keyword+".*");
    File file = null;
    try {
        file = new File(url.toURI());
    } catch (Exception e) {
        file = new File(url.getPath());
    } 
    return file;
}  

Answer 2

The NPE occurs because the provided path does not contain any files and isn'ta directory. Your input string must be a valid URL to the „mappings“ directory. The path to use depends on your selected working dir of the application.

Answer 3

You'll need to examine the URL and have specific code for each protocol. This answer shows how to interpret the URL. Something like:

try (JarFile jar = ((JarURLConnection) url.openConnection()).getJarFile()) {
    JarEntry entry =
        jar.stream()
        .filter(t -> ...)
        .findFirst().orElse(null);

    try (InputStream in = jar.getInputStream(entry)) {
        ...
    }
}