Path of the folder inside the jar file

Question

I have jar file langdetect.jar. It has a hierarchy shown in image

There is a class LanguageDetection at com/langdetect package. I need to access the path of the profiles.sm folder from above class while executing the jar file.

Thanks in advance.

Answer 1

Jars are nothing else than Zip files and Java provides support for handling those.

Java 6 (and earlier)

You can open the jar file as a ZipFile and iterate over the entries of it. Each entry has a full path name inside the file, there is no such thing as relative path names. Though you have to take care, that all entries - although being absolute in the zip file - do not start with a '/', if you need this, you have to add it. The following snippet will get you the path of a class file. The className has to end with .class, i.e. LanguageDetection.class

String getPath(String jar, String className) throws IOException {
    final ZipFile zf = new ZipFile(jar);
    try {
        for (ZipEntry ze : Collections.list(zf.entries())) {
            final String path = ze.getName();
            if (path.endsWith(className)) {
                final StringBuilder buf = new StringBuilder(path);
                buf.delete(path.lastIndexOf('/'), path.length()); //removes the name of the class to get the path only
                if (!path.startsWith("/")) { //you may omit this part if leading / is not required
                    buf.insert(0, '/');
                }
                return buf.toString();
            }
        }
    } finally {
        zf.close();
    }
    return null;
}

Java 7/8

You may open the JAR file using the Java7 FileSystem support for JAR files. This allows you to operate on the jar file as if it would be normal FileSystem. So you could walk the fileTree until you have found your file and the get the Path from it. The following example uses Java8 Streams and Lambdas, a version for Java7 could be derived from this but would be a bit larger.

Path jarFile = ...;

Map<String, String> env = new HashMap<String, String>() {{
        put("create", "false");
    }};

try(FileSystem zipFs = newFileSystem(URI.create("jar:" + jarFileFile.toUri()), env)) {
  Optional<Path> path = Files.walk(zipFs.getPath("/"))
                             .filter(p -> p.getFileName().toString().startsWith("LanguageDetection"))
                             .map(Path::getParent)
                             .findFirst();
  path.ifPresent(System.out::println);
}

Your particular Problem

The above solutions are for finding the path inside a Jar or Zip, but may possibly not be the solution to your problem.

Im not sure, whether I understand your problem correctly. As far as I see it, you'd like to have access to the path inside the classfolder for any purpose. The problem with that is, that the Class/Resource lookup mechanism doesn't apply to folders, only files. The concept that is close is a package, but that is always bound to a class.

So you always need a concrete file to be accessed via getResource() method. For example MyClass.class.getResource(/path/to/resource.txt).

If the resources are located in a profiles.sm folder relative to a class and its package, i.e. in /com/languagedetect/profile.sm/ you could build the path from the reference class, for example the class LanguageDetection in that package and derive the absolute path from this to the profiles.sm path:

String basePath = "/" + LanguageDetection.class.getPackage().getName().replaceAll("\\.", "/") + "/profiles.sm/";
URL resource = LanguageDetection.class.getResource(basePath + "myResource.txt");

If there is only one profiles.sm in the root of the jar, simply go for

String basePath = "/profiles.sm/";
URL resource = LanguageDetection.class.getResource(basePath + "myResource.txt");

If you have multiple jars with a resource in /profiles.sm, you could gain access to all of those via the classloader and then extract the Jar file from the URL of the class

for(URL u : Collections.list(LanguageDetection.class.getClassLoader().getResources("/profiles.sm/yourResource"))){
        System.out.println(u);
    }

In any case it's not possible without accessing the zip/jar file to browse the contents of this path or folder because Java does not support browsing for classes or resources inside a package/folder in classpath. You may use the Reflections lib for that or extend the ClassLoader example above by additionally reading the content of the detected jars using the zip example from above.