Access resources from another jar file

Question

I have a simple structure: A data jar file which contains a batch of data, and a service jar file, which runs a service using the data. To make the data easy to replace, I have them separate, and service.jar's classpath contains the directory which data.jar is in.

Within service.jar, I use getResource to load the data files. This works if the data files are directly within the folder, but fails when they are inside data.jar;

This fails:

all
+ globalclasspath
| + data.jar
|   + mine.properties
+ daemons
  + service.jar

jsvc -cp globalclasspath:daemons/service.jar (...)

MyClass.class.getClassLoader( ).getResource( "mine.properties" ); // <-- null

But this works:

all
+ globalclasspath
| + mine.properties
+ daemons
  + service.jar

jsvc -cp globalclasspath:daemons/service.jar (...)

MyClass.class.getClassLoader( ).getResource( "mine.properties" ); // <-- not null

I don't want to change the classpath (unless I can change it to something generic which doesn't depend on the name of the data jar file), but I'm fine with changing the getResource string (I've tried /data/mine.properties and /data.jar/mine.properties to no avail). Is there a change I can make so that the resources can be loaded from within the jar?

Answer 1

Solution 1

Use a classpath wildcard.

jsvc -cp globalclasspath/*:daemons/service.jar (...)

See "How to use a wildcard in the classpath to add multiple jars?"

Solution 2

To read data in JARs not on the classpath, use URLClassLoader. The general algorithm is this:

1. Find the list of JARs in the globalclasspath directory.
2. Create a URLClassLoader from this list of JARs.
3. Look up the resource you want from the URLClassLoader instance.

To find JARs on the classpath, I used ResourceList from the StackOverflow article "Get a list of resources from classpath directory."

public class MyClass {
    /**
     * Creates a {@code URLClassLoader} from JAR files found in the
     * globalclasspath directory, assuming that globalclasspath is in
     * {@code System.getProperty("java.class.path")}.
     */
    private static URLClassLoader createURLClassLoader() {
        Collection<String> resources = ResourceList.getResources(Pattern.compile(".*\\.jar"));
        Collection<URL> urls = new ArrayList<URL>();
        for (String resource : resources) {
            File file = new File(resource);
            // Ensure that the JAR exists
            // and is in the globalclasspath directory.
            if (file.isFile() && "globalclasspath".equals(file.getParentFile().getName())) {
                try {
                    urls.add(file.toURI().toURL());
                } catch (MalformedURLException e) {
                    // This should never happen.
                    e.printStackTrace();
                }
            }
        }
        return new URLClassLoader(urls.toArray(new URL[urls.size()]));
    }

    public static void main(String[] args) {
        URLClassLoader classLoader = createURLClassLoader();
        System.out.println(classLoader.getResource("mine.properties"));
    }
}

I ran the following command:

java -cp globalclasspath:daemons/service.jar MyClass

The terminal output:

jar:file:/workspace/all/globalclasspath/data.jar!/mine.properties