Access resources from another jar file
classpath, java, jsvc
Solution
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:
- Find the list of JARs in the `globalclasspath` directory.
- Create a `URLClassLoader` from this list of JARs.
- 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
Problem
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?