How to Include a JAR File within Another JAR in Java Classpath?

Can a Java classpath be set to include a JAR file that is nested inside another JAR file?

Including a JAR file within another JAR and specifying it in the Java classpath is not directly supported by the Java ClassLoader. However, there are workarounds to access classes from the nested JAR file.

// Example of custom ClassLoader to load nested JAR
public class NestedJarClassLoader extends ClassLoader {
    public Class<?> loadClassFromNestedJar(String jarPath, String className) throws Exception {
        try (JarFile jarFile = new JarFile(jarPath)) {
            JarEntry entry = jarFile.getJarEntry(className.replace('.', '/') + ".class");
            if (entry != null) {
                InputStream inputStream = jarFile.getInputStream(entry);
                byte[] classBytes = inputStream.readAllBytes();
                return defineClass(className, classBytes, 0, classBytes.length);
            }
        }
        return null;
    }
}

Causes

• Java does not support hierarchical class paths out of the box, which means it won't resolve a JAR inside another JAR automatically.
• The ClassLoader treats the JAR files as flat structures, hence inner JARs are not recognized.

Solutions

• Use the 'JAR in JAR' technique with a custom ClassLoader that can extract the contained JARs and load classes as needed.
• Utilize build tools such as Maven or Gradle that can handle dependencies in a more sophisticated way, often flattening dependencies into a single executable JAR.
• Unzip the nested JAR files manually and add their classes to the parent JAR's structure, avoiding nested JARs altogether.

Mistake: Assuming nested JARs will be automatically included when specifying the outer JAR in the classpath.

Solution: Manually extract the inner JAR or modify your build process to handle dependencies instead.

Mistake: Not using a custom ClassLoader when trying to access classes from a nested JAR.

Solution: Implement a custom ClassLoader that can handle nested JAR loading.