How to load JAR files dynamically at Runtime?

Question

Why is it so hard to do this in Java? If you want to have any kind of module system you need to be able to load JAR files dynamically. I\'m told there\'s a way of doing it b

Answer 1

With Java 9, the answers with URLClassLoader now give an error like:

java.lang.ClassCastException: java.base/jdk.internal.loader.ClassLoaders$AppClassLoader cannot be cast to java.base/java.net.URLClassLoader

This is because the class loaders used have changed. Instead, to add to the system class loader, you can use the Instrumentation API through an agent.

Create an agent class:

package ClassPathAgent;

import java.io.IOException;
import java.lang.instrument.Instrumentation;
import java.util.jar.JarFile;

public class ClassPathAgent {
    public static void agentmain(String args, Instrumentation instrumentation) throws IOException {
        instrumentation.appendToSystemClassLoaderSearch(new JarFile(args));
    }
}

Add META-INF/MANIFEST.MF and put it in a JAR file with the agent class:

Manifest-Version: 1.0
Agent-Class: ClassPathAgent.ClassPathAgent

Run the agent:

This uses the byte-buddy-agent library to add the agent to the running JVM:

import java.io.File;

import net.bytebuddy.agent.ByteBuddyAgent;

public class ClassPathUtil {
    private static File AGENT_JAR = new File("/path/to/agent.jar");

    public static void addJarToClassPath(File jarFile) {
        ByteBuddyAgent.attach(AGENT_JAR, String.valueOf(ProcessHandle.current().pid()), jarFile.getPath());
    }
}