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

Another working solution using Instrumentation that works for me. It has the advantage of modifying the class loader search, avoiding problems on class visibility for dependent classes:

Create an Agent Class

For this example, it has to be on the same jar invoked by the command line:

package agent;

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

public class Agent {
   public static Instrumentation instrumentation;

   public static void premain(String args, Instrumentation instrumentation) {
      Agent.instrumentation = instrumentation;
   }

   public static void agentmain(String args, Instrumentation instrumentation) {
      Agent.instrumentation = instrumentation;
   }

   public static void appendJarFile(JarFile file) throws IOException {
      if (instrumentation != null) {
         instrumentation.appendToSystemClassLoaderSearch(file);
      }
   }
}

Modify the MANIFEST.MF

Adding the reference to the agent:

Launcher-Agent-Class: agent.Agent
Agent-Class: agent.Agent
Premain-Class: agent.Agent

I actually use Netbeans, so this post helps on how to change the manifest.mf

Running

The Launcher-Agent-Class is only supported on JDK 9+ and is responsible for loading the agent without explicitly defining it on the command line:

 java -jar 

The way that works on JDK 6+ is defining the -javaagent argument:

java -javaagent: -jar 

Adding new Jar at Runtime

You can then add jar as necessary using the following command:

Agent.appendJarFile(new JarFile());

I did not find any problems using this on documentation.