How to load JAR files dynamically at Runtime?

java

reflection

classloaders

dynamic-jar-loading

byNikita Barsukov·Jan 12, 2025

Have a situation where you need to load classes from external JAR files dynamically at runtime? Fret not, Java's URLClassLoader comes to the rescue. Here's the ace up your sleeve:

URL jarUrl = new URL("file:///path/to/your.jar"); // the "where's Waldo" of your JAR
URLClassLoader loader = new URLClassLoader(new URL[]{jarUrl}); // your JAR-chariot 
Class<?> clazz = loader.loadClass("mypackage.MyClass"); // it's morphin' time!
// Now, use clazz.newInstance() or clazz.getDeclaredMethod() to interact

This approach assumes you are able to handle any rogue exceptions and have your JAR's file path and class name at your fingertips.

Not all battles are won with standard weapons, though. Complex class loading situations may call for custom classloaders or the strategically built Java Module System.

Dynamic JAR loading - The next level

Getting the hang of loading JAR files? Time to take it to the next level with advanced methods such as reflection and custom classloaders:

Reflective loading of JARs

Sometimes, you’ll need your URLClassLoader to lift some JARs after Pilates. Use reflection to ‘jack up’ (invoke) the addURL method and bring those new JARs onboard:

Method method = URLClassLoader.class.getDeclaredMethod("addURL", URL.class); // "Open Sesame" for URLClassLoader
method.setAccessible(true); // The Force is strong with you
method.invoke(loader, jarUrl); // Flexing the reflection muscle

Loading JARs with custom classloaders

For those times when you need to demonstrate your sophisticated palate, custom classloaders let you load JARs with unparalleled elegance and flexibility. Just remember to uphold family ties by accepting a parent classloader in the constructor. Maybe an add method would be the cherry on top?

public class DynamicClassLoader extends ClassLoader {
    public DynamicClassLoader(ClassLoader parent) {
        super(parent); // Like father, like son
    }

    public void add(URL url) {
        // That JAR-loading magic goes here
    }
}

Consider advanced tools like OSGi and JCL's JarClassLoader, when JAR loading feels like wrestling an octopus.

Deep dive into reflection and custom classloaders

When vanilla Java just isn't cutting the mustard, let's go gourmet with reflection and custom classloaders:

Game of Reflection Thrones

Force open the addURL method for your URLClassLoader and drop JARs like they’re hot.
Dynamically call methods and constructors with Class.getMethod and Class.newInstance.

Custom classloader: A MasterChef recipe

Sprinkle in some error handling for a dash of resilience against spicy code
Season with security considerations to avoid a nasty classpath pollution.

How to navigate the stormy seas of Dynamic JAR Loading

Alright, back to the action. The ocean of dynamic JAR loading is vast and deep, let's make sure you've got the best route mapped out:

Opt for the Java Module System

The Java Module System is your advanced GPS system, helping you neatly organise your code into interacting sections instead of a messy JAR-ball.

Sail with the OSGi Framework

Deploy the OSGi Framework if runtime updates and modularization make your heart sing. It treats JAR files as bundles and babysits their lifecycles like a pro.

Don't ignore the pirates: Security and Immutability of ClassLoaders

Navigating the open sea of dynamic class loading, you need to stay sharp for security threats and classloader hierarchy issues. Keep a close eye on your treasure map and maintain your application's integrity.

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