How to Dynamically Load and Reload JAR Files in Java at Runtime?

What are the best practices for loading and reloading JAR files dynamically in a running Java application?

// Example of loading a JAR file dynamically in Java
URLClassLoader classLoader = new URLClassLoader(new URL[]{new File("path/to/your.jar").toURI().toURL()});
Class<?> cls = classLoader.loadClass("com.example.YourClass");

Dynamically loading JAR files in Java allows applications to extend their functionality without downtime. This is particularly useful for systems that need to hot-swap new features or updates. However, doing this involves careful considerations regarding class loaders and resource management.

// Example of creating a custom ClassLoader
class CustomClassLoader extends URLClassLoader {
    public CustomClassLoader(URL[] urls) {
        super(urls);
    }
}

Causes

• ClassLoader limitations that prevent unloading of classes once instantiated.
• Potential memory leaks from lingering references to previously loaded classes.
• Compatibility issues arising from changes in class signatures between different JAR versions.

Solutions

• Utilize a separate ClassLoader for each JAR to isolate the classes and allow unloading when necessary.
• Implement a mechanism to track and manage loaded classes and their dependencies responsibly.
• Use frameworks like OSGi for robust dynamic module loading.

Mistake: Not managing class loader references properly, leading to memory leaks.

Solution: Ensure to dereference the old ClassLoader and its objects so that they can be garbage collected.

Mistake: Assuming old classes can be reloaded normally with a simple reload command.

Solution: Create a new instance of ClassLoader to instantiate classes from the new JAR.