How to Dynamically Load JAR Files at Runtime in Java?

What are the methods for dynamically loading JAR files at runtime in Java?

// Example code for loading JAR files dynamically
import java.net.URL;
import java.net.URLClassLoader;

public class DynamicJarLoader {
    public static void loadJar(String jarPath) throws Exception {
        URL jarUrl = new URL("file://" + jarPath);
        URLClassLoader loader = new URLClassLoader(new URL[]{jarUrl});
        Class<?> cls = loader.loadClass("com.example.YourClass"); // Replace with your class
        Object instance = cls.getDeclaredConstructor().newInstance();
        // Now you can use the instance as you wish.
    }
}

Dynamically loading JAR files in Java involves using a custom ClassLoader which can locate and load classes from external JAR files at runtime. This capability is vital for implementing modular architectures, plugins, or any scenario where you need flexibility in loading resources without restarting the application.

public class CustomClassLoader extends ClassLoader {
    public Class<?> loadJarClass(String jarPath, String className) throws Exception {
        // Logic to load classes from the specified JAR file
        try (URLClassLoader urlClassLoader = new URLClassLoader(new URL[]{new File(jarPath).toURI().toURL()})) {
            return urlClassLoader.loadClass(className);
        }
    }
}

Causes

• Java's static class loading mechanism complicates dynamic loading without a custom approach.
• Standard class loaders cannot dynamically load classes from JAR files once the application is running.

Solutions

• Use a URLClassLoader for dynamically loading classes from JAR files.
• Implement a custom ClassLoader if you need more advanced loading capabilities, such as isolation of loaded classes.

Mistake: Not closing ClassLoader resources and leaving open references which can lead to memory leaks.

Solution: Ensure to close the ClassLoader or use try-with-resources to automatically close it.

Mistake: Hardcoding class names or paths which makes the loader inflexible.

Solution: Utilize configuration files or parameters to specify class names and paths.