How can I implement a custom class loader in Java that dynamically loads and executes Java code at runtime?

Question

I am building a Java-based application that needs to dynamically load and execute plugin classes at runtime. Each plugin is represented by a separate Java class file that implements a common interface Plugin. To achieve this, I need to implement a custom class loader (PluginClassLoader) that can load and instantiate plugin classes from external JAR files.

I tried implementing a custom class loader in my Java-based application to dynamically load and execute plugin classes at runtime. I expected the plugin classes to be successfully loaded and instantiated from external JAR files, and for them to be available for execution within my application's context.

Here's an example of my attemted custom class loader:

public class PluginClassLoader extends ClassLoader {

    public Class<?> loadPluginClass(String pluginFilePath) throws IOException {
        byte[] pluginBytes = readPluginFile(pluginFilePath);
        return defineClass(null, pluginBytes, 0, pluginBytes.length);
    }

    private byte[] readPluginFile(String pluginFilePath) throws IOException {
        // Code to read the plugin file from external JAR
    }
}

I expected that the loadPluginClass method would load the plugin class from the specified file path and return a Class object that I could then use to instantiate and execute the plugin. However, when I tested it, I encountered class loading issues and ClassNotFoundExceptions for the plugin classes that I tried to load.

Answer 1

You have added a method to your custom class loader to load a single class on your own request. However, if the plugin consists of more than one class, i.e. the class you’re trying to load has dependencies to other classes within the plugin, the JVM will request those classes from your loader. Those requests won’t use your loadPluginClass method which the JVM can’t know. Instead, it will invoke the inherited method loadClass(String name, boolean resolve).

Since you didn’t override neither this method nor the findClass(String name) method (which would be the preferred way), this method will throw a ClassNotFoundException for every class not found through the parent loader. A failure to resolve a dependency may cause a failure for the dependent class, so even your initial call to loadPluginClass can fail with a ClassNotFoundException at the defineClass call.

Since you said that you are using jar files for your plugins, there is no need to implement your own ClassLoader at all. Just use the already existing URLClassLoader and pass the URL of the jar file to the constructor. Or subclass it, to be able to call addURL on it. Mind that the correct way to convert a File to a URL is to call toURI().toURL() on the File object instead of toURL(), to avoid problems with spaces or other special characters in path names.

Further keep in mind to specify your main application’s loader as parent loader when the main application has not been loaded through the default application class loader.