NoClassDefFoundError. Why??? How can I fix it?

Question

I wrote my classloader:

package ru.sberbank.school.homework8;

import ru.sberbank.school.homework8.plugin.Plugin;

import java.io.File;
import java.io.FileInputStream;
import java.io.IOException;

public class PluginManager extends ClassLoader {
private final String pluginRootDirectory;

public PluginManager(String pluginRootDirectory) {
    this.pluginRootDirectory = pluginRootDirectory;
}

public Plugin load(String pluginName, String pluginClassName) {
    String name = pluginName + "." + pluginClassName;
    try {
        Class clazz;
        try {
            clazz = super.findSystemClass(name);
        } catch (ClassNotFoundException e) {
            String fileName = pluginRootDirectory + "\\" + pluginName + "\\" + pluginClassName + ".class";
            try (FileInputStream fin = new FileInputStream(fileName)) {
                byte[] buffer = new byte[(int) (new File(fileName).length())];
                fin.read(buffer);
                clazz = defineClass(name, buffer, 0, buffer.length);
            }
        }
        return (Plugin)clazz.newInstance();

    } catch (IOException | InstantiationException | IllegalAccessException ignored) {
        return null;
    }
}

}

When I run it:

package ru.sberbank.school.homework8;

import ru.sberbank.school.homework8.plugin.Plugin;

public class PluginManagerTest {
    public static void main(String[] args) {
        String pluginRootDirectory = "D:\\sbt\\target\\classes\\ru\\sberbank\\school\\homework8";
        PluginManager pluginManager = new PluginManager(pluginRootDirectory);
        Plugin plugin = pluginManager.load("plugin", "PluginImpl");
        if (plugin != null) {
            plugin.doUseful();
        }
    }
}

Exception in thread "main" java.lang.NoClassDefFoundError: plugin/PluginImpl (wrong name: ru/sberbank/school/homework8/plugin/PluginImpl) at java.lang.ClassLoader.defineClass1(Native Method)

I get NoClassDefFoundError. Why??? How can I fix it???

Help me, please!

package ru.sberbank.school.homework8.plugin;

public class PluginImpl implements Plugin {
    @Override
    public void doUseful() {
        System.out.println("My plugin!");
    }
}

Answer 1

You get this error because you don't provide the correct FQN of your class, indeed in your load method, you try to find the class corresponding to pluginName + "." + pluginClassName pluginName + "." + pluginClassName that will be in your case plugin.PluginImpl but the package name of your class PluginImpl is actually ru.sberbank.school.homework8.plugin such that the real FQN of your class is ru.sberbank.school.homework8.plugin.PluginImpl .

To fix this problem, you need to replace:

Plugin plugin = pluginManager.load("plugin", "PluginImpl");

With:

Plugin plugin = pluginManager.load("ru.sberbank.school.homework8.plugin", "PluginImpl");

Or you could modify your method load to add a prefix assuming that you will always retrieve your plugins from the same root package:

public Plugin load(String pluginName, String pluginClassName) {
    String name = "ru.sberbank.school.homework8." + pluginName + "." + pluginClassName;