NoClassDefFoundError。 为什么？？？ 我该如何解决？

Question

我写了我的类加载器：

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;
    }
}

}

当我运行它时：

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();
        }
    }
}

线程“main”中的异常 java.lang.NoClassDefFoundError: plugin/PluginImpl（错误名称：ru/sberbank/school/homework8/plugin/PluginImpl）在 java.lang.ClassLoader.defineClass1(Native Method)

我得到 NoClassDefFoundError。 为什么？？？ 我该如何修复它？？？

请帮帮我！

package ru.sberbank.school.homework8.plugin;

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

Answer 1

你得到这个错误是因为你没有提供你的类的正确 FQN，确实在你的load方法中，你试图找到对应于pluginName + "." + pluginClassName的类pluginName + "." + pluginClassName pluginName + "." + pluginClassName在你的情况下是plugin.PluginImpl但你的类PluginImpl的包名实际上是ru.sberbank.school.homework8.plugin这样你的类的真正 FQN 是ru.sberbank.school.homework8.plugin.PluginImpl .

要解决此问题，您需要更换：

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

和：

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

或者你可以修改你的方法load来添加一个前缀，假设你总是从同一个根包中检索你的插件：

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