NoClassDefFoundError。为什么？？？我该如何解决？

debugcn 发表于 Dev

雅科夫·布尔采夫（Yakov Burtsev）

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“主要” java.lang.NoClassDefFoundError中的异常：java.lang.ClassLoader.defineClass1（本机方法）上的plugin / PluginImpl（错误名称：ru / sberbank / school / homework8 / plugin / PluginImpl）

我收到NoClassDefFoundError。为什么？？？我该如何解决？？？

请帮帮我！

package ru.sberbank.school.homework8.plugin;

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

尼古拉斯·菲洛托（Nicolas Filotto）

因为没有提供正确的类的FQN，所以会收到此错误，确实是在您的load方法中，您尝试找到pluginName + "." + pluginClassName与之对应的类，plugin.PluginImpl但是您的类的包名称PluginImpl实际上ru.sberbank.school.homework8.plugin是真正的FQN您的班级是ru.sberbank.school.homework8.plugin.PluginImpl。

要解决此问题，您需要替换：

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

和：

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

或者，您可以修改方法load以添加前缀，前提是您始终将始终从同一根包中检索插件：

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

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