从包中加载类会产生 NoClassDefFoundError

Question

我有一个小程序试图允许插件通过 class 文件复制到特定的 ext 目录。

该程序源自 https://javaranch.com/journal/200607/Plugins.html，我试图对其进行简化并添加目录扫描功能以扫描原始代码缺少的包和目录。

当运行原始代码时，它起作用了。当我添加我的目录和包扫描功能并在演示包上测试它时，它失败了。以下是示例。

接受动态加载class文件作为插件的系统目录布局：

 testpack-+
          |
          +---PluginDemo.java
          |
          +---PluginFunction.java

测试插件的目录布局：

b-+
  |
  +---Fibonacci.java
  
testpack-+
         |
         +---PluginFunction.java

带有自定义 class 加载程序的 PluginDemo 代码：

package testpack;

import java.io.DataInputStream;
import java.io.File;
import java.io.FileInputStream;
import java.util.*;

public class PluginDemo extends ClassLoader {

static String pluginsDir = "ext";
static File basePluginDir = null;
File directory;

static List plugins;

public static void main(String args[]) {
    PluginDemo demo = new PluginDemo();
    basePluginDir = new File(System.getProperty("user.dir") + File.separator + pluginsDir);
    demo.getPlugins(pluginsDir, "");
}

PluginDemo() {
    plugins = new ArrayList();
}

protected void getPlugins(String directory, String parent) {
    File dir = new File(System.getProperty("user.dir") + File.separator + directory);
    if (dir.exists() && dir.isDirectory()) {
        String[] files = dir.list();
        for (int i = 0; i < files.length; i++) {
            try {
                // Allows recursive targetting of nested directories
                String newTargetFile = System.getProperty("user.dir") + File.separator + directory + File.separator
                        + files[i];
                System.out.println("Targetting: " + newTargetFile);

                File currentTarget = new File(newTargetFile);
                if (currentTarget.isDirectory()) {
                    String newDirectoryTarget = directory + File.separator + files[i];
                    getPlugins(newDirectoryTarget, files[i]);
                }

                if (!files[i].endsWith(".class"))
                    continue;

                String childFile = parent + File.separator + files[i].substring(0, files[i].indexOf("."));
                Class c = loadClass(childFile);
                Class[] intf = c.getInterfaces();
                for (int j = 0; j < intf.length; j++) {
                    if (intf[j].getName().equals("PluginFunction")) {
                        PluginFunction pf = (PluginFunction) c.newInstance();
                        plugins.add(pf);
                        continue;
                    }
                }
            } catch (Exception ex) {
                System.err.println("File " + files[i] + " does not contain a valid PluginFunction class.");
                ex.printStackTrace();
            }
        }
    }
}

public Class loadClass(String name) throws ClassNotFoundException {
    return loadClass(name, true);
}

public Class loadClass(String classname, boolean resolve) throws ClassNotFoundException {
    try {
        Class c = findLoadedClass(classname);
        if (c == null) {
            try {
                c = findSystemClass(classname);
            } catch (Exception ex) {
            }
        }
        if (c == null) {
            String filename = classname.replace('.', File.separatorChar) + ".class";

            // Create a File object. Interpret the filename relative to the
            // directory specified for this ClassLoader.
            File baseDir = new File(System.getProperty("user.dir"));
            File f = new File(baseDir, PluginDemo.pluginsDir + File.separator + filename);
            int length = (int) f.length();
            byte[] classbytes = new byte[length];
            DataInputStream in = new DataInputStream(new FileInputStream(f));
            in.readFully(classbytes);
            in.close();
            c = defineClass(classname, classbytes, 0, length);
        }

        if (resolve)
            resolveClass(c);

        return c;
    } catch (Exception ex) {
        throw new ClassNotFoundException(ex.toString());
    }
}
}

PluginFunction接口代码：

package testpack;

public interface PluginFunction {

    // let the application pass in a parameter
    public void setParameter (int param);

    // retrieve a result from the plugin
    public int getResult();

    // return the name of this plugin
    public String getPluginName();

    // can be called to determine whether the plugin
    // aborted execution due to an error condition
    public boolean hasError();
}

Fibonacci.java代码：

package b;

import testpack.PluginFunction;

public class Fibonacci implements PluginFunction {

    int parameter = 0;
    boolean hasError = false;

    public boolean hasError() {
        return hasError;
    }

    public void setParameter (int param) {
        parameter = param;
    }

    public int getResult() {
        hasError = false;
        return fib(parameter);
    }

    protected int fib (int n) {
        if (n < 0) {
            hasError = true;
            return 0;
        }

        if (n == 0)
            return 0;
        else if (n == 1)
            return 1;
        else
            return fib(n-1) + fib(n-2); 
    }

    public String getPluginName() {
        return "Fibonacci";
    }
}

有错误的输出：

Targetting: C:\Users\Administrator\eclipse-workspace\TestPluginSystem\ext\b\Fibonacci.class
Exception in thread "main" java.lang.NoClassDefFoundError: b\Fibonacci (wrong name: b/Fibonacci)
    at java.lang.ClassLoader.defineClass1(Native Method)
    at java.lang.ClassLoader.defineClass(ClassLoader.java:760)
    at java.lang.ClassLoader.defineClass(ClassLoader.java:642)
    at testpack.PluginDemo.loadClass(PluginDemo.java:89)
    at testpack.PluginDemo.loadClass(PluginDemo.java:65)
    at testpack.PluginDemo.getPlugins(PluginDemo.java:47)
    at testpack.PluginDemo.getPlugins(PluginDemo.java:40)
    at testpack.PluginDemo.main(PluginDemo.java:19)

我需要帮助才能让这个包和目录扫描功能 classloader 工作。谢谢。

Answer 1

查看错误和方法ClassLoader.defineClass，我认为name参数必须有.作为包分隔符，而不是/或\。

在您的 getPlugins 代码中，childFile 是使用 File.separator

构建的

String childFile = parent + File.separator + files[i].substring(0, files[i].indexOf("."));
Class c = loadClass(childFile);

从包中加载类会产生 NoClassDefFoundError

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