从包类加载产生NoClassDefFoundError

问题描述

我有一个小程序，试图通过复制到特定ext目录的类文件允许插件。

该程序派生自https://javaranch.com/journal/200607/Plugins.html，我曾尝试对其进行简化，并增加了目录扫描功能，以扫描原始代码缺乏的软件包和目录。

运行原始代码时，它可以工作。当我添加目录和程序包扫描功能并在演示程序包上对其进行测试时，它将失败。下面是示例。

接受动态加载的类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

带有自定义类加载器的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,length);
        }

        if (resolve)
            resolveClass(c);

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

PluginFunction界面代码：

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";
    }
}

输出错误：

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)

我需要帮助来使此程序包和具有目录扫描功能的类加载器正常工作。谢谢。

解决方法

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

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

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