使用URLClassLoader加载外部class文件报错NoClassDefFoundError与ClassNotFoundException的解决方法前言一、使用URLClassLoader加载外部class文件报错"NoClassDefFoundError"二、使用URLClassLoader加载外部class文件报错"ClassNotFoundException"三、使用URLClassLoader加载多个不同的外部class文件，但实际加载了相同的class文件四、保险方法：使用自定义

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//实际的class路径为：F:/demo1/server/MyServlet.class
//这个class中有：package server;
//因此className为[server.MyServlet];
//classPath为[F:/demo1/],不能加server。

public void function2(String classPath, String className){

            //File xFile=new File("F:/demo1/");
            File xFile=new File(classPath);

            URL xUrl= null;
            try {
                xUrl = xFile.toURL();
            } catch (MalformedURLException e) {
                e.printStackTrace();
            }
            URL  urls[] = new URL[]{ xUrl };
            URLClassLoader uLoad = new URLClassLoader(urls);
            try {
                //Class c = uLoad.loadClass("server.MyServlet");
                Class c = uLoad.loadClass(className);
                //把class实例化，赋值给变量。
                //类里的一个变量private HttpServlet httpServlet;
                this.httpServlet = (HttpServlet)c.newInstance();
            } catch (ClassNotFoundException e) {
                e.printStackTrace();
            } catch (IllegalAccessException e) {
                e.printStackTrace();
            } catch (InstantiationException e) {
                e.printStackTrace();
            }
        }

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package test;

import java.io.ByteArrayOutputStream;
import java.io.File;
import java.io.FileInputStream;

public class MyClassLoader extends ClassLoader{
    private String classPath;

    //    F:/demo1/
    //    完整路径为：F:/demo1/server/MyServlet.class
    public MyClassLoader(String classPath){
        this.classPath = classPath;
    }

    @Override
    protected Class<?> findClass(String name) throws ClassNotFoundException {
        byte[] bytes;
        Class<?> clazz;
        try{
            bytes = getClassByte(name);
            //继承ClassLoader是为了用defineClass方法。
            clazz = defineClass(name, bytes, 0, bytes.length);
            return clazz;
        }catch (Exception e){

        }
        return super.findClass(name);
    }

    //server.MyServlet
    //完整路径为：F:/demo1/server/MyServlet.class
    private byte[] getClassByte(String name){
        name = name.replaceAll("\.","/");
        String realPath = this.classPath+name+".class";
        File f = new File(realPath);
        if(f.exists()){
            byte[] bytes = new byte[0];

            try {
                FileInputStream fis = new FileInputStream(f);
                ByteArrayOutputStream baos = new ByteArrayOutputStream();
                int len;
                byte[] buffer = new byte[1024];
                while ((len = fis.read(buffer)) != -1) {
                    baos.write(buffer, 0, len);
                }
                bytes = baos.toByteArray();
                fis.close();
                baos.close();
            } catch (Exception e) {
                e.printStackTrace();
            }
            System.out.println("从本地读取【"+realPath+"】完成");
            return  bytes;
        }else{
            return null;
        }
    }
}

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 public void function1(String classPath, String className){

            try {
                MyClassLoader myClassLoader = new MyClassLoader(classPath);
                Class c = myClassLoader.findClass(className);
                this.httpServlet = (HttpServlet)c.newInstance();
            } catch (InstantiationException e) {
                e.printStackTrace();
            } catch (IllegalAccessException e) {
                e.printStackTrace();
            } catch (ClassNotFoundException e) {
                e.printStackTrace();
            }
        }