在上一章中我们加载Servlet用的是URLClassLoader，在这一章中我们使用自定义的类加载器来替换URLClassLoader。关于类加载器的文章请参考这里：深入探讨 Java 类加载器。这篇文章写得很棒。
具体的代码如下：

package com.zkn.imitate.tomcat.secondchapter.first;

import com.zkn.imitate.tomcat.secondchapter.Request;
import com.zkn.imitate.tomcat.secondchapter.Response;
import com.zkn.imitate.tomcat.secondchapter.StaticResourceProcessor;
import com.zkn.imitate.tomcat.utils.Constants;
import com.zkn.imitate.tomcat.utils.StringUtil;

import java.io.IOException;
import java.net.InetAddress;
import java.net.ServerSocket;
import java.net.Socket;

/**
 * Created by wb-zhangkenan on 2016/12/29.
 */
public class HttpServer02 {

    public static void main(String[] args){

        await();
    }

    private static void await() {

        ServerSocket serverSocket = null;
        try {
            boolean shutDown = false;
            serverSocket = new ServerSocket(8004,1, InetAddress.getByName("127.0.0.1"));
            while (!shutDown){
                Socket socket = serverSocket.accept();
                Request request = new Request(socket.getInputStream());
                request.parseRequest();
                Response response = new Response(socket.getOutputStream());
                String uri = request.getUri();
                if(uri !=null && uri.startsWith("/favicon.ico")){

                }else if(!StringUtil.isEmpty(uri) && uri.startsWith("/static/")){
                    StaticResourceProcessor resouce = new StaticResourceProcessor();
                    resouce.process(request,response);
                }else{
                    ServletProcessor01 servletProcessor = new ServletProcessor01();
                    servletProcessor.process(request,response);
                }
                socket.close();
                shutDown = Constants.SHUT_DOWN.equals(request.getUri());
            }
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

public class ServletProcessor01 {

    public void process(Request request, Response response) {

        String str = request.getUri();
        if(!StringUtil.isEmpty(str) && str.lastIndexOf("/") >= 0){
            str = str.substring(str.lastIndexOf("/")+1);
        }else {
            return;
        }
        FileSystemClassLoader fileClassLoader =
                new FileSystemClassLoader(Constants.WEB_ROOT);
        try {
            Class clazz = fileClassLoader.findClass(str);
            Servlet servlet = (Servlet) clazz.newInstance();
            servlet.service(request,response);
        } catch (ClassNotFoundException e) {
            e.printStackTrace();
        } catch (InstantiationException e) {
            e.printStackTrace();
        } catch (IllegalAccessException e) {
            e.printStackTrace();
        } catch (ServletException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

自定义的类加载器：

package com.zkn.imitate.tomcat.utils;

import java.io.*;
import java.util.Arrays;

/**
 * Created by wb-zhangkenan on 2017/1/3.
 */
public class FileSystemClassLoader extends ClassLoader{
    /**
     * 应用根路径
     */
    private String rootDir;

    public FileSystemClassLoader(String rootDir) {
        this.rootDir = rootDir;
    }

    public FileSystemClassLoader() {
    }

    @Override
    public Class<?> findClass(String name) throws ClassNotFoundException {
        byte[] bytes = getByteData(name);
        System.out.println(Arrays.toString(bytes));
        return defineClass(null,bytes,0,bytes.length);
    }

    private byte[] getByteData(String path){

        String className = getClassName(path);
        byte[] bytes = new byte[1024];
        try {
            FileInputStream fis = new FileInputStream(className);
            //ByteArrayOutputStream内部有一个数组用来保存读取的字节流
            ByteArrayOutputStream baiStream = new ByteArrayOutputStream();
            int flag = 0;
            while ((flag = fis.read(bytes)) != -1){
                /**
                 * 这里需要注意：写入的时候，写入的范围一定是0，flag。
                 * 原因是：有可能读取的bytes不够1024个字节，这个时候如果不写入读取范围的话，
                 *       则会把bytes中存留的上次读取的数据也写入到ByteArrayOutputStream中。
                 *       这样在defineClass的时候会出现异常。
                 *       异常信息如下：
                 *Exception in thread "main" java.lang.ClassFormatError:
                 *      Extra bytes at the end of class file FirstServlet
                 */
                baiStream.write(bytes,0,flag);
            }
            return baiStream.toByteArray();
        } catch (FileNotFoundException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }
        return bytes;
    }

    private String getClassName(String className){

        if(StringUtil.isEmpty(className)){
             throw new RuntimeException("Class名称不能为空");
        }
        return rootDir+ File.separatorChar+className.replace('.',File.separatorChar)+".class";
    }

}

TomCat中的应用加载器是WebappLoader。WebappLoader创建一个org.apache.catalina.loader.WebappClassLoader类的实例作为它的类加载器。有时间的话我会写一个关于WebAppLoader的一个分析。