深入剖析tomcat之一个简单的servlet容器

上一篇，我们讲解了如果开发一个简单的Http服务器，这一篇，我们扩展一下，让我们的服务器具备servlet的解析功能。

简单介绍下Servlet接口
如果我们想要自定义一个Servlet，那么我们必须继承Servlet，并且实现下面几个重要的方法

public void init(ServletConfig config) throws ServletException
public void service(ServletRequest request,ServletResponse response) throws ServletException,java.io.IOException
public void destroy()
public ServletConfig getServletConfig()
public String getServletInfo() 

五个方法中，init,destroy,service都是和servlet的生命周期相关的方法。当实例化某个servlet类之后，servlet会调用init进行初始化，当servlet的请求到达之后，就会调用service方法，并将servletRequest和servletResponse对象作为参数传入，前者包含客户端的Http请求的信息，后者包含服务器的响应信息。

这个简单的Servlet容器的流程如下

• 等待http请求
• 对应的servletRequest对象和servletResponse对象，
• 判断请求的类型，如果是请求静态资源，则找到静态资源的文件，返回给客户端
• 如果是Servlet请求，载入servlet类，调用service()方法，传入servletRequest对象和servletResponse对象

涉及到的主要的类

• SimpleServletContainerServer
• Request
• Response
• Servlet
• PrimitiveServlet
• StaticProcessor
• ServletProcessor

关于Request和Response的定义在上一篇幅有定义，这里我们稍微扩展了一下，碍于篇幅，不在这里展示。

PrimitiveServlet类，继承自Servlet，Servlet请求的处理类
类定义：

package servletContainer;

import java.io.IOException;

import base.Request;
import base.Response;
import base.ServletConfig;
import interf.Servlet;

public class PrimitiveServlet implements Servlet {

    @Override
    public void init(ServletConfig config) {
        System.out.println("PrimitiveServlet init");
    }

    @Override
    public void service(Request request, Response response) throws IOException {
        response.getOutput().write("Primitive Servlet".getBytes());
    }

    @Override
    public void destroy() {

    }

    @Override
    public ServletConfig getServletConfig() {
        return null;
    }

    @Override
    public String getServletInfo() {
        return null;
    }

}

SimpleServletContainerServer 类
功能：程序入口，监听Http请求，并且负责创建Request和Response
类定义

package servletContainer;

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

import base.Request;
import base.Response;
import servletContainer.processor.ServletProcessor;
import servletContainer.processor.StaticProcessor;

public class SimpleServletContainerServer {
    private static final String SHUT_DOWN = "/SHUTDOWN";

    private boolean shutdown = false;

    private ServletProcessor servletProcessor = new ServletProcessor();

    private StaticProcessor staticProcessor = new StaticProcessor();

    public static void main(String args[]){
        SimpleServletContainerServer server = new SimpleServletContainerServer();
        server.init();
        server.await();
    }

    public void init(){
        servletProcessor.init();
        staticProcessor.init();
    }

    public void await(){

        ServerSocket serverSocket = null;
        int port = 8080;

        try{
            serverSocket = new ServerSocket(port, 1, InetAddress.getByName("127.0.0.1"));
        }
        catch (IOException e){
            e.printStackTrace();
            System.exit(-1);
        }

        while(!shutdown){
            Socket socket = null;
            InputStream input = null;
            OutputStream output = null;

            try{

                socket = serverSocket.accept();
                input = socket.getInputStream();
                output = socket.getOutputStream();

                Request request = new Request(input);
                request.parse();

                Response response = new Response(output);
                response.setRequest(request);

                if(request.getUri().startsWith("/servlet/")){
                    servletProcessor.process(request, response);
                }
                else{
                    staticProcessor.process(request, response);
                }

                socket.close();
                shutdown = request.getUri().equals(SHUT_DOWN);

            }
            catch (Exception e){
                e.printStackTrace();
                System.exit(1);
            }
        }

    }

}

我们引入了StaticProcessor和ServletProcessor进行逻辑的处理，我们看下这两个类的定义
首先这两个类都继承自IProcessor接口

package servletContainer.processor;

import base.Request;
import base.Response;

public interface IProcessor {

    public void init();

    public void process(Request request,
            Response response);
}

StaticProcessor类主要是处理静态资源请求
类定义

package servletContainer.processor;

import base.Request;
import base.Response;

public class StaticProcessor implements IProcessor {

    @Override
    public void process(Request request, Response response) {
        try{
            response.sendStaticResource();
        }
        catch (Exception e){
            e.printStackTrace();
        }
    }

    @Override
    public void init() {

    }

}

ServeletProcessor主要负责处理Servlet请求，初始化的时候，初始化所有的Servlet子类，接收到servlet的http请求之后，根据请求名称，调用对应的service函数。
类定义

package servletContainer.processor;

import java.util.HashMap;
import java.util.Map;

import base.Request;
import base.Response;
import interf.Servlet;
import servletContainer.PrimitiveServlet;

public class ServletProcessor implements IProcessor {

    private Map<String,Servlet> map = new HashMap<String,Servlet>();

    public ServletProcessor() {

    }

    public void init(){
        PrimitiveServlet servlet = new PrimitiveServlet();
        servlet.init(null);
        map.put("PrimitiveServlet", servlet);
    }

    @Override
    public void process(Request request, Response response) {
        String uri = request.getUri();
        String servletName = uri.substring(uri.lastIndexOf("/") + 1);

        Servlet servlet = map.get(servletName);
        try{
            if(servlet != null){
                servlet.service(request, response);
            }
            else{
                String errorMessage = "HTTP/1.1 404 File Not Found\r\n" +
                        "Content-Type: text/html\r\n" +
                        "Content-Length:23\r\n" +
                        "\r\n" +
                        "<h1>File Not Found</h1>";
                response.getWriter().print(errorMessage.getBytes());
            }
        }
        catch (Exception e){
            e.printStackTrace();
        }
        catch (Throwable e){
            e.printStackTrace();
        }

    }

}

结果
我们在eclipse里运行结果