需求分析
根据Tomcat的基本功能分析，基本需求包括：
- 监听端口，接受外部请求
- 多线程并发处理多个请求
- 解析HTTP请求，根据URL找到对应的Servlet
- 扫描Web目录，解析web.xml配置，加载开发者实现的Servlet类，生成对象并调用其service方法得到response返回给客户端
代码实现
接受请求的服务端
通过scoket监听端口，将接受到的请求提交到线程池处理。线程池中的任务为具体的处理逻辑。包括解析HTTP请求，匹配内部的Servlet。这个过程中的难点在于实现自己的HttpServletResponse类。查资料目前只看到Tomcat内部有实现，可以参考实现一个简化的版本。
public void server() throws Exception {
ExecutorService executor = Executors.newCachedThreadPool();
ServerSocket serverSocket = new ServerSocket(8080);
System.out.println("tomcat start");
while (!serverSocket.isClosed()) {
final Socket request = serverSocket.accept();
executor.execute(() -> {
try {
InputStream inputStream = request.getInputStream();
System.out.println("accecpt request");
BufferedReader reader = new BufferedReader(new InputStreamReader(inputStream, "utf-8"));
String msg = null;
List<String> msgList = new LinkedList<>();
while ((msg = reader.readLine()) != null) {
if (msg.length() == 0) {
break;
}
msgList.add(msg);
// System.out.println(msg);
}
HttpServletRequest servletRequest = null; //fixme
HttpServletResponse servletResponse = null; //fixme
Servlet servlet = new ProjectUtil().servletMapping.get("my-servlet-1.0-SNAPSHOT");
servlet.service(servletRequest, servletResponse);
// 可以通过outputStream来模拟返回数据
OutputStream outputStream = request.getOutputStream();
outputStream.write("Hello World".getBytes());
outputStream.flush();
} catch (IOException|ServletException e) {
e.printStackTrace();
}finally {
}
});
}
serverSocket.close();
}
加载开发者的Servlet类
配置文件用来表明Web目录名称和对应的Servlet类的位置，此处省略。只加载指定的Servlet类。使用URLClassLoader加载之后，MyServlet类就可以在当前JVM中访问到。
private void loadServlet() throws Exception {
String filePath = "/home/ubuntu/space/test/myservlet/target/my-servlet-1.0-SNAPSHOT/WEB-INF/classes/";
File classPath = new File(filePath);
String repository = (new URL("file", null, classPath.getCanonicalPath() + File.separator)).toString();
URLClassLoader loader = new URLClassLoader(new URL[]{new URL(repository)});
for (Map.Entry<Object, Object> entry : properties.entrySet()) {
String servletName = entry.getKey().toString();
String servletClassName = entry.getValue().toString();
Class servletClass = loader.loadClass("controller.MyServlet");
System.out.println(servletClass.getName());
Servlet servlet = (Servlet) servletClass.newInstance();
servletMapping.put(servletName, servlet);
}
}
上述代码中，将加载到的类通过Java的反射机制生成对象存储在ServletMapping中，供请求处理时进行选择使用。
实现自己的Servlet
继承HttpServlet类，实现自己的MyServlet类。通过Maven打包成war包，部署到上述Tomcat指定的目录下。主要代码如下：
public class MyServlet extends HttpServlet {
@Override
protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
System.out.println("MyServlet doGet");
OutputStream outputStream = resp.getOutputStream();
String name = req.getParameter("name");
String hello = "Hello " + name;
outputStream.write(hello.getBytes());
}
@Override
protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
System.out.println("MyServlet doPost");
}
@Override
protected void service(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
if ("GET".equals(req.getMethod())) {
this.doGet(req, resp);
} else {
this.doPost(req, resp);
}
}
}
部署完成后，运行Tomcat，在浏览器中输入请求，可以在控制台中看到MyServlet的输出。要实现根据浏览器输入，执行特定的逻辑，需要实现自己的HttpServletResponse类。
总结
主要用到了Java中的Socket、线程池、类加载器、反射等内容。
