基于Java web服务器简单实现一个Servlet容器

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上篇写了一个简单的Java web服务器实现，只能处理一些静态资源的请求，本篇文章实现的Servlet容器基于前面的服务器做了个小改造，增加了Servlet请求的处理。

程序执行步骤
1.创建一个ServerSocket对象；
2.调用ServerSocket对象的accept方法，等待连接，连接成功会返回一个Socket对象，否则一直阻塞等待；
3.从Socket对象中获取InputStream和OutputStream字节流，这两个流分别对应request请求和response响应；
4.处理请求：读取InputStream字节流信息，转成字符串形式，并解析，这里的解析比较简单，仅仅获取uri(统一资源标识符)信息;
5.处理响应（分两种类型，静态资源请求响应或servlet请求响应）：如果是静态资源请求，则根据解析出来的uri信息，从WEB_ROOT目录中寻找请求的资源资源文件, 读取资源文件，并将其写入到OutputStream字节流中；如果是Servlet请求，则首先生成一个URLClassLoader类加载器，加载请求的servlet类，创建servlet对象，执行service方法（往OutputStream写入响应的数据）；
6.关闭Socket对象；
7.转到步骤2，继续等待连接请求；

代码实现：
添加依赖：

 <!-- https://mvnrepository.com/artifact/javax.servlet/servlet-api -->
 <dependency>
 <groupId>javax.servlet</groupId>
 <artifactId>servlet-api</artifactId>
 <version>2.3</version>
 </dependency>

服务器代码：

package ex02.pyrmont.first;

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

import ex02.pyrmont.Request;
import ex02.pyrmont.Response;
import ex02.pyrmont.StaticResourceProcessor;

public class HttpServer1 {

 // 关闭服务命令
 private static final String SHUTDOWN_COMMAND = "/SHUTDOWN";

 public static void main(String[] args) {
 HttpServer1 server = new HttpServer1();
 //等待连接请求
 server.await();
 }

 public void await() {
 ServerSocket serverSocket = null;
 int port = 8080;
 try {
 //服务器套接字对象
 serverSocket = new ServerSocket(port, 1, InetAddress.getByName("127.0.0.1"));
 } catch (IOException e) {
 e.printStackTrace();
 System.exit(1);
 }

 // 循环等待请求
 while (true) {
 Socket socket = null;
 InputStream input = null;
 OutputStream output = null;
 try {
 //等待连接，连接成功后，返回一个Socket对象
 socket = serverSocket.accept();
 input = socket.getInputStream();
 output = socket.getOutputStream();

 // 创建Request对象并解析
 Request request = new Request(input);
 request.parse();
 // 检查是否是关闭服务命令
 if (request.getUri().equals(SHUTDOWN_COMMAND)) {
  break;
 }
 
 // 创建 Response 对象
 Response response = new Response(output);
 response.setRequest(request);

 if (request.getUri().startsWith("/servlet/")) {
  //请求uri以/servlet/开头，表示servlet请求
  ServletProcessor1 processor = new ServletProcessor1();
  processor.process(request, response);
 } else {
  //静态资源请求
  StaticResourceProcessor processor = new StaticResourceProcessor();
  processor.process(request, response);
 }

 // 关闭 socket
 socket.close();

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

常量类：

package ex02.pyrmont;

import java.io.File;

public class Constants {
 public static final String WEB_ROOT = System.getProperty("user.dir")
 + File.separator + "webroot";
 public static final String WEB_SERVLET_ROOT = System.getProperty("user.dir")
 + File.separator + "target" + File.separator + "classes";
 
}

Request：

package ex02.pyrmont;

import java.io.InputStream;
import java.io.IOException;
import java.io.BufferedReader;
import java.io.UnsupportedEncodingException;
import java.util.Enumeration;
import java.util.Locale;
import java.util.Map;
import javax.servlet.RequestDispatcher;
import javax.servlet.ServletInputStream;
import javax.servlet.ServletRequest;

public class Request implements ServletRequest {

 private InputStream input;
 private String uri;

 public Request(InputStream input) {
 this.input = input;
 }

 public String getUri() {
 return uri;
 }
 /**
 * 
 * requestString形式如下：
 * GET /index.html HTTP/1.1
 * Host: localhost:8080
 * Connection: keep-alive
 * Cache-Control: max-age=0
 * ...
 * 该函数目的就是为了获取/index.html字符串
 */
 private String parseUri(String requestString) {
 int index1, index2;
 index1 = requestString.indexOf(' ');
 if (index1 != -1) {
 index2 = requestString.indexOf(' ', index1 + 1);
 if (index2 > index1)
 return requestString.substring(index1 + 1, index2);
 }
 return null;
 }
 //从InputStream中读取request信息，并从request中获取uri值
 public void parse() {
 // Read a set of characters from the socket
 StringBuffer request = new StringBuffer(2048);
 int i;
 byte[] buffer = new byte[2048];
 try {
 i = input.read(buffer);
 } catch (IOException e) {
 e.printStackTrace();
 i = -1;
 }
 for (int j = 0; j < i; j++) {
 request.append((char) buffer[j]);
 }
 System.out.print(request.toString());
 uri = parseUri(request.toString());
 }

 /* implementation of the ServletRequest */
 public Object getAttribute(String attribute) {
 return null;
 }

 public Enumeration<?> getAttributeNames() {
 return null;
 }

 public String getRealPath(String path) {
 return null;
 }

 public RequestDispatcher getRequestDispatcher(String path) {
 return null;
 }

 public boolean isSecure() {
 return false;
 }

 public String getCharacterEncoding() {
 return null;
 }

 public int getContentLength() {
 return 0;
 }

 public String getContentType() {
 return null;
 }

 public ServletInputStream getInputStream() throws IOException {
 return null;
 }

 public Locale getLocale() {
 return null;
 }

 public Enumeration<?> getLocales() {
 return null;
 }

 public String getParameter(String name) {
 return null;
 }

 public Map<?, ?> getParameterMap() {
 return null;
 }

 public Enumeration<?> getParameterNames() {
 return null;
 }

 public String[] getParameterValues(String parameter) {
 return null;
 }

 public String getProtocol() {
 return null;
 }

 public BufferedReader getReader() throws IOException {
 return null;
 }

 public String getRemoteAddr() {
 return null;
 }

 public String getRemoteHost() {
 return null;
 }

 public String getScheme() {
 return null;
 }

 public String getServerName() {
 return null;
 }

 public int getServerPort() {
 return 0;
 }

 public void removeAttribute(String attribute) {
 }

 public void setAttribute(String key, Object value) {
 }

 public void setCharacterEncoding(String encoding)
 throws UnsupportedEncodingException {
 }

}

Response：

package ex02.pyrmont;

import java.io.OutputStream;
import java.io.IOException;
import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.File;
import java.io.PrintWriter;
import java.util.Locale;
import javax.servlet.ServletResponse;
import javax.servlet.ServletOutputStream;

public class Response implements ServletResponse {

 private static final int BUFFER_SIZE = 1024;
 Request request;
 OutputStream output;
 PrintWriter writer;

 public Response(OutputStream output) {
 this.output = output;
 }

 public void setRequest(Request request) {
 this.request = request;
 }

 //将web文件写入到OutputStream字节流中
 public void sendStaticResource() throws IOException {
 byte[] bytes = new byte[BUFFER_SIZE];
 FileInputStream fis = null;
 try {
 /* request.getUri has been replaced by request.getRequestURI */
 File file = new File(Constants.WEB_ROOT, request.getUri());
 fis = new FileInputStream(file);
 /*
 * HTTP Response = Status-Line(( general-header | response-header |
 * entity-header ) CRLF) CRLF [ message-body ] Status-Line =
 * HTTP-Version SP Status-Code SP Reason-Phrase CRLF
 */
 int ch = fis.read(bytes, 0, BUFFER_SIZE);
 while (ch != -1) {
 output.write(bytes, 0, ch);
 ch = fis.read(bytes, 0, BUFFER_SIZE);
 }
 } catch (FileNotFoundException e) {
 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>";
 output.write(errorMessage.getBytes());
 } finally {
 if (fis != null)
 fis.close();
 }
 }

 /** implementation of ServletResponse */
 public void flushBuffer() throws IOException {
 }

 public int getBufferSize() {
 return 0;
 }

 public String getCharacterEncoding() {
 return null;
 }

 public Locale getLocale() {
 return null;
 }

 public ServletOutputStream getOutputStream() throws IOException {
 return null;
 }

 public PrintWriter getWriter() throws IOException {
 // autoflush is true, println() will flush,
 // but print() will not.
 writer = new PrintWriter(output, true);
 return writer;
 }

 public boolean isCommitted() {
 return false;
 }

 public void reset() {
 }

 public void resetBuffer() {
 }

 public void setBufferSize(int size) {
 }

 public void setContentLength(int length) {
 }

 public void setContentType(String type) {
 }

 public void setLocale(Locale locale) {
 }
}

静态资源请求处理：

package ex02.pyrmont;

import java.io.IOException;

public class StaticResourceProcessor {

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

Servlet请求处理：

package ex02.pyrmont.first;

import java.net.URL;
import java.net.URLClassLoader;
import java.net.URLStreamHandler;
import java.io.IOException;

import javax.servlet.Servlet;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;

import ex02.pyrmont.Constants;
import ex02.pyrmont.Request;
import ex02.pyrmont.Response;

public class ServletProcessor1 {

 public void process(Request request, Response response) {

 String uri = request.getUri();
 String servletName = uri.substring(uri.lastIndexOf("/") + 1);
 
 //类加载器，用于从指定JAR文件或目录加载类
 URLClassLoader loader = null;
 try {
 URLStreamHandler streamHandler = null;
 //创建类加载器
 loader = new URLClassLoader(new URL[]{new URL(null, "file:" + Constants.WEB_SERVLET_ROOT, streamHandler)});
 } catch (IOException e) {
 System.out.println(e.toString());
 }
 
 Class<?> myClass = null;
 try {
 //加载对应的servlet类
 myClass = loader.loadClass(servletName);
 } catch (ClassNotFoundException e) {
 System.out.println(e.toString());
 }

 Servlet servlet = null;

 try {
 //生产servlet实例
 servlet = (Servlet) myClass.newInstance();
 //执行ervlet的service方法
 servlet.service((ServletRequest) request,(ServletResponse) response);
 } catch (Exception e) {
 System.out.println(e.toString());
 } catch (Throwable e) {
 System.out.println(e.toString());
 }

 }
}

Servlet类：

import javax.servlet.*;
import java.io.IOException;
import java.io.PrintWriter;

public class PrimitiveServlet implements Servlet {

 public void init(ServletConfig config) throws ServletException {
 System.out.println("init");
 }

 public void service(ServletRequest request, ServletResponse response)
 throws ServletException, IOException {
 System.out.println("from service");
 PrintWriter out = response.getWriter();
 out.println("Hello. Roses are red.");
 out.print("Violets are blue.");
 }

 public void destroy() {
 System.out.println("destroy");
 }

 public String getServletInfo() {
 return null;
 }

 public ServletConfig getServletConfig() {
 return null;
 }

}

结果测试：
静态资源请求：

servlet请求（因为只是第一个字符串被刷新到浏览器，所以你不能看到第二个字符串Violets are blue。我们将在后续完善该容器）：

改进

前面实现的Servlet容器有一个严重的问题，用户在servlet里可以直接将ServletRequest、ServletResponse向下转型为Request和Response类型，并直接调用其内部的public方法，这是一个不好的设计，改进方法是给Request、Response 增加外观类，这样，用户只能访问外观类里定义的public方法。
Request外观类

package ex02.pyrmont.second;

import java.io.IOException;
import java.io.BufferedReader;
import java.io.UnsupportedEncodingException;
import java.util.Enumeration;
import java.util.Locale;
import java.util.Map;

import javax.servlet.RequestDispatcher;
import javax.servlet.ServletInputStream;
import javax.servlet.ServletRequest;

import ex02.pyrmont.Request;

public class RequestFacade implements ServletRequest {

 private ServletRequest request = null;

 public RequestFacade(Request request) {
 this.request = request;
 }

 /* implementation of the ServletRequest */
 public Object getAttribute(String attribute) {
 return request.getAttribute(attribute);
 }

 public Enumeration<?> getAttributeNames() {
 return request.getAttributeNames();
 }

 @SuppressWarnings("deprecation")
 public String getRealPath(String path) {
 return request.getRealPath(path);
 }

 public RequestDispatcher getRequestDispatcher(String path) {
 return request.getRequestDispatcher(path);
 }

 public boolean isSecure() {
 return request.isSecure();
 }

 public String getCharacterEncoding() {
 return request.getCharacterEncoding();
 }

 public int getContentLength() {
 return request.getContentLength();
 }

 public String getContentType() {
 return request.getContentType();
 }

 public ServletInputStream getInputStream() throws IOException {
 return request.getInputStream();
 }

 public Locale getLocale() {
 return request.getLocale();
 }

 public Enumeration<?> getLocales() {
 return request.getLocales();
 }

 public String getParameter(String name) {
 return request.getParameter(name);
 }

 public Map<?, ?> getParameterMap() {
 return request.getParameterMap();
 }

 public Enumeration<?> getParameterNames() {
 return request.getParameterNames();
 }

 public String[] getParameterValues(String parameter) {
 return request.getParameterValues(parameter);
 }

 public String getProtocol() {
 return request.getProtocol();
 }

 public BufferedReader getReader() throws IOException {
 return request.getReader();
 }

 public String getRemoteAddr() {
 return request.getRemoteAddr();
 }

 public String getRemoteHost() {
 return request.getRemoteHost();
 }

 public String getScheme() {
 return request.getScheme();
 }

 public String getServerName() {
 return request.getServerName();
 }

 public int getServerPort() {
 return request.getServerPort();
 }

 public void removeAttribute(String attribute) {
 request.removeAttribute(attribute);
 }

 public void setAttribute(String key, Object value) {
 request.setAttribute(key, value);
 }

 public void setCharacterEncoding(String encoding)
  throws UnsupportedEncodingException {
 request.setCharacterEncoding(encoding);
 }

}

Response外观类

package ex02.pyrmont.second;

import java.io.IOException;
import java.io.PrintWriter;
import java.util.Locale;

import javax.servlet.ServletResponse;
import javax.servlet.ServletOutputStream;

import ex02.pyrmont.Response;

public class ResponseFacade implements ServletResponse {

 private ServletResponse response;

 public ResponseFacade(Response response) {
 this.response = response;
 }

 public void flushBuffer() throws IOException {
 response.flushBuffer();
 }

 public int getBufferSize() {
 return response.getBufferSize();
 }

 public String getCharacterEncoding() {
 return response.getCharacterEncoding();
 }

 public Locale getLocale() {
 return response.getLocale();
 }

 public ServletOutputStream getOutputStream() throws IOException {
 return response.getOutputStream();
 }

 public PrintWriter getWriter() throws IOException {
 return response.getWriter();
 }

 public boolean isCommitted() {
 return response.isCommitted();
 }

 public void reset() {
 response.reset();
 }

 public void resetBuffer() {
 response.resetBuffer();
 }

 public void setBufferSize(int size) {
 response.setBufferSize(size);
 }

 public void setContentLength(int length) {
 response.setContentLength(length);
 }

 public void setContentType(String type) {
 response.setContentType(type);
 }

 public void setLocale(Locale locale) {
 response.setLocale(locale);
 }

}

处理Servlet请求类：

package ex02.pyrmont.second;

import java.net.URL;
import java.net.URLClassLoader;
import java.net.URLStreamHandler;
import java.io.IOException;

import javax.servlet.Servlet;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;

import ex02.pyrmont.Constants;
import ex02.pyrmont.Request;
import ex02.pyrmont.Response;

public class ServletProcessor2 {

 public void process(Request request, Response response) {

 String uri = request.getUri();
 String servletName = uri.substring(uri.lastIndexOf("/") + 1);
 // 类加载器，用于从指定JAR文件或目录加载类
 URLClassLoader loader = null;
 try {
  URLStreamHandler streamHandler = null;
  // 创建类加载器
  loader = new URLClassLoader(new URL[] { new URL(null, "file:"
   + Constants.WEB_SERVLET_ROOT, streamHandler) });
 } catch (IOException e) {
  System.out.println(e.toString());
 }

 Class<?> myClass = null;
 try {
  // 加载对应的servlet类
  myClass = loader.loadClass(servletName);
 } catch (ClassNotFoundException e) {
  System.out.println(e.toString());
 }

 Servlet servlet = null;
 //给request、response增加外观类，安全性考虑，防止用户在servlet里直接将ServletRequest、ServletResponse向下转型为Request和Response类型，
 //并直接调用其内部的public方法，因为RequestFacade、ResponseFacade里不会有parse、sendStaticResource等方法；
 RequestFacade requestFacade = new RequestFacade(request);
 ResponseFacade responseFacade = new ResponseFacade(response);
 try {
  servlet = (Servlet) myClass.newInstance();
  servlet.service((ServletRequest) requestFacade, (ServletResponse) responseFacade);
 } catch (Exception e) {
  System.out.println(e.toString());
 } catch (Throwable e) {
  System.out.println(e.toString());
 }

 }
}

其它代码与前面实现的Servlet容器基本一致。
验证程序，分别请求静态资源和Servlet，发现结果与前面实现的容器一致；

参考资料：《深入剖析Tomcat》

@author 风一样的码农

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