【0】README

0.1）本文部分文字转自　“深入剖析Tomcat”，旨在学习  一个简单的servlet容器  的基础知识；

0.2）for complete source code, please visit https://github.com/pacosonTang/HowTomcatWorks/tree/master/chapter2

0.3）温馨建议：建议阅读本文之前，已阅读过 tomcat(1)的系列文章，因为它们是环环相扣的；

【1】javax.servlet.Servlet接口

2.0）init， service， destroy方法是与 servlet的生命周期相关的方法；

2.1）init方法：当实例化某个servlet类后；servlet容器会调用其 init() 方法进行初始化，servlet只调用该方法一次；且servlet 程序员可以覆盖此方法，在其中编写仅需要执行一次的初始化代码；

2.2）service方法：当一个servlet客户端请求到达后，servlet容器就调用相应的servlet的service方法，并将 servletRequest 和 servletResponse 对象作为参数传入；

2.3）destroy方法：当servlet容器关闭或servlet容器要释放内存时，才会将 servlet实例移除，而且只有当servlet实例的service方法中的所有线程都退出或执行超时后，才会调用 destroy方法；

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;}
}

t1）当第一次调用某个 servlet 时，要载入该 servlet类，并调用其 init() 方法；（干货——servlet仅调用该方法一次）

t2）针对每个request请求， 创建一个 javax.servlet.ServletRequest 实例 和 一个 javax.servlet.ServletResponse 实例；

t3）调用该 servlet 的 service() 方法，将 servletRequest对象和 servletResponse对象作为参数传入；（参见上述PrimitiveServlet的定义）

t4）当关闭该servlet类时，调用其 destroy() 方法，并卸载该servlet类；

2.1）printing results

2.2）soruce code at a glance

// HttpServer1 类 既可以对静态资源请求，也可以对servlet资源请求
public class HttpServer1 { private static final String SHUTDOWN_COMMAND = "/SHUTDOWN";  private boolean shutdown = false;public static void main(String[] args) {HttpServer1 server = new HttpServer1();server.await();}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);}// Loop waiting for a requestwhile (!shutdown) {Socket socket = null;InputStream input = null;OutputStream output = null;try {socket = serverSocket.accept();input = socket.getInputStream();output = socket.getOutputStream();// create Request object and parseRequest request = new Request(input);request.parse();// create Response objectResponse response = new Response(output);response.setRequest(request);// check if this is a request for a servlet or a static resource// a request for a servlet begins with "/servlet/"if (request.getUri().startsWith("/servlet/")) { // 若HTTP请求的是servlet(以servlet打头)ServletProcessor1 processor = new ServletProcessor1();processor.process(request, response);} // 若http请求的是静态资源else {StaticResourceProcessor processor = new StaticResourceProcessor();processor.process(request, response);}// Close the socketsocket.close();//check if the previous URI is a shutdown commandshutdown = request.getUri().equals(SHUTDOWN_COMMAND);}catch (Exception e) {e.printStackTrace();System.exit(1);}}}
}

package com.tomcat.chapter2;import java.io.File;public class Constants { // 常数类，封装了 WEB_ROOT 常量public static final String WEB_ROOT =System.getProperty("user.dir") + File.separator  + "webroot";	
}

// Request: HTTP请求对象，其parse方法用于读取HTTP请求头；其parseUri方法用于解析client 请求的 uri

public class Request implements ServletRequest {private InputStream input;private String uri;public Request(InputStream input) {this.input = input;}public String getUri() {return uri;}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;}public void parse() {// Read a set of characters from the socketStringBuffer 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());}//...... some barren functions

// Response：<span style="color: rgb(85, 85, 85); font-family: 宋体;">HTTP响应对象，其sendStaticResource方法 读取request解析出的uri对应的资源文件，并发送该文件数据到client端；

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;}/* This method is used to serve a static page */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 PrintWriter getWriter() throws IOException {// autoflush is true, println() will flush,// but print() will not.writer = new PrintWriter(output, true);return writer;}

// （servlet 处理器）用于处理对servlet 资源的HTTP 请求（干货代码，特别是里面的类加载器）
public class ServletProcessor1 {public void process(Request request, Response response) {String uri = request.getUri();String servletName = uri.substring(uri.lastIndexOf("/") + 1);URLClassLoader loader = null; // 类载入器try {// create a URLClassLoader, 创建类载入器（类加载器是干货代码 ）URL[] urls = new URL[1];URLStreamHandler streamHandler = null;File classPath = new File(Constants.WEB_ROOT);// the forming of repository is taken from the createClassLoader method in// org.apache.catalina.startup.ClassLoaderFactoryString repository = (new URL("file", null, classPath.getCanonicalPath() + File.separator)).toString() ;// file:E:\bench-cluster\cloud-data-preprocess\HowTomcatWorks\webroot\// the code for forming the URL is taken from the addRepository method in// org.apache.catalina.loader.StandardClassLoader class.urls[0] = new URL(null, repository, streamHandler);// urls[0] = file:E:/bench-cluster/cloud-data-preprocess/HowTomcatWorks/webroot/loader = new URLClassLoader(urls);}catch (IOException e) {System.out.println(e.toString() );}Class myClass = null;try {myClass = loader.loadClass("servlet."+servletName); // 载入 servlet类}catch (ClassNotFoundException e) {System.out.println(e.toString());}Servlet servlet = null;try {servlet = (Servlet) myClass.newInstance(); // 会创建已载入的servlet类的一个实例servlet.service((ServletRequest) request, (ServletResponse) response);}catch (Exception e) {System.out.println(e.toString());}catch (Throwable e) {System.out.println(e.toString());}}
}

public class StaticResourceProcessor {public void process(Request request, Response response) {try {response.sendStaticResource(); // 直接调用response的 sendStaticResource 方法.....}catch (IOException e) {e.printStackTrace();}}
}

补充）本文总结了上述应用程序的调用流程图

1.1）这是不安全的做法： 一旦我们有了Request 实例后，我们就可以调用其parse方法，而有了Response实例后，我们就可以调用其 sendStaticResource方法。不能将parse() 和 sendStaticResource() 设置为私有方法，因为它们会被其他类调用，但这两个方法在servlet中不应该是可用的；

2.1）构建外观类 RequestFacade 和 ResponseFacade：分别实现 ServletRequest 和 ServletResponse ，然后在其构造函数中指定Request 和 Response对象（private私有访问权限） ，外观类中的实现方法均调用 Request 或 Response对象的相应方法；

4.1）打印结果同 HttpServer1，故 omit it；

4.2）源代码实现（我这里给给出外观类代码，其他和HttpServer1 中的类似）

public class HttpServer2 {// shutdown commandprivate static final String SHUTDOWN_COMMAND = "/SHUTDOWN";// the shutdown command receivedprivate boolean shutdown = false;public static void main(String[] args) {HttpServer2 server = new HttpServer2();server.await();}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);}// Loop waiting for a requestwhile (!shutdown) {Socket socket = null;InputStream input = null;OutputStream output = null;try {socket = serverSocket.accept();input = socket.getInputStream();output = socket.getOutputStream();// create Request object and parseRequest request = new Request(input);request.parse();// create Response objectResponse response = new Response(output);response.setRequest(request);//check if this is a request for a servlet or a static resource//a request for a servlet begins with "/servlet/"if (request.getUri().startsWith("/servlet/")) {ServletProcessor2 processor = new ServletProcessor2();processor.process(request, response);}else {StaticResourceProcessor processor = new StaticResourceProcessor();processor.process(request, response);}// Close the socketsocket.close();//check if the previous URI is a shutdown commandshutdown = request.getUri().equals(SHUTDOWN_COMMAND);}catch (Exception e) {e.printStackTrace();System.exit(1);}}}
}

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();}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);}}

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);}}

public class ServletProcessor2 {public void process(Request request, Response response) {String uri = request.getUri();String servletName = uri.substring(uri.lastIndexOf("/") + 1);URLClassLoader loader = null;try {URL[] urls = new URL[1];URLStreamHandler streamHandler = null;File classPath = new File(Constants.WEB_ROOT);String repository = (new URL("file", null, classPath.getCanonicalPath() + File.separator)).toString() ;urls[0] = new URL(null, repository, streamHandler);loader = new URLClassLoader(urls);}catch (IOException e) {System.out.println(e.toString() );}Class myClass = null;try {myClass = loader.loadClass("servlet." + servletName);}catch (ClassNotFoundException e) {System.out.println(e.toString());}Servlet servlet = null;RequestFacade requestFacade = new RequestFacade(request);  // attend this lineResponseFacade responseFacade = new ResponseFacade(response); <span style="font-family: 宋体;">// attend this line</span>try {servlet = (Servlet) myClass.newInstance();servlet.service((ServletRequest) requestFacade, (ServletResponse) responseFacade); // and this line}catch (Exception e) {System.out.println(e.toString());}catch (Throwable e) {System.out.println(e.toString());}}
}