一个简单的servlet容器

【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）文件目录如下：

【1】javax.servlet.Servlet接口

1）intro Servlet：Servlet编程需要使用到 javax.servlet 和 javax.servlet.http 两个包下的接口和类。在所有的类和接口中， javax.servlet.servlet 接口是最为重要的。所有的servlet 程序都必须实现该接口或继承自实现了该接口的类；

2）Servlet接口中声明了5个方法： init， service， destroy， getServletConfig， getServletInfo 方法；

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；

3）看个荔枝：

 

【2】应用程序1

1）一个功能齐全的 servlet容器有以下几件事情要做（things）：

t1）当第一次调用某个 servlet 时，要载入该 servlet类，并调用其 init() 方法；（仅此一次）

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；

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

2）servlet容器的实现源码和访问结果

2.1）printing results

2.2）source code at a glance

package com.tomcat.chapter2;

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

// HttpServer1 类 既可以对静态资源请求，也可以对servlet资源请求
public class HttpServer1 {

  /** WEB_ROOT is the directory where our HTML and other files reside.
   *  For this package, WEB_ROOT is the "webroot" directory under the working directory.
   *  The working directory is the location in the file system
   *  from where the java command was invoked.
   */
  // shutdown command
  private static final String SHUTDOWN_COMMAND = "/SHUTDOWN";

  // the shutdown command received
  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 request
    while (!shutdown) {
      Socket socket = null;
      InputStream input = null;
      OutputStream output = null;
      try {
        socket = serverSocket.accept();
        input = socket.getInputStream();
        output = socket.getOutputStream();

        // create Request object and parse
        Request request = new Request(input);
        request.parse();

        // create Response object
        Response 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 socket
        socket.close();
        //check if the previous URI is a shutdown command
        shutdown = request.getUri().equals(SHUTDOWN_COMMAND);
      }
      catch (Exception e) {
        e.printStackTrace();
        System.exit(1);
      }
    }
  }
}

package com.tomcat.chapter2;

import java.net.URL;
import java.net.URLClassLoader;
import java.net.URLStreamHandler;
import java.io.File;
import java.io.IOException;
import javax.servlet.Servlet;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;

// 用于处理对servlet 资源的HTTP 请求
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.ClassLoaderFactory
      String 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());
    }

  }
}

package com.tomcat.chapter2;

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;
  }

  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 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 {
  }

}

package com.tomcat.chapter2;

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;
  }

  /* 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 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) {
  }
}

package com.tomcat.chapter2;

import java.io.IOException;

public class StaticResourceProcessor {

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

package com.tomcat.chapter2;

import java.io.File;

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

---

【3】应用程序2

1）problem：ServletProcessor1 中必须将Request 转型为 javax.servlet.ServletRequest，将Response 转型为 javax.servlet.ServletResponse 实例，将它们作为参数传递给 service方法；

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）solution：使用外观类（Facade）来解决

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；

3）Conclusion：引入外观类的目的是， 不向外界提供访问到Request 和 Response 的接口；

4）源代码实现和打印结果

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）

package com.tomcat.chapter2;

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

public class HttpServer2 {

  // shutdown command
  private static final String SHUTDOWN_COMMAND = "/SHUTDOWN";

  // the shutdown command received
  private 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 request
    while (!shutdown) {
      Socket socket = null;
      InputStream input = null;
      OutputStream output = null;
      try {
        socket = serverSocket.accept();
        input = socket.getInputStream();
        output = socket.getOutputStream();

        // create Request object and parse
        Request request = new Request(input);
        request.parse();

        // create Response object
        Response 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 socket
        socket.close();
        //check if the previous URI is a shutdown command
        shutdown = request.getUri().equals(SHUTDOWN_COMMAND);
      }
      catch (Exception e) {
        e.printStackTrace();
        System.exit(1);
      }
    }
  }
}

package com.tomcat.chapter2;

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 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);
  }

}

package com.tomcat.chapter2;

import java.io.IOException;
import java.io.PrintWriter;
import java.util.Locale;
import javax.servlet.ServletResponse;
import javax.servlet.ServletOutputStream;

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);
  }

}

package com.tomcat.chapter2;

import java.net.URL;
import java.net.URLClassLoader;
import java.net.URLStreamHandler;
import java.io.File;
import java.io.IOException;
import javax.servlet.Servlet;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;

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 {
      // create a URLClassLoader
      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.ClassLoaderFactory
      String repository = (new URL("file", null, classPath.getCanonicalPath() + File.separator)).toString() ;
      // 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);
      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 line
    ResponseFacade responseFacade = new ResponseFacade(response); // attend this line
    try {
      servlet = (Servlet) myClass.newInstance();
      servlet.service((ServletRequest) requestFacade, (ServletResponse) responseFacade); // attend this line
    }
    catch (Exception e) {
      System.out.println(e.toString());
    }
    catch (Throwable e) {
      System.out.println(e.toString());
    }

  }
}