How Tomcat Works（三）

上文中描述的简单的服务器是不符合Servlet规范的，所以本文进一步描述一个简单的Servlet容器是怎么实现的

所以我们首先要明白Servlet接口规范，规范有不同版本，本人就先一视同仁了：

public interface Servlet {

    public void init(ServletConfig config) throws ServletException;      

    public ServletConfig getServletConfig();

    public void service(ServletRequest req, ServletResponse res)
        throws ServletException, IOException;
    public String getServletInfo();      

    public void destroy();
}

上面的方法中，init() 、service()和 destroy()是与Servlet的生命周期密切相关的方法，熟悉Servlet生命周期的童鞋是比较清楚的

Servlet容器通常只调用Servlet实例的init()方法一次，用于初始化相关信息；

service()方法用于响应客户端请求，传入ServletRequest和ServletResponse参数，service()方法会被多次调用

当Servlet容器关闭或Servlet容器需要释放内存时，会调用Servlet实例的destroy()方法，用于清理自身持有的资源，如内存、文件句柄和线程等，确保所有的持久化状态与内存中该Servlet对象的当前状态同步。

下面我们来看一个简单的Servlet容器怎么实现：

HttpServer1类：

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/")) {
          ServletProcessor1 processor = new ServletProcessor1();
          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);
      }
    }
  }
}

上面方法中，Servlet容器根据请求的路径分发到不同的处理类进行处理，servlet请求交给ServletProcessor1类处理，静态资源交给StaticResourceProcessor类处理

注：本文中的Servlet容器跟上文相比，将响应请求的功能解耦， 由处理器类（ServletProcessor1类和StaticResourceProcessor类）来承担

Request类（注意我们这里的Request类已经实现了ServletRequest 接口，已经是按照规范来搞的了）

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;
  }

  //省略后面的代码

}

Response类（实现ServletResponse接口）

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

上面实现了获取 PrintWriter对象的方法

StaticResourceProcessor类（静态资源处理）

public class StaticResourceProcessor {

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

方法中仅仅简单的调用了response对象的sendStaticResource()方法

ServletProcessor1类(servlet资源处理类)

public class ServletProcessor1 {

  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(servletName);
    }
    catch (ClassNotFoundException e) {
      System.out.println(e.toString());
    }

    Servlet servlet = null;

    try {
      servlet = (Servlet) myClass.newInstance();
      servlet.service((ServletRequest) request, (ServletResponse) response);
    }
    catch (Exception e) {
      System.out.println(e.toString());
    }
    catch (Throwable e) {
      System.out.println(e.toString());
    }

  }
}

上面的步骤是首先根据客户端请求路径获取请求的servlet名称，然后根据servlet类路径（类载入器仓库）创建类载入器，进一步根据servlet名称载入该servlet类并实例化，最后调用该servlet的serice()方法

其中Constants类保持工作目录常量（Servlet类路径）

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

我们继续分析，其实上面的ServletProcessor1类的process()方法是存在问题的，在下面的代码段

Servlet servlet = null;

    try {
      servlet = (Servlet) myClass.newInstance();
      servlet.service((ServletRequest) request, (ServletResponse) response);
    }
    catch (Exception e) {
      System.out.println(e.toString());
    }
    catch (Throwable e) {
      System.out.println(e.toString());
    }

这里的Request对象与Resposne对象分别向上转型为ServletRequest实例和ServletResponse实例

如果了解这个servlet容器内部原理的servlet程序员就可以（在自己实现的serlet类中）将ServletRequest实例和 ServletResponse实例分别向下转型为真实的Request实例和Response实例，就可以调用各自的公有方法了（Request实例的 parse()方法和Response实例的sendStaticResource()方法），而servlet容器又不能将这些公有方法私有化，因为其 他外部类还要调用它们，一个比较完美的解决方法是分别为Request类和Response类创建外观类，分别为RequestFacade类与 ResponseFacade类，与前者实现共同的接口，然后保持对前者的引用，相关的接口实现方法分别调用其引用实例的方法，于是世界从此清静了

注：其实本人认为这里不应该叫做外观类，可能叫包装器类更合适吧（因为本人没听过外观类有实现共同接口的说法）

RequestFacade类

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);
  }
 //省略后面的代码

}

ResponseFacade类

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();
  }
   //省略后面的代码
}

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本文链接 http://www.cnblogs.com/chenying99/p/3231637.html