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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