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;
try {
i = input.read(buffer);
}
catch (IOException e) {
e.printStackTrace();
i = -1;
}
for (int j=0; j<i; j++) {
request.append((char) buffer);
}
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;
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;
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 = 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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