《How tomcat works》读书笔记_一个简单的Servlet容器
这个Servlet容器可以处理简单的servlet和静态资源，通过url请求来判断客户端请求的是什么资源。
首先写一个供测试用的Servlet：
package ex02.pyrmont;
import javax.servlet.*;
import java.io.IOException;
import java.io.PrintWriter;
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;
}
}Servlet是Server+Applet的缩写，表示一个服务器应用。Servlet是一套规范，我们按照这套规范写的代码就可以在Java的服务器上运行，既然Servlet是一套规范，那么最重要的当然是接口了，Servlet3.1中的接口定义如下
package javax.servlet;
import java.io.IOException;
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();
}Servlet接口中5个方法中init,service和destroy是生命周期方法，在servlet类被初始化之后，会调用init方法(当load-on-startup设置为负数或不设置时，init方法会在servlet第一次用到时才调用)，init方法在servlet整个生命周期中只被调用一次。getServletConfig方法用于获取ServletConfig，一般地一个servlet实现Servlet接口的同时也会实现ServletConfig接口，它保存的是这个servlet的上下文信息。getServletInfo方法可以获取一些servlet相关的信息，如作者版权等，这个方法需要自己实现，默认返回空字符串。destroy方法主要用于在servlet销毁(一般指关闭服务器)时释放一些资源，也只会调用一次。
Tomcat中init方法被调用时会接收到一个ServletConfig的参数，是由容器传进去的，ServletConfig顾名思义指的是Servlet的配置，我们在web.xml中定义Servlet时通过init-param标签配置的参数就是通过ServletConfig来保存的。ServletConfig是一个servlet的配置信息，这个接口中getServletContext方法可以一个ServletContext对象，这个对象非常重要，它代表的是我们这个应用本身，ServletContext中设置的信息可以被当前应用的所有servlet所共享。
ServletConfig和ServletContext最常见的使用之一就是传递初始化参数。
现在，让我们从一个servlet容器的角度来研究一下servlet编程。总的来说，一个全功能的servlet容器会为servlet的每个HTTP请求做下面一些工作：
- 当第一次调用servlet的时候，加载该servlet类并调用servlet的init方法(仅仅一次)。
- 对每次请求，构造一个javax.servlet.ServletRequest实例和一个javax.servlet.ServletResponse实例
- 调用servlet的service方法，同时传递ServletRequest和ServletResponse对象
- 当servlet类被关闭的时候，调用servlet的destroy方法并卸载servlet类
本章的第一个servlet容器不是全功能的。因此，她不能运行什么除了非常简单的servlet，而且也不调用servlet的init方法和destroy方法。相反它做了下面的事情：
- 等待HTTP请求
- 构造一个ServletRequest对象和一个ServletResponse对象
- 假如该请求需要一个静态资源的话，调用StaticResourceProcessor实例的process方法，同时传递ServletRequest和ServletResponse对象
- 假如该请求需要一个servlet的话，加载servlet类并调用servlet的service方法，同时传递ServletRequest和ServletResponse对象
注意：在这个servlet容器中，每一次servlet被请求的时候，servlet类都会被加载。 第一个应用程序由6个类组成
- HttpServer2
- Request
- RequestFacade
- Response
- ResponseFacade
- StaticResourceProcessor
- ServletProcessor2
- Constants
package ex02.pyrmont;
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);
}
}
}
}HttpServer2中的await方法等待Http请求，通过请求的Url判断是返回静态资源或是调用Servlet。
- 请求类
package ex02.pyrmont;
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.AsyncContext;
import javax.servlet.DispatcherType;
import javax.servlet.RequestDispatcher;
import javax.servlet.ServletContext;
import javax.servlet.ServletInputStream;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;
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;
}
。。。。
}servlet的service方法从servlet容器中接收一个javax.servlet.ServletRequest实例和一个javax.servlet.ServletResponse实例。这就是说对于每一个HTTP请求，servlet容器必须构造一个ServletRequest对象和一个ServletResponse对象并把它们传递给正在服务的servlet的service方法。
这里只使用一些简单的功能，很多方法都没有实现。
- Response
package ex02.pyrmont;
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;
}
。。。。
}在getWriter方法中，PrintWriter类的构造方法的第二个参数是一个布尔值表明是否允许自动刷新。传递true作为第二个参数将会使任何println方法的调用都会刷新输出(output)。不过，print方法不会刷新输出。 因此，任何print方法的调用都会发生在servlet的service方法的最后一行，输出将不会被发送到浏览器。
Response类还拥有在第1章中谈到的sendStaticResource方法。
由于Servlet程序员在编写servlet程序时通过向下转型可以获得request和response对象，从而调用其中的方法，拥有一个Request实例，它们就可以调用parse方法。拥有一个Response实例，就可以调用sendStaticResource方法。而我们并不希望把这两个方法暴露给用户，所以为Request对象和Response对象各自创建了一个门面类，将门面类封装的request和response对象设置成private，这个Servlet程序员只能够转型为门面类，从而访问不到具体对象中不在公共接口中的方法。
package ex02.pyrmont;
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.AsyncContext;
import javax.servlet.DispatcherType;
import javax.servlet.RequestDispatcher;
import javax.servlet.ServletContext;
import javax.servlet.ServletInputStream;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;
@SuppressWarnings({"unchecked", "rawtypes", "deprecation"})
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);
}
@Override
public long getContentLengthLong() {
// TODO Auto-generated method stub
return request.getContentLengthLong();
}
@Override
public int getRemotePort() {
// TODO Auto-generated method stub
return request.getRemotePort();
}
@Override
public String getLocalName() {
// TODO Auto-generated method stub
return request.getLocalName();
}
@Override
public String getLocalAddr() {
// TODO Auto-generated method stub
return request.getLocalAddr();
}
@Override
public int getLocalPort() {
// TODO Auto-generated method stub
return request.getLocalPort();
}
@Override
public ServletContext getServletContext() {
// TODO Auto-generated method stub
return request.getServletContext();
}
@Override
public AsyncContext startAsync() throws IllegalStateException {
// TODO Auto-generated method stub
return request.startAsync();
}
@Override
public AsyncContext startAsync(ServletRequest servletRequest,
ServletResponse servletResponse) throws IllegalStateException {
// TODO Auto-generated method stub
return request.startAsync(servletRequest, servletResponse);
}
@Override
public boolean isAsyncStarted() {
// TODO Auto-generated method stub
return request.isAsyncStarted();
}
@Override
public boolean isAsyncSupported() {
// TODO Auto-generated method stub
return request.isAsyncSupported();
}
@Override
public AsyncContext getAsyncContext() {
// TODO Auto-generated method stub
return request.getAsyncContext();
}
@Override
public DispatcherType getDispatcherType() {
// TODO Auto-generated method stub
return request.getDispatcherType();
}
}package ex02.pyrmont;
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);
}
@Override
public String getContentType() {
// TODO Auto-generated method stub
return response.getContentType();
}
@Override
public void setCharacterEncoding(String charset) {
// TODO Auto-generated method stub
response.setCharacterEncoding(charset);
}
@Override
public void setContentLengthLong(long length) {
// TODO Auto-generated method stub
response.setContentLengthLong(length);
}
}- ServletProcessor
package ex02.pyrmont;
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(servletName);
}
catch (ClassNotFoundException e) {
System.out.println(e.toString());
}
Servlet servlet = null;
RequestFacade requestFacade = new RequestFacade(request);
ResponseFacade responseFacade = new ResponseFacade(response);
try {
servlet = (Servlet) myClass.newInstance();
servlet.service((ServletRequest) requestFacade, (ServletResponse) responseFacade);
}
catch (Exception e) {
System.out.println(e.toString());
}
catch (Throwable e) {
System.out.println(e.toString());
}
}
}用来加载Servlet并调用它的service方法。
运行程序：
从结果中看，PrimitiveServlet中的两条信息只输出了一条。这是因为在getWriter方法中，PrintWriter类的构造方法的第二个参数是一个布尔值表明是否允许自动刷新。传递true作为第二个参数将会使任何println方法的调用都会刷新输出(output)。不过，print方法不会刷新输出。