我们这些可怜虫，只有沿着大神的思路，这样我们才能进步得更快；因为我们不是跟大神处于同一级别上。所以我这里是参考《How Tomcat Works》这本英文版的大作来理解tomcat的工作原理
本人认为，Tomcat容器源码是学习java编程以及设计模式等的绝佳范例，深入理解其源码对我辈开发人员的编程水平提高大有裨益！
我们可以从该书指定的官方网址下载相关示例源码 http://www.brainysoftware.com，本文顺着作者的思路，介绍一个简单的web服务器
我们知道，web服务器是使用http协议与客户端进行通信，所以读者有必要先了解http协议格式；基于java的web服务器会使用两个重要的类 java.net.Socket与java.net.ServerSocket（服务器端与客户端通过Socket通信）
关于http协议，网上的资料汗牛充栋，本人在这里加上简略的描述（http协议基于tcp协议）
http客户端请求包括如下部分：
Method-URI-Protocol/Version 方法-地址-版本
Request header 请求头
Entity body 请求实体
比如http://www.outletss.com/ 是本人以前帮客户做的网站，如果我们在浏览器打开该url地址，实际上客户端向服务器发送了如下格式的消息
GET http://www.outletss.com/ HTTP/1.1
Host: www.outletss.com
Connection: keep-alive
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
User-Agent: Mozilla/5.0 (Windows NT 5.2) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/27.0.1453.116 Safari/537.36
Accept-Encoding: gzip,deflate,sdch
Accept-Language: zh-CN,zh;q=0.8
Cookie: lzstat_uv=12863370662865423613|2989608; CKFinder_Path=Images%3A%2F%3A1; JSESSIONID=D7F9EC74149CB674D19A253B46273A77; lzstat_ss=1366758708_0_1375562495_2989608
http服务器端响应包括如下部分：
Protocol-Status code-Description 协议状态 描述代码
Response headers 响应头
Entity body 响应实体
然后服务器端向客户端响应了如下格式的消息
HTTP/1.1 200 OK
Connection: close
Date: Sat, 03 Aug 2013 15:00:30 GMT
Server: Microsoft-IIS/6.0
X-UA-Compatible: IE=EmulateIE7
X-Powered-By: ASP.NET
Set-Cookie: JSESSIONID=0A5B07FF5661CA6F8D87937A54B4EEF5; Path=/; HttpOnly
Content-Type: text/html;charset=UTF-8
Content-Language: zh-CN <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
//这里省略了html代码
</html>
基于java的Socket编程，可以参考java网络编程相关资料，Socket服务器端与客户端的网络交互与本地文件系统I/O具有一致的编程模型，基本上也是输入流与输出流的概念（如果你不懂输入流输出流的概念，基本上还要去修炼）
Socket分为客户端与服务器端，Socket表示客户端套接字，ServerSocket表示服务器端套接字，我们参考书中示例，看一个简单的服务器怎么实现
HttpServer类表示一个web服务器，示例代码如下：
public class HttpServer {
/** 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.
*/
public static final String WEB_ROOT =
System.getProperty("user.dir") + File.separator + "webroot";
// shutdown command
private static final String SHUTDOWN_COMMAND = "/SHUTDOWN";
// the shutdown command received
private boolean shutdown = false;
public static void main(String[] args) {
HttpServer server = new HttpServer();
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);
response.sendStaticResource();
// 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();
continue;
}
}
}
}
在上面代码里面，首先创建一个ServerSocket实例，然后用一个while循环监听客户端请求，接收到客户端请求后，通过ServerSocket实例的accept方法返回Socket实例，将该Socket实例的输入流与输出流封装成Request实例与Response实例，并调用Response实例的void sendStaticResource()方法响应请求。
Request类代码如下：
public class Request {
private InputStream input;
private String uri;
public Request(InputStream input) {
this.input = input;
}
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());
}
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 String getUri() {
return uri;
}
}
上面的void parse()方法是解析客户端的请求参数，这里是解析客户端请求的URL地址
Response类的代码如下：
/*
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
*/ public class Response { private static final int BUFFER_SIZE = 1024;
Request request;
OutputStream output; public Response(OutputStream output) {
this.output = output;
} public void setRequest(Request request) {
this.request = request;
} public void sendStaticResource() throws IOException {
byte[] bytes = new byte[BUFFER_SIZE];
FileInputStream fis = null;
try {
File file = new File(HttpServer.WEB_ROOT, request.getUri());
if (file.exists()) {
fis = new FileInputStream(file);
int ch = fis.read(bytes, 0, BUFFER_SIZE);
while (ch!=-1) {
output.write(bytes, 0, ch);
ch = fis.read(bytes, 0, BUFFER_SIZE);
}
}
else {
// file not found
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());
}
}
catch (Exception e) {
// thrown if cannot instantiate a File object
System.out.println(e.toString() );
}
finally {
if (fis!=null)
fis.close();
}
}
}
这里最重要的是void sendStaticResource()方法，用于向输出流写入数据（这里是静态文件），响应客户端请求
本文介绍的是一个最简单的web服务器，Tomcat容器的复杂性远不止如此简单，待后文接着分析
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