How Tomcat Works - A Simple Web Server
1. Http协议
1.1 Http 请求
一个http请求包含了三个部分：
1）方法 - 统一资源定位符（uri） - 协议
2）请求头
3）请求实体
一个例子：
POST /examples/default.jsp HTTP/1.1 Accept: text/plain; text/html Accept-Language: en-gb Connection: Keep-Alive Host: localhost User-Agent: Mozilla/4.0 (compatible; MSIE 4.01; Windows 98) Content-Length: 33 Content-Type: application/x-www-form-urlencoded Accept-Encoding: gzip, deflate lastName=Franks&firstName=Michael
这里的方法是POST，其他支持的方法还有GET，OPTIONS，HEAD，PUT，DELETE。URI是 /examples/default.jsp表示请求资源的路径。协议是HTTP/1.1
第二部分就是请求头，各项之间用回车/换行(CRLF)分割。
最后一部分是请求实体，这里就是
lastName=Franks&firstName=Michael
请求实体和请求头之间是用一个只包含CRLF的空行分割。
1.2 Http响应
和http请求类似，http响应也包含了三部分：
1）协议-状态码-描述
2）响应头
3）响应实体
一个例子：
HTTP/1.1 200 OK Server: Microsoft-IIS/4.0 Date: Mon, 5 Jan 2004 13:13:33 GMT Content-Type: text/html Last-Modified: Mon, 5 Jan 2004 13:13:12 GMT Content-Length: 112 <html> <head> <title>HTTP Response Example</title> </head> <body> Welcome to Brainy Software </body> </html>
协议是http/1.1，状态码200，描述是OK。响应头各项之间也是用CRLF分割。响应头和响应实体之间也是一个包含CRLF的空行分割。
2. 一个简单的web服务器
这个例子实现了一个简单的web服务器，包含HttpServer，Response和Request三个java类和一些静态的资源文件。
HttpServer.java负责监听请求:
import java.net.Socket;
import java.net.ServerSocket;
import java.net.InetAddress;
import java.io.InputStream;
import java.io.OutputStream;
import java.io.IOException;
import java.io.File;
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;
}
}
}
}
Request.java:
import java.io.InputStream;
import java.io.IOException;
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;
}
}
Response.java:
import java.io.OutputStream;
import java.io.IOException;
import java.io.FileInputStream;
import java.io.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
*/
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();
}
}
}
HttpServer 接受web请求然后将封装成一个request对象，request对请求的uri进行解析，在webroot里面找到对应的静态文件，然后写到socket的outputstream。如果文件不存在，则写回一个404错误信息。
测试会发现在各个浏览器上测试结果不一样。
IE上如果访问的页面没有找到，会直接显示404，不会显示我们响应的错误信息。如果是访问localhost:8080/index.html 可以正常显示。
Firefox如果访问的页面没有找到，会显示我们响应的404错误信息，如果访问localhost:8080/index.html会显示HTML文件的源码。
注意Response sendStaticResource 这个方法是有问题的，因为根据http协议，响应体之前还有方法，状态码，响应头等信息。但是这里是直接把文件内容给写回去了。为了能把这部分内容加上去，对HttpServer做了一些修改；
import java.io.File;
import java.io.IOException;
import java.io.OutputStream;
import java.nio.file.Files;
/*
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
*/
class Response {
Request request;
OutputStream output;
public Response(OutputStream output) {
this.output = output;
}
public void setRequest(Request request) {
this.request = request;
}
public void sendStaticResource() throws IOException {
try {
File file = new File(HttpServer.WEB_ROOT, request.getUri());
if (file.exists()) {
StringBuilder response = new StringBuilder();
response.append("HTTP/1.1 200 OK\r\n");
// Hack it
if (request.getUri().startsWith("/images")) {
response.append("Content-Type: image/png\r\n");
} else {
response.append("Content-Type: text/html\r\n");
}
response.append("Content-Length: ").append(file.length()).append("\r\n");
response.append("\r\n");
output.write(response.toString().getBytes());
// Write file
output.write(Files.readAllBytes(file.toPath()));
} 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());
}
}
}
现在在firfox和chrome上都可以正常工作。
