Servlet也是JavaEE的一种规范，位于javax.servlet下，Servlet规范还包含Filter。该包下分为两部分：servlet有关和http有关。
为什么会有两部分？设计该规范时认为Servlet是一种服务模型，不应与协议耦合，因此就抽象出了一个 javax.servlet，同时提供一套基于HTTP协议上的Servlet扩展，当然就现在看，还没有基于其他协议的流行的Servlet实现。也是因为HttpServlet的普遍性，本文以此为例说明。
从宏观上来看，HttpServlet基于“请求-响应”模型：用户发出请求，服务端响应用户。说到这点，可以先来看看CGI。Common Gateway Interface，通用网关接口，执行过程为：
Servlet的出现弥补了以上缺点，我们知道HttpServlet：
下面来看一下HttpServlet的执行流程：
解释一下：
而各种V层框架仅仅是对这两个类的进一步封装，再怎么操作，最终还是操作的HttpServletRequest和HttpServletResponse。
public interface Servlet
{
public abstract void init(ServletConfig servletconfig)
throws ServletException;
public abstract ServletConfig getServletConfig();
public abstract void service(ServletRequest servletrequest, ServletResponse servletresponse)
throws ServletException, IOException;
public abstract String getServletInfo();
public abstract void destroy();
} Servlet定义了生命周期有关的和读取配置的接口，它的作用就是规定声明周期管理、接受ServletRequest和ServletResponse。package javax.servlet;
import java.io.IOException;
import java.io.Serializable;
import java.util.Enumeration;
import java.util.ResourceBundle;
// Referenced classes of package javax.servlet:
// Servlet, ServletConfig, ServletException, ServletContext,
// ServletRequest, ServletResponse
public abstract class GenericServlet
implements Servlet, ServletConfig, Serializable
{
public void destroy()
{
}
public void init(ServletConfig config)
throws ServletException
{
this.config = config;
init();
}
public void init()
throws ServletException
{
}
//省略其他函数
}package javax.servlet.http;
import java.io.IOException;
import java.io.Serializable;
import java.lang.reflect.Method;
import java.text.MessageFormat;
import java.util.Enumeration;
import java.util.ResourceBundle;
import javax.servlet.*;
// Referenced classes of package javax.servlet.http:
// NoBodyResponse, HttpServletRequest, HttpServletResponse
public abstract class HttpServlet extends GenericServlet
implements Serializable
{
public HttpServlet()
{
}
//省略部分函数
public void service(ServletRequest req, ServletResponse res)
throws ServletException, IOException
{
HttpServletRequest request;
HttpServletResponse response;
try
{
request = (HttpServletRequest)req;
response = (HttpServletResponse)res;
}
catch(ClassCastException e)
{
throw new ServletException("non-HTTP request or response");
}
service(request, response);
}
protected void service(HttpServletRequest req, HttpServletResponse resp)
throws ServletException, IOException
{
String method = req.getMethod();
if(method.equals("GET"))
{
long lastModified = getLastModified(req);
if(lastModified == -1L)
{
doGet(req, resp);
} else
{
long ifModifiedSince = req.getDateHeader("If-Modified-Since");
if(ifModifiedSince < (lastModified / 1000L) * 1000L)
{
maybeSetLastModified(resp, lastModified);
doGet(req, resp);
} else
{
resp.setStatus(304);
}
}
} else
if(method.equals("HEAD"))
{
long lastModified = getLastModified(req);
maybeSetLastModified(resp, lastModified);
doHead(req, resp);
} else
if(method.equals("POST"))
doPost(req, resp);
else
if(method.equals("PUT"))
doPut(req, resp);
else
if(method.equals("DELETE"))
doDelete(req, resp);
else
if(method.equals("OPTIONS"))
doOptions(req, resp);
else
if(method.equals("TRACE"))
{
doTrace(req, resp);
} else
{
String errMsg = lStrings.getString("http.method_not_implemented");
Object errArgs[] = new Object[1];
errArgs[0] = method;
errMsg = MessageFormat.format(errMsg, errArgs);
resp.sendError(501, errMsg);
}
}
protected void doGet(HttpServletRequest req, HttpServletResponse resp)
throws ServletException, IOException
{
String protocol = req.getProtocol();
String msg = lStrings.getString("http.method_get_not_supported");
if(protocol.endsWith("1.1"))
resp.sendError(405, msg);
else
resp.sendError(400, msg);
}
protected void doPost(HttpServletRequest req, HttpServletResponse resp)
throws ServletException, IOException
{
String protocol = req.getProtocol();
String msg = lStrings.getString("http.method_post_not_supported");
if(protocol.endsWith("1.1"))
resp.sendError(405, msg);
else
resp.sendError(400, msg);
}
//省略doHead、doDelete等方法
}原文：http://blog.csdn.net/lidatgb/article/details/38347055