Web开发11：Servlet

最新推荐文章于 2025-11-25 12:11:25 发布

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本文深入探讨了Servlet的工作机制，特别是HttpServlet类中的service方法如何处理不同的HTTP请求方法，如GET、POST等，并介绍了如何通过Tomcat获取HttpServlet的源码。

我们编写的Servlet一般继承HttpServlet

要查看HttpServlet的源码需到tomcat网站上下载

http://tomcat.apache.org/download-60.cgi

选择下载最下方的Source Code Distributions 的zip文件,下载后解压，能过eclispe的关联，关联到

apache-tomcat-6.0.32-src\apache-tomcat-6.0.32-src\java目录

如下代码所示：HttpServlet.java

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);
    }

转换完毕后，会调用HttpServlet类中自己定义的service方法，如下所示：

protected void service(HttpServletRequest req, HttpServletResponse resp)
        throws ServletException, IOException {

所谓请求的方法名：是指HttpServlet类中的getMethod方法，根据http协议来获得的，比如http的get协议或post协议等。

 protected void service(HttpServletRequest req, HttpServletResponse resp)
        throws ServletException, IOException {

        String method = req.getMethod();

        if (method.equals(METHOD_GET)) {
            long lastModified = getLastModified(req);
            if (lastModified == -1) {
                // servlet doesn't support if-modified-since, no reason
                // to go through further expensive logic
                doGet(req, resp);
            } else {
                long ifModifiedSince = req.getDateHeader(HEADER_IFMODSINCE);
                if (ifModifiedSince < (lastModified / 1000 * 1000)) {
                    // If the servlet mod time is later, call doGet()
                    // Round down to the nearest second for a proper compare
                    // A ifModifiedSince of -1 will always be less
                    maybeSetLastModified(resp, lastModified);
                    doGet(req, resp);
                } else {
                    resp.setStatus(HttpServletResponse.SC_NOT_MODIFIED);
                }
            }

        } else if (method.equals(METHOD_HEAD)) {
            long lastModified = getLastModified(req);
            maybeSetLastModified(resp, lastModified);
            doHead(req, resp);

        } else if (method.equals(METHOD_POST)) {
            doPost(req, resp);
            
        } else if (method.equals(METHOD_PUT)) {
            doPut(req, resp);        
            
        } else if (method.equals(METHOD_DELETE)) {
            doDelete(req, resp);
            
        } else if (method.equals(METHOD_OPTIONS)) {
            doOptions(req,resp);
            
        } else if (method.equals(METHOD_TRACE)) {
            doTrace(req,resp);
            
        } else {
            //
            // Note that this means NO servlet supports whatever
            // method was requested, anywhere on this server.
            //

            String errMsg = lStrings.getString("http.method_not_implemented");
            Object[] errArgs = new Object[1];
            errArgs[0] = method;
            errMsg = MessageFormat.format(errMsg, errArgs);
            
            resp.sendError(HttpServletResponse.SC_NOT_IMPLEMENTED, 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(HttpServletResponse.SC_METHOD_NOT_ALLOWED, msg);
        } else {
            resp.sendError(HttpServletResponse.SC_BAD_REQUEST, msg);
        }
    }

PS：request中的setAttribute,getAttribute是由HashMap实现的

Servlet核心API之间的关系UML图

新建一个InitServlet.java，在启动TOMCAT后，第一次访问InitServlet时，init invoked会被调用。

package com.test.servlet;

import java.io.IOException;
import java.io.PrintWriter;

import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

public class InitServlet extends HttpServlet {

	public void doGet(HttpServletRequest request, HttpServletResponse response)
			throws ServletException, IOException {
		System.out.println("get invoked");
	}

	public void doPost(HttpServletRequest request, HttpServletResponse response)
			throws ServletException, IOException {
		super.doPost(request, response);
	}

	public void init() throws ServletException {
		System.out.println("init invoked");
	}

}

在工程的Web.xml配置文件中，加入如下<load-on-startup>配置信息，表示在服务器启动时，会自动装载该Servlet

<servlet>
    <servlet-name>InitServlet</servlet-name>
    <servlet-class>com.test.servlet.InitServlet</servlet-class>
    <load-on-startup>2</load-on-startup>
  </servlet>

在启动服务器时，会打印出如下信息

注意上图中的：在Servlet被修改，重新加载后重新调用一次destroy，再执行init。

ServletContext从session中取出，如下

public void doGet(HttpServletRequest request, HttpServletResponse response)
			throws ServletException, IOException {
		ServletContext context = request.getSession().getServletContext();
		if(null == context.getAttribute("count")){
			context.setAttribute("count", "1");
		}else{
			int count = (Integer)context.getAttribute("count");
			context.setAttribute("count", count+1);
		}
		
	}