Servlet可以被认为是服务端的applet，它被WEB服务器加载和执行，前端可以显示页面和获得页面数据，后台可以操纵数据库，能完成JavaBean的很多功能。在这里我较为具体的说说Servlet在Cookie，Session和上传文件上的应用，在说明时我给出一些能编绎运行的小例子，最后给出一个文件上传例子以加深印象。
我们先来看看SERVLET程序的基本构架：
式1：
package test;
import javax.servlet.*;
import javax.servlet.http.*;
import java.io.*;
import java.util.*;
public class test extends HttpServlet {
public void init(ServletConfig config) throws ServletException {
super.init(config);
}
public void service(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
int f =1; switch(f){
case 1:firstMothed(request,respponse);break;
}
}
public void firstMothed(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
response.setContentType("text/Html");
OutputStreamWriter osw = new
OutputStreamWriter(response.getOutputStream());
PrintWriter out = new PrintWriter (response.getOutputStream());
out.println("< html>");
out.println("< head>< title>Servlet1< /title>< /head>");
out.println("< body>你好！");
out.println("< /body>< /html>");
out.close();
}
}
式2：
package test;
import javax.servlet.*;
import javax.servlet.http.*;
import java.io.*;
import java.util.*;
public class test extends HttpServlet {
public void init(ServletConfig config) throws ServletException {
super.init(config);
}
public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
response.setContentType("text/html");
OutputStreamWriter osw = new OutputStreamWriter(response.getOutputStream());
PrintWriter out = new PrintWriter (response.getOutputStream());
out.println("< html>");
out.println("< head>< title>Servlet1< /title>< /head>");
out.println("< body>你好！");
out.println("< /body>< /html>");
out.close();
}
public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
response.setContentType("text/html");
OutputStreamWriter osw = new OutputStreamWriter(response.getOutputStream());
PrintWriter out = new PrintWriter (response.getOutputStream());
out.println("< html>");
out.println("< head>< title>Servlet1< /title>< /head>");
out.println("< body>你好！");
out.println("< /body>< /html>");
out.close();
}
}
式1适合于作总控模块，此SERVLET作中间调度，根据不同的f值调用不同的SERVLET或方法。
式2适合于对html的get和post有不同要求的情况。
但这并不是绝对的，式2就完全可以代替式1，只要在doGet()方法中写上doPost就与式1完全一样。
在init方法中执行的语句，只要这个servlet被启动了就一直有效，比如，我们在init()中new了一个对象，那么这个对象的内存空间就永远存在，除非显式地把这个对象赋为null，或重启服务。
HttpServletRequest和HttpServletResponse两个对象实现http请求，它们有很多有用的方法，在下面的cookie和session治理中会细加描述。
1， cookie治理 cookie用于在客户端保存个人所特有的信息，它采取在客户机写临时文件的机制。
package test;
import javax.servlet.*;
import javax.servlet.http.*;
import java.io.*;
import java.util.*;
public class test extends HttpServlet {
public void init(ServletConfig config) throws ServletException {
super.init(config);
}
public void service(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
//写cookie
String CookieName ="js79"; //若是汉字则需编码
String CookieValue = "yesky";//若是汉字则需编码
Cookie cookie = new Cookie(CookieName,CookieValue);
cookie.setMaxAge(age); // age = Integer.MAX_VALUE 永不过期
cookie.setPath("/");
//读cookie
String value = null;
Cookie[] cookies = request.getCookies();
if (cookies != null) {
for (int i=0; i< cookies.length; i++) {
if (cookies[i].getName().equals(CookieName))
value = cookies[i].getValue();
break;
}
}
}
response.setContentType("text/html");
OutputStreamWriter osw = new OutputStreamWriter(response.getOutputStream());
PrintWriter out = new PrintWriter (response.getOutputStream());
out.println("< html>");
out.println("< head>< title>test< /title>< /head>");
out.println("cookie键:"+CookieName+"< br>");
out.println("cookie值: "+value);
out.println("< /body>< /html>");
out.close();
}
}
2,session治理
Session在Servlet中是很有用的，它比cookie安全可靠灵活，但是治理起来有点麻烦，用得不好会造成服务器的开销很大，浪费资源。下面是一个基于Session治理一个对象的简单例子。
一个简单的bean对象TestObject
package test;
public class TestObject extends Object {
int id = 0; public String cur="";
}
package test;
import javax.servlet.*;
import javax.servlet.http.*;
import java.io.*;
import java.util.*;
public class TestMan extends HttpServlet {
public void init(ServletConfig config) throws ServletException {
super.init(config);
}
public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
int f = 1;
if(request.getParameter("f")!=null)
f =
Integer.parseInt(request.getParameter("f"));
switch(f){
case 1: this.getResult(request,response);
break;
case 2:
this.setSession(request,response);
break;
}
}
public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
doGet(request,response);
}
public void getResult(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
TestObject testObject = null;
testObject = getStatus(request,response);
String html = testObject.id; doWrite( response,html);
}
public void setSession(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
HttpSession session = request.getSession();
TestObject testObject = null;
testObject = getStatus(request,response);
String tmp = null;
tmp = request.getParameter("id");
if(tmp != null) testObject.id = tmp;
session.putValue("testObject ",article);
getResult(request,response);
}
private TestObject getStatus(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
HttpSession session = request.getSession();
TestObject testObject = null;
if(session!=null){
if(session.getValue("testObject ")!=null){
testObject = (TestObject)session.getValue("testObject ");
}
else{
testObject = new TestObject ();
}
}
else{
testObject = new TestObject ();
}
return testObject;
}
private void doWrite(HttpServletResponse response,String html) throws ServletException, IOException {
PrintWriter out = response.getWriter();
out.println(html);
out.close();
}
/////////////////////////////////////////////////////////
}
若能轻松搞定上面的例子，相信读者对SERVLET已有了较为深刻的理解。
下面再介绍一个上传文件例子，其中汲及到了下载的免费JavaBean (如有感爱好的朋友，可来函索要免费JavaBean源代码,Email:js79@yesky.com)
上传基本原理：由页面发出一个http请求，服务端得到请求后，解析多媒体协议，读出文件内容，写文件内容到服务器，所有的这些功能都封装到JavaBean中。
上传文件的必需条件：Browser端< form>表单的ENCTYPE属性值必须为 multipart/form-data，它告诉我们传输的数据要用到多媒体传输协议，由于多媒体传输的都是大量的数据，所以规定上传文件必须是post方法，< input>的type属性必须是file。
package upload;
import javax.servlet.*;
import javax.servlet.http.*;
import java.io.*;
import java.util.*;
public class UpLoadServlet extends HttpServlet {
public void init(ServletConfig config) throws ServletException{
super.init(config);
}
public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
PrintWriter out = response.getWriter();
out.println("< HTML>< HEAD>< TITLE>UpLoad< /TITLE>"
+"< meta http-equiv=´Content-Type´ content=´text/html; charset=gb2312´>"
+"< /HEAD>"
+"< body>");
out.println("< div align=´center´ valign=´top´>"
+"< span class=´nava´>请你选择上传的文件(请注重文件大小只能在20K之内)< /span>< BR>"
+"< form ENCTYPE=´multipart/form-data´ method=post action=´´>"
+"< input type=´file´ name=´file´>"
+"< input type=´submit´ value=´发送´>"
+"< /form>"
+"< /div>");
out.println("< /body>< /html>");
out.close();
}
////////
public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
PrintWriter out = response.getWriter();
int tmpID = 1; try {
MultipartRequest multi = new MultipartRequest(request,"/home/js79/html/", 5 * 1024 * 1024);
}
catch(Exception e){
tmpID = -1; System.out.println(e);
}
if(tmpID == 1){
out.println("< HTML>< HEAD>< TITLE>UpLoad< /TITLE>"
+"< meta http-equiv=´Content-Type´ content=´text/html; charset=gb2312´>"
+"< /HEAD>"
+"< body>");
out.println("上传成功！< /body>< /html>");
}
else{
out.println("< HTML>< HEAD>< TITLE>UpLoad< /TITLE>"
+"< meta http-equiv=´Content-Type´ content=´text/html; charset=gb2312´>"
+"< /HEAD>"
+"< body>");
out.println("上传不成功！< /body>< /html>");
}
out.close();
}
}
