cookie原理：
在访问第一个servlet的时候创建一个cookie写给浏览器，然后浏览器存储在缓存或者硬盘，下次访问servlet2的时候通过request带过去
demo1：（通过cookie得到上次访问时间）
@WebServlet("/cookieDemo1")
public class cookieDemo1 extends HttpServlet {
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
response.setCharacterEncoding("UTF_8");
response.setContentType("text/html;charset=UTF-8");
PrintWriter out=response.getWriter();
out.print("您上次访问时间</br>");
//获得用户的时间的cookie
Cookie cookies[]=request.getCookies();//从客户端获得cookie
for(int i=0;cookies!=null&&i<cookies.length;i++){
if(cookies[i].getName().equals("lastAccessTime")){
Long cookieValue=Long.parseLong(cookies[i].getValue());//得到了用户的上次访问时间
Date date=new Date(cookieValue);
out.print(date.toLocaleString());
}
}
//创建用户的cookie
Cookie cookie=new Cookie("lastAccessTime", System.currentTimeMillis()+"");
cookie.setMaxAge(1*30*24*3600);//按照毫秒设置有效期（生命周期）不设置默认是关闭浏览器就失效---（设置为0，则是命令删除该cookie）
cookie.setPath("/day06");//能带进去的有效路径，给浏览器看的 不设置默认是／day06/servlet setDomain是表示能带入cookie的域（www.snowing.com）
response.addCookie(cookie);//写给浏览器
}
protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
doGet(request, response);
}
}
浏览器输出：
cookie细节一------->
浏览器一般只允许存放300个cookie，每个站点一般存放20个，每个cookie大小限制为4kb
cookie模仿购物车推荐你曾经阅读过的东西：demo
首页分两部分，一部分是现实全部产品，另一部分是曾经阅读过的，当点击链接去阅读想阅读的书的时候把书id带过来，然后创建cookie发送给浏览器
下次访问首页，通过cookie把id读到，在曾阅读部分下显示
@WebServlet("/cookieDemo2")
public class cookieDemo2 extends HttpServlet {
//代表首页的servlet
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
response.setCharacterEncoding("UTF_8");
response.setContentType("text/html;charset=UTF-8");
PrintWriter out=response.getWriter();
//1.输出网站所有商品
out.write("本网站有如下商品<br/>");
Map<String, Book> map=Db.getAll();
for(Map.Entry<String,Book> entry:map.entrySet()){
Book book=entry.getValue();
out.print("<a href='/day06/cookieDemo3?id="+book.getId()+"' target='_blank'>"+book.getName() +"</a><br/>");
}
//2.显示用户曾经看过的商品
out.print("<br/>您曾经看过如下商品：<br/>");
Cookie cookies[]=request.getCookies();
for(int i=0;cookies!=null&&i<cookies.length;i++){
if(cookies[i].getName().equals("bookHistory")){
String ids[]=cookies[i].getValue().split("\,");
String idss[]=new String[3];
if(ids.length>=3){
for(int j=0;j<3;j++){
idss[j]=ids[j].toString();
}
for(String id:idss){
Book book2=map.get(id);
out.print("<a href='/day06/cookieDemo3?id="+book2.getId()+"' target='_blank'>"+book2.getName() +"</a><br/>");
}
}
else{
for(String id:ids){
Book book2=map.get(id);
out.print("<a href='/day06/cookieDemo3?id="+book2.getId()+"' target='_blank'>"+book2.getName() +"</a><br/>");
}
}
}
}
}
protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
doGet(request, response);
}
}
class Db{
private static Map<String,Book> map=new LinkedHashMap();
static{
map.put("1",new Book("1","java1111开发","老张","一本好书"));
map.put("2",new Book("2","java2222开发","老张","一本好书"));
map.put("3",new Book("3","java3333开发","老张","一本好书"));
map.put("4",new Book("4","java4444开发","老张","一本好书"));
}
public static Map getAll(){
return map;
}
}
class Book{
private String id;
private String name;
private String author;
private String description;
public Book() {
super();
// TODO Auto-generated constructor stub
}
public Book(String id, String name, String author, String description) {
super();
this.id = id;
this.name = name;
this.author = author;
this.description = description;
}
public String getId() {
return id;
}
public void setId(String id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public String getAuthor() {
return author;
}
public void setAuthor(String author) {
this.author = author;
}
public String getDescription() {
return description;
}
public void setDescription(String description) {
this.description = description;
}
}
@WebServlet("/cookieDemo3")
//显示商品详细信息的servlet
public class cookieDemo3 extends HttpServlet {
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
response.setCharacterEncoding("UTF_8");
response.setContentType("text/html;charset=UTF-8");
//根据用户带过来的id号，显示商品详细信息
String id=request.getParameter("id");
Map<String, Book> map=Db.getAll();
PrintWriter out=response.getWriter();
Book book=map.get(id);
out.print(book.getId()+book.getName() +"<br/>");
//构建cookie回写给浏览器
String cookieValue=buildCookie(request,id);
Cookie cookie=new Cookie("bookHistory",cookieValue);
cookie.setMaxAge(1*30*24*3600);//按照毫秒设置有效期（生命周期）不设置就是关闭浏览器就失效
cookie.setPath("/day06");//能带进去的有效路径，给浏览器看的
response.addCookie(cookie);//写给浏览器
}
private String buildCookie(HttpServletRequest request, String id) {
Cookie cookies[]=request.getCookies();
String cookieValue = null;
int flag=0;
for(int i=0;null!=cookies&&i<cookies.length;i++){
if(cookies[i].getName().equals("bookHistory")){
String ids[]=cookies[i].getValue().split("\,");
for(String id1:ids){
if(id1.equals(id)){
flag=1+flag;
cookieValue=id1+","+cookieValue;
}
else{
//cookieValue=request.getCookies()[i].getValue();
if(cookieValue==null){
cookieValue=id1;
}
else{
cookieValue=cookieValue+","+id1;}
}
}
}
}
if(flag==1){
return cookieValue;
}
if(flag==0){
if(request.getCookies()==null){
return id;
}
cookieValue=id+","+cookieValue;
return cookieValue;
}
return request.getCookies()+"";
}
protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
doGet(request, response);
}
}