練習cookie做一個瀏覽商品記錄顯示，第一次會顯示瀏覽過的商品，然后在瀏覽第二次就出現空指針異常

2016-6-16 16:51:48 org.apache.catalina.core.StandardWrapperValve invoke

嚴重: Servlet.service() for servlet [CookieDemo2] in context with path [/testHttp] threw exception

java.lang.NullPointerException

at cn.itcast.cookie.CookieDemo2.doGet(CookieDemo2.java:43)

at javax.servlet.http.HttpServlet.service(HttpServlet.java:624)

at javax.servlet.http.HttpServlet.service(HttpServlet.java:731)

at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:303)

at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)

at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:220)

at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:122)

at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:505)

at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:169)

at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:103)

at org.apache.catalina.valves.AccessLogValve.invoke(AccessLogValve.java:956)

at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:116)

at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:436)

at org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1078)

at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:625)

at org.apache.tomcat.util.net.JIoEndpoint$SocketProcessor.run(JIoEndpoint.java:316)

at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:886)

at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:908)

at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)

at java.lang.Thread.run(Thread.java:619)

CookieDemo2.java

package cn.itcast.cookie;

import java.io.IOException;

import java.io.PrintWriter;

import java.util.LinkedHashMap;

import java.util.Map;

import javax.servlet.ServletException;

import javax.servlet.http.Cookie;

import javax.servlet.http.HttpServlet;

import javax.servlet.http.HttpServletRequest;

import javax.servlet.http.HttpServletResponse;

public class CookieDemo2 extends HttpServlet {

public 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.輸出網站所有商品

out.write("本網站有如下商品：
");

Map map = Db.getAll();

for(Map.Entry entry:map.entrySet()){

Book book = entry.getValue();

out.print(""+book.getName()+"
");

}

//2.顯示用戶瀏覽過的商品

out.print("
您曾經瀏覽過的商品：
");

Cookie cookies[] = request.getCookies();

for(int i=0;cookies!=null && i

if(cookies[i].getName().equals("bookHistory")){

String ids[] = cookies[i].getValue().split("\\,");

for(String id:ids){

Book book = (Book) Db.getAll().get(id);

out.print(book.getName()+"
");

}

}

}

}

public void doPost(HttpServletRequest request, HttpServletResponse response)

throws ServletException, IOException {

doGet(request, response);

}

}

class Db{

private static Map map = new LinkedHashMap();

static{

map.put("1",new Book("1","javaweb開發","老張","一本好書！！"));

map.put("2",new Book("2","jdbc開發","老張","一本好書！！"));

map.put("3",new Book("3","spring開發","老黎","一本好書！！"));

map.put("4",new Book("4","struts開發","老畢","一本好書！！"));

map.put("5",new Book("5","android開發","老黎","一本好書！！"));

}

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;

}

}

CookieDemo3.java

package cn.itcast.cookie;

import java.io.IOException;

import java.io.PrintWriter;

import java.util.Arrays;

import java.util.LinkedList;

import javax.servlet.ServletException;

import javax.servlet.http.Cookie;

import javax.servlet.http.HttpServlet;

import javax.servlet.http.HttpServletRequest;

import javax.servlet.http.HttpServletResponse;

public class CookieDemo3 extends HttpServlet {

public 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.根據用戶帶過來的id，顯示相應商品的詳細信息

String id = request.getParameter("id");

Book book = (Book) Db.getAll().get(id);

out.write(book.getId()+"
");

out.write(book.getName()+"
");

out.write(book.getAuthor()+"
");

out.write(book.getDescription()+"
");

//構建cookie，回寫給瀏覽器

String cookieValue = buildCookie(id,request);

Cookie cookie = new Cookie("bookHistory",cookieValue);

cookie.setMaxAge(1*30*24*60*60);

cookie.setPath("/testHttp");

response.addCookie(cookie);

}

private String buildCookie(String id, HttpServletRequest request) {

String bookHistory = null;

Cookie cookies[] = request.getCookies();

for(int i=0;cookies!=null && i

if(cookies[i].getName().equals("bookHistory")){

bookHistory = cookies[i].getValue();

}

}

if(bookHistory==null){

return id;

}

LinkedList list = new LinkedList(Arrays.asList(bookHistory.split("//,")));

if(list.contains(id)){

list.remove(id);

}else{

if(list.size()>=3){

list.removeLast();

}

}

list.addFirst(id);

StringBuffer sb = new StringBuffer();

for(String bid : list){

sb.append(bid+",");

}

sb.deleteCharAt(sb.length()-1).toString();

return null;

}

public void doPost(HttpServletRequest request, HttpServletResponse response)

throws ServletException, IOException {

doGet(request, response);

}

}

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