创建一个使用 cookie 来存储用户访问次数的 servlet？-这就是我试图解决的问题
这是我的 html
<html>
<head>
</head>
<body>
<form action="NewServlet">
<input type="text" name="text1">
<input type="submit"/>
</form>
</body>
</html>
这是NewServlet.java
public class NewServlet extends HttpServlet {
public void service(HttpServletRequest req,HttpServletResponse res)throws
IOException,ServletException
{
res.setContentType("text/html");
Cookie[] cook=req.getCookies();
String s=req.getParameter("text1");
int a=0;
if(cook.length>0)
{
for(Cookie cookie :cook)
{
if(cookie.getName().equals(s))
{
int b=Integer.parseInt(cookie.getValue());
b=b+1;
cookie.setValue(String.valueOf(b));
a++;
}
}
}
if(a>0)
{
res.sendRedirect("NewServlet1");
}
else
{
Cookie cok=new Cookie(s,"1");
res.addCookie(cok);
req.setAttribute("user", s);
RequestDispatcher r=req.getRequestDispatcher("NewServlet1");
r.forward(req,res);
}
}
}
这是 NewServlet1.java
public class NewServlet1 extends HttpServlet {
protected void service(HttpServletRequest req, HttpServletResponse res)
throws ServletException, IOException {
res.setContentType("text/html;charset=UTF-8");
Cookie[] cookie=req.getCookies();
String s=String.valueOf(req.getAttribute("user"));
for(Cookie cok:cookie)
{
if(cok.getName().equals(s))
{
int a=Integer.parseInt(cok.getValue());
res.getWriter().println("number of times visited"+a);
}
}
}
}
我正在做的是检查 cookie 是否存在，如果不存在，则在 NewServlet.java 上创建 cookie。但是我收到了空指针异常(错误 500) 单击提交按钮时
堆栈跟踪
at First.NewServlet.service(NewServlet.java:28)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:729)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:291)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:239)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
at org.netbeans.modules.web.monitor.server.MonitorFilter.doFilter(MonitorFilter.java:393)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:239)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:217)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:106)
at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:502)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:142)
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:79)
at org.apache.catalina.valves.AbstractAccessLogValve.invoke(AbstractAccessLogValve.java:616)
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:88)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:518)
at org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1091)
at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:673)
at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1500)
at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.run(NioEndpoint.java:1456)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)
at java.lang.Thread.run(Thread.java:748)
最佳答案
将 if(cook.length>0) 更改为 if(cook !=null && cook.length > 0)。这样，如果 cook 为 null，则它不会检查 .length，从而避免 NPE
关于java - Servet - 为什么我会收到空指针异常？，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/52847253/