JSP錯誤java.lang.NullPointerException

Question

我正在使用if {.. return} else語句制作一個簡單的jsp頁面，用於處理來自表單的用戶輸入並得到以下錯誤，請告訴如何解決此錯誤？

org.apache.jasper.JasperException: An exception occurred processing JSP page 
/create_user.jsp at line 21

18:         String username=request.getParameter("username");
19:         String password=request.getParameter("password");
20:         String email=request.getParameter("e-mail");
21:         if(username.length()<2 || password.length()<6 || email.length()
            <5){
22:             out.print("Username must be atleast 2 characters long, 
Password must be atleast 6 characters long, E-mail must be atleast 5 
characters long");
23:             return;
24:         }else{


Stacktrace:
org.apache.jasper.servlet.JspServletWrapper.handleJspException(JspServletWra
pper.java:568)
org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:4
70)
org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:395)
org.apache.jasper.servlet.JspServlet.service(JspServlet.java:339)
javax.servlet.http.HttpServlet.service(HttpServlet.java:727)
org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)

root cause

java.lang.NullPointerException
org.apache.jsp.create_005fuser_jsp._jspService(create_005fuser_jsp.java:81)
org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:70)
javax.servlet.http.HttpServlet.service(HttpServlet.java:727)
org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:4
32)
org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:395)
org.apache.jasper.servlet.JspServlet.service(JspServlet.java:339)
javax.servlet.http.HttpServlet.service(HttpServlet.java:727)
org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)

Answer 1

我懷疑您的表單“電子郵件”字段的參數名稱是“ 電子郵件”而不是“電子郵件”。

於是

String email=request.getParameter("e-mail"); --> This will return null

和

email.length() will throw Null Pointer Exception

我個人遵循的優良作法是在訪問之前始終進行Null檢查。