Question

Code is something like 

    2: <%
    3: String rolename = (String)session.getAttribute("roles");
    4: out.println("role is " + rolename);
    5: if(rolename.equals("MY_ROLE")){
    6: out.println("tension");
    7: }
    8: %>

问题是：第4行正确打印了rolename的值，即“MY_ROLE”，但我在第5行得到空指针异常，我假设因为rolename为null。

Error is as below :

org.apache.jasper.JasperException：在第10行处理JSP页面/WEB-INF/views/fragments/menu.jspf时发生异常

    7: <%
    8:    String rolename = (String)session.getAttribute("roles");
    9:    out.println("role is " + rolename);
    10:   if(rolename.equals("MY_ROLE")){
    11:       out.println("tension");
    12:   }
    13:%>

Stacktrace:
    org.apache.jasper.servlet.JspServletWrapper.handleJspException(JspServletWrapper.java:521)
    org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:430)
    org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:313)
    org.apache.jasper.servlet.JspServlet.service(JspServlet.java:260)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:717)
    org.springframework.web.servlet.view.InternalResourceView.renderMergedOutputModel(InternalResourceView.java:229)
    org.springframework.web.servlet.view.AbstractView.render(AbstractView.java:262)
    org.springframework.web.servlet.DispatcherServlet.render(DispatcherServlet.java:1180)
    org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:950)
    org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:852)
    org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:882)
    org.springframework.web.servlet.FrameworkServlet.doGet(FrameworkServlet.java:778)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:617)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:717)


root cause 

java.lang.NullPointerException
    org.apache.jsp.WEB_002dINF.views.pages.view_jsp._jspService(view_jsp.java:106)
    org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:70)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:717)
    org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:388)
    org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:313)
    org.apache.jasper.servlet.JspServlet.service(JspServlet.java:260)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:717)
    org.springframework.web.servlet.view.InternalResourceView.renderMergedOutputModel(InternalResourceView.java:229)
    org.springframework.web.servlet.view.AbstractView.render(AbstractView.java:262)
    org.springframework.web.servlet.DispatcherServlet.render(DispatcherServlet.java:1180)
    org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:950)
    org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:852)
    org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:882)
    org.springframework.web.servlet.FrameworkServlet.doGet(FrameworkServlet.java:778)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:617)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:717)

Please help.

Answer 1

执行String rolename = (String)session.getAttribute("roles");时，会话参数可以是null，也就是说，它可能从未设置过。

如果是这种情况，那么rolename现在将null和if(rolename.equals("MY_ROLE")){将抛出NullPointerException，因为您在null变量上使用点运算符，这基本上等同于执行{ {1}}。

有几种方法可以解决它：

null.equals("MY_ROLE")
if(rolename!=null && rolename.equals("MY_ROLE")){

也就是说，要么显式检查null，要么颠倒比较的顺序，并对字符串文字执行点运算符。

无法比较jsp scriptlet中的字符串值

1 个答案: