Tomcat reports exception org. apache. catalina. core. standardwrappervalve invoke

First time
This error is the most depressing, because there is no syntax error at all, and everything is correct,
Req. getSession (). setAttribute ("isaudit", audit );
Req. getSession (). setAttribute ("notaudit", notaudit );
Req. getRequestDispatcher ("proposer/applyHistory. jsp"). forward (req, resp );

I send messages through the session on the servlet end, and the other party receives the messages. It seems that there is no error at all, that is, the above exception will occur,
Finally, I searched for it online. As a result, I changed the first parameter name isaudit in setAttribute, which is difficult to find.

Online statement:
Org. apache. catalina. core. StandardWrapperValve invoke
Today, I wrote the code and encountered this exception several times in succession. Debug takes a few hours to figure it out, dizzy.
I found some pieces to make a summary on the Internet, but it is not guaranteed to be accurate. I will write it down for your reference only.
The following are error causes:
1. In servlet or jsp, call the getOutputStream () and response. getWriter () Methods of response repeatedly.
2. The form does not contain the method = "post" action = "" parameter
3. Check the JDBC database Code. For example, the property of the called object is null.
Connection conn = null; then, conn.? is called .?? Method
I am wrong here.
For reference only!

 

Second
My error is that the SQL statement is incorrectly written.