An error that has plagued me for a long time-it turned out to be a JSP connection to the MySQL database.

When running a JSP system, the following error is always reported:
HTTP status 500-

TypeException report

Message

Description The server encountered an internal error () that prevented it from fulfilling this request.

Exception

org.apache.jasper.JasperException
org.apache.jasper.servlet.JspServletWrapper.handleJspException(JspServletWrapper.java:512)
org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:395)
org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:314)
org.apache.jasper.servlet.JspServlet.service(JspServlet.java:264)
javax.servlet.http.HttpServlet.service(HttpServlet.java:802)

Root Cause

java.lang.NullPointerException
org.apache.jsp.first_jsp._jspService(first_jsp.java:98)
org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:97)
javax.servlet.http.HttpServlet.service(HttpServlet.java:802)
org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:334)
org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:314)
org.apache.jasper.servlet.JspServlet.service(JspServlet.java:264)
javax.servlet.http.HttpServlet.service(HttpServlet.java:802)

Note The full stack trace of the root cause is available in the Apache Tomcat/5.5.20 logs.

However, after refresh several times, or after a period of time, the system runs normally. After finding a lot of information, we finally got the result. The solution benefited from the following prompt:
1.

Quote: Java. Lang. nullpointerexception
Note the full stack trace of the root cause is available in the Tomcat logs.

View log
Check your JSP page
2. java. Lang. nullpointerexception is a runtime exception. It indicates that your code is accessing an object with a null value during running.
Org. Apache. jsp. pass_jsp. _ jspservice (pass_jsp.java: 66) indicates the location of the error statement.
Find the file pass_jsp.java In the tomcat installation directory to see what line 66th is.
3. after reading the code, I think con is null during running, so statement stmt = con. createstatement: The exception occurs. The reason con is null is probably connection con = yy. getconn (); here. That is to say, the getconn method of YY returns a null value. Solution: Determine the con before using con, or catch exceptions using try catch.

Later, I checked Tomcat's logs and found a stdout_20070104.log file in logs, which reported the log of the current day, which contains the following information:
Java. SQL. sqlexception: communication failure during handshake. Is there a server running on localhost: 3306?
Unable to load driver
Java. SQL. sqlexception: communication failure during handshake. Is there a server running on localhost: 3306?
Unable to load driver
Java. SQL. sqlexception: communication failure during handshake. Is there a server running on localhost: 3306?
Unable to load driver
Java. SQL. sqlexception: communication failure during handshake. Is there a server running on localhost: 3306?
Unable to load driver

After thinking about the database connection file opendb. Java, rewrite this class and write the code as follows:
Import java. SQL .*;
Import java. Io .*;

Public class opendb
{
String host = "localhost ";
String database = "netshop ";
String username = "root ";
String userpassword = "*****"; // The password username must be consistent with the password and username of the database in use.
Private string drivername = "org. gjt. Mm. MySQL. Driver ";
Private string url = "JDBC: mysql: //" + host + "/" + database + "? Useunicode = true & characterencoding = gb2312 ";

Connection dbconn;
Public opendb ()
{

}
Public connection getconnection ()
{
Try
{
Class. forname (drivername). newinstance ();
Dbconn = drivermanager. getconnection (URL, username, userpassword );
}
Catch (exception ex)
{
System. Out. println (ex. tostring ());
Dbconn = NULL;
System. Out. println ("unable to load driver ");
}

Return dbconn;
}

}

After exploration, it is found that the system will run normally every time the opendb. Java file is re-compiled after Tomcat is started or started.