Tech Per

Today I was battling an issue for some of my colleagues. They were getting this exception from JBoss/Mysql ConnectorJ driver:

java.sql.SQLException: Ping failed: java.lang.IllegalArgumentException: object is not an instance of declaring class

in the JBoss console.

A look at the stacktrace told me, that it was coming from inside the method com.mysql.jdbc.integration.jboss.MysqlValidConnectionChecker.isValidConnection, so naturally, I dived into the sources for that file (aah, the wonders of open source). This is from the 3.1.14 version of ConnectorJ:

	public MysqlValidConnectionChecker() {
		....
			Class mysqlConnection = Thread.currentThread()
					.getContextClassLoader().loadClass(
							"com.mysql.jdbc.Connection");
			pingMethod = mysqlConnection.getMethod("ping", null);
		....
	}

	public SQLException isValidConnection(Connection conn) {
		if (pingMethod != null) {
			try {
				this.pingMethod.invoke(conn, NO_ARGS_OBJECT_ARRAY);
				return null;
			} catch (Exception ex) {
				if (ex instanceof SQLException) {
					return (SQLException) ex;
				}

				return new SQLException("Ping failed: " + ex.toString());
			}
		}
        ...
	}

The interesting part is the this.pingMethod.invoke(conn, NO_ARGS_OBJECT_ARRAY) call, which was failing. Sadly, if you look at the catch block, it does not preserve or log the stacktrace of that exception (the ex instance), so I had to built a version of the driver myself.

After some trials, what I found was, that the Connection class, that is loaded in the constructor (the class from which the cached Method instance pingMethod is obtained), was coming from a different classloader (a webapp classloader), than the classloader that loaded the conn instance coming in as parameter in the call to isValidConnection. This made the reflection call bomb out.

Why the classloader was different I believe had to do with the way JBoss was started in our setup. It was being started from inside IDEA, which seemed to include the classpath of all the modules of the project.

A simple fix for the driver would be to do this:

   Method method = conn.getClass().getMethod("ping", null);
   method.invoke(conn, NO_ARGS_OBJECT_ARRAY);

instead of caching the Method instance.

But, who would want to patch the mysql-driver, when we can upgrade?

In ConnectorJ v5.1.x, this has been completely rewritten, to simply do SELECT 1 against the server, instead of trying the ping call on the connection. In v.5.0.x, there is a slightly altered version, though it has the same problem, as the v3.x does.

One other way to avoid the bug, if you are unable to upgrade the driver version, is to disable the connection-checker altogether. You do this by editing your datasource definition deployed in JBoss (typically named mysql-ds.xml) and remove/comment out the element:

   <valid-connection-checker-class-name>com.mysql.jdbc.integration.jboss.MysqlValidConnectionChecker</valid-connection-checker-class-name>