I tried to turn on logging for SQL warnings when using Hibernate (4.3.6.Final) with MySql.
When they did not show up, I debugged through the code and found the condition check is incorrect as shown below.

Class: org.hibernate.engine.jdbc.spi.SqlExceptionHelper
Method: walkWarnings

Code:

248	public void walkWarnings(
249			SQLWarning warning,
250			WarningHandler handler) {
251		if ( warning == null || handler.doProcess() ) {
252			return;
253		}
254		handler.prepare( warning );
255		while ( warning != null ) {
256			handler.handleWarning( warning );
257			warning = warning.getNextWarning();
258		}
259	}

The condition handler.doProcess() returns true when the log level is at WARN. And yet, the method returns. I believe the condition should be "!handler.doProcess()".