OAF - Write Diagnostic statements for debug Purposes:

--OAF - Write Diagnostic statements for debug Purposes:
From your contorller :
import oracle.apps.fnd.framework.OAFwkConstants;

String testmsg = "This is a test diagnostic message";
pageContext.writeDiagnostics(this,"TESTDIAG: = " + testmsg ,OAFwkConstants.STATEMENT);

Then set profile option at user level
FND: Diagnostics - set to Yes
FND: Debug Log Enabled - set to Yes --default to NO
FND: Debug Log Level - set to Statement
FND: Debug Log Filename for Middle-Tier - give a path to a log file on the server. (test.log)
FND: Debug Log Level ---%

run the application and search for your diagnostic message in test.log file.