Loggers expose the interface that application code directly uses.
Handlers send the log records (created by loggers) to the appropriate destination.
Filters provide a finer grained facility for determining which log records to output.
Formatters specify the layout of log records in the final output.
Multiple calls to with the same name will return a reference to the same logger object.
bug mutiple call to addHandler for same logger will cause duplicate entries in log file. To solve the problem , make sure only one addHandler to on logger name. in other way, set propagate to false in logger config save the work too.
