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# :stopdoc:
#
# It happens sometimes that it is very expensive to construct a logging
# message; for example, if a large object structure has to be traversed
# during execution of an `object.to_s` method. It would be convenient to
# delay creation of the message until the log event actually takes place.
#
# For example, with a logger configured only to show WARN messages and higher,
# creating the log message for an INFO message would be wasteful. The INFO log
# event would never be generated in this case.
#
# Log message creation can be performed lazily by wrapping the expensive
# message generation code in a block and passing that to the logging method.
require 'logging'
Logging.logger.root.appenders = Logging.appenders.stdout
Logging.logger.root.level = :info
# We use this dummy method in order to see if the method gets called, but in practice,
# this method might do complicated string operations to construct a log message.
def expensive_method
puts "Called!"
"Expensive message"
end
log = Logging.logger['Lazy']
# If you log this message the usual way, expensive_method gets called before
# debug, hence the Logging framework has no chance to stop it from being executed
# immediately.
log.info("Normal")
log.debug(expensive_method)
# If we put the message into a block, then the block is not executed, if
# the message is not needed with the current log level.
log.info("Block unused")
log.debug { expensive_method }
# If the log message is needed with the current log level, then the block is of
# course executed and the log message appears as expected.
log.info("Block used")
log.warn { expensive_method }
# :startdoc:
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