Inserting log statements into your code is a low-tech method
for debugging it. It may also be the only way because
debuggers are not always available or applicable. This is
often the case for distributed applications.
On the other hand, some people argue that log statements
pollute source code and decrease legibility. (We believe that
the contrary is true). In the Java language where a
preprocessor is not available, log statements increase the
size of the code and reduce its speed, even when logging is
turned off. Given that a reasonably sized application may
contain thousands of log statements, speed is of particular
With log4j it is possible to enable logging at runtime
without modifying the application binary. The log4j package is
designed so that these statements can remain in shipped code
without incurring a heavy performance cost. Logging behavior
can be controlled by editing a configuration file, without
touching the application binary.
Logging equips the developer with detailed context for
application failures. On the other hand, testing provides
quality assurance and confidence in the application. Logging
and testing should not be confused. They are
complementary. When logging is wisely used, it can prove to be
an essential tool.
One of the distinctive features of log4j is the notion of
inheritance in loggers. Using a logger
hierarchy it is possible to control which log
statements are output at arbitrarily fine granularity but also
great ease. This helps reduce the volume of logged output and
minimize the cost of logging.
The target of the log output can be a file, an
remote log4j server, a remote Unix Syslog daemon, or even a NT
Event logger among many other output targets.
On an AMD Duron clocked at 800Mhz running JDK 1.3.1, it costs
about 5 nanoseconds to determine if a logging statement should
be logged or not. Actual logging is also quite fast, ranging
from 21 microseconds microseconds using the SimpleLayout, 37
microseconds using the TTCCLayout. The performance of the
PatternLayout is almost as good as the dedicated layouts,
except that it is much more flexible.
The package is being constantly improved thanks to input from
users and code contributed by authors in the community.