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 importance.

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 categories. Using a category 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 OutputStream, a java.io.Writer, a remote log4j server, a remote Unix Syslog daemon or even a NT Event logger.

On a 233 MHz ThinkPad running JDK 1.1.7B, it costs about 46 nanoseconds to determine if that statement should be logged or not. Actual logging is also quite fast, ranging from 79 microseconds using the SimpleLayout, 164 microseconds using the TTCCLayout and around a millisecond when printing exceptions. The performance of the PatternLayout is almost as good as the dedicated layouts, except that it is a lot more flexible.

The package is being constantly improved thanks to input from users and code contributed by authors in the community.