Loggers should be declared to be static and final.
It is good programming practice to share a single logger object between all of the instances of a particular class and to use the same logger for the duration of the program.
The following statement errantly declares a non-static logger.
private final Logger logger =
Logger.getLogger(MyClass.class);
Multiple Loggers
It is a poor logging practice to use multiple loggers rather than logging levels in a single class.
Good logging practice dictates the use of a single logger that supports different logging levels for each class.
The following code errantly declares multiple loggers.
public class MyClass {
private final static Logger GOOD =
Logger.getLogger(MyClass.class);
private final static Logger BAD =
Logger.getLogger(MyClass.class);
private final static Logger UGLY =
Logger.getLogger(MyClass.class);
...
}
Use of a System Output Stream
Using System.out or System.err rather than a dedicated logging facility makes it difficult to monitor the behavior of the program. It can also cause log messages to accidentally be returned to the end users, revealing internal information to attackers.
The first Java program that a developer learns to write often looks like this:
public class MyClass
public static void main(String[] args) {
System.out.println("hello world");
}
}
While most programmers go on to learn many nuances and subtleties about Java, a surprising number hang on to this first lesson and never give up on writing messages to standard output using System.out.println().
The problem is that writing directly to standard output or standard error is often used as an unstructured form of logging. Structured logging facilities provide features like: Logging levels, uniform formatting, a logger identifier, timestamps, and perhaps most critically; the ability to direct the log messages to the right place. When the use of system output streams is jumbled together with code that uses loggers properly, the result is often a well-kept log that is missing critical information. In addition, using system output streams and can also cause log messages to accidentally be returned to end users, revealing an application’s internal information to attackers.
Developers widely accept the need for structured logging, but many continue to use system output streams in their “pre-production” development. If the code you are reviewing is past the initial phases of development, use of System.out or System.err may indicate an oversight in the move to a structured logging system.
