Logger vs. System.out.println

Question

I'm using the PMD plugin for eclipse and it gives me an error when using System.out.println() with the explanation:

System.(out|err).print is used, consider using a logger.

My question is - What is a Logger? How is it used to print to the screen? Why is it better?

Answer 1

See this short introduction to log4j.

The issue is in using System.out to print debugging or diagnostic information. It is a bad practice because you cannot easily change log levels, turn it off, customize it, etc.

However if you are legitimately using System.out to print information to the user, then you can ignore this warning.

Answer 2

If you are using System.out|err.println(..) to print out user-information on console in your application's main()-method, you do nothing wrong. You can get rid of the message via inserting a comment "//NOPMD".

System.out.println("Fair use of System.out.println(..).");// NOPMD 

There is a "Mark as reviewed"-Option in the PMD-Violations Outline for this purpose.

Of course you can trick PMD with following code snippet:

PrintStream out=System.out; out.println("I am fooling PMD.");   

Outside of your main()-Method use a Log-System like eg Log4j.

UPDATE:

You can also modify the PMD-Rule "SystemPrintln" to use the following XPath:

//MethodDeclaration[@MethodName!="main"]//Name[ starts-with(@Image, 'System.out.print') or starts-with(@Image, 'System.err.print') ] | //Initializer//Name[ starts-with(@Image, 'System.out.print') or starts-with(@Image, 'System.err.print') ] 

This will ignore System.out.println etc in any method named 'main' in your code, but check for System.out.println in initializer code. I like this, because from my point of view, System.out.println is safe in method 'main(String args[])'. But use with caution, I have to check, where in the AST a System.out.println can occur also and have to adapt the XPath.