Loggers should:

• private: never be accessible outside of its parent class. If another class needs to log something, it should instantiate its own logger.
• static: not be dependent on an instance of a class (an object). When logging something, contextual information can of course be provided in the messages but the logger should be created at class level to prevent creating a logger along with each object.
• (final|const): be created once and only once per class.

Noncompliant Code Example

public Logger logger = LoggerFactory.getLogger(Foo.class);  

Compliant Solution

private static final Logger LOGGER = LoggerFactory.getLogger(Foo.class);