[LOG4J2-710] Documentation for Custom Levels and Custom Loggers

• Type: Bug
• Status: Resolved
• Priority: Major
• Resolution: Fixed
• Affects Version/s: 2.0
• Fix Version/s: 2.0.1
• Component/s: Documentation
• Assignee: Remko Popma
• Reporter: Remko Popma
• Created: 11/Jul/14 16:24
• Updated: 09/Apr/15 19:55
• Resolved: 18/Jul/14 03:19

Description

Documentation preview. Feedback welcome.
Side-nav menu:

• Custom Log Levels
  • Adding or Replacing Levels
  • Generating Logger Wrappers
  • Example Usage
  • Tool Usage

Custom Loggers page:

Custom Log Levels
Log4J 2 supports custom log levels. It is possible to log messages at any self-defined custom level by passing this level to the Logger.log() method:

final Logger logger = LogManager.getLogger();
final Level VERBOSE = Level.forName("VERBOSE", 550);

logger.log(VERBOSE, "a verbose message");
logger.log(VERBOSE, "another message");

When defining a custom log level, the intLevel parameter (550 in the example above) determines where the custom level exists in relation to the standard levels built in to Log4J 2. For reference, the table below shows the intLevel of the built-in log levels.

Standard Level	intLevel
OFF	0
FATAL	100
ERROR	200
WARN	300
INFO	400
DEBUG	500
TRACE	600
ALL	Integer.MAX_VALUE

The Standard Logger Interface
The built-in levels have a set of convenience methods on the Logger interface that makes them easier to use. For example, the Logger interface has 14 debug methods that support the DEBUG level:

// convenience methods for the built-in DEBUG level
debug(Marker, Message)
debug(Marker, Message, Throwable)
debug(Marker, Object)
debug(Marker, Object, Throwable)
debug(Marker, String)
debug(Marker, String, Object...)
debug(Marker, String, Throwable)
debug(Message)
debug(Message, Throwable)
debug(Object)
debug(Object, Throwable)
debug(String)
debug(String, Object...)
debug(String, Throwable)

Similar method sets exist for the other built-in levels.

Adding or Replacing Levels

It would be nice to have the same ease of use with custom levels, so that after declaring a custom VERBOSE level, we would be able to use code like this:

logger.verbose("a verbose message"); // no need to pass the VERBOSE level as a parameter
logger.verbose("another message");

In the above example, a convenience method was added to the Logger interface, in addition to the existing trace(), debug(), info(), ... methods for the built-in log levels.

There is another use case, Domain Specific Language loggers, where we want to replace the existing trace(), debug(), info(), ... methods with all-custom methods.

For example, for medical devices we could have only critical(), warning(), and advisory() methods. Another example could be a game that has only defcon1(), defcon2(), and defcon3() levels.

If it were possible to hide existing log levels, users could customize the Logger interface to match their requirements. Some people may not want to have a FATAL or a TRACE level, for example. They would like to be able to create a custom Logger that only has debug(), info(), warn() and error() methods.

Generating source code for a Logger wrapper
Common Log4J usage is to get an instance of the Logger interface from the LogManager and call the methods on this interface. However, the custom log Levels are not known in advance, so Log4J cannot provide an interface with convenience methods for these custom log Levels.

To solve this, Log4J ships with a tool that generates source code for a Logger wrapper. The generated wrapper class has convenience methods for each custom log level, making custom levels just as easy to use as the built-in levels.

There are two flavors of wrappers: ones that extend the Logger API (adding methods to the built-in levels) and ones that customize the Logger API (replacing the built-in methods).

When generating the source code for a wrapper class, you need to specify:

• the fully qualified name of the class to generate
• the list of custom levels to support and their intLevel relative strength
• whether to extend Logger (and keep the existing built-in methods) or have only methods for the custom log levels

You would then include the generated source code in the project where you want to use custom log levels.

Example Usage of a Generated Logger Wrapper
Here is an example of how one would use a generated logger wrapper for custom levels DIAG, NOTICE and VERBOSE:

// ExtLogger is a generated logger wrapper
import com.mycompany.myproject.ExtLogger;

public class MyService {
    // instead of Logger logger = LogManager.getLogger(MyService.class):
    private static final ExtLogger logger = ExtLogger.create(MyService.class);
    
    public void someMethod() {
        // ...
        logger.trace("the built-in TRACE level");
        logger.verbose("a custom level: a VERBOSE message");
        logger.debug("the built-in DEBUG level");
        logger.notice("a custom level: a NOTICE message");
        logger.info("the built-in INFO level");
        logger.diag("a custom level: a DIAG message");
        logger.warn("the built-in WARN level");
        logger.error("the built-in ERROR level");
        logger.fatal("the built-in FATAL level");
        // ...
    }
    ...
}

Generating Extended Loggers

Use the following command to generate a wrapper that adds methods to the built-in ones:

java -cp log4j-core-2.0.jar org.apache.logging.log4j.core.tools.Generate$ExtendedLogger com.mycomp.ExtLogger DIAG=350 NOTICE=450 VERBOSE=550 > com/mycomp/ExtLogger.java

This will generate source code for a logger wrapper that has the convenience methods for the built-in levels as well as the specified custom levels. The tool sends the generated source code to the console, so we redirected the output to a file.

Generating Custom Loggers

Use the following command to generate a wrapper that hides the built-in levels and has only custom levels:

java -cp log4j-core-2.0.jar org.apache.logging.log4j.core.tools.Generate$CustomLogger com.mycomp.MyLogger DEFCON1=350 DEFCON2=450 DEFCON3=550 > com/mycomp/MyLogger.java

This will generate source code for a logger wrapper that has only convenience methods for the specified custom levels, not for the built-in levels. The tool sends the generated source code to the console, so we redirected the output to a file.

Issue Links

• supercedes: LOG4J2-519 Custom/Extended Loggers