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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:
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.
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:
Similar method sets exist for the other built-in 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:
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.
Use the following command to generate a wrapper that adds methods to the built-in ones:
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.
Use the following command to generate a wrapper that hides the built-in levels and has only custom levels:
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.