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From "Remko Popma (JIRA)" <j...@apache.org>
Subject [jira] [Updated] (LOG4J2-519) Custom/Extended Loggers
Date Wed, 02 Jul 2014 15:24:25 GMT

     [ https://issues.apache.org/jira/browse/LOG4J2-519?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel

Remko Popma updated LOG4J2-519:

    Attachment:     (was: Generate.java)

> Custom/Extended Loggers
> -----------------------
>                 Key: LOG4J2-519
>                 URL: https://issues.apache.org/jira/browse/LOG4J2-519
>             Project: Log4j 2
>          Issue Type: New Feature
>          Components: API
>    Affects Versions: 2.0-rc1
>            Reporter: Remko Popma
> Please try out the attached [^Generate.java] tool that provides the functionality described
below and give feedback if you notice any issues.
> {anchor:Extending}
> *[#Extending] the Logger interface*
> LOG4J2-41 added support for custom log Levels. Users can now log messages at the new
custom level by passing the custom level to the {{Logger.log()}} method:
> {code}
> final Logger logger = LogManager.getLogger();
> final Level VERBOSE = Level.forName("VERBOSE", 550);
> logger.log(VERBOSE, "a verbose message");
> logger.log(VERBOSE, "another message");
> {code}
> However, custom levels are not as easy to use as the built-in levels: one needs to call
the generic {{log()}} method and pass a {{Level}} parameter. Built-in levels on the other
hand have a set of convenience methods on the Logger interface. For example, the Logger interface
has 14 debug methods that support the DEBUG level:
> {code}
> 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)
> {code}
> Similar method sets exist for the other built-in levels.
> Several people have expressed the desire to have the same ease of use with custom levels,
so after declaring a custom VERBOSE level, we would like to be able to use code like this:
> {code}
> logger.verbose("a verbose message"); // no need to pass the VERBOSE level as a parameter
> logger.verbose("another message");
> {code}
> {anchor:Customizing}
> *[#Customizing] the Logger interface*
> In the above use case, convenience methods were _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.
> Finally, 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.
> {anchor:wrapper}
> *Proposal: Generate 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. This makes it hard to achieve the above customization;
especially taking away existing methods is not possible.
> An alternative is for the user to create a wrapper class that exposes only the convenience
methods for the desired log levels. When _extending_ the {{Logger}} API (_adding_ methods),
this wrapper class could subclass {{org.apache.logging.log4j.spi.AbstractLoggerWrapper}}.
When _customizing_ the {{Logger}} API (_removing_ built-in methods), the wrapper class would
simply not extend AbstractLoggerWrapper, so the only public methods would be the methods for
the custom log levels.
> As the custom log Levels are not known in advance, Log4J cannot provide pre-built wrapper
classes for these custom log Levels. However, we don't want to ask the users to hand-code
such a wrapper class: this is cumbersome and error-prone: there are 14 methods for each built-in
level. To provide comparable functionality for custom log Levels one would need to provide
14 methods for each custom log Level.
> The proposal is to solve this by providing a tool that generates the source code for
a wrapper class. The user can 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
> and the tool generates the source code for the wrapper class that has exactly the required
methods. Users would include this source code in the project where they want to use custom
log levels.
> {anchor:GeneratedWrapperUsage}
> Note that no Log4J API changes are required to support this functionality.
> Users would create instances of the wrapper by calling a factory method on the wrapper
class, instead of calling the {{LogManager.getLogger()}} methods.
> For example, instead of writing:
> {code}
> import org.apache.logging.log4j.Level;
> import org.apache.logging.log4j.LogManager;
> import org.apache.logging.log4j.Logger;
> final Logger logger = LogManager.getLogger(MyClass.class); // standard log4j logger
> final Level VERBOSE = Level.forName("VERBOSE", 550);
> logger.log(VERBOSE, "a verbose message");
> {code}
> users would instead write:
> {code}
> // MyLogger is a generated customized logger wrapper
> import com.mycompany.myproject.MyLogger;
> final MyLogger logger = MyLogger.create(MyClass.class);
> logger.verbose("a verbose message");
> {code}
> Creating an extended or customized Logger is as easy as creating a standard Logger (one
line of code). Extended Loggers are drop-in replacements for standard Loggers - they only
add more convenience methods.

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