Implementing Logging with a Mixin in Java

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Implementing Logging with a Mixin
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An example of a feature that could usefully be incorporated in many different classes is logging Logging is commonly performed by creating an instance of the javautilloggingLogger class and invoking one of its log() methods when something needs to be recorded Here, for example, is an example of a class that uses this technique to log the fact that its init block has been called:
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1 2 3 4 5 6 7 package javafxclasses; import javautilloggingLevel; import javautilloggingLogger; public class LoggedClass { var level = LevelWARNING;
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var logger = LoggergetLogger("javafxclasses"); init { loggerlog(level, "In init"); } }
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The logger that the class will use is created on line 8The Logging API allows you to create multiple named loggers that can be individually configured to write output to different locations It is common practice for every class in a package to share the same loggerWe use that pattern here by supplying the name of the owning package as the name of the logger when it is created on line 8The logger itself is used on line 10 to record the fact that the init block has been invoked Here is the result of running this example:
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26-May-2010 20:22:40 javafxclassesLoggedClass userInit$ WARNING: In init
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There is nothing especially complicated about this pattern, but there is a downside to it every class that wants to log something has to create a Logger instance and needs to adhere to the rule that the logger should be named after the package that owns that class It would be better if this functionality could be encapsulated so that the class itself could rely on the presence of a logger variable that refers to a correctly configured loggerThis is an ideal case for using a mixin because
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It is a well-defined task that can be completely implemented in a separate class The functionality can usefully be used in many classes that are otherwise unrelated (and, in particular, do not have a common base class where this code could otherwise be placed)
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The implementation of this functionality as a mixin is shown in Listing 11-5
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Listing 11-5
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Implementing Logging in a Mixin Class
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package javafxclasses; import javalangThrowable; import javautilloggingLevel; import javautilloggingLogger; public mixin class Logging { protected var loggerName = getPackageName(); protected var logger = LoggergetLogger(loggerName); public var level = LevelINFO; public function isLogging():Boolean { level != LevelOFF; } protected function log(level:Level, text:String) { loggerlog(level, text);
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} protected function log(level:Level, text:String, t:Throwable) { loggerlog(level, text, t); } protected function log(text:String) { loggerlog(level, text); } protected function log(text:String, t:Throwable) { loggerlog(level, text, t); } public override function toString():String { "Logging mixin, log level is {level}" } function getPackageName():String { var className = getClass()getName(); var index = classNamelastIndexOf(""); return if (index == -1) "default" else classNamesubstring(0, index); } }
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Apart from the word mixin in the class declaration on line 7, this is just like any other class definitionThis class declares three variables and a set of functions that use them:
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The loggerName variable is the name of the Logger that will be used It is set to the name of the package of the hosting class In a mixin, as in any other class, the getClass() method returns the Class object for the actual class of which the mixin is a part, not that of the mixin class itself In the getPackageName() method, we strip off the trailing part of the classname to leave only the name of the package7 The logger variable is the actual Logger that the mixin functions will use The level variable is the default level at which log entries will be written by those functions that do not have an explicit level parameterThis variable has a default value of LevelINFO, but is public and can be changed by application code at any time
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The expression getClass()getPackage()getName() returns the same value, but under some circumstances, the getPackage() method returns null, so we get the package name by string manipulation instead