svn commit: r1111 - in logback/trunk: logback-examples/src/main/java/chapter3 logback-examples/src/main/java/joran logback-site/src/site/xdocTemplates logback-site/src/site/xdocTemplates/manual
Author: seb Date: Mon Dec 18 14:50:48 2006 New Revision: 1111 Added: logback/trunk/logback-examples/src/main/java/chapter3/ - copied from r1098, /logback/trunk/logback-examples/src/main/java/joran/ logback/trunk/logback-site/src/site/xdocTemplates/manual/joran.xml Removed: logback/trunk/logback-examples/src/main/java/joran/ Modified: logback/trunk/logback-site/src/site/xdocTemplates/documentation.xml logback/trunk/logback-site/src/site/xdocTemplates/manual/index.xml Log: Renamed joran directory to chapter3. Copied joran.xml to the manual directory Modified: logback/trunk/logback-site/src/site/xdocTemplates/documentation.xml ============================================================================== --- logback/trunk/logback-site/src/site/xdocTemplates/documentation.xml (original) +++ logback/trunk/logback-site/src/site/xdocTemplates/documentation.xml Mon Dec 18 14:50:48 2006 @@ -26,9 +26,6 @@ and Tomcat</a> </li> <li> - <a href="joran.html">A introduction to Joran</a> - </li> - <li> <a href="faq.html">A Frequently Asked Questions list (FAQ)</a> </li> <li> Modified: logback/trunk/logback-site/src/site/xdocTemplates/manual/index.xml ============================================================================== --- logback/trunk/logback-site/src/site/xdocTemplates/manual/index.xml (original) +++ logback/trunk/logback-site/src/site/xdocTemplates/manual/index.xml Mon Dec 18 14:50:48 2006 @@ -61,6 +61,10 @@ <ul> <li><p> + <a href="joran.html"><b>Chapter 3: Logback configuration with Joran</b></a> + </p></li> + + <li><p> <a href="appenders.html"><b>Chapter 4: Appenders</b></a> </p></li> Added: logback/trunk/logback-site/src/site/xdocTemplates/manual/joran.xml ============================================================================== --- (empty file) +++ logback/trunk/logback-site/src/site/xdocTemplates/manual/joran.xml Mon Dec 18 14:50:48 2006 @@ -0,0 +1,449 @@ +<?xml version="1.0"?> +<document> + <!-- + + Warning: do not use any auto-format function on this file. + Since "source" divs use pre as white-space, it affects the + look of the code parts in this document. + + --> +<body> + <h2>Chapter 3: Logback configuration with Joran</h2> + <div class="author"> + Authors: Ceki Gülcü, Sébastien Pennec + </div> + +<p>Joran stands for a cold north-west wind which, every now and then, +blows force-fully on Lake Leman, a.k.a lake Geneva. Located right in +the middle of Europe, the Leman happens to be the continent's largest +sweet water reserve. +</p> + +<h2>Introduction</h2> + +<p>For it's configuration, logback relies on Joran, a +mature, flexible and powerful configuration framework. Many of the +capabilities offered by logback modules are possible thanks to Joran. +</p> + +<p>Joran is actually a generic configuration system which can be used +independently of logging. To emphaises this point, we should mention +that the logback-core module does not have a notion of loggers. In +that sprit, many of the examples related to this tutorial, have +nothing to do with loggers, appenders or layouts. +</p> + +<p>The examples for this tutorial can be found under +<em>LOGBACK_HOME/logback-examples/src/main/java/chapter3</em>. +</p> + +<p>To install joran, simply <a href="download.html">download</a> +logback and add <em>logback-core-VERSION.jar</em> to your classpath.</p> + +<h2>Historical perspective</h2> + +<p>One of the most powerful features of the Java language is +reflection. Reflection makes it possible to configure software systems +declaratively. For example, many important properties of an EJB are +configured with the <em>ejb.xml</em> file. While EJBs are written in Java, many +of their properties are specified within the <em>ejb.xml</em> file. Similarly, +logback settings can be specified in a configuration file, expressed +in XML format. +</p> + +<p>In log4j, logback's predecessor, <code>DOMConfigurator</code> that +shipped with log4j version 1.2.x can parse configuration files written +in XML. The <code>DOMConfigurator</code> was written in a way that +forced to tweak it each time the structure of the configuration file +changed. The modified code had to be recompiled and redeployed. Just +as importantly, the code of the DOMConfigurator consists of loops +dealing with children elements containing many interspersed if/else +statements. One could not help but notice that that particular code +reeked of redundancy. The <a +href="http://jakarta.apache.org/commons/digester/">digester +project</a> has shown that it is possible to parse XML files using +pattern matching rules. At parse time, digester will apply the rules +that match previously stated patterns. Rule classes are usually quite +small and specialized. Consequently, they are relatively easy to +understand and to maintain. +</p> + +<p>Joran is heavily inspired by the commons-digester project but uses +a slightly different terminology. In commons-digester, a rule can be +seen as consisting of a pattern and a rule, as shown by the +<code>Digester.addRule(String pattern, Rule rule)</code> method. We +find it unnecessarily confusing to have a rule to consist of itself, +not recursively but with a different meaning. In Joran, a rule +consists of a pattern and an action. An action is invoked when a match +occurs for the corresponding pattern. This relation between patterns +and actions lies at the core of Joran. Quite remarkably, one can deal +with quite complex requirements by using simple patterns, or more +precisely with exact matches and wildcard matches. For example, the +pattern "a/b" will match a <code><b></code> element nested within +an <code><a></code> element but not a <code><c></code> element, +even if nested within a <code><b></code> element. It is also +possible to match a particular XML element, regardless of its nesting +level, by using the "*" wildcard character. For example, the pattern +"*/a" will match an <code><a></code> element at any nesting +position within the document. Other types of patterns, for example +"a/*", are not currently supported by Joran. +</p> + +<h2>SAX or DOM?</h2> + +<p>Due to the event-based architecture of the SAX API, a tool based on +SAX cannot easily deal with forward references, that is, references to +elements which are defined later than the current element being +processed. Elements with cyclical references are equally +problematic. More generally, the DOM API allows the user to perform +searches on all the elements and make forward jumps. +</p> + +<p>This extra flexibility initially led us to choose the DOM API as +the underlying parsing API for Joran. After some experimentation, it +quickly became clear that dealing with jumps to distant elements while +parsing the DOM tree did not make sense when the interpretation rules +were expressed in the form of patterns and actions. <em>Joran only +needs to be given the elements in the XML document in a sequential, +depth-first order.</em> +</p> + +<p>Joran was first implemented in DOM. However, the author migrated to +SAX in order to benefit form the location information provided to the +user, that is, to an <code>org.w3.sax.ContentHandler</code>. With the +help of location information, it becomes possible to display essential +error reports to the user which include exact line and column. This +extra information turns out to be handy in hunting down problems. +</p> + + +<h2>Actions</h2> + +<p>Actions extend the +<code>ch.qos.logback.core.joran.action.Action</code> class which +consists of the following abstract methods. +</p> + + +<div class="source"><pre>package ch.qos.logback.core.joran.action; + +import org.xml.sax.Attributes; +import ch.qos.logback.core.joran.spi.ExecutionContext; + +public abstract class Action { + + + /** + * Called when the parser first encounters an element. + */ + public abstract void begin(ExecutionContext ec, + String name, + Attributes attributes); + + /** + * Called when the parser encounters the element end. At + * this stage, we can assume that child elements, if any, + * have been processed. + */ + public abstract void end(ExecutionContext ec, String name); +}</pre></div> + +<p>Thus, every action must implement the begin and end methods.</p> + + +<h2>Execution context</h2> + +<p>To allow various actions to collaborate, the invocation of begin +and end methods include an execution context as the first +parameter. The execution context includes an object stack, an object +map, an error list and a reference to the Joran interpreter invoking +the action. Please see the +<code>ch.qos.logback.core.joran.spi.ExecutionContext</code> class for +the exact list of fields contained in the execution context. +</p> + +<p>Actions can collaborate together by fetching, pushing or popping +objects from the common object stack, or by putting and fetching keyed +objects on the common object map. Actions can report any error +conditions by adding error items on the execution context's +<code>StatusManager</code>. +</p> + +<a name="helloWorld" /> +<h3>A hello world example</h3> + +<p>The <em>logback-examples/src/main/java/joran/helloWorld/</em> directory includes a +trivial action and Joran interpreter setup which just diaplays "Hello +World" when a <hello-world> element is encountered in an XML file. +It also includes the basic steps which are +necessary to set up and invoke a Joran interpreter. +</p> +<p> +The <em>hello.xml</em> file contains only one element, without any +other nested elements. The <code>HelloWorldAction</code> class is +a trivial implementation: it only prints "Hello World" in the console when +it's <code>begin()</code> method is called. +</p> +<p> +<code>HelloWorld</code> is a class that sets up the Joran interpreter, +with the minimal steps necessary: +</p> + +<ul> + <!-- Pretty dirty, thanks site-generation... --> + <p>It creates a <code>RuleStore</code> and a <code>Context</code></p> + <p>It adds the <em>hello-world</em> pattern, with it's corresponding action</p> + <p>It creates a Joran interpreter, and passes the <code>RuleStore</code></p> + <p>It creates a SAX parser and parses the given file, specifying the newly created + Joran interpreter as the <code>ContentHandler</code></p> +</ul> + +<p> +It's last step is to print the content of the <code>Context</code>. +Since Joran uses logback's powerfull <code>Status</code> objects for +error reporting, one can have a good feedback on what happened during +the parsing. +</p> + +<p> +In this example, the parsing is rather simple. The <em>hello-world</em> element +will activate <code>HelloWorldAction</code>'s <code>begin()</code> and +<code>end()</code> methods. +In the first method, a simple call to <code>System.out.println()</code> +will be issued, displaying "Hello World" in the console. +</p> + +<a name="calculator" /> +<h3>Collaborating actions</h3> +<p> +The logback-examples/src/main/java/joran/calculator/ directory includes several actions +which collaborate together through the common object stack in order +to accomplish simple computations. +</p> +<p> +The <em>calculator1.xml</em> file contains a <code>computation</code> element, +with a nested <code>literal</code> element. +</p> +<p> +In the <code>Calculator1</code> class, we declare various patterns and actions, +that will collaborate and calculate a result based on the xml file. The simple +<em>calculator1.xml</em> file only creates a computation and declares a literal +value. The resulting parsing is pretty simple: +</p> +<ul> + <p>The <code>ComputationAction1</code> class' <code>begin()</code> method + is called</p> + <p>The <code>LiteralAction</code> class' <code>begin()</code> and <code>end()</code> + methods are called</p> + <p>The <code>ComputationAction1</code> class' <code>end()</code> method + is called</p> +</ul> +<p> +What is interesting here is the way that the Actions collaborate. +The <code>LiteralAction</code> reads a literal value and pushes it in the +object stack maintained by the <code>ExecutionContext</code>. Once done, +any other action can pop the value to read or modify it. Here, the +<code>end()</code> method of the <code>ComputationAction1</code> class pops +the value from the stack and prints it. +</p> +<p>The <em>calculator2.xml</em> file is a bit more complex, and much more interesting.</p> +<p>It contains the following elements:</p> +<div class="source"><pre><computation name="toto"> + <literal value="7"/> + <literal value="3"/> + <add/> + <literal value="3"/> + <multiply/> +</computation></pre></div> +<p> +Here, there are obviously more actions that will be part of the computation. +</p> +<p>When called, the <code>AddAction</code> class will remove the two integers at +the bottom of the stack, add them and push the resulting integer at the +top of the stack, for further use.</p> +<p>Later in the computation, the <code>MultiplyAction</code> class will be called. +It will take the last two integers from the stack, multiply them and +push the result in the stack.</p> +<p>We have here two examples of action whose <code>begin()</code> method behaves in +a certain, predictable way, but whose <code>end()</code> methods are empty.</p> + +<p>Finally, a <em>calculator3.xml</em> is also provided, to demonstrate the possibility +elements that contain instances of the same element. Here's the content of +<em>calculator3.xml</em>:</p> +<div class="source"><pre><computation name="toto"> + <computation> + <literal value="7"/> + <literal value="3"/> + <add/> + </computation> + + <literal value="3"/> + <multiply/> +</computation></pre></div> + +<p>Much like the use of parentheses in an algebrical equation, the presence of +a <code>computation</code> element nested in another is managed by the +<code>ComputationAction2</code> class using an internal stack. The well-formedness +of XML will guarantee that a value saved by one begin() will be consumed +only by the matching end() method.</p> + +<a name="newRule" /> +<h3>New-rule action</h3> +<p>Joran includes an action which allows the Joran interpreter to lean +new rules on the fly while interpreting the XML file containing the +new rules. See the <em>logback-examples/src/main/java/joran/newRule/</em> +directory for sample code. +</p> +<p>In this package, the <code>NewRuleCalculator</code> class contains +the same setup as we have seen so far, but for one line:</p> + +<source>ruleStore.addRule(new Pattern("/computation/new-rule"), new NewRuleAction());</source> + +<p>By adding this line, we ask Joran to allow new rules to be learnt +at parsing time. It works pretty much like the other rules: it has a +<code>begin()</code> and <code>end()</code> method, and is called each time +the parser finds a <em>new-rule</em> element.</p> + +<p>When called, the <code>begin()</code> method looks for a <em>pattern</em> +and a <em>actionClass</em> attribute. The action class is then instanciated +and added to the <code>RuleStore</code>, along with its corresponding pattern.</p> + +<p>Here is how new rules can be declared in an xml file:</p> + +<div class="source"><pre><new-rule pattern="*/computation/literal" actionClass="joran.calculator.LiteralAction"/></pre></div> + +<p>Using new rule declarations, the preceding example, involving the calculation, could be +expressed this way:</p> + +<div class="source"><pre><computation name="toto"> + <new-rule pattern="*/computation/literal" + actionClass="joran.calculator.LiteralAction"/> + <new-rule pattern="*/computation/add" + actionClass="joran.calculator.AddAction"/> + <new-rule pattern="*/computation/multiply" + actionClass="joran.calculator.MultiplyAction"/> + + <computation> + <literal value="7"/> + <literal value="3"/> + <add/> + </computation> + + <literal value="3"/> + <multiply/> +</computation></pre></div> + +<a name="implicit" /> +<h3>Implicit actions </h3> +<p>The rules defined thus far are called explicit rules because they +require an explicit pattern, hence fixing the tag name of the elements +for which they apply. +</p> + +<p>In highly extensible systems, the number and type of components to +handle are innumerable so that it would become very tedious or even +impossible to list all the applicable patterns by name. +</p> + +<p>At the same time, even in highly extensible systems one can observe +well-defined patterns linking the various parts together. Implicit +rules come in very handy when processing components composed of +sub-components unknown ahead of time. For example, Apache Ant is +capable of handling tasks which contain tags unknown at compile time +by looking at methods whose names start with add, as in addFile, or +addClassPath. When Ant encounters an embedded tag within a task, it +simply instantiates an object that matches the signature of the task +class' add method and attaches the resulting object to the parent. +</p> + +<p>Joran includes similar capability in the form of implicit +actions. Joran keeps a list of implicit actions which can be applied +if no explicit pattern matches the current XML element. However, +applying an implicit action may not be always appropriate. Before +executing the implicit action, Joran asks an implicit action whether +it is appropriate in the current context. Only if the action replies +affirmatively does Joran interpreter invoke the (implicit) +action. This extra step makes it possible to support multiple implicit +actions or obviously none, if no implicit action is appropriate for a +given situation. +</p> + +<p>For example, the NestedComponentIA extending ImplicitAction , will +instantiate the class specified in a nested component and attach it +to the parent component by using setter method of the parent +component and the nested element's name. Under certain circumstances, +a nested action needs to be applied to an element say <a> and also +to another element <b> nested within <a>. The current +implementation of <code>NestedComponentIA</code> is capable of +handling multiply nested elements requiring intervention by the same +implicit action. +</p> + +<p>Both ImplicitAction and NestedComponentIA are located in the +<code>ch.qos.logback.core.joran.action</code> package. +</p> + +<p>Refer to the <em>logback-examples/src/main/java/joran/implicit</em> +directory for an example of an implicit action. +</p> + +<p>In that directory, you will find two actions classes, one xml file and one +class containing the setup of Joran.</p> + +<p>The <code>NOPAction</code> class does nothing. It is used to set +the context of the <em>foo</em> element, using this line:</p> + +<source>ruleStore.addRule(new Pattern("*/foo"), new NOPAction());</source> + +<p>After that, the implicit action, namely <code>PrintMeImplicitAction</code>, +is added to the <code>RuleStore</code>. This is done by simply adding a new +instance of the action to the <code>Joran interpreter</code></p> + +<source>ji.addImplicitAction(new PrintMeImplicitAction());</source> + +<p>When called, the <code>isApplicable()</code> method of <code>PrintMeImplicitAction</code> +checks the value of the <em>printme</em> attribute. If the value is <code>true</code>, +the implicit action is applicable: its <code>begin()</code> method will be called.</p> + +<p>The <em>implicit1.xml</em> file contains the following lines:</p> + +<div class="source"><pre><foo> + + <xyz printme="true"> + <abc printme="true"/> + </xyz> + + <xyz/> + + <foo printme="true"/> + +</foo></pre></div> + +<p>As one can see, the first element will be printed, since it has a <em>printme</em> +attribute, which bears the value <code>true</code>.</p> + +<p>The second element will not be printed, because no <em>printme</em> attibute is present.</p> + +<p>The last element will not be printed, although the required attribute is present. +This is because implicit rules are called only if no explicit rules are defined. Since +we added a <code>NOPAction</code> with the <em>*/foo</em> pattern, it will be used instead +of the <code>PrintMeImplicitAction</code>.</p> + +<p>Running the example yields the following output:</p> + +<div class="source"><pre>Element <xyz> asked to be printed. +Element <abc> asked to be printed. +ERROR in ch.qos.logback.core.joran.spi.ExecutionContext@1c5c1 - no applicable action \ +for <xyz>, current pattern is [/foo/xyz]</pre></div> + +<p>The last line was printed because of a call to <code>StatusPrinter</code> at the end +of the main class.</p> + +<h3>Non goals</h3> + +<p>The Joran API is not intended to be used to parse documents with +thousands of elements. +</p> + +</body> +</document> \ No newline at end of file
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