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<?xml version="1.0"?> <!-- Licensed to the Apache Software Foundation (ASF) under one or more contributor license agreements. See the NOTICE file distributed with this work for additional information regarding copyright ownership. The ASF licenses this file to you under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with the License. You may obtain a copy of the License at http://www.apache.org/licenses/LICENSE-2.0 Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions and limitations under the License. --> <document xmlns="http://maven.apache.org/XDOC/2.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://maven.apache.org/XDOC/2.0 https://maven.apache.org/xsd/xdoc-2.0.xsd"> <properties> <title>Mojo API Specification</title> <author email="jdcasey@apache.org">John Casey</author> </properties> <body> <section name="Introduction"> <p>Maven plugins can be written in Java or any of a number of scripting languages. Plugins consists of one or more Mojos, each one being the implementation for one of the plugin's <b>goals</b>.</p> <p>NOTE: this documentation covers Mojo API introduced in Maven 2 and used by Maven 3. It does not cover the <a href="https://maven.apache.org/ref/4-LATEST/api/index.html">new Maven 4 API</a>.</p> <p>Although the requirements on Mojos are minimal by design, there are still a very few requirements that Mojo developers must keep in mind:</p> <ol> <li>a Mojo must have a method named <code>execute</code> which declares no parameters, and has a void return type. If this method throws an exception, that exception must either be a derivative of <code>java.lang.RuntimeException</code>, or a derivative of <a href="/ref/current/maven-plugin-api/apidocs/org/apache/maven/plugin/MojoExecutionException.html"><code>org.apache.maven.plugin.MojoExecutionException</code></a>. It goes without saying that in the latter case, the execute method must declare that it throws this exception.</li> <li> Additionally, Mojos must declare a field for each goal <b>parameter</b> they specify, and these parameter fields will be populated before <code>execute()</code> is called.</li> <li>Finally, all Mojos must be accompanied by metadata in <code>META-INF/maven/plugin.xml</code> describing parameters, lifecycle bindings, etc. This descriptor will be covered in more detail below.</li> </ol> <subsection name="Mojo Code"> <p>Basically, these Mojo requirements are embodied by the <a href="/ref/current/maven-plugin-api/apidocs/org/apache/maven/plugin/Mojo.html"><code>org.apache.maven.plugin.Mojo</code> interface</a>, which the Mojo must implement.<br /> Usually this is done by extending its <a href="/ref/current/maven-plugin-api/apidocs/org/apache/maven/plugin/AbstractMojo.html">abstract base class counterpart <code>org.apache.maven.plugin.AbstractMojo</code></a>.<br /> The Mojo will have access to the standard Maven user-feedback mechanism, <a href="/ref/current/maven-plugin-api/apidocs/org/apache/maven/plugin/logging/Log.html"><code>org.apache.maven.plugin.logging.Log</code></a>, so the Mojo can communicate important events to the console or other log sink. </p> </subsection> <subsection name="The Descriptor and Annotations"> <p>As mentioned before, each Plugin - or packaged set of Mojos - must provide a <a href="/ref/current/maven-plugin-api/plugin.html"><code>META-INF/maven/plugin.xml</code> descriptor file</a> inside the Plugin jar file.<br /> Fortunately, Maven also provides a set of Java annotations (named <a href="/plugin-tools/maven-plugin-tools-annotations/index.html">Maven Plugin Tools Java5 Annotations</a>) and tools (named <a href="/plugin-tools/">Plugin Tools</a>) to generate this descriptor, so developers don't have to worry about directly authoring or maintaining a separate XML metadata file. </p> </subsection> <subsection name="Project Descriptor (POM) Requirements"> <p>The POM must declare a packaging element which describes this project as a Maven plugin project: <code>&lt;packaging&gt;maven-plugin&lt;/packaging&gt;</code></p> </subsection> </section> <!--section name="The Descriptor and Annotations"> <p>In addition to the normal Java requirements in terms of interfaces and/or abstract base classes which need to be implemented, a plugin descriptor must accompany these classes inside the plugin jar. This descriptor file is used to provide metadata about the parameters and other component requirements for a set of Mojos so that Maven can initialize the Mojo and validate its configuration before executing it. As such, the plugin descriptor has a certain set of information that is required for each Mojo specification to be valid, as well as requirements for the overall plugin descriptor itself.</p> <p>NOTE: In the following discussion, bolded items are the descriptor's element name along with a Mojo Javadoc tag (if applicable) supporting that piece of the plugin descriptor. A couple of examples are: <b>someElement (@annotation parameterName="parameterValue")</b> or <b>someOtherElement (@annotation &lt;rawAnnotationValue&gt;)</b> . </p> <p><b>NOTE:</b> since maven-plugin-plugin 3.0, it is now possible to use <a href="/plugin-tools/maven-plugin-tools-annotations/index.html">Maven Plugin Tools Java 5 Annotations</a> equivalent to <a href="/plugin-tools/maven-plugin-tools-java/index.html">Mojo Javadoc tags</a>. See <a href="/plugin-tools/maven-plugin-plugin/examples/using-annotations.html">Using annotations documentation</a>. </p> <p>The plugin descriptor (see <a href="/ref/current/maven-plugin-api/plugin.html">descriptor reference</a>) must be provided in a jar resource with the path: <code>META-INF/maven/plugin.xml</code>, and it must contain the following: </p> <table> <tr> <th>Descriptor Element</th> <th>Required?</th> <th>Notes</th> </tr> <tr> <td><code>mojos</code></td> <td>Yes</td> <td>Descriptors for each Mojo provided by the plugin, each inside a <b><code>mojo</code></b> sub-element. Mojo descriptors are covered in detail below. Obviously, a plugin without any declared Mojos doesn't make sense, so the <b><code>mojos</code></b> element is required, along with at least one <b><code>mojo</code></b> sub-element. </td> </tr> <tr> <td><code>dependencies</code></td> <td>Yes</td> <td>A set of dependencies which the plugin requires in order to function. Each dependency is provided inside a <b><code>dependency</code></b> sub-element. Dependency specifications are covered below. Since all plugins must have a dependency on <code>maven-plugin-api</code> , this element is effectively required. <i>Using the plugin toolset, these dependencies can be extracted from the POM's dependencies.</i> </td> </tr> </table> <p>Each Mojo specified inside a plugin descriptor must provide the following (annotations specified here are at the class level):</p> <table> <!== Annotations listed by specific, autodetect and Javadoc, all alphabetical ==> <tr> <th>Descriptor Element</th> <th>Mojo Javadoc tag</th> <th>Required?</th> <th>Notes</th> </tr> <tr> <td><code>aggregator</code></td> <td><code>@aggregator</code></td> <td>No</td> <td>Flags this Mojo to run it in a multi module way, i.e. aggregate the build with the set of projects listed as modules.</td> </tr> <tr> <td><code>configurator</code></td> <td><code>@configurator &lt;roleHint&gt;</code></td> <td>No</td> <td>The configurator type to use when injecting parameter values into this Mojo. The value is normally deduced from the Mojo's implementation language, but can be specified to allow a custom ComponentConfigurator implementation to be used. <i>NOTE: This will only be used in very special cases, using a highly controlled vocabulary of possible values. (Elements like this are why it's a good idea to use the descriptor tools.)</i> </td> </tr> <tr> <td><code>execute</code></td> <td> <ul> <li><code>@execute phase=&quot;&lt;phaseName&gt;&quot; lifecycle=&quot;&lt;lifecycleId&gt;&quot;</code></li> <li><code>@execute phase=&quot;&lt;phaseName&gt;&quot;</code></li> <li><code>@execute goal=&quot;&lt;goalName&gt;&quot;</code></li> </ul> </td> <td>No</td> <td>When this goal is invoked, it will first invoke a parallel lifecycle, ending at the given phase. If a goal is provided instead of a phase, that goal will be executed in isolation. The execution of either will not affect the current project, but instead make available the <code>${executedProject}</code> expression if required. An alternate lifecycle can also be provided: for more information see the documentation on the <a href="../guides/introduction/introduction-to-the-lifecycle.html">build lifecycle</a>.</td> </tr> <tr> <td><code>executionStrategy</code></td> <td><code>@executionStrategy &lt;strategy&gt;</code></td> <td>No</td> <td>Specify the execution strategy. <i>NOTE: Unsupported since Maven 3.0.</i></td> </tr> <tr> <td><code>goal</code></td> <td><code>@goal &lt;goalName&gt;</code></td> <td>Yes</td> <td>The name for the Mojo that users will reference from the command line to execute the Mojo directly, or inside a POM in order to provide Mojo-specific configuration.</td> </tr> <tr> <td><code>inheritByDefault</code></td> <td><code>@inheritByDefault &lt;true|false&gt;</code></td> <td>No. Default: <code>true</code></td> <td>Specify that the Mojo is inherited. <i>NOTE: Unsupported since Maven 3.0.</i></td> </tr> <tr> <td><code>instantiationStrategy</code></td> <td><code>@instantiationStrategy &lt;per-lookup&gt;</code></td> <td>No. Default: <code>per-lookup</code></td> <td>Specify the instantiation strategy.</td> </tr> <tr> <td><code>phase</code></td> <td><code>@phase &lt;phaseName&gt;</code></td> <td>No</td> <td> Defines a default phase to bind a mojo execution to if the user does not explicitly set a phase in the POM. <em>Note:</em> This annotation will not automagically make a mojo run when the plugin declaration is added to the POM. It merely enables the user to omit the <code>&lt;phase&gt;</code> element from the surrounding <code>&lt;execution&gt;</code> element. </td> </tr> <tr> <td><code>requiresDependencyResolution</code></td> <td><code>@requiresDependencyResolution &lt;requiredClassPath&gt;</code></td> <td>No</td> <td> Flags this Mojo as requiring the dependencies in the specified class path to be resolved before it can execute. The matrix below illustrates which values for <i>&lt;requiredClassPath&gt;</i> (first column) are supported and which dependency scopes (first row) they will request to resolve: <table> <tr> <td></td> <td>system</td> <td>provided</td> <td>compile</td> <td>runtime</td> <td>test</td> </tr> <tr> <td><code>compile</code></td> <td>X</td> <td>X</td> <td>X</td> <td>-</td> <td>-</td> </tr> <tr> <td><code>runtime</code></td> <td>-</td> <td>-</td> <td>X</td> <td>X</td> <td>-</td> </tr> <tr> <td><code>compile+runtime</code> (since Maven 3.0)</td> <td>X</td> <td>X</td> <td>X</td> <td>X</td> <td>-</td> </tr> <tr> <td><code>test</code></td> <td>X</td> <td>X</td> <td>X</td> <td>X</td> <td>X</td> </tr> </table> If this annotation is present but no scope is specified, the scope defaults to <code>runtime</code>. If the annotation is not present at all, the mojo must not make any assumptions about the artifacts associated with a Maven project. </td> </tr> <tr> <td><code>requiresDependencyCollection</code></td> <td><code>@requiresDependencyCollection &lt;requiredClassPath&gt;</code></td> <td>No</td> <td> Flags this mojo as requiring information about the dependencies that would make up the specified class path. As the name suggests, this annotation is similar to <code>@requiresDependencyResolution</code> and supports the same values for <i>&lt;requiredClassPath&gt;</i>. The important difference is that this annotation will not resolve the files for the dependencies, i.e. the artifacts associated with a Maven project can lack a file. As such, this annotation is meant for mojos that only want to analyze the set of transitive dependencies, in particular during early lifecycle phases where full dependency resolution might fail due to projects which haven't been built yet. A mojo may use both this annotation and <code>@requiresDependencyResolution</code> at the same time. The resolution state of any dependency that is collected but not requested to be resolved is undefined. Since Maven 3.0. </td> </tr> <tr> <td><code>requiresDirectInvocation</code></td> <td><code>@requiresDirectInvocation &lt;true|false&gt;</code></td> <td>No. Default: <code>false</code></td> <td>Flags this Mojo to be invoke directly. <i>NOTE: Unsupported since Maven 3.0.</i></td> </tr> <tr> <td><code>requiresOnline</code></td> <td><code>@requiresOnline &lt;true|false&gt;</code></td> <td>No. Default: <code>false</code></td> <td>Flags this Mojo to require online mode for its operation.</td> </tr> <tr> <td><code>requiresProject</code></td> <td><code>@requiresProject &lt;true|false&gt;</code></td> <td>No. Default: <code>true</code></td> <td>Flags this Mojo to run inside of a project.</td> </tr> <tr> <td><code>requiresReports</code></td> <td><code>@requiresReports &lt;true|false&gt;</code></td> <td>No. Default: <code>false</code></td> <td>Flags this Mojo to require reports. <i>NOTE: Unsupported since Maven 3.0.</i></td> </tr> <tr> <td><code>threadSafe</code></td> <td><code>@threadSafe &lt;true|false&gt;</code></td> <td>No. Default: <code>false</code></td> <td> Marks this mojo as being thread-safe, i.e. the mojo safely supports concurrent execution during parallel builds. Mojos without this annotation will make Maven output a warning when used during a parallel build session. The short-hand notation <code>@threadSafe</code> without a tag value is equivalent to <code>@threadSafe true</code>. Since Maven 3.0. </td> </tr> <!== Autodetect ==> <tr> <td><code>description</code></td> <td>none (detected)</td> <td>No</td> <td>The description of this Mojo's functionality. <i>Using the toolset, this will be the class-level Javadoc description provided. NOTE: While this is not a required part of the Mojo specification, it SHOULD be provided to enable future tool support for browsing, etc. and for clarity.</i> </td> </tr> <tr> <td><code>implementation</code></td> <td>none (detected)</td> <td>Yes</td> <td>The Mojo's fully-qualified class name (or script path in the case of non-Java Mojos).</td> </tr> <tr> <td><code>language</code></td> <td>none (detected)</td> <td>No. Default: <code>java</code></td> <td>The implementation language for this Mojo (Java, beanshell, etc.).</td> </tr> <!== Javadoc ==> <tr> <td><code>deprecated</code></td> <td><code>@deprecated &lt;deprecated-text&gt;</code></td> <td>No</td> <td>Description with the reason of Mojo deprecation. Similar to Javadoc deprecated. This will trigger a warning when a user tries to use a Mojo marked as deprecated. </td> </tr> <tr> <td><code>since</code></td> <td><code>@since &lt;since-text&gt;</code></td> <td>No</td> <td>Specify the version when the Mojo was added to the API. Similar to Javadoc since.</td> </tr> </table> <p>Each Mojo specifies the parameters that it expects in order to work. These parameters are the Mojo's link to the outside world, and will be satisfied through a combination of POM/project values, plugin configurations (from the POM and configuration defaults), and System properties.</p> <p>NOTE[1]: For this discussion on Mojo parameters, a single annotation may span multiple elements in the descriptor's specification for that parameter. Duplicate annotation declarations in this section will be used to detail each parameter of an annotation separately.</p> <p>NOTE[2]: In many cases, simply annotating a Mojo field with <b>@parameter</b> will be enough to allow injection of a value for that parameter using POM configuration elements. The discussion below shows advanced usage for this annotation, along with others. </p> <p>Each parameter for a Mojo must be specified in the plugin descriptor as follows:</p> <table> <!== Annotations listed by specific, autodetect and Javadoc, all alphabetical ==> <tr> <th>Descriptor Element</th> <th>Mojo Javadoc tag</th> <th>Required?</th> <th>Notes</th> </tr> <tr> <td><code>alias</code></td> <td><code>@parameter alias=&quot;myAlias&quot;</code></td> <td>No</td> <td>Specifies an alias which can be used to configure this parameter from the POM. This is primarily useful to improve user-friendliness, where Mojo field names are not intuitive to the user or are otherwise not conducive to configuration via the POM.</td> </tr> <tr> <td><code>configuration</code></td> <td><code>@component role=&quot;...&quot; roleHint=&quot;...&quot;</code></td> <td>No</td> <td>Populates the field with an instance of a Plexus component. This is like declaring a <i>requirement</i> in a Plexus component. The default requirement will have a role equal to the declared type of the field, and will use the role hint "default". You can customise either of these by providing a <code>role</code> and/or <code>roleHint</code> parameter. <i>e.g.</i> <code>@component role=&quot;org.apache.maven.artifact.ArtifactHandler&quot; roleHint=&quot;ear&quot;</code>. <b>Note:</b> This is identical to the deprecated form of parameter: <code>@parameter expression=&quot;${component.yourpackage.YourComponentClass#roleHint}&quot;</code>. </td> </tr> <tr> <td><code>configuration</code></td> <td>maven-plugin-plugin 2.x:<br /><code>@parameter expression=&quot;${aSystemProperty}&quot; default-value=&quot;${anExpression}&quot;</code><br /><br /> maven-plugin-plugin 3.x:<br /><code>@parameter property=&quot;aSystemProperty&quot; default-value=&quot;${anExpression}&quot;</code></td> <td>No</td> <td><p>Specifies the expressions used to calculate the value to be injected into this parameter of the Mojo at buildtime.</p> <p>The expression given by <code>default-value</code> is commonly used to refer to specific elements in the POM, such as <code>${project.resources}</code>, which refers to the list of resources meant to accompany the classes in the resulting JAR file. Of course, the default value need not be an expression but can also be a simple constant like <code>true</code> or <code>1.5</code>. And for parameters of type <code>String</code> one can mix expressions with literal values, e.g. <code>${project.artifactId}-${project.version}-special</code>.</p> <p>The system property given by <code>property</code> in maven-plugin-plugin 3.x or <code>expression</code> in maven-plugin-plugin 2.x enables users to override the default value from the command line via <code>-DaSystemProperty=value</code>.</p> <p><i>NOTE: If neither <code>default-value</code> nor <code>property</code> or <code>expression</code> are specified, the parameter can only be configured from the POM. The use of '${' and '}' in default value is required to delimit actual expressions which may be evaluated.</i></p> </td> </tr> <tr> <td><code>editable</code></td> <td><code>@readonly</code></td> <td>No</td> <td>Specifies that this parameter cannot be configured directly by the user (as in the case of POM-specified configuration). This is useful when you want to force the user to use common POM elements rather than plugin configurations, as in the case where you want to use the artifact's final name as a parameter. In this case, you want the user to modify &lt;build&gt;&lt;finalName/&gt;&lt;/build&gt; rather than specifying a value for finalName directly in the plugin configuration section. It is also useful to ensure that - for example - a List-typed parameter which expects items of type Artifact doesn't get a List full of Strings. <i>NOTE: Specification of this annotation flags the parameter as non-editable; there is no true/false value.</i> </td> </tr> <tr> <td><code>required</code></td> <td><code>@required</code></td> <td>No</td> <td>Whether this parameter is required for the Mojo to function. This is used to validate the configuration for a Mojo before it is injected, and before the Mojo is executed from some half-state. <i>NOTE: Specification of this annotation flags the parameter as required; there is no true/false value.</i> </td> </tr> <!== Autodetect ==> <tr> <td><code>description</code></td> <td>none (detected)</td> <td>No</td> <td>The description of this parameter's use inside the Mojo. <i>Using the toolset, this is detected as the Javadoc description for the field. NOTE: While this is not a required part of the parameter specification, it SHOULD be provided to enable future tool support for browsing, etc. and for clarity.</i> </td> </tr> <tr> <td><code>name</code></td> <td>none (detected)</td> <td>Yes</td> <td>The name of the parameter, to be used in configuring this parameter from the Mojo's declared defaults (discussed below) or from the POM. <i>Using the toolset, this is detected as the Java field name.</i> </td> </tr> <tr> <td><code>type</code></td> <td>none (detected)</td> <td>Yes</td> <td>The Java type for this parameter. This is used to validate the result of any expressions used to calculate the value which should be injected into the Mojo for this parameter. <i>Using the toolset, this is detected as the class of the Java field corresponding to this parameter.</i> </td> </tr> <!== Javadoc ==> <tr> <td><code>deprecated</code></td> <td><code>@deprecated &lt;deprecated-text&gt;</code></td> <td>No</td> <td>Description with the reason of parameter deprecation. Similar to Javadoc deprecated. This will trigger a warning when a user tries to configure a parameter marked as deprecated. </td> </tr> <tr> <td><code>since</code></td> <td><code>@since &lt;since-text&gt;</code></td> <td>No</td> <td>Specify the version when the Mojo was added to the API. Similar to Javadoc since.</td> </tr> </table> <p>The final component of a plugin descriptor is the dependencies. This enables the plugin to function independently of its POM (or at least to declare the libraries it needs to run). Dependencies are taken from the <b>runtime</b> scope of the plugin's calculated dependencies (from the POM). Dependencies are specified in exactly the same manner as in the POM, except for the &lt;scope&gt; element (all dependencies in the plugin descriptor are assumed to be runtime, because this is a runtime profile for the plugin). </p> </section--> <!--section name="Plugin Tools"> <p>By now, we've mentioned the plugin tools several times without telling you what they are or how to use them. Instead of manually writing (and maintaining) the metadata detailed above, Maven ships with some tools to aid in this task. In fact, the only thing a plugin developer needs to do is declare his project to be a plugin from within the POM. Once this is done, Maven will call the appropriate descriptor generators, etc. to produce an artifact that is ready for use within Maven builds. Optional metadata can be injected via Javadoc annotation (and possibly JDK5 annotations in the future) as described above, enabling richer interactions between the Mojo and the user. The section below describes the changes to the POM which are necessary to create plugin artifacts.</p> </section--> <!--section name="Project Descriptor (POM) Requirements"> <p>From the POM, Maven plugin projects look quite similar to any other project. For pure Java plugins, the differences are even smaller than for script-based plugins. The following details the POM elements which are necessary to build a Maven plugin artifact.</p> <table> <tr> <th>POM Element</th> <th>Required for Java Mojos?</th> <th>Sample Declaration</th> <th>Notes</th> </tr> <tr> <td>packaging</td> <td>Yes</td> <td> <code>&lt;packaging&gt; maven-plugin &lt;/packaging&gt;</code> </td> <td>The POM must declare a packaging element which describes this project as a Maven plugin project.</td> </tr> <tr> <td>scriptSourceDirectory</td> <td>No</td> <td> <code>&lt;scriptSourceDirectory&gt; src/main/scripts &lt;/scriptSourceDirectory&gt;</code> </td> <td>In the case of script-based Mojos (which are not covered in detail within this document), the POM must include an additional element to distinguish script sources from (optional) Java supporting classes. This element is <code>scriptSourceDirectory</code>, inside the <code>build</code> section. This directory is included in the list of resources which accompany any compiled code in the resulting artifact. It is specified separately from the resources in the build section to denote its special status as an alternate source directory for scripts.</td> </tr> </table> <p>After making the changes above, the developer can simply call</p> <code>mvn install</code> <p> to install the plugin to the local repository. (Any of the other standard lifecycle targets like package, deploy, etc. are also available in like fashion.) </p> </section--> <!--section name="IDE integration"> <p>If you're using JetBrains IntelliJ IDEA to develop your plugin, you can use the following to configure the javadoc annotations as live templates.</p> <ol> <li>Download <a href="./maven.xml">this file</a>, and place it in <code>$USER_HOME/.IntelliJIdea/config/templates</code></li> <li>(re)startup IntelliJ IDEA (templates are loaded on startup)</li> <li>add the following list to Settings -> IDE -> Errors -> General -> Unknown javadoc tags -> Additional javadoc tags <ul> <li>aggregator, execute, goal, phase, requiresDirectInvocation, requiresProject, requiresReports, requiresOnline, parameter, component, required, readonly</li> </ul> </li> </ol> </section--> <section name="Resources"> <p>This section simply gives a listing of pointers for more information.</p> <ul> <li>Maven Plugin API [ <a href="/ref/current/maven-plugin-api/apidocs/index.html">link</a> ] </li> <li>MojoDescriptor API [ <a href="/ref/current/maven-plugin-api/apidocs/org/apache/maven/plugin/descriptor/MojoDescriptor.html">link</a> ] </li> </ul> </section> </body> </document>