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o Adding a piece about dependency management.
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Trygve Laugstol
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<properties>
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<title>Dependency Mechanism</title>
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<author email="brett@apache.org">Brett Porter</author>
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<author email="trygvis@apache.org">Trygve Laugstol </author>
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</properties>
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<body>
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<section name="Dependency Mechanism">
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<p>
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There are many aspects to working with dependencies in Maven. While it is not the sole focus of Maven, it
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does comprise a large and important part of the system.
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does comprise a large and important part of the system. Learn more about:
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<ul>
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<li>
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<a href="#transitive_dependencies">Transitive Dependencies</a>
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</li>
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<li>
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<a href="#dependency_scope">Dependency Scope</a>
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</li>
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<li>
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<a href="#dependency_management">Dependency Management</a>
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</li>
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</ul>
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</p>
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<p>
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This document is currently in the process of being written, so not all facets are covered.
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<i>This document is currently in the process of being written, so not all facets are covered.</i>
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</p>
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<a name="transitive_dependencies"/>
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<subsection name="Transitive Dependencies">
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<p>
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Transitive dependencies are a new feature in Maven 2.0. This allows you to avoid needing to discover and
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@ -33,12 +46,14 @@
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</p>
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<ul>
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<li>
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<i>Dependency mediation</i>- this determines what version of a dependency will be used when multiple
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<i>Dependency mediation</i>
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- this determines what version of a dependency will be used when multiple
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different ones are encountered. Currently, Maven 2.0 only supports using the "nearest definition" - so
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you can always guarantee a version by declaring it explicitly in the project's POM.
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</li>
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<li>
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<i>Dependency scope</i>- this allows you to only include dependencies appropriate for the current stage
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<i>Dependency scope</i>
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- this allows you to only include dependencies appropriate for the current stage
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of the build. This is described in more detail below.
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</li>
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</ul>
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@ -49,6 +64,7 @@
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</p>
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</subsection>
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<subsection name="Dependency Scope">
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<a name="dependency_scope"/>
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<p>
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Dependency scope is used to limit the transitivity of a depedency, and also to affect the classpath used for
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various build tasks.
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@ -58,20 +74,24 @@
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</p>
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<ul>
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<li>
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<b>compile</b>- this is the default scope, used if none is specified. Compile dependencies are available
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<b>compile</b>
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- this is the default scope, used if none is specified. Compile dependencies are available
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in all classpaths.
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</li>
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<li>
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<b>provided</b>- this is much like compile, but indicates you expect the JDK or a container to provide it.
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<b>provided</b>
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- this is much like compile, but indicates you expect the JDK or a container to provide it.
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It is only available on the compilation classpath, and is not transitive.
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</li>
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<li>
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<b>runtime</b>- this scope indicates that the dependency is not required for compilation, but is for
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<b>runtime</b>
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- this scope indicates that the dependency is not required for compilation, but is for
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execution.
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It is in the runtime and test classpaths, but not the compile classpath.
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</li>
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<li>
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<b>test</b>- this scope indicates that the dependency is not required for normal use of the application, and
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<b>test</b>
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- this scope indicates that the dependency is not required for normal use of the application, and
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is only available for the test compilation and execution phases.
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</li>
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</ul>
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@ -119,13 +139,140 @@
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</tr>
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</table>
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<p>
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<b>(*) Note:</b>it is intended that this should be runtime instead, so that all compile dependencies must
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<b>(*) Note:</b>
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it is intended that this should be runtime instead, so that all compile dependencies must
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be explicitly listed - however, there is the case where the library you depend on extends a class from another
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library, forcing you to have available at compile time. For this reason, compile time dependencies remain
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as compile scope even when they are transitive.
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</p>
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</subsection>
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<a name="dependency_management"/>
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<subsection name="Dependency Management">
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<p>
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The dependency management section is a mechanism for centralizing dependency information. When you have
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a set of projects that inherits a common parent it's possible to put all information about the dependency
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in the common POM and have simpler references to the artifacts in the child POMs. The mechanism is best
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illustrated through some examples. Given these two POMs which extend the same parent:
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</p>
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<p>
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Project A:
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<source><![CDATA[
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<project>
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...
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<dependencies>
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<dependency>
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<groupId>group-a</groupId>
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<artifactId>artifact-a</artifactId>
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<version>1.0</version>
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<exclusions>
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<exclusion>
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<groupId>group-c</groupId>
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<artifactId>excluded-artifact</artifactId>
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</exclusion>
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</exclusions>
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</dependency>
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<dependency>
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<groupId>group-a</groupId>
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<artifactId>artifact-b</artifactId>
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<version>1.0</version>
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<type>bar</type>
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<scope>runtime</scope>
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</dependency>
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</dependencies>
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</project>]]></source>
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Project B:
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<source><![CDATA[
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<project>
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...
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<dependencies>
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<dependency>
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<groupId>group-c</groupId>
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<artifactId>artifact-b</artifactId>
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<version>1.0</version>
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<type>war</type>
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<scope>runtime</scope>
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</dependency>
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<dependency>
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<groupId>group-a</groupId>
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<artifactId>artifact-b</artifactId>
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<version>1.0</version>
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<type>bar</type>
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<scope>runtime</scope>
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</dependency>
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</dependencies>
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</project>]]></source>
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</p>
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<p>
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These two example POMs share a common dependencies and each one non-trivial dependency. This information
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can be put in the parent POM like this:
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<source><![CDATA[
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<project>
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...
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<dependencies>
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<dependency>
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<groupId>group-a</groupId>
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<artifactId>artifact-a</artifactId>
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<version>1.0</version>
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<exclusions>
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<exclusion>
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<groupId>group-c</groupId>
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<artifactId>excluded-artifact</artifactId>
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</exclusion>
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</exclusions>
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</dependency>
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<dependency>
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<groupId>group-c</groupId>
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<artifactId>artifact-b</artifactId>
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<version>1.0</version>
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<type>war</type>
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<scope>runtime</scope>
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</dependency>
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<dependency>
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<groupId>group-a</groupId>
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<artifactId>artifact-b</artifactId>
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<version>1.0</version>
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<type>bar</type>
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<scope>runtime</scope>
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</dependency>
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</dependencies>
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</project>]]></source>
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And then the two child poms would become much simpler:
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</p>
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<p>
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<source><![CDATA[
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<project>
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...
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<dependencies>
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<dependency>
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<groupId>group-a</groupId>
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<artifactId>artifact-a</artifactId>
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</dependency>
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<dependency>
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<groupId>group-a</groupId>
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<artifactId>artifact-b</artifactId>
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</dependency>
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</dependencies>
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</project>]]></source>
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<source><![CDATA[
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<project>
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...
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<dependencies>
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<dependency>
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<groupId>group-c</groupId>
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<artifactId>artifact-b</artifactId>
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</dependency>
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<dependency>
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<groupId>group-a</groupId>
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<artifactId>artifact-b</artifactId>
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</dependency>
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</dependencies>
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</project>]]></source>
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</p>
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<p>
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The reference information about the dependency management tags is available from the
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<a href="maven-model/maven.html#class_DependencyManagement">project descriptor reference</a>.
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</p>
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</subsection>
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</section>
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</body>
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</document>
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