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Dependencies in MavenA project may have dependency on some project which may have dependency on several others and so on. It may be a nightmare to manage all these dependencies manually, so maven provides a cool way to do this. cd ~/.m2/repository/org/apache/maven/plugins find . -name \*.pom ./maven-antrun-plugin/1.3/maven-antrun-plugin-1.3.pom ./maven-archetype-plugin/2.2/maven-archetype-plugin-2.2.pom ./maven-assembly-plugin/2.2-beta-5/maven-assembly-plugin-2.2-beta-5.pom ./maven-clean-plugin/2.5/maven-clean-plugin-2.5.pom . . . ./maven-surefire-plugin/2.12.4/maven-surefire-plugin-2.12.4.pom ./maven-war-plugin/2.1.1/maven-war-plugin-2.1.1.pom cat maven-jar-plugin/2.4/maven-jar-plugin-2.4.pom
.
.
.
<dependencies>
<dependency>
<groupId>org.apache.maven</groupId>
<artifactId>maven-plugin-api</artifactId>
<version></version>
</dependency>
<dependency>
<groupId>org.apache.maven</groupId>
<artifactId>maven-project</artifactId>
<version></version>
</dependency>
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.
.
<dependency>
<groupId>org.codehaus.plexus</groupId>
<artifactId>plexus-utils</artifactId>
<version>3.0</version>
</dependency>
<dependency>
<groupId>org.apache.maven.plugin-testing</groupId>
<artifactId>maven-plugin-testing-harness</artifactId>
<version>1.2</version>
<scope>test</scope>
</dependency>
<dependency>
<groupId>junit</groupId>
<artifactId>junit</artifactId>
<version>3.8.2</version>
<scope>test</scope>
</dependency>
</dependencies>
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.
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So, every plugin in maven has a pom.xml which defines the dependencies for that project. Due to this, you need not manually define all dependencies. A developer can just include the top-level dependencies and rest assured that the whole dependency tree will be managed automatically by maven! For example, to add a dependency on log4j, the following needs to be added inside the <dependencies> element. <dependency> <groupId>org.apache.logging.log4j.adapters</groupId> <artifactId>log4j-1.2-api</artifactId> <version>2.0-beta4</version> </dependency> Once this is done, maven will download whatever is needed to download log4j for your application. To see what dependencies are in use, run the command mvn dependency:resolve to get the following output: [INFO] ------------------------------------------------------------------------ [INFO] Building simple 1.0-SNAPSHOT [INFO] ------------------------------------------------------------------------ [INFO] [INFO] --- maven-dependency-plugin:2.8:resolve (default-cli) @ simple --- [INFO] [INFO] The following files have been resolved: [INFO] org.apache.logging.log4j:log4j-api:jar:2.0-beta4:compile [INFO] org.apache.logging.log4j.adapters:log4j-1.2-api:jar:2.0-beta4:compile [INFO] org.apache.logging.log4j:log4j-core:jar:2.0-beta4:compile [INFO] junit:junit:jar:3.8.1:test [INFO] [INFO] ------------------------------------------------------------------------ [INFO] BUILD SUCCESS [INFO] ------------------------------------------------------------------------ Another cool command is mvn dependency:tree which outputs the dependencies in a tree format: [INFO] Scanning for projects... [INFO] [INFO] ------------------------------------------------------------------------ [INFO] Building simple 1.0-SNAPSHOT [INFO] ------------------------------------------------------------------------ [INFO] [INFO] --- maven-dependency-plugin:2.8:tree (default-cli) @ simple --- [INFO] org.companyname.simple:simple:jar:1.0-SNAPSHOT [INFO] +- junit:junit:jar:3.8.1:test [INFO] \- org.apache.logging.log4j.adapters:log4j-1.2-api:jar:2.0-beta4:compile [INFO] +- org.apache.logging.log4j:log4j-api:jar:2.0-beta4:compile [INFO] \- org.apache.logging.log4j:log4j-core:jar:2.0-beta4:compile [INFO] ------------------------------------------------------------------------ [INFO] BUILD SUCCESS [INFO] ------------------------------------------------------------------------ |
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