Maven Dependency Mechanism

Dependency Scope

Dependency scope is used to limit the transitivity of a dependency, and also to affect the classpath used for various build tasks.
There are dependency 6 scopes available:
  • compile - This is the default scope, used if none is specified. Compile dependencies are available in all classpaths of a project. Furthermore, those dependencies are propagated to dependent projects.
  • provided - This is much like compile, but indicates you expect the JDK or a container to provide the dependency at runtime. For example, when building a web application for the Java Enterprise Edition, you would set the dependency on the Servlet API and related Java EE APIs to scope provided because the web container provides those classes. This scope is only available on the compilation and test classpath, and is not transitive.
  • runtime - This scope indicates that the dependency is not required for compilation, but is for execution. It is in the runtime and test classpaths, but not the compile classpath.
  • test - This scope indicates that the dependency is not required for normal use of the application, and is only available for the test compilation and execution phases. This scope is not transitive.
  • system - This scope is similar to provided except that you have to provide the JAR which contains it explicitly. The artifact is always available and is not looked up in a repository.
  • import (only available in Maven 2.0.9 or later) - This scope is only supported on a dependency of type pom in the section. It indicates the dependency to be replaced with the effective list of dependencies in the specified POM's section. Since they are replaced, dependencies with a scope of import do not actually participate in limiting the transitivity of a dependency.

Dependency Management

The dependency management is a mechanism for centralizing dependency information. When you have a set of projects that inherit a common parent it's possible to put all information about the dependency in the common POM and have simpler references to the artifacts in the child POMs. 

The mechanism is best illustrated by some examples. Given these two POMs which extend the same parent:

Project A:
<project>
  ...
  <dependencies>
    <dependency>
      <groupId>group-a</groupId>
      <artifactId>artifact-a</artifactId>
      <version>1.0</version>
      <exclusions>
        <exclusion>
          <groupId>group-c</groupId>
          <artifactId>excluded-artifact</artifactId>
        </exclusion>
      </exclusions>
    </dependency>
    <dependency>
      <groupId>group-a</groupId>
      <artifactId>artifact-b</artifactId>
      <version>1.0</version>
      <type>bar</type>
      <scope>runtime</scope>
    </dependency>
  </dependencies>
</project>
Project B:
<project>
  ...
  <dependencies>
    <dependency>
      <groupId>group-c</groupId>
      <artifactId>artifact-b</artifactId>
      <version>1.0</version>
      <type>war</type>
      <scope>runtime</scope>
    </dependency>
    <dependency>
      <groupId>group-a</groupId>
      <artifactId>artifact-b</artifactId>
      <version>1.0</version>
      <type>bar</type>
      <scope>runtime</scope>
    </dependency>
  </dependencies>
</project>
These two example POMs share a common dependency and each has one non-trivial dependency. This information can be put in the parent POM like this:
<project>
  ...
  <dependencyManagement>
    <dependencies>
      <dependency>
        <groupId>group-a</groupId>
        <artifactId>artifact-a</artifactId>
        <version>1.0</version>
 
        <exclusions>
          <exclusion>
            <groupId>group-c</groupId>
            <artifactId>excluded-artifact</artifactId>
          </exclusion>
        </exclusions>
 
      </dependency>
 
      <dependency>
        <groupId>group-c</groupId>
        <artifactId>artifact-b</artifactId>
        <version>1.0</version>
        <type>war</type>
        <scope>runtime</scope>
      </dependency>
 
      <dependency>
        <groupId>group-a</groupId>
        <artifactId>artifact-b</artifactId>
        <version>1.0</version>
        <type>bar</type>
        <scope>runtime</scope>
      </dependency>
    </dependencies>
  </dependencyManagement>
</project>
And then the two child poms would become much simpler:
<project>
  ...
  <dependencies>
    <dependency>
      <groupId>group-a</groupId>
      <artifactId>artifact-a</artifactId>
    </dependency>
 
    <dependency>
      <groupId>group-a</groupId>
      <artifactId>artifact-b</artifactId>
      <!-- This is not a jar dependency, so we must specify type. -->
      <type>bar</type>
    </dependency>
  </dependencies>
</project>
<project>
  ...
  <dependencies>
    <dependency>
      <groupId>group-c</groupId>
      <artifactId>artifact-b</artifactId>
      <!-- This is not a jar dependency, so we must specify type. -->
      <type>war</type>
    </dependency>
 
    <dependency>
      <groupId>group-a</groupId>
      <artifactId>artifact-b</artifactId>
      <!-- This is not a jar dependency, so we must specify type. -->
      <type>bar</type>
    </dependency>
  </dependencies>
</project>
A second and very important use of the dependency management section is to control the versions of artifacts used in transitive dependencies. As an example consider these projects:
Project A:
<project>
 <modelVersion>4.0.0</modelVersion>
 <groupId>maven</groupId>
 <artifactId>A</artifactId>
 <packaging>pom</packaging>
 <name>A</name>
 <version>1.0</version>
 <dependencyManagement>
   <dependencies>
     <dependency>
       <groupId>test</groupId>
       <artifactId>a</artifactId>
       <version>1.2</version>
     </dependency>
     <dependency>
       <groupId>test</groupId>
       <artifactId>b</artifactId>
       <version>1.0</version>
       <scope>compile</scope>
     </dependency>
     <dependency>
       <groupId>test</groupId>
       <artifactId>c</artifactId>
       <version>1.0</version>
       <scope>compile</scope>
     </dependency>
     <dependency>
       <groupId>test</groupId>
       <artifactId>d</artifactId>
       <version>1.2</version>
     </dependency>
   </dependencies>
 </dependencyManagement>
</project>
Project B:
<project>
  <parent>
    <artifactId>A</artifactId>
    <groupId>maven</groupId>
    <version>1.0</version>
  </parent>
  <modelVersion>4.0.0</modelVersion>
  <groupId>maven</groupId>
  <artifactId>B</artifactId>
  <packaging>pom</packaging>
  <name>B</name>
  <version>1.0</version>
  <dependencyManagement>
    <dependencies>
      <dependency>
        <groupId>test</groupId>
        <artifactId>d</artifactId>
        <version>1.0</version>
      </dependency>
    </dependencies>
  </dependencyManagement>
  <dependencies>
    <dependency>
      <groupId>test</groupId>
      <artifactId>a</artifactId>
      <version>1.0</version>
      <scope>runtime</scope>
    </dependency>
    <dependency>
      <groupId>test</groupId>
      <artifactId>c</artifactId>
      <scope>runtime</scope>
    </dependency>
  </dependencies>
</project>
When maven is run on project B version 1.0 of artifacts a, b, c, and d will be used regardless of the version specified in their pom.
a and c both are declared as dependencies of the project so version 1.0 is used due to dependency mediation. Both will also have runtime scope since it is directly specified. b is defined in B's parent's dependency management section and since dependency management takes precedence over dependency mediation for transitive dependencies, version 1.0 will be selected should it be referenced in a or c's pom. b will also have compile scope.

Finally, since d is specified in B's dependency management section, should d be a dependency (or transitive dependency) of a or c, version 1.0 will be chosen - again because dependency management takes precedence over dependency mediation and also because the current pom's declaration takes precedence over its parent's declaration. The reference information about the dependency management tags is available from the project descriptor reference.
Importing Dependencies

The examples in the previous section describe how to specify managed dependencies through inheritance. However, in larger projects, it may be impossible to accomplish this since a project can only inherit from a single parent.

To accommodate this, projects can import managed dependencies from other projects. This is accomplished by declaring a pom artifact as a dependency with a scope of "import".

Project B:
<project>
  <modelVersion>4.0.0</modelVersion>
  <groupId>maven</groupId>
  <artifactId>B</artifactId>
  <packaging>pom</packaging>
  <name>B</name>
  <version>1.0</version>
  <dependencyManagement>
    <dependencies>
      <dependency>
        <groupId>maven</groupId>
        <artifactId>A</artifactId>
        <version>1.0</version>
        <type>pom</type>
        <scope>import</scope>
      </dependency>
      <dependency>
        <groupId>test</groupId>
        <artifactId>d</artifactId>
        <version>1.0</version>
      </dependency>
    </dependencies>
  </dependencyManagement>
  <dependencies>
    <dependency>
      <groupId>test</groupId>
      <artifactId>a</artifactId>
      <version>1.0</version>
      <scope>runtime</scope>
    </dependency>
    <dependency>
      <groupId>test</groupId>
      <artifactId>c</artifactId>
      <scope>runtime</scope>
    </dependency>
  </dependencies>
</project>
Assuming A is the pom defined in the preceding example, the end result would be the same. All of A's managed dependencies would be incorporated into B except for d since it is defined in this pom.
Project X:
<project>
 <modelVersion>4.0.0</modelVersion>
 <groupId>maven</groupId>
 <artifactId>X</artifactId>
 <packaging>pom</packaging>
 <name>X</name>
 <version>1.0</version>
 <dependencyManagement>
   <dependencies>
     <dependency>
       <groupId>test</groupId>
       <artifactId>a</artifactId>
       <version>1.1</version>
     </dependency>
     <dependency>
       <groupId>test</groupId>
       <artifactId>b</artifactId>
       <version>1.0</version>
       <scope>compile</scope>
     </dependency>
   </dependencies>
 </dependencyManagement>
</project>
Project Y:
<project>
 <modelVersion>4.0.0</modelVersion>
 <groupId>maven</groupId>
 <artifactId>Y</artifactId>
 <packaging>pom</packaging>
 <name>Y</name>
 <version>1.0</version>
 <dependencyManagement>
   <dependencies>
     <dependency>
       <groupId>test</groupId>
       <artifactId>a</artifactId>
       <version>1.2</version>
     </dependency>
     <dependency>
       <groupId>test</groupId>
       <artifactId>c</artifactId>
       <version>1.0</version>
       <scope>compile</scope>
     </dependency>
   </dependencies>
 </dependencyManagement>
</project>
Project Z:
<project>
  <modelVersion>4.0.0</modelVersion>
  <groupId>maven</groupId>
  <artifactId>Z</artifactId>
  <packaging>pom</packaging>
  <name>Z</name>
  <version>1.0</version>
  <dependencyManagement>
    <dependencies>
      <dependency>
        <groupId>maven</groupId>
        <artifactId>X</artifactId>
        <version>1.0</version>
        <type>pom</type>
        <scope>import</scope>
      </dependency>
      <dependency>
        <groupId>maven</groupId>
        <artifactId>Y</artifactId>
        <version>1.0</version>
        <type>pom</type>
        <scope>import</scope>
      </dependency>
    </dependencies>
  </dependencyManagement>
</project>
In the example above Z imports the managed dependencies from both X and Y. However, both X and Y contain dependency a. Here, version 1.1 of a would be used since X is declared first and a is not declared in Z's dependencyManagement.
This process is recursive. For example, if X imports another pom, Q, when Z is processed it will simply appear that all of Q's managed dependencies are defined in X.
Imports are most effective when used for defining a "library" of related artifacts that are generally part of a multi-project build. It is fairly common for one project to use one or more artifacts from these libraries. However, it has sometimes been difficult to keep the versions in the project using the artifacts in sync with the versions distributed in the library. The pattern below illustrates how a "bill of materials" (BOM) can be created for use by other projects.

The root of the project is the BOM pom. It defines the versions of all the artifacts that will be created in the library. Other projects that wish to use the library should import this pom into the dependencyManagement section of their pom.
<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
  <modelVersion>4.0.0</modelVersion>
  <groupId>com.test</groupId>
  <artifactId>bom</artifactId>
  <version>1.0.0</version>
  <packaging>pom</packaging>
  <properties>
    <project1Version>1.0.0</project1Version>
    <project2Version>1.0.0</project2Version>
  </properties>
  <dependencyManagement>
    <dependencies>
      <dependency>
        <groupId>com.test</groupId>
        <artifactId>project1</artifactId>
        <version>${project1Version}</version>
      </dependency>
      <dependency>
        <groupId>com.test</groupId>
        <artifactId>project2</artifactId>
        <version>${project1Version}</version>
      </dependency>
    </dependencies>
  </dependencyManagement>
  <modules>
    <module>parent</module>
  </modules>
</project>
The parent sub-project has the BOM pom as its parent. It is a normal multi-project pom.
<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
     xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
  <modelVersion>4.0.0</modelVersion>
  <parent>
    <groupId>com.test</groupId>
    <version>1.0.0</version>
    <artifactId>bom</artifactId>
  </parent>
 
  <groupId>com.test</groupId>
  <artifactId>parent</artifactId>
  <version>1.0.0</version>
  <packaging>pom</packaging>
 
  <dependencyManagement>
    <dependencies>
      <dependency>
        <groupId>log4j</groupId>
        <artifactId>log4j</artifactId>
        <version>1.2.12</version>
      </dependency>
      <dependency>
        <groupId>commons-logging</groupId>
        <artifactId>commons-logging</artifactId>
        <version>1.1.1</version>
      </dependency>
    </dependencies>
  </dependencyManagement>
  <modules>
    <module>project1</module>
    <module>project2</module>
  </modules>
</project>
Next are the actual project poms.
<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
     xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
  <modelVersion>4.0.0</modelVersion>
  <parent>
    <groupId>com.test</groupId>
    <version>1.0.0</version>
    <artifactId>parent</artifactId>
  </parent>
  <groupId>com.test</groupId>
  <artifactId>project1</artifactId>
  <version>${project1Version}</version>
  <packaging>jar</packaging>
 
  <dependencies>
    <dependency>
      <groupId>log4j</groupId>
      <artifactId>log4j</artifactId>
    </dependency>
  </dependencies>
</project>
 
<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
     xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
  <modelVersion>4.0.0</modelVersion>
  <parent>
    <groupId>com.test</groupId>
    <version>1.0.0</version>
    <artifactId>parent</artifactId>
  </parent>
  <groupId>com.test</groupId>
  <artifactId>project2</artifactId>
  <version>${project2Version}</version>
  <packaging>jar</packaging>
 
  <dependencies>
    <dependency>
      <groupId>commons-logging</groupId>
      <artifactId>commons-logging</artifactId>
    </dependency>
  </dependencies>
</project>
The project that follows shows how the library can now be used in another project without having to specify the dependent project's versions.
<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
     xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
  <modelVersion>4.0.0</modelVersion>
  <groupId>com.test</groupId>
  <artifactId>use</artifactId>
  <version>1.0.0</version>
  <packaging>jar</packaging>
 
  <dependencyManagement>
    <dependencies>
      <dependency>
        <groupId>com.test</groupId>
        <artifactId>bom</artifactId>
        <version>1.0.0</version>
        <type>pom</type>
        <scope>import</scope>
      </dependency>
    </dependencies>
  </dependencyManagement>
  <dependencies>
    <dependency>
      <groupId>com.test</groupId>
      <artifactId>project1</artifactId>
    </dependency>
    <dependency>
      <groupId>com.test</groupId>
      <artifactId>project2</artifactId>
    </dependency>
  </dependencies>
</project>
Finally, when creating projects that import dependencies beware of the following:
Do not attempt to import a pom that is defined in a sub-module of the current pom. Attempting to do that will result in the build failing since it won't be able to locate the pom. Never declare the pom importing a pom as the parent (or grandparent, etc) of the target pom. There is no way to resolve the circularity and an exception will be thrown. When referring to artifacts whose poms have transitive dependencies the project will need to specify versions of those artifacts as managed dependencies.

Not doing so will result in a build failure since the artifact may not have a version specified.

In the next article, we will look into 3 types of maven repository.

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