Understanding Apache Maven (Part 5): Dependencies in MavenOctober 22, 2021
In this, part 5 of the series, a walkthrough of the topic of Maven dependencies is covered!
What are dependencies
Dependencies are the basic building blocks of a Maven project.
Imagine writing code that requires logging output or using String utilities or parsing JSON text. The logic can be coded into the project... or a library can be used. Most of the time, it makes sense to harness an existing library to minimize the amount of code needed. This also encourages reuse.
The libraries, required to compile, run, and test the project in a Maven ecosystem are referred to as dependencies.
The project in question could potentially be a library used as a dependency in some other consumer POM.
How are dependencies located?
In Part 3 of the series, dependency coordinates and distinguishers were covered. As a recap: a dependency location can be reached via its groupId, artifactId and version (G-A-V or GAV) coordinates and furthermore the type and classifier can be specified to pinpoint the exact dependency needed in the project. Together these can be referred to as location coordinates.
A future article will be dedicated to a walkthrough of the POM file, while this article is focused on a deep dive on dependencies specifically.
An sample of a dependency block in an XML format POM file is listed below:
<project> ... <dependencies> <dependency> <groupId>a.group-id</groupId> <artifactId>an-artifact</artifactId> <version>1.0</version> <exclusions> <exclusion> <groupId>transitive.group-id</groupId> <artifactId>excluded-artifact</artifactId> </exclusion> </exclusions> <optional>true</optional> </dependency> <dependency> <groupId>another.group.id</groupId> <artifactId>another-artifact</artifactId> <version>1.0.0-SNAPSHOT</version> <type>zip</type> <scope>runtime</scope> </dependency> </dependencies> </project>
This excerpt is not exhaustive in how a dependency excerpt can look. Time to dig in!
A dependency in a POM
Dependencies for a project are declared in a dependencies element. This element represents a set of unique dependency elements. As exemplified above and described in earlier blogs, a dependency can contain the G-A-V coordinates and additional optional distinguishers as needed. In addition to the location coordinates, a dependency can contain exclusions, a scope and an optional tag.
As mentioned earlier, a POM has dependencies. The project itself can be a dependency for some other consumer project. The current project’s dependencies are then considered transitive dependencies for the other project. When Maven pulls in a dependency from the location coordinates, it also attempts to pull in the transitive dependencies for it. Put in different words, if project A depends on dependency (another project) B and this B depends on dependencies C and D, then Maven attempts to resolve and pull in B, C and D when creating an effective POM for project A. More on this in a bit.
The depth of transitive dependencies is not limited. Traversal continues until the level where there are no further transitives for each dependency listed. This entire structure of a dependency and its complete transitive graph is known as its dependency tree.
In some cases, it may not be necessary to pull one or more transitive dependencies (and their entire further depth). A means to instruct Maven to ignore certain “branches” of the tree is via an exclusion. As the excerpt suggests, exclusions are a set of rejection criteria. An exclusion requires a groupId and artifactId (more on this in a bit). It is possible to use a wildcard (*) in the exclusion elements (functional since Apache Maven 3.2.1).
A dependency element can have one or more exclusion elements nested within an exclusions element.
A dependency may be required to compile a project or to run a project or to only run the project’s tests. A scope instructs Maven on how the said dependency is used in the project lifecycle. There are a few scopes enumerated for usage in dependencies. A tabulated summary:
|compile||the default scope. These dependencies will be available on the classpath of the project. Also, any project that identifies this project as a dependency will find compile scope dependencies propagated in the dependency tree.|
|provided||a scope that determines that the dependency will be made available for use external to the project’s build artifacts. For instance a container or server will furnish the dependency at runtime and is available on the classpath during execution or tests. These dependency is not propagated as transitive.|
|runtime||a scope that determines that a dependency is only required at runtime and not at compile time. Typical usecases are when an API and its implementation are produced as separate dependencies. The compilation may only need the API dependency while the execution at runtime will require an actual implementation as well. The dependency is propagated as a runtime transitive dependency when the project artifact itself becomes another project’s dependency.|
|test||a scope that determines that a dependency is only required for compiling and running tests and not during a normal compilation nor execution of the project. the dependency is not propagated as a transitive.|
|system||a scope that stops Maven from resolving a dependency from a repository. The scope requires an additional |
|import||a special scope used exclusively in a |
Tabulated scope values with notes on each
The project may need some dependencies that need not be passed on to any other projects that use the current project as a dependency. Such dependencies can be of any scope. An element in the dependency structure is
optional that marks the said dependency as only needed for the current project’s Maven executions.
An anecdotal example of depending on a metrics library: The current project may need a metrics library for execution and testing, however when the project is used as a dependency, there may be no need for the consumer project to rely on this metrics library. Such a dependency can be tagged as optional.
A graphical representation
How to view the dependency tree
It is possible to view the dependency tree of the project POM via a command line as well as via most modern IDEs. Command line options for viewing the dependency tree:
View full dependency tree of the POM
View a verbose dependency tree of the POM
mvn dependency:tree -Dverbose=true
mvn dependency:tree -Dverbose
NOTE: The verbose flag is true if the option is mentioned, so an “=true” can be removed.
PERSONAL OPINION: Prefer the usage of -D<option>=<value> over -D<option>.
CAUTION: This produces a lot of output !
View a verbose dependency tree of the POM for a specific dependency
mvn dependency:tree -Dverbose=true -Dincludes=<groupId>
mvn dependency:tree -Dverbose=true -Dincludes=<groupId>:<artifactId>
How Maven resolves transitive dependency versions
A project POM can include several dependencies, which may further have varying depths of transitive dependencies. It is very possible that a few dependencies share transitive dependencies but depend on different versions. Maven is thus tasked with electing the right transitive dependency to use for its effective POM, to avoid duplication. Since Maven cannot sort version strings (versions are arbitrary strings and may not follow a strict semantic sequence), Maven takes the approach of nearest transitive dependency in the tree depth. This is very similar to how Java picks up the first jar in the class path when looking for a fully qualified class name.
to illustrate with an example, let us look at the transitive dependency Dx in the example below.
POM P1 has a few dependencies listed below (with dummy Group, Artifact and Version numbers) with transitive dependencies shown as
- Dependency D1 (G1:A1:V1)
- Dependency D2 (G2:A2:V2)
- Dependency D3 (G3:A3:V3)
- Dependency D4 (G4:A4:V4)
Maven creates a dependency tree during its effective POM generation that is illustrated below:
the above example shows V1.2.0 of Dx as the transitive dependency of choice since it is nearest in depth and first in resolution in this dependency tree.
Helping Maven pick a different version
Add a direct dependency
Adding the desired transitive dependency version as a direct dependency in the project POM will result in such a dependency being the nearest in depth, and thus the dependency version to be selected. In the above example, if the desired version to be used was v1.3.0, then adding a dependency D5 (Gx:Vx:V1.3.0) would ensure its selection.
A project may contain several modules as was highlighted in Part 3 of this series. Often times, both for compatibility enforcement and POM hygiene, it is necessary to ensure the same version of the dependency be used across all child modules. In addition, the ability to override the nearest depth selection by selecting a specific version requires a lookup section in the POM. A dependencyManagement section in a POM is such a lookup.
Adding dependencies in a dependencyManagement does not include them in the dependency graph, rather provides a lookup table for Maven to help determine the selected or chosen version of the transitive dependency that is listed.
A dependencyManagement section contains a dependencies element. Each dependency listed under is a lookup reference used either in the current POM or in any POM that inherits (either any POM that identifies the current POM as a parent or any POM that imports the project POM as a bill-of-materials).
Inheriting a dependencyManagement implies a few items:
- Once a dependency is listed in the section, any inheriting POMs can skip the version attribute when declaring the dependency. A version is no longer required, since the dependencyManagement provides one. Deliberately adding a version will override what the managed section defines, so standard Maven version nearest depth kicks in.
- A project POM can acquire the a managed dependency version by either declaring parentage or by importing a bill-of-materials.
- Maven uses the dependencyManagement during the effective POM generation phase.
- Declaring a dependency in the dependencyManagement structure is just for a lookup reference.
- If a dependency defined in the dependencyManagement is never encountered in the actual dependency tree for the current project, it is ignored when generating the effective POM.
- A bill-of-materials POM is typically a large dependencyManagement block of compatible versions of several potential transitive dependencies that may (or may not) be required in the current project.
- A bill-of-materials (BOM) POM is a special POM of packaging type of
pom. The BOM POM is
imported into the project POM as a dependency with a scope of
An amazing resource to find out more about best practices for Maven can be found at: https://jlbp.dev/.
That is a wrap on this article! There is a lot more to cover on this topic including version ranges and enforcing version rules. These topics will be covered as separate upcoming articles in the series!