Chapter 8. Dependency Management Basics

Very roughly, dependency management is made up of two pieces. Firstly, Gradle needs to know about the things that your project needs to build or run, in order to find them. We call these incoming files the dependencies of the project. Secondly, Gradle needs to build and upload the things that your project produces. We call these outgoing files the publications of the project. Let's look at these two pieces in more detail:

Most projects are not completely self-contained. They need files built by other projects in order to be compiled or tested and so on. For example, in order to use Hibernate in my project, I need to include some Hibernate jars in the classpath when I compile my source. To run my tests, I might also need to include some additional jars in the test classpath, such as a particular JDBC driver or the Ehcache jars.

These incoming files form the dependencies of the project. Gradle allows you to tell it what the dependencies of your project are, so that it can take care of finding these dependencies, and making them available in your build. The dependencies might need to be downloaded from a remote Maven or Ivy repository, or located in a local directory, or may need to be built by another project in the same multi-project build. We call this process dependency resolution.

Note that this feature provides a major advantage over Ant. With Ant, you only have the ability to specify absolute or relative paths to specific jars to load. With Gradle, you simply declare the “names” of your dependencies, and other layers determine where to get those dependencies from. You can get similar behavior from Ant by adding Apache Ivy, but Gradle does it better.

Often, the dependencies of a project will themselves have dependencies. For example, Hibernate core requires several other libraries to be present on the classpath with it runs. So, when Gradle runs the tests for your project, it also needs to find these dependencies and make them available. We call these transitive dependencies.

The main purpose of most projects is to build some files that are to be used outside the project. For example, if your project produces a java library, you need to build a jar, and maybe a source jar and some documentation, and publish them somewhere.

These outgoing files form the publications of the project. Gradle also takes care of this important work for you. You declare the publications of your project, and Gradle take care of building them and publishing them somewhere. Exactly what “publishing” means depends on what you want to do. You might want to copy the files to a local directory, or upload them to a remote Maven or Ivy repository. Or you might use the files in another project in the same multi-project build. We call this process publication.

Let's look at some dependency declarations. Here's a basic build script:

apply plugin: 'java'

repositories {
    mavenCentral()
}

dependencies {
    compile group: 'org.hibernate', name: 'hibernate-core', version: '3.6.7.Final'
    testCompile group: 'junit', name: 'junit', version: '4.+'
}

What's going on here? This build script says a few things about the project. Firstly, it states that Hibernate core 3.6.7.Final is required to compile the project's production source. By implication, Hibernate core and its dependencies are also required at runtime. The build script also states that any junit >= 4.0 is required to compile the project's tests. It also tells Gradle to look in the Maven central repository for any dependencies that are required. The following sections go into the details.

In Gradle dependencies are grouped into configurations. A configuration is simply a named set of dependencies. We will refer to them as dependency configurations. You can use them to declare the external dependencies of your project. As we will see later, they are also used to declare the publications of your project.

The Java plugin defines a number of standard configurations. These configurations represent the classpaths that the Java plugin uses. Some are listed below, and you can find more details in Table 23.5, “Java plugin - dependency configurations”.

Various plugins add further standard configurations. You can also define your own custom configurations to use in your build. Please see Section 51.3, “Dependency configurations” for the details of defining and customizing dependency configurations.

There are various types of dependencies that you can declare. One such type is an external dependency. This a dependency on some files built outside the current build, and stored in a repository of some kind, such as Maven central, or a corporate Maven or Ivy repository, or a directory in the local file system.

To define an external dependency, you add it to a dependency configuration:

dependencies {
    compile group: 'org.hibernate', name: 'hibernate-core', version: '3.6.7.Final'
}

An external dependency is identified using group, name and version attributes. Depending on which kind of repository you are using, group and version may be optional.

The shortcut form for declaring external dependencies looks like “group:name:version”.

dependencies {
    compile 'org.hibernate:hibernate-core:3.6.7.Final'
}

To find out more about defining and working with dependencies, have a look at Section 51.4, “How to declare your dependencies”.

How does Gradle find the files for external dependencies? Gradle looks for them in a repository. A repository is really just a collection of files, organized by group, name and version. Gradle understands several different repository formats, such as Maven and Ivy, and several different ways of accessing the repository, such as using the local file system or HTTP.

By default, Gradle does not define any repositories. You need to define at least one before you can use external dependencies. One option is use the Maven central repository:

repositories {
    mavenCentral()
}

Or a remote Maven repository:

repositories {
    maven {
        url "http://repo.mycompany.com/maven2"
    }
}

Or a remote Ivy repository:

repositories {
    ivy {
        url "http://repo.mycompany.com/repo"
    }
}

You can also have repositories on the local file system. This works for both Maven and Ivy repositories.

repositories {
    ivy {
        // URL can refer to a local directory
        url "../local-repo"
    }
}

A project can have multiple repositories. Gradle will look for a dependency in each repository in the order they are specified, stopping at the first repository that contains the requested module.

To find out more about defining and working with repositories, have a look at Section 51.6, “Repositories”.

Dependency configurations are also used to publish files.^[3] We call these files publication artifacts, or usually just artifacts.

The plugins do a pretty good job of defining the artifacts of a project, so you usually don't need to do anything special to tell Gradle what needs to be published. However, you do need to tell Gradle where to publish the artifacts. You do this by attaching repositories to the uploadArchives task. Here's an example of publishing to a remote Ivy repository:

uploadArchives {
    repositories {
        ivy {
            credentials {
                username "username"
                password "pw"
            }
            url "http://repo.mycompany.com"
        }
    }
}

Now, when you run gradle uploadArchives, Gradle will build and upload your Jar. Gradle will also generate and upload an ivy.xml as well.

You can also publish to Maven repositories. The syntax is slightly different.^[4] Note that you also need to apply the Maven plugin in order to publish to a Maven repository. when this is in place, Gradle will generate and upload a pom.xml.

apply plugin: 'maven'

uploadArchives {
    repositories {
        mavenDeployer {
            repository(url: "file://localhost/tmp/myRepo/")
        }
    }
}

To find out more about publication, have a look at Chapter 52, Publishing artifacts.

For all the details of dependency resolution, see Chapter 51, Dependency Management, and for artifact publication see Chapter 52, Publishing artifacts.

If you are interested in the DSL elements mentioned here, have a look at Project.configurations{}, Project.repositories{} and Project.dependencies{}.

Otherwise, continue on to some of the other tutorials.