Gradle Dependencies and Dependency Configurations

External

• https://docs.gradle.org/current/userguide/core_dependency_management.html
• https://docs.gradle.org/current/userguide/dependency_management_terminology.html
• https://docs.gradle.org/current/userguide/dependency_management_for_java_projects.html

Internal

• Gradle Project
• Gradle Java Plugin

TO DEPLETE

Gradle Dependencies and Dependency Configurations TODEPLETE

TODO

• https://docs.gradle.org/current/userguide/dependency_management_terminology.html

Overview

The vast majority of Java projects rely on external libraries so managing a project's dependencies is an important part of building a Java project.

Specifying a dependency for a Java project requires just three pieces of information:

• Which dependency - specified by the library name and version. This is specified in the dependencies {} block.
• What is needed for - compilation or running. This is specified in the dependencies {} block.
• Where to look for it. This is specified in the repositories {...} block.

Dependencies for a project can be managed with Project.dependencies(Closure), Project.getDependencies() and the "dependencies" build.gradle DSL element:

dependencies {
  // configuration closure
}

Configurations for a project can be managed with Project.configurations(Closure), Project.getConfigurations() and the "configurations" build.gradle DSL element:

configurations {
  // configuration closure
}

Concepts

Configuration

DSL Reference https://docs.gradle.org/current/dsl/org.gradle.api.artifacts.Configuration.html

A configuration represents a named collection of artifacts and their dependencies, grouped together for a specific goal such as compiling and running the code from a source set. A configuration is similar to a Maven scope.

Configuration, in this context, does not refer to a configuration file of any sort, as in a settings configuration settings.gradle or a build configuration build.gradle. It refers to named set of artifacts.

Each source set points to several configurations required to compile and run the code included in the source set. Also see:

Source Set

Default Configurations added by the Java Plugin

compileOnly

The Java plugin "compileOnly" configuration defines dependencies that are necessary to compile the main source set code, but that shouldn't be part of the runtime classpath.

runtimeOnly

The Java plugin "runtimeOnly" configuration defines dependencies that are necessary to run the main source set code, but not for compilation.

implementation

The Java plugin "implementation" configuration defines dependencies that are necessary to compile and run the main source set code. "implementation" supersedes the "compile" configuration.

testCompileOnly

The Java plugin "testCompileOnly" configuration defines dependencies that are necessary to compile the test source set code, but that shouldn't be part of the runtime classpath.

testRuntimeOnly

The Java plugin "testRuntimeOnly" configuration defines dependencies that are necessary to run the test source set code, but not for compilation.

testImplementation

The Java plugin "testImplementation" configuration defines dependencies that are necessary to compile and run the test source set code.

api

A Java Library plugin configuration.

compileOnlyApi

A Java Library plugin configuration.

Deprecated Configurations

compile

This is a configuration that was used for dependencies that are required to both compile and run the main source set code. It is deprecated and it will issue a warning when it is used. It does not distinguish between dependencies that impact the public API of a java library project.

build.gradle Syntax

dependencies {
  implementation 'io.someproject:someartifact:0.1.0'
}

The Dependency Cache

https://docs.gradle.org/current/userguide/dependency_cache.html

Once a dependency is resolved as result of the dependency resolution, Gradle stores the associated artifacts in a local cache, known as the dependency cache. One of the goals of storing artifacts locally in the cache is to minimize the number of remote requests made during the dependency resolution process.

The dependency cache is located under $GRADLE_USER_HOME/caches (typically ~/.gradle/caches). The directory contains two storage types: file-based storage for downloaded artifacts and raw metadata files, and a binary store of resolved module meta-data. This cache is also known as the metadata cache. The metadata cache contains, among other things, the result of resolving dynamic versions to concrete versions, the resolved module metadata, including module artifacts and module dependencies, the resolved artifact metadata, including a pointer to the downloaded file in the file-based storage, the absence of a particular module or artifact, eliminating repeated attempts to access a resource that does not exist.

For a specific dependency version, the files are maintained under .gradle/caches/modules-2/files-* and the corresponding binary metadata is maintained in .gradle/caches/modules-2/metadata-*. For example, the binary artifacts for SLF4J API 1.7.25, where "org.slf4j" is the group ID and "slf4j-api" is the artifact ID, are stored as follows:

.gradle/caches/modules-2/files-2.1/org.slf4j/slf4j-api/1.7.25
                                                         |
                                                         + df51c4a85dd6acf8b6cdc9323596766b3d577c28/slf4j-api-1.7.25.pom
                                                         |
                                                         + da76ca59f6a57ee3102f8f9bd9cee742973efa8a/slf4j-api-1.7.25.jar
                                                         |
                                                         + 962153db4a9ea71b79d047dfd1b2a0d80d8f4739/slf4j-api-1.7.25-sources.jar

The corresponding binary metadata is stored as:

.gradle/caches/modules-2/metadata-2.58/descriptors/org.slf4j/slf4j-api/1.7.25
                                                                         |
                                                                         + 1dd858de07b774d6be9d3e38c5646087/descriptor.bin
                                                                         |
                                                                         + 33ddd5448b9e1258ce46034f20c899a8/descriptor.bin
                                                                         |
                                                                         + f8e6315c37eb56998f7a5ba08e30db71/descriptor.bin

Dependency resolution will fail if the required artifacts are not available in any repository specified in the build, even if the local cache has a copy of the artifact that was retrieved from a different repository.

Before downloading an artifact from a repository, Gradle tries to get its checksum first by attempting to download the SHA file associated with the artifact. If the checksum can be retrieved, and Gradle determines that a file with the same id and checksum exists in the cache. If the checksum cannot be retrieved, the artifact will be downloaded (and ignored if it matches an existing artifact).

The cache is periodically scanned (at most every 24 hours) for artifacts that have not been used for more than 30 days and obsolete artifacts are deleted, to ensure the cache does not grow indefinitely.

Forced Cache Update

Gradle can be told to ignore all cached entries for resolved modules and artifacts with the --refresh-dependencies command line option. When used, a fresh resolve will be performed against all configured repositories, with dynamic versions recalculated, modules refreshed, and artifacts downloaded. However, where possible Gradle will check if the previously downloaded artifacts are valid before downloading again. This is done by comparing published SHA1 values in the repository with the SHA1 values for existing downloaded artifacts. A new snapshot repository will produce the following log:

gradle clean build --refresh-dependencies
...
Download s3://my-repo/snapshots/com/example/my-artifact/1.0.0-SNAPSHOT/maven-metadata.xml
...
Download s3://my-repo/snapshots/com/example/my-artifact/1.0.0-SNAPSHOT/my-artifact-1.0.0-20181008.231746-18.jar

Low-Level Cache Manipulation

It is sometimes necessary to remove individual dependency from cache. TODO.