The following graph describes the main configurations setup when the Java Library plugin is in use.

• The configurations in green are the ones a user should use to declare dependencies
• The configurations in pink are the ones used when a component compiles, or runs against the library
• The configurations in blue are internal to the component, for its own use
• The configurations in white are configurations inherited from the Java plugin

And the next graph describes the test configurations setup:

The compile, testCompile, runtime and testRuntime configurations inherited from the Java plugin are still available but are deprecated. You should avoid using them, as they are only kept for backwards compatibility.

The role of each configuration is described in the following tables:

Table 1. Java Library plugin – configurations used to declare dependencies

Table 2. Java Library plugin — configurations used by consumers

Table 3. Java Library plugin – configurations used by the library itself

Link: https://docs.gradle.org/current/userguide/java_library_plugin.html

The key difference between the standard Java plugin and the Java Library plugin is that the latter introduces the concept of an APIexposed to consumers. A library is a Java component meant to be consumed by other components. It’s a very common use case in multi-project builds, but also as soon as you have external dependencies.

The plugin exposes two configurations that can be used to declare dependencies: api and implementation. The apiconfiguration should be used to declare dependencies which are exported by the library API, whereas the implementationconfiguration should be used to declare dependencies which are internal to the component.

Example 2. Declaring API and implementation dependencies

dependencies {
    api("commons-httpclient:commons-httpclient:3.1")
    implementation("org.apache.commons:commons-lang3:3.5")
}

Dependencies appearing in the api configurations will be transitively exposed to consumers of the library, and as such will appear on the compile classpath of consumers. Dependencies found in the implementation configuration will, on the other hand, not be exposed to consumers, and therefore not leak into the consumers’ compile classpath. This comes with several benefits:

• dependencies do not leak into the compile classpath of consumers anymore, so you will never accidentally depend on a transitive dependency
• faster compilation thanks to reduced classpath size
• less recompilations when implementation dependencies change: consumers would not need to be recompiled
• cleaner publishing: when used in conjunction with the new maven-publish plugin, Java libraries produce POM files that distinguish exactly between what is required to compile against the library and what is required to use the library at runtime (in other words, don’t mix what is needed to compile the library itself and what is needed to compile against the library).

If your build consumes a published module with POM metadata, the Java and Java Library plugins both honor api and implementation separation through the scopes used in the pom. Meaning that the compile classpath only includes compilescoped dependencies, while the runtime classpath adds the runtime scoped dependencies as well.

This often does not have an effect on modules published with Maven, where the POM that defines the project is directly published as metadata. There, the compile scope includes both dependencies that were required to compile the project (i.e. implementation dependencies) and dependencies required to compile against the published library (i.e. API dependencies). For most published libraries, this means that all dependencies belong to the compile scope. However, as mentioned above, if the library is published with Gradle, the produced POM file only puts api dependencies into the compile scope and the remaining implementation dependencies into the runtime scope.

link: https://docs.gradle.org/current/userguide/java_library_plugin.html