Gradle Configurations Explained: What is the difference between API and Implementation?

Available configurations when java-library plugin is applied to gradle. Source: Gradle Java Library Plugin

Introduction

Gradle dependencies are grouped into sets called configurations. Different configurations are used for building classpath for the major two tasks — compile classpath is used for compilation and runtime classpath is used for running the application. The usage of the new configurations — api, implementation, and runtimeOnly is still a bit of a mystery for many developers, so let’s explain it on a very trivial example.

TL;DR

In case of exposing dependencies to the consumers, you need to focus on the pink boxes. Configurations included in apiElements are exposed to the consumers for their compile classpath. Analogously, configurations included in runtimeElements are exposed to the consumers for their runtime classpath. Therefore

  • api configuration is used for compile and runtime classpaths and it is also exposed to the consumers for their compile and runtime classpaths
  • implementation configuration is used for compile and runtime classpath but it is only exposed to the consumers for their runtime classpath
  • runtimeOnly configuration is only used for the runtime classpath and it is also exposed to the consumers for their runtime classpath
  • compileOnly configuration is only used for the compile classpath and it is not exposed to any consumer
  • compileOnlyApi configuration is only used for the compile classpath and it is also exposed to the consumers for their compile classpath when Gradle metadata is consumed

Christmas Dinner (Application Project)

There are some similarities between cooking and programming so let's make a traditional Czech Christmas dinner — potato salad with something deep-fried and breaded, originally a fish. If it were a Java project then it could be described as a following set of dependencies.

plugins {
id 'application'
}

dependencies {
api project(':dish')
api project(':cutlery')
api project(':potato-salad')
api project(':breaded-dish')

implementation project(':potato-salad-with-ham')
implementation project(':breaded-carp')

runtimeOnly project(':plate')
}

mainClassName = 'christmas_dinner.ChristmasDinner'

The traditional Christmas dish consists of a potato salad and some breaded dishes. These two dishes are generic ones and we will have to search for specific recipes. Therefore these can be analogous to the project's API and therefore they are declared as api configuration.

The potato salad with ham and breaded carp are these particular recipes being used for the dinner this year but can be easily altered with, for example, light wiener potato salad and wienerschnitzel. We won't announce these to your guests. Similar to this, we use implementation configuration in Gradle to hide the internals of our project. Typically, if some classes from a library are only used within the method body then we can use implementation configuration. They are required to compile the project but they are not exposed for the compilation to the depending projects.

The last group contains resources that are only required for serving the dish. You won't drop the dishes directly on the table so you need the plate. Analogously, there are the dependencies that are only used in runtime when serving the application to either run it or to enhance it. Typical examples could be the server implementation and the management library.

Here is a pseudocode for serving the dish:

In the following example, you can see that you cannot use any of dependencies on the runtime classpath in your code:

Plate cannot be reached because it has been declared as runtimeOnly dependency as well asOnion which is part of the implementation configuration of the potato salad with ham. Pictures like this feel counter-intuitive and may scare you as it looks like dependencies in the implementation configuration has not been taken into an account and it may look like Plate and Onion will be missing from the final distribution but as we see in section Shopping List (Runtime Classpath), all the necessary dependencies will be present in the runtime classpath.

You can also test it by executing the application:

./gradlew :christmas-dinner:run# result:Serving:
[plate with potato salad with ham, breaded carp]
Cutlery:
[fork, knife]

You can see all the runtime dependencies such as plate, fork and knife are present and found while running the application.

Potato Salad with Ham (Library Project)

In the end-projects such as the application that are not used as a dependency to any other project, the way how configurations work is pretty clear. What is not so clear is how the different types of dependencies behave transitively. Let's dig deeper into one of the dishes — the potato salad with ham:

dependencies {
api project(':dish')
api project(':potato-salad')
api project(':ham')

implementation project(':boiled-potatoes')
implementation project(':mayo')
implementation project(':onion')
implementation project(':pickles')
implementation project(':peppers')
implementation project(':bowl')
implementation project(':cutting-board')
implementation project(':vegetable-knife')
implementation project(':potato-slicer')

runtimeOnly project(':fork')
}

We can adopt a different point of view here. The groups are now bit different

  • first group contains the ingredients that should be exposed to the consumers such as the ham and potato salad
  • second group contains everything else required to prepare the dish
  • the last group contains the kitchenware required to serve the dish

Similar to that, we add to api configuration the dependencies that need to be visible to the consumer for the compilation. Anything else required for compilation is added into implementation configuration. No worries as these dependencies will be also available in the runtime. At last, runtimeOnly configuration only contains dependencies required in runtime.

If we return to our example we can see that we can actually use Ham directly into our final recipe as we declared it as api dependency but we cannot use Fork which is runtimeOnly dependnecy.

If we remove the offending Fork and run the application again, we can see ham is now also on the plate.

Serving:
[plate with potato salad with ham, breaded carp, ham]
Cutlery:
[fork, knife]

Shopping List (Runtime Classpath)

Let's imagine that the dinner should be served tonight in a brand new apartment. You want to be sure that you have everything ready for cooking as well as for serving the dish. You're planning to visit the department store nearby and you want to be sure that don't forget everything. This is how would the shopping list look like.

You can see the shopping list contains everything regardless how deep we had declared the ingredients or kitchenwere.

You can see that the image refers to the runtime classpath of the application. If you use the application plugin that all the JARs listed above will be exported into the lib folder of the generated bundle. Regardless using api, implementation and runtimeOnly configurations, all the required runtime dependencies will be included.

If we compare the runtime classpath to the compile classpath then we can see that only dependencies declared as api are available in the consumer projects.

You can see that none of projects declared as implementation or runtimeOnly are present on the compile classpath of the depending project.

Summary

If you are creating Java library which then you should think twice which of your dependencies are required for the compilation of the depending projects. If any type is exposed publicly into the signature of your classes and interfaces then you need to declare it as api. The dependencies used internally within method bodes can be declared as implementation. The dependencies only required in runtime can be declared as runtime.

There is a Gradle Lint project which can help you decide the right configuration to be used:

You can dig deeper into the Christmas Dinner example here:

Or inspect just the build scan:

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