# Binary Plugin Testing

version 9.4.1-20260311015558+0000

Gradle provides a robust mechanism for testing binary plugins.

Gradle’s `TestKit` allows you to programmatically execute synthetic builds that use the plugin under development, all within the plugin’s own build.

A well-tested plugin typically includes two levels of testing:

1. Unit Tests

2. Functional Tests

The directory of our plugin looks as follows:

    .
    └── plugin
        ├── settings.gradle.kts
        ├── build.gradle.kts
        └── src
           ├── main
           │   └── java/org/example
           │       ├── FileSizeDiffTask.java
           │       ├── FileSizeDiffPlugin.java
           │       └── FileSizeDiffExtension.java
           ├── test
           │   └── java/org/example
           │       └── FileSizeDiffPluginTest.java
           └── functionalTest
               └── java/org/example
                   └── FileSizeDiffPluginFunctionalTest.java

    .
    └── plugin
        ├── settings.gradle
        ├── build.gradle
        └── src
           ├── main
           │   └── java/org/example
           │       ├── FileSizeDiffTask.java
           │       ├── FileSizeDiffPlugin.java
           │       └── FileSizeDiffExtension.java
           ├── test
           │   └── java/org/example
           │       └── FileSizeDiffPluginTest.java
           └── functionalTest
               └── java/org/example
                   └── FileSizeDiffPluginFunctionalTest.java

## 1. Unit Test

Unit tests validate the internal behavior of your plugin in a lightweight, isolated project. These tests simulate applying your plugin without needing a full Gradle execution.

They test:

- The plugin applies without errors

- Tasks or extensions are registered correctly

The unit test for the `filesizediff` plugin looks as follows:

src/test/java/org/example/FileSizeDiffPluginTest

    package org.example;

    import org.gradle.testfixtures.ProjectBuilder;
    import org.gradle.api.Project;
    import org.junit.jupiter.api.Test;
    import static org.junit.jupiter.api.Assertions.*;

    class FileSizeDiffPluginTest {
        @Test void pluginRegistersATask() {
            // Create a test project and apply the plugin
            Project project = ProjectBuilder.builder().build();
            project.getPlugins().apply("org.example.filesizediff");

            // Verify the result
            assertNotNull(project.getTasks().findByName("fileSizeDiff"));
        }
    }

This test checks that the `org.example.filesizediff` plugin is applied and a `fileSizeDiff` task is added.

Unit tests are fast and useful for verifying the basic mechanics of your plugin logic.

## 2. Functional Test

Functional tests (also known as end-to-end plugin tests) use Gradle’s `TestKit` to launch real Gradle builds in a temporary directory. This is how you verify your plugin works correctly in real-world usage.

They test:

- The plugin can be applied to a real build

- The plugin task runs correctly and produces the expected output

- Users can configure the plugin via the DSL

The functional test for the `filesizediff` plugin looks as follows:

src/functionalTest/java/org/example/FileSizeDiffPluginFunctionalTest.java

    package org.example;

    import org.gradle.testkit.runner.BuildResult;
    import org.gradle.testkit.runner.GradleRunner;
    import org.gradle.testkit.runner.TaskOutcome;
    import org.junit.jupiter.api.BeforeEach;
    import org.junit.jupiter.api.Test;
    import org.junit.jupiter.api.io.TempDir;

    import java.io.File;
    import java.io.IOException;
    import java.nio.file.Files;

    import static org.junit.jupiter.api.Assertions.assertEquals;
    import static org.junit.jupiter.api.Assertions.assertTrue;

    public class FileSizeDiffPluginFunctionalTest {
        // Temporary directory for each test, automatically cleaned up after the test run
        @TempDir
        File projectDir;

        // Helper to get reference to build.gradle in the temp project
        private File getBuildFile() {
            return new File(projectDir, "build.gradle");
        }

        // Helper to get reference to settings.gradle in the temp project
        private File getSettingsFile() {
            return new File(projectDir, "settings.gradle");
        }

        // Create minimal build and settings files before each test
        @BeforeEach
        void setup() throws IOException {
            // Empty settings.gradle
            writeString(getSettingsFile(), "");

            // Apply the plugin and configure the extension in build.gradle
            writeString(getBuildFile(), """
                plugins {
                    id("org.example.filesizediff")
                }
                diff {
                    file1 = file("a.txt")
                    file2 = file("b.txt")
                }
            """
            );
        }

        // Test case: both input files have the same size (empty)
        @Test
        void canDiffTwoFilesOfTheSameSize() throws IOException {
            // Create empty file a.txt
            writeString(new File(projectDir, "a.txt"), "");
            // Create empty file b.txt
            writeString(new File(projectDir, "b.txt"), "");

            // Run the build with the plugin classpath and invoke the fileSizeDiff task
            BuildResult result = GradleRunner.create()
                    .withProjectDir(projectDir)
                    .withPluginClasspath()
                    .withArguments("fileSizeDiff")
                    .build();

            // Verify the output message and successful task result
            assertTrue(result.getOutput().contains("Files have the same size"));
            assertEquals(TaskOutcome.SUCCESS, result.task(":fileSizeDiff").getOutcome());
        }

        // Test case: first file is larger than second file
        @Test
        void canDiffTwoFilesOfDiffSize() throws IOException {
            // File a.txt has 7 bytes
            writeString(new File(projectDir, "a.txt"), "dsdsdad");
            // File b.txt is empty
            writeString(new File(projectDir, "b.txt"), "");

            // Run the build and invoke the plugin task
            BuildResult result = GradleRunner.create()
                    .withProjectDir(projectDir)
                    .withPluginClasspath()
                    .withArguments("fileSizeDiff")
                    .build();

            // Verify the output message indicates a.txt is larger
            assertTrue(result.getOutput().contains("a.txt was larger: 7 bytes"));
            assertEquals(TaskOutcome.SUCCESS, result.task(":fileSizeDiff").getOutcome());
        }

        // Helper method to write string content to a file
        private static void writeString(File file, String string) throws IOException {
            Files.writeString(file.toPath(), string);
        }
    }

`GradleRunner` is the core API provided by `TestKit` for executing builds in a test environment. `GradleRunner` simulates a Gradle invocation. Each test creates a temporary project with a `build.gradle` file, input files, and runs the plugin task using `GradleRunner`.

Unlike unit tests in `src/test/java` that test individual classes, functional tests live in `src/functionalTest/java` and verify that your plugin behaves correctly in a real build. Gradle doesn’t automatically recognize custom test source sets, so you need to declare a `functionalTest` source set and configure a task to run it.

Fortunately, `gradle init` can scaffold this setup for you:

plugin/build.gradle.kts

    plugins {
        `java-gradle-plugin`    (1)
    }

    group = "org.example"   (3)
    version = "1.0.0"

    repositories {
        mavenCentral()
    }

    dependencies {  (2)
        testImplementation("org.junit.jupiter:junit-jupiter-api:5.10.0")
        testRuntimeOnly("org.junit.jupiter:junit-jupiter-engine:5.10.0")
        testRuntimeOnly("org.junit.platform:junit-platform-launcher")
    }

    gradlePlugin {  (3)
        plugins {
            create("filesizediff") {
                id = "org.example.filesizediff"
                implementationClass = "org.example.FileSizeDiffPlugin"
            }
        }
    }

    // Created by gradle init

    // Add a source set for the functional test suite
    val functionalTestSourceSet = sourceSets.create("functionalTest") {
    }

    configurations["functionalTestImplementation"].extendsFrom(configurations["testImplementation"])
    configurations["functionalTestRuntimeOnly"].extendsFrom(configurations["testRuntimeOnly"])

    // Add a task to run the functional tests
    val functionalTest by tasks.registering(Test::class) {
        description = "Runs functional tests."
        group = "verification"
        testClassesDirs = functionalTestSourceSet.output.classesDirs
        classpath = functionalTestSourceSet.runtimeClasspath
        useJUnitPlatform()
    }

    gradlePlugin.testSourceSets.add(functionalTestSourceSet)

    tasks.named<Task>("check") {
        // Run the functional tests as part of `check`
        dependsOn(functionalTest)
    }

    tasks.named<Test>("test") {
        // Use JUnit Jupiter for unit tests.
        useJUnitPlatform()
    }

plugin/build.gradle

    plugins {
        id('java-gradle-plugin')    (1)
    }

    group = "org.example"   (3)
    version = "1.0.0"

    repositories {
        mavenCentral()
    }

    dependencies {  (2)
        testImplementation('org.junit.jupiter:junit-jupiter-api:5.10.0')
        testRuntimeOnly('org.junit.jupiter:junit-jupiter-engine:5.10.0')
        testRuntimeOnly('org.junit.platform:junit-platform-launcher')
    }

    gradlePlugin {  (3)
        plugins {
            filesizediff {
                id = 'org.example.filesizediff'
                implementationClass = 'org.example.FileSizeDiffPlugin'
            }
        }
    }

    // Created by gradle init

    // Add a source set for the functional test suite
    sourceSets {
        functionalTest {
        }
    }

    configurations {
        functionalTestImplementation.extendsFrom(testImplementation)
        functionalTestRuntimeOnly.extendsFrom(testRuntimeOnly)
    }

    // Add a task to run the functional tests
    tasks.register('functionalTest', Test) {
        description = 'Runs functional tests.'
        group = 'verification'
        testClassesDirs = sourceSets.functionalTest.output.classesDirs
        classpath = sourceSets.functionalTest.runtimeClasspath
        useJUnitPlatform()
    }

    // Include functional test source set in plugin validation
    gradlePlugin.testSourceSets.add(sourceSets.functionalTest)

    // Run functional tests as part of check
    tasks.named('check', Task) {
        dependsOn tasks.named('functionalTest')
    }

    // Use JUnit Platform for unit tests
    tasks.named('test', Test) {
        useJUnitPlatform()
    }

## Consumer Project

You can optionally create a consumer project that uses your plugin:

    .
    ├── settings.gradle.kts // Include custom plugin
    ├── build.gradle.kts    // Applies custom plugin
    │
    └── plugin
        ├── settings.gradle.kts
        ├── build.gradle.kts
        └── src
           ...

    .
    ├── settings.gradle.kts // Include custom plugin
    ├── build.gradle.kts    // Applies custom plugin
    │
    └── plugin
        ├── settings.gradle
        ├── build.gradle
        └── src
           ...

First you can point to the plugin as an included build in the `consumer` settings file:

settings.gradle.kts

    rootProject.name = "consumer"

    includeBuild("plugin")

settings.gradle

    rootProject.name = 'consumer'

    includeBuild("plugin")

Then in the build file of the `consumer` project, you apply the plugin and test it:

build.gradle.kts

    plugins {
        id("org.example.filesizediff")
    }

    diff {
        file1 = file("a.txt")
        file2 = file("b.txt")
    }

build.gradle

    plugins {
        id("org.example.filesizediff")
    }

    diff {
        file1 = file('a.txt')
        file2 = file('b.txt')
    }

In the `consumer` project, you can create dummy `a.txt` and `b.txt` files and run `./gradlew fileSizeDiff`:

    $ ./gradlew fileSizeDiff

    > Task :fileSizeDiff
    Files have the same size: 0 bytes
    Wrote diff result to /home/user/gradle/samples/build/diff-result.txt

    BUILD SUCCESSFUL in 0s

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