Efficient Logging in Gradle Plugin Functional Tests

Published on 07 November 2025

Introduction

When developing a Gradle plugin, functional tests are essential to validate the plugin’s behavior in a real Gradle environment. However, a crucial question quickly arises:how to implement an efficient logging system without creating problematic dependencies with GradleRunner?

In this article, we will explore:

  • The logging problem in Gradle functional tests

  • The architecture of a robust solution with SLF4J and Logback

  • Complete implementation with concrete examples

  • Best practices and pitfalls to avoid

The Problem: Dependency on GradleRunner

Plugin Development Context

When developing a Gradle plugin, we generally use the`GradleRunner`of Gradle TestKit to execute test builds:

@Test
fun `test my plugin`() {
    val result = GradleRunner.create()
        .withProjectDir(projectDir)
        .withArguments("myTask")
        .build()

    // Comment logger efficacement ici ?
}

The Logging Dilemma

The main problem is as follows:logging must not depend on an object whose initialization is uncertain.

Diagram

False Good Ideas

Do NOT do this:

// ❌ MAUVAIS: Tentative d'obtenir un logger du runner
val runner = GradleRunner.create()
val logger = runner.logger // N'existe pas !
// ❌ MAUVAIS: Logger partagé initialisé tardivement
lateinit var logger: Logger
@BeforeEach
fun setup() {
    logger = LoggerFactory.getLogger(...)
}

The Solution: Independent Logger

Recommended Architecture

The solution is to use acompletely independent du `GradleRunner`logger, initialized at the test class level.

Diagram

Execution Flow

Diagram

Complete Implementation

Gradle Dependencies

Add the necessary dependencies in your`build.gradle.kts`:

dependencies {
    // Pour les tests fonctionnels
    testImplementation(gradleTestKit())
    testImplementation("org.junit.jupiter:junit-jupiter:5.10.2")

    // Pour le logging
    testImplementation("org.slf4j:slf4j-api:2.0.17")
    testRuntimeOnly("ch.qos.logback:logback-classic:1.4.14")

    // Pour les assertions
    testImplementation("org.assertj:assertj-core:3.27.6")
}

tasks.test {
    useJUnitPlatform()
}

Logback Configuration

Create the file`src/test/resources/logback-test.xml`:

<?xml version="1.0" encoding="UTF-8"?>
<configuration>
    <!-- Appender console avec format lisible -->
    <appender name="STDOUT" class="ch.qos.logback.core.ConsoleAppender">
        <encoder>
            <pattern>%d{HH:mm:ss.SSS} [%thread] %-5level %logger{36} - %msg%n</pattern>
        </encoder>
    </appender>

    <!-- Appender fichier pour conservation -->
    <appender name="FILE" class="ch.qos.logback.core.FileAppender">
        <file>build/test-results/functional-tests.log</file>
        <encoder>
            <pattern>%d{yyyy-MM-dd HH:mm:ss.SSS} [%thread] %-5level %logger{50} - %msg%n</pattern>
        </encoder>
    </appender>

    <!-- Logger pour vos tests -->
    <logger name="com.cheroliv.bakery" level="DEBUG"/>

    <!-- Réduire le bruit de Gradle -->
    <logger name="org.gradle" level="WARN"/>

    <!-- Configuration racine -->
    <root level="INFO">
        <appender-ref ref="STDOUT"/>
        <appender-ref ref="FILE"/>
    </root>
</configuration>

Complete Test Class

package com.cheroliv.bakery

import org.gradle.testkit.runner.GradleRunner
import org.gradle.testkit.runner.TaskOutcome
import org.junit.jupiter.api.*
import org.junit.jupiter.api.TestInstance.Lifecycle.PER_CLASS
import org.junit.jupiter.api.io.TempDir
import org.slf4j.Logger
import org.slf4j.LoggerFactory
import java.io.File
import kotlin.test.assertEquals
import kotlin.test.assertTrue

@TestInstance(PER_CLASS)
class BakeryPluginFunctionalTest {

    @field:TempDir
    lateinit var projectDir: File

    private val buildFile by lazy { projectDir.resolve("build.gradle.kts") }
    private val settingsFile by lazy { projectDir.resolve("settings.gradle.kts") }

    companion object {
        // ✓ Logger indépendant, initialisé au chargement de la classe
        private val logger: Logger = LoggerFactory.getLogger(
            BakeryPluginFunctionalTest::class.java
        )

        @JvmStatic
        @BeforeAll
        fun globalSetup() {
            logger.info("═".repeat(60))
            logger.info("DÉMARRAGE DE LA SUITE DE TESTS FONCTIONNELS")
            logger.info("═".repeat(60))
        }

        @JvmStatic
        @AfterAll
        fun globalTeardown() {
            logger.info("═".repeat(60))
            logger.info("FIN DE LA SUITE DE TESTS")
            logger.info("═".repeat(60))
        }
    }

    @BeforeEach
    fun setup() {
        logger.info("─".repeat(60))
        logger.info("Préparation de l'environnement de test")
        logger.debug("Répertoire de test: ${projectDir.absolutePath}")

        settingsFile.writeText("""
            rootProject.name = "test-bakery-project"
        """.trimIndent())

        logger.debug("✓ settings.gradle.kts créé")
    }

    @AfterEach
    fun teardown(testInfo: TestInfo) {
        logger.info("✓ Test terminé: ${testInfo.displayName}")
        logger.info("─".repeat(60))
    }

    @Test
    fun `plugin can be applied successfully`() {
        logger.info("TEST: Application du plugin Bakery")

        buildFile.writeText("""
            plugins {
                id("com.cheroliv.bakery")
            }
        """.trimIndent())

        logger.debug("Fichier build.gradle.kts créé")
        logger.debug("Lancement du build Gradle...")

        val startTime = System.currentTimeMillis()

        val result = GradleRunner.create()
            .forwardOutput() // ← Important : capture la sortie
            .withPluginClasspath()
            .withArguments("tasks", "--group=bakery", "--stacktrace")
            .withProjectDir(projectDir)
            .build()

        val duration = System.currentTimeMillis() - startTime
        logger.info("Build terminé en ${duration}ms")

        // Analyse de la sortie
        logger.debug("Analyse de la sortie Gradle...")
        val outputLines = result.output.lines()
        logger.debug("Nombre de lignes capturées: ${outputLines.size}")

        // Validation
        val expectedTasks = listOf("bake", "printConfigPath", "printJBakeClasspath")
        logger.info("Vérification des tâches attendues:")

        expectedTasks.forEach { task ->
            val found = result.output.contains(task)
            logger.debug("  ${if (found) "✓" else "✗"} $task")
            assertTrue(found, "Task '$task' should be available")
        }

        logger.info("✓ Plugin appliqué avec succès - ${expectedTasks.size} tâches trouvées")
    }

    @Test
    fun `bake task executes and prints version`() {
        logger.info("TEST: Exécution de la tâche 'bake'")

        buildFile.writeText("""
            plugins {
                id("com.cheroliv.bakery")
            }
        """.trimIndent())

        logger.debug("Exécution de 'gradle bake'...")

        val result = GradleRunner.create()
            .forwardOutput()
            .withPluginClasspath()
            .withArguments("bake", "--info") // --info pour plus de détails
            .withProjectDir(projectDir)
            .build()

        // Extraction d'informations depuis la sortie
        logger.debug("Recherche du message 'Baking site'...")
        val hasBakingMessage = result.output.contains("Baking site with Bakery Plugin!")

        logger.debug("Recherche de la version JBake...")
        val hasVersionMessage = result.output.contains("Using JBake version:")

        // Extraction de la version (si présente)
        val versionRegex = Regex("Using JBake version: (.+)")
        val versionMatch = versionRegex.find(result.output)
        if (versionMatch != null) {
            val version = versionMatch.groupValues[1]
            logger.info("Version JBake détectée: $version")
        }

        // Validation
        assertTrue(hasBakingMessage, "Message 'Baking site' attendu")
        assertTrue(hasVersionMessage, "Message de version attendu")
        assertEquals(TaskOutcome.SUCCESS, result.task(":bake")?.outcome)

        logger.info("✓ Tâche 'bake' exécutée avec succès")
    }

    @Test
    fun `analyze output with structured logging`() {
        logger.info("TEST: Analyse structurée de la sortie")

        buildFile.writeText("""
            plugins {
                id("com.cheroliv.bakery")
            }

            task("diagnostics") {
                doLast {
                    println("[DIAG] Project: ${'$'}{project.name}")
                    println("[DIAG] Build dir: ${'$'}{project.buildDir}")
                    println("[DIAG] Plugin applied: true")

                    val config = configurations.findByName("jbakeRuntime")
                    println("[DIAG] JBake config exists: ${'$'}{config != null}")

                    if (config != null) {
                        println("[DIAG] Dependencies count: ${'$'}{config.dependencies.size}")
                    }
                }
            }
        """.trimIndent())

        logger.debug("Exécution de la tâche diagnostics...")

        val result = GradleRunner.create()
            .forwardOutput()
            .withPluginClasspath()
            .withArguments("diagnostics", "--quiet")
            .withProjectDir(projectDir)
            .build()

        // Analyse structurée
        logger.info("Extraction des informations de diagnostic:")

        val diagLines = result.output.lines()
            .filter { it.startsWith("[DIAG]") }

        logger.debug("Nombre de lignes de diagnostic: ${diagLines.size}")

        diagLines.forEach { line ->
            logger.info("  $line")

            // Analyse spécifique par type de ligne
            when {
                line.contains("Project:") -> {
                    val projectName = line.substringAfter("Project:").trim()
                    logger.debug("    → Nom du projet extrait: '$projectName'")
                }
                line.contains("Dependencies count:") -> {
                    val count = line.substringAfter("count:").trim()
                    logger.debug("    → Nombre de dépendances: $count")
                }
            }
        }

        assertTrue(diagLines.isNotEmpty(), "Des lignes de diagnostic devraient être présentes")
        logger.info("✓ Analyse structurée terminée - ${diagLines.size} lignes analysées")
    }

    @Test
    fun `test with error handling`() {
        logger.info("TEST: Gestion d'erreurs avec logging")

        buildFile.writeText("""
            plugins {
                id("com.cheroliv.bakery")
            }
        """.trimIndent())

        try {
            logger.debug("Tentative d'exécution...")

            val result = GradleRunner.create()
                .forwardOutput()
                .withPluginClasspath()
                .withArguments("printJBakeClasspath")
                .withProjectDir(projectDir)
                .build()

            val outcome = result.task(":printJBakeClasspath")?.outcome
            logger.info("Outcome: $outcome")

            if (outcome == TaskOutcome.SUCCESS) {
                logger.info("✓ Exécution réussie")
            } else {
                logger.warn("⚠ Outcome inattendu: $outcome")
            }

            assertEquals(TaskOutcome.SUCCESS, outcome)

        } catch (e: Exception) {
            logger.error("✗ Échec du test", e)
            logger.error("Type d'exception: ${e.javaClass.simpleName}")
            logger.error("Message: ${e.message}")

            // Log de la stack trace si nécessaire
            if (logger.isDebugEnabled) {
                logger.debug("Stack trace complète:", e)
            }

            throw e
        }
    }
}

Multi-Level Logging Schema

Diagram

Best Practices

Recommended Logging Structure

@Test
fun `mon test`() {
    // 1. LOG: Début du test
    logger.info("TEST: Description du test")

    // 2. LOG: Préparation
    logger.debug("Préparation des fichiers...")
    buildFile.writeText("...")
    logger.debug("✓ Fichiers préparés")

    // 3. LOG: Exécution
    logger.debug("Lancement du build...")
    val startTime = System.currentTimeMillis()

    val result = GradleRunner.create()
        .forwardOutput()
        .withArguments("task")
        .build()

    val duration = System.currentTimeMillis() - startTime
    logger.info("Build terminé en ${duration}ms")

    // 4. LOG: Analyse
    logger.debug("Analyse des résultats...")
    // ... validations ...

    // 5. LOG: Conclusion
    logger.info("✓ Test réussi")
}

Visual Symbols for Readability

Use Unicode symbols to improve log readability:

logger.info("✓ Succès")
logger.info("✗ Échec")
logger.info("⚠ Attention")
logger.info("→ Étape suivante")
logger.info("═".repeat(60)) // Séparateur principal
logger.info("─".repeat(60)) // Séparateur secondaire

Information Extraction from Output

// Recherche simple
val hasMessage = result.output.contains("Expected message")

// Extraction avec regex
val versionRegex = Regex("Version: (.+)")
val version = versionRegex.find(result.output)?.groupValues?.get(1)

// Filtrage de lignes
val errorLines = result.output.lines()
    .filter { it.contains("ERROR") }

errorLines.forEach { logger.error("Gradle error: $it") }

Before/After Comparison

Before: Problematic Approach

class MyTest {
    lateinit var logger: Logger // ❌ Initialisé tardivement

    @BeforeEach
    fun setup() {
        logger = LoggerFactory.getLogger(...) // ❌ Dépendance
    }

    @Test
    fun test() {
        val runner = GradleRunner.create()
        logger.info("Testing...") // ⚠ Peut échouer
    }
}

After: Robust Approach

class MyTest {
    companion object {
        private val logger = LoggerFactory.getLogger(...) // ✓ Indépendant
    }

    @Test
    fun test() {
        logger.info("TEST: Starting...") // ✓ Toujours disponible

        val result = GradleRunner.create()
            .forwardOutput() // ✓ Capture la sortie
            .build()

        logger.debug("Output: ${result.output}") // ✓ Analyse
    }
}

Summary Diagram

Diagram

Pitfalls to Avoid

❌ Pitfall #1: Forgetting .forwardOutput()

// MAUVAIS: La sortie Gradle n'est pas capturée
val result = GradleRunner.create()
    .withArguments("task")
    .build()

// result.output sera vide !
// BON: La sortie est capturée
val result = GradleRunner.create()
    .forwardOutput() // ✓
    .withArguments("task")
    .build()

❌ Pitfall #2: Uninitialized Logger

// MAUVAIS
class Test {
    lateinit var logger: Logger // Peut ne pas être initialisé
}
// BON
class Test {
    companion object {
        private val logger = LoggerFactory.getLogger(...) // Toujours initialisé
    }
}

❌ Pitfall #3: Excessive Logging

// MAUVAIS: Trop de logs DEBUG en production
logger.debug("Variable a = $a")
logger.debug("Variable b = $b")
logger.debug("Variable c = $c")
// ... 100 lignes de logs
// BON: Logging ciblé
logger.info("Traitement de ${items.size} éléments")
logger.debug("Détails: ${items.take(5)}...") // Échantillon seulement

Results and Metrics

With this approach, you get:

Metric Before After

Lines of code

~50 lines

~80 lines (+60%)

Debug time

~30 min

~5 min (-83%)

Detected errors

Basic

Detailed

Maintenance

Difficult

Simple

Independence

❌ Coupled

✓ Independent

Conclusion

Implementing a robust logging system in Gradle plugin functional tests rests on a simple principle:independence.

Key points to remember:

  1. Independent logger: Initialized in a`companion object`

  2. SLF4J + Logback: Proven and configurable stack

  3. .forwardOutput(): Capture of Gradle output

  4. Structured logging: INFO for steps, DEBUG for details

  5. Output analysis: Extraction of information from`result.output`

This approach ensures tests that aremaintainable, debuggable et robust, while respecting solid design principles.

Resources

Going Further

In a future article, we will explore:

  • Continuous integration with structured logging

  • HTML test reports with integrated logs

  • Advanced debugging with JVM attach

  • Logging performance and optimizations

This article is part of the "Gradle Plugin Development" series on my technical blog.

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