Render rich assertion diff

documentation/documentation.gradle.kts

@@ -54,6 +54,12 @@ val tools by sourceSets.creating
 val toolsImplementation by configurations.getting
 
 dependencies {
+	repositories {
+		// TODO: Remove
+		mavenLocal()
+		mavenCentral()
+	}
+
 	implementation(projects.junitJupiterApi) {
 		because("Jupiter API is used in src/main/java")
 	}

gradle/libs.versions.toml

@@ -12,7 +12,7 @@ junit4 = "4.13.2"
 junit4Min = "4.12"
 ktlint = "1.8.0"
 log4j = "2.25.3"
-opentest4j = "1.3.0"
+opentest4j = "1.4.0-SNAPSHOT"
 openTestReporting = "0.2.5"
 snapshotTests = "1.11.0"
 surefire = "3.5.5"
@@ -67,6 +67,7 @@ openTestReporting-events = { module = "org.opentest4j.reporting:open-test-report
 openTestReporting-tooling-core = { module = "org.opentest4j.reporting:open-test-reporting-tooling-core", version.ref = "openTestReporting" }
 openTestReporting-tooling-spi = { module = "org.opentest4j.reporting:open-test-reporting-tooling-spi", version.ref = "openTestReporting" }
 picocli = { module = "info.picocli:picocli", version = "4.7.7" }
+javadiffutils = { module = "io.github.java-diff-utils:java-diff-utils", version = "4.15" }
 roseau-cli = { module = "io.github.alien-tools:roseau-cli", version = "0.5.0" }
 slf4j-julBinding = { module = "org.slf4j:slf4j-jdk14", version = "2.0.17" }
 snapshotTests-junit5 = { module = "de.skuzzle.test:snapshot-tests-junit5", version.ref = "snapshotTests" }

junit-jupiter-api/src/main/java/org/junit/jupiter/api/AssertionFailureBuilder.java

@@ -192,20 +192,37 @@ public void buildAndThrow() throws AssertionFailedError {
 	 */
 	public AssertionFailedError build() {
 		String reason = nullSafeGet(this.reason);
+		String message = nullSafeGet(this.message);
+		var assertionFailedError = new AssertionFailedError( //
+			formatExceptionMessage(reason, message), //
+			formatReason(reason, message), //
+			expected, //
+			actual, //
+			cause //
+		);
+		maybeTrimStackTrace(assertionFailedError);
+		return assertionFailedError;
+	}
+
+	private @Nullable String formatReason(@Nullable String reason, @Nullable String message) {
+		if (mismatch) {
+			// TODO: The suffix is implicit due to mismatch, but how to make explicit?
+			reason = (reason == null ? "" : reason + ", ") + "expected did not match actual";
+		}
+		if (reason != null) {
+			message = buildPrefix(message) + reason;
+		}
+		return message;
+	}
+
+	private @Nullable String formatExceptionMessage(@Nullable String reason, @Nullable String message) {
 		if (mismatch && includeValuesInMessage) {
 			reason = (reason == null ? "" : reason + ", ") + formatValues(expected, actual);
 		}
-		String message = nullSafeGet(this.message);
 		if (reason != null) {
 			message = buildPrefix(message) + reason;
 		}
-
-		var assertionFailedError = mismatch //
-				? new AssertionFailedError(message, expected, actual, cause) //
-				: new AssertionFailedError(message, cause);
-
-		maybeTrimStackTrace(assertionFailedError);
-		return assertionFailedError;
+		return message;
 	}
 
 	private void maybeTrimStackTrace(Throwable throwable) {

junit-platform-console/junit-platform-console.gradle.kts

@@ -18,6 +18,8 @@ dependencies {
 	compileOnlyApi(libs.jspecify)
 
 	shadowed(libs.picocli)
+	// TODO: Sort out relocation, licence, ect
+	shadowed(libs.javadiffutils)
 
 	osgiVerification(projects.junitJupiterEngine)
 	osgiVerification(projects.junitPlatformLauncher)
@@ -29,7 +31,8 @@ tasks {
 		options.compilerArgs.addAll(listOf(
 			"-Xlint:-module", // due to qualified exports
 			"--add-modules", "info.picocli",
-			"--add-reads", "${javaModuleName}=info.picocli"
+			"--add-reads", "${javaModuleName}=info.picocli",
+			"-Xlint:-requires-automatic", // Java diff utils
 		))
 		options.errorprone.nullaway {
 			excludedFieldAnnotations.addAll(

junit-platform-console/src/main/java/module-info.java

@@ -23,6 +23,7 @@
 	requires org.junit.platform.engine;
 	requires org.junit.platform.launcher;
 	requires org.junit.platform.reporting;
+	requires io.github.javadiffutils;
 
 	exports org.junit.platform.console.output to org.junit.start;
 

junit-platform-console/src/main/java/org/junit/platform/console/output/RichDiffFormatter.java

@@ -0,0 +1,72 @@
+/*
+ * Copyright 2015-2026 the original author or authors.
+ *
+ * All rights reserved. This program and the accompanying materials are
+ * made available under the terms of the Eclipse Public License v2.0 which
+ * accompanies this distribution and is available at
+ *
+ * https://www.eclipse.org/legal/epl-v20.html
+ */
+
+package org.junit.platform.console.output;
+
+import java.util.function.Function;
+
+import com.github.difflib.text.DiffRowGenerator;
+
+import org.opentest4j.AssertionFailedError;
+
+final class RichDiffFormatter {
+	public String format(AssertionFailedError assertionFailed) {
+		StringBuilder builder = new StringBuilder();
+
+		if (assertionFailed.isReasonDefined()) {
+			builder.append(": ");
+			builder.append(assertionFailed.getReason());
+		}
+		builder.append(System.lineSeparator());
+
+		builder.append("+ actual - expected");
+		builder.append(System.lineSeparator());
+
+		var generator = DiffRowGenerator.create() //
+				.lineNormalizer(Function.identity()) // Don't normalize lines
+				.showInlineDiffs(false) //
+				.build();
+
+		var diffRows = generator.generateDiffRows( //
+			assertionFailed.getExpected().getStringRepresentation().lines().toList(), //
+			assertionFailed.getActual().getStringRepresentation().lines().toList() //
+		);
+
+		diffRows.forEach(diffRow -> {
+			switch (diffRow.getTag()) {
+				case INSERT -> {
+					builder.append("+ ");
+					builder.append(diffRow.getNewLine());
+					builder.append(System.lineSeparator());
+				}
+				case DELETE -> {
+					builder.append("- ");
+					builder.append(diffRow.getOldLine());
+					builder.append(System.lineSeparator());
+				}
+				case CHANGE -> {
+					builder.append("+ ");
+					builder.append(diffRow.getOldLine());
+					builder.append(System.lineSeparator());
+					builder.append("- ");
+					builder.append(diffRow.getNewLine());
+					builder.append(System.lineSeparator());
+				}
+				case EQUAL -> {
+					builder.append("  ");
+					builder.append(diffRow.getNewLine());
+					builder.append(System.lineSeparator());
+				}
+			}
+		});
+
+		return builder.toString();
+	}
+}

junit-platform-console/src/main/java/org/junit/platform/console/output/TreePrinter.java

@@ -22,6 +22,7 @@
 import org.junit.platform.engine.TestExecutionResult.Status;
 import org.junit.platform.engine.reporting.FileEntry;
 import org.junit.platform.engine.reporting.ReportEntry;
+import org.opentest4j.AssertionFailedError;
 
 /**
  * @since 1.0
@@ -123,11 +124,19 @@ private void printThrowable(String indent, TestExecutionResult result) {
 			return;
 		}
 		Throwable throwable = result.getThrowable().get();
+		String message = formatThrowable(throwable);
+		printMessage(Style.FAILED, indent, message);
+	}
+
+	private static String formatThrowable(Throwable throwable) {
+		if (throwable instanceof AssertionFailedError assertionFailedError) {
+			return new RichDiffFormatter().format(assertionFailedError);
+		}
 		String message = throwable.getMessage();
 		if (StringUtils.isBlank(message)) {
 			message = throwable.toString();
 		}
-		printMessage(Style.FAILED, indent, message);
+		return message;
 	}
 
 	private void printReportEntry(String indent, ReportEntry reportEntry) {

platform-tests/platform-tests.gradle.kts

@@ -30,6 +30,11 @@ val woodstoxRuntimeClasspath = configurations.resolvable("woodstoxRuntimeClasspa
 }
 
 dependencies {
+	repositories {
+		mavenLocal()
+		mavenCentral()
+	}
+
 	// --- Things we are testing --------------------------------------------------
 	testImplementation(projects.junitPlatformCommons)
 	testImplementation(projects.junitPlatformConsole)

platform-tests/src/test/java/org/junit/platform/console/output/RichDiffFormatterTests.java

@@ -0,0 +1,92 @@
+/*
+ * Copyright 2015-2026 the original author or authors.
+ *
+ * All rights reserved. This program and the accompanying materials are
+ * made available under the terms of the Eclipse Public License v2.0 which
+ * accompanies this distribution and is available at
+ *
+ * https://www.eclipse.org/legal/epl-v20.html
+ */
+
+package org.junit.platform.console.output;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+
+import org.junit.jupiter.api.Test;
+import org.opentest4j.AssertionFailedError;
+
+class RichDiffFormatterTests {
+
+	@Test
+	void lineAdded() {
+		assertRichDiffEquals("""
+				{
+				  "speaker": "world"
+				}
+				""", """
+				{
+				  "speaker": "world"
+				  "message": "hello"
+				}
+				""", """
+				expected did not match actual
+				+ actual - expected
+				  {
+				    "speaker": "world"
+				+   "message": "hello"
+				  }
+				""");
+	}
+
+	@Test
+	void lineRemoved() {
+		assertRichDiffEquals("""
+				{
+				  "speaker": "world"
+				  "message": "hello"
+				}
+				""", """
+				{
+				  "speaker": "world"
+				}
+				""", """
+				expected did not match actual
+				+ actual - expected
+				  {
+				    "speaker": "world"
+				-   "message": "hello"
+				  }
+				""");
+	}
+
+	@Test
+	void lineChanged() {
+		assertRichDiffEquals("""
+				{
+				  "speaker": "world"
+				  "message": "hello"
+				}
+				""", """
+				{
+				  "speaker": "you"
+				  "message": "hello"
+				}
+				""", """
+				expected did not match actual
+				+ actual - expected
+				  {
+				+   "speaker": "world"
+				-   "speaker": "you"
+				    "message": "hello"
+				  }
+				""");
+	}
+
+	private static void assertRichDiffEquals(String expected, String actual, String expectedDiff) {
+		var formatter = new RichDiffFormatter();
+		var assertionFailed = assertThrows(AssertionFailedError.class, () -> assertEquals(expected, actual));
+		assertEquals(expectedDiff, formatter.format(assertionFailed));
+	}
+
+}

settings.gradle.kts

@@ -15,6 +15,8 @@ plugins {
 dependencyResolutionManagement {
 	repositories {
 		mavenCentral()
+		// TODO: Remove
+		mavenLocal()
 	}
 }