add a diff section to report of AssertionFailedError

With this commit, a dependency to https://github.com/java-diff-utils/java-diff-utils is introduced.
java-diff-utils is used to create a diff from the expected and the actual result and report it.

See #3139

Author: bechte

gradle/libs.versions.toml

@@ -41,6 +41,7 @@ gradle-versions = { module = "com.github.ben-manes:gradle-versions-plugin", vers
 groovy4 = { module = "org.apache.groovy:groovy", version = "4.0.13" }
 groovy2-bom = { module = "org.codehaus.groovy:groovy-bom", version = "2.5.21" }
 hamcrest = { module = "org.hamcrest:hamcrest", version = "2.2" }
+java-diff-utils = { module = "io.github.java-diff-utils:java-diff-utils", version = "4.12" }
 jfrunit = { module = "org.moditect.jfrunit:jfrunit-core", version = "1.0.0.Alpha2" }
 jimfs = { module = "com.google.jimfs:jimfs", version = "1.3.0" }
 jmh-core = { module = "org.openjdk.jmh:jmh-core", version.ref = "jmh" }

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

@@ -15,6 +15,8 @@ dependencies {
 
 	compileOnly(libs.openTestReporting.events)
 
+	implementation(libs.java.diff.utils)
+
 	shadowed(libs.picocli)
 
 	osgiVerification(projects.junitJupiterEngine)
@@ -27,7 +29,9 @@ tasks {
 			"--add-modules", "org.opentest4j.reporting.events",
 			"--add-reads", "${project.projects.junitPlatformReporting.dependencyProject.javaModuleName}=org.opentest4j.reporting.events",
 			"--add-modules", "info.picocli",
-			"--add-reads", "${javaModuleName}=info.picocli"
+			"--add-reads", "${javaModuleName}=info.picocli",
+			"--add-modules", "io.github.javadiffutils",
+			"--add-reads", "${javaModuleName}=io.github.javadiffutils"
 		))
 	}
 	shadowJar {

junit-platform-console/src/main/java/org/junit/platform/console/tasks/ConsoleTestExecutor.java

@@ -21,6 +21,8 @@
 import java.util.Optional;
 import java.util.function.Supplier;
 
+import com.github.difflib.text.DiffRowGenerator;
+
 import org.apiguardian.api.API;
 import org.junit.platform.commons.JUnitException;
 import org.junit.platform.commons.util.ClassLoaderUtils;
@@ -143,6 +145,7 @@ private SummaryGeneratingListener registerListeners(PrintWriter out, Optional<Pa
 	private Optional<DetailsPrintingListener> createDetailsPrintingListener(PrintWriter out) {
 		ColorPalette colorPalette = getColorPalette();
 		Theme theme = outputOptions.getTheme();
+
 		switch (outputOptions.getDetails()) {
 			case SUMMARY:
 				// summary listener is always created and registered

junit-platform-console/src/main/java/org/junit/platform/console/tasks/FlatPrintingListener.java

@@ -11,12 +11,20 @@
 package org.junit.platform.console.tasks;
 
 import java.io.PrintWriter;
+import java.util.Arrays;
+import java.util.List;
+import java.util.stream.Collectors;
+
+import com.github.difflib.text.DiffRow;
+import com.github.difflib.text.DiffRowGenerator;
 
 import org.junit.platform.commons.util.ExceptionUtils;
+import org.junit.platform.commons.util.StringUtils;
 import org.junit.platform.engine.TestExecutionResult;
 import org.junit.platform.engine.reporting.ReportEntry;
 import org.junit.platform.launcher.TestIdentifier;
 import org.junit.platform.launcher.TestPlan;
+import org.opentest4j.AssertionFailedError;
 
 /**
  * @since 1.0
@@ -27,10 +35,19 @@ class FlatPrintingListener implements DetailsPrintingListener {
 
 	private final PrintWriter out;
 	private final ColorPalette colorPalette;
+	private final DiffRowGenerator diffRowGenerator;
 
 	FlatPrintingListener(PrintWriter out, ColorPalette colorPalette) {
 		this.out = out;
 		this.colorPalette = colorPalette;
+		this.diffRowGenerator = DiffRowGenerator.create() //
+				.showInlineDiffs(true) //
+				.mergeOriginalRevised(true) //
+				.inlineDiffByWord(true) //
+				.oldTag(f -> "~") //
+				.newTag(f -> "**") //
+				.build();
+		;
 	}
 
 	@Override
@@ -78,9 +95,26 @@ private void printlnTestDescriptor(Style style, String message, TestIdentifier t
 	}
 
 	private void printlnException(Style style, Throwable throwable) {
+		if (throwable instanceof AssertionFailedError) {
+			AssertionFailedError assertionFailedError = (AssertionFailedError) throwable;
+			String expected = assertionFailedError.getExpected().getStringRepresentation();
+			String actual = assertionFailedError.getActual().getStringRepresentation();
+			String diff = calculateDiff(expected, actual);
+
+			printlnMessage(style, "Expected ", expected);
+			printlnMessage(style, "Actual   ", actual);
+			printlnMessage(style, "Diff     ", diff);
+		}
 		printlnMessage(style, "Exception", ExceptionUtils.readStackTrace(throwable));
 	}
 
+	private String calculateDiff(String expected, String actual) {
+		List<String> expectedLines = Arrays.asList(StringUtils.nullSafeToString(expected));
+		List<String> actualLines = Arrays.asList(StringUtils.nullSafeToString(actual));
+		List<DiffRow> diffRows = diffRowGenerator.generateDiffRows(expectedLines, actualLines);
+		return diffRows.stream().map(DiffRow::getOldLine).collect(Collectors.joining("\n"));
+	}
+
 	private void printlnMessage(Style style, String message, String detail) {
 		println(style, INDENTATION + "=> " + message + ": %s", indented(detail));
 	}

platform-tests/src/test/java/org/junit/platform/console/tasks/FlatPrintingListenerTests.java

@@ -26,6 +26,7 @@
 import org.junit.platform.engine.reporting.ReportEntry;
 import org.junit.platform.fakes.TestDescriptorStub;
 import org.junit.platform.launcher.TestIdentifier;
+import org.opentest4j.AssertionFailedError;
 
 /**
  * @since 1.0
@@ -71,6 +72,38 @@ void executionFinishedWithFailure() {
 			() -> assertEquals(INDENTATION + "=> Exception: java.lang.AssertionError: Boom!", lines[1]));
 	}
 
+	@Nested
+	class DiffOutputTests {
+		@Test
+		void printDiffForAssertionFailedErrors() {
+			var stringWriter = new StringWriter();
+			listener(stringWriter).executionFinished(newTestIdentifier(),
+				failed(new AssertionFailedError("Detail Message", "Expected content", "Actual content")));
+			var lines = lines(stringWriter);
+
+			assertTrue(lines.length >= 5, "At least 5 lines are expected in failure report!");
+			assertAll("lines in the output", //
+				() -> assertEquals("Finished:    demo-test ([engine:demo-engine])", lines[0]), //
+				() -> assertEquals(INDENTATION + "=> Expected : Expected content", lines[1]), //
+				() -> assertEquals(INDENTATION + "=> Actual   : Actual content", lines[2]), //
+				() -> assertEquals(INDENTATION + "=> Diff     : ~Expected~**Actual** content", lines[3]), //
+				() -> assertEquals(INDENTATION + "=> Exception: org.opentest4j.AssertionFailedError: Detail Message",
+					lines[4]));
+		}
+
+		@Test
+		void ignoreDiffForAnyAssertionErrors() {
+			var stringWriter = new StringWriter();
+			listener(stringWriter).executionFinished(newTestIdentifier(), failed(new AssertionError("Detail Message")));
+			var lines = lines(stringWriter);
+
+			assertTrue(lines.length >= 2, "At least 2 lines are expected in failure report!");
+			assertAll("lines in the output", //
+				() -> assertEquals("Finished:    demo-test ([engine:demo-engine])", lines[0]), //
+				() -> assertEquals(INDENTATION + "=> Exception: java.lang.AssertionError: Detail Message", lines[1]));
+		}
+	}
+
 	@Nested
 	class ColorPaletteTests {