Render rich diffs for expected and actual values

Author: mpkorstanje

documentation/documentation.gradle.kts

@@ -57,8 +57,9 @@ dependencies {
 	repositories {
 		// TODO: Remove
 		mavenLocal()
+		mavenCentral()
 	}
-	
+
 	implementation(projects.junitJupiterApi) {
 		because("Jupiter API is used in src/main/java")
 	}

gradle/libs.versions.toml

@@ -67,7 +67,9 @@ 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" }
+roseau-cli = { module = "io.github.alien-tools:roseau-cli", version = "0.4.0" }
 slf4j-julBinding = { module = "org.slf4j:slf4j-jdk14", version = "2.0.17" }
 snapshotTests-junit5 = { module = "de.skuzzle.test:snapshot-tests-junit5", version.ref = "snapshotTests" }
 snapshotTests-xml = { module = "de.skuzzle.test:snapshot-tests-xml", version.ref = "snapshotTests" }

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

@@ -193,19 +193,19 @@ public void buildAndThrow() throws AssertionFailedError {
 	public AssertionFailedError build() {
 		String reason = nullSafeGet(this.reason);
 		var assertionFailedError = new AssertionFailedError( //
-				formatExceptionMessage(reason),  //
-				formatReason(reason), //
-				expected, // 
-				actual, //
-				cause //
+			formatExceptionMessage(reason), //
+			formatReason(reason), //
+			expected, //
+			actual, //
+			cause //
 		);
 		maybeTrimStackTrace(assertionFailedError);
 		return assertionFailedError;
 	}
 
 	private @Nullable String formatReason(@Nullable String reason) {
 		if (mismatch) {
-			return  (reason == null ? "" : reason + ", ") + "expected did not match actual";
+			return (reason == null ? "" : reason + ", ") + "expected did not match actual";
 		}
 		return reason;
 	}

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

@@ -18,6 +18,8 @@ dependencies {
 	compileOnlyApi(libs.jspecify)
 
 	shadowed(libs.picocli)
+	// TODO: Sort out
+	shadowed(libs.javadiffutils)
 
 	osgiVerification(projects.junitJupiterEngine)
 	osgiVerification(projects.junitPlatformLauncher)

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 +1,45 @@
+/*
+ * 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 com.github.difflib.text.DiffRowGenerator;
+
+import org.junit.platform.commons.util.ExceptionUtils;
+import org.opentest4j.AssertionFailedError;
+
+final class RichDiffFormatter {
+	public String format(AssertionFailedError assertionFailed) {
+		if (!(assertionFailed.isActualDefined() && assertionFailed.isExpectedDefined())) {
+			return ExceptionUtils.readStackTrace(assertionFailed);
+		}
+
+		StringBuilder builder = new StringBuilder();
+
+		builder.append(assertionFailed.getClass().getSimpleName());
+		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().mergeOriginalRevised(true).build();
+
+		// TODO: But how to render the stacktrace?
+
+		builder.append(generator.generateDiffRows(assertionFailed.getExpected().toString().lines().toList(),
+			assertionFailed.getActual().toString().lines().toList()));
+
+		return builder.toString();
+	}
+}

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 +1,48 @@
+/*
+ * 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 test() {
+		var expected = """
+				{
+				  "speaker": "world"
+				  "message": "hello"
+				}
+				""";
+		var actuall = """
+				{
+				  "speaker": "you"
+				  "message": "hello"
+				}
+				""";
+
+		var assertionFailed = assertThrows(AssertionFailedError.class, () -> assertEquals(expected, actuall));
+
+		var formatter = new RichDiffFormatter();
+
+		String message = formatter.format(assertionFailed);
+
+		assertEquals("""
+
+				""", message);
+
+	}
+
+}