feat: Implement conversion of diff content to ReviewDog format

lib-extra/src/main/java/com/diffplug/spotless/extra/middleware/ReviewDogGenerator.java

@@ -0,0 +1,149 @@
+/*
+ * Copyright 2022-2025 DiffPlug
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package com.diffplug.spotless.extra.middleware;
+
+import java.util.List;
+
+import com.diffplug.spotless.FormatterStep;
+import com.diffplug.spotless.Lint;
+
+/**
+ * Utility class for generating ReviewDog compatible output in the rdjsonl format.
+ * This class provides methods to create diff and lint reports that can be used by ReviewDog.
+ */
+public final class ReviewDogGenerator {
+
+	private static final String SOURCE = "spotless";
+
+	private ReviewDogGenerator() {
+		// Prevent instantiation
+	}
+
+	/**
+	 * Generates a ReviewDog compatible JSON line (rdjsonl) for a diff between
+	 * the actual content and the formatted content of a file.
+	 *
+	 * @param path             The file path
+	 * @param actualContent    The content as it currently exists in the file
+	 * @param formattedContent The content after formatting is applied
+	 * @return A string in rdjsonl format representing the diff
+	 */
+	public static String rdjsonlDiff(String path, String actualContent, String formattedContent) {
+		if (actualContent.equals(formattedContent)) {
+			return "";
+		}
+
+		String diff = createUnifiedDiff(path, actualContent, formattedContent);
+
+		return String.format(
+				"{\"message\":{\"path\":\"%s\",\"message\":\"File requires formatting\",\"diff\":\"%s\"}}",
+				escapeJson(path),
+				escapeJson(diff));
+	}
+
+	/**
+	 * Generates ReviewDog compatible JSON lines (rdjsonl) for lint issues
+	 * identified by formatting steps.
+	 *
+	 * @param path             The file path
+	 * @param steps            The list of formatter steps applied
+	 * @param lintsPerStep     The list of lints produced by each step
+	 * @return A string in rdjsonl format representing the lints
+	 */
+	public static String rdjsonlLints(String path, List<FormatterStep> steps, List<List<Lint>> lintsPerStep) {
+		if (lintsPerStep == null || lintsPerStep.isEmpty()) {
+			return "";
+		}
+
+		StringBuilder builder = new StringBuilder();
+
+		for (int i = 0; i < lintsPerStep.size(); i++) {
+			List<Lint> lints = lintsPerStep.get(i);
+			if (lints == null || lints.isEmpty()) {
+				continue;
+			}
+
+			String stepName = (i < steps.size()) ? steps.get(i).getName() : "unknown";
+			for (Lint lint : lints) {
+				builder.append(formatLintAsJson(path, lint, stepName)).append('\n');
+			}
+		}
+
+		return builder.toString().trim();
+	}
+
+	/**
+	 * Creates a unified diff between two text contents.
+	 */
+	private static String createUnifiedDiff(String path, String actualContent, String formattedContent) {
+		String[] actualLines = actualContent.split("\\r?\\n", -1);
+		String[] formattedLines = formattedContent.split("\\r?\\n", -1);
+
+		StringBuilder diff = new StringBuilder();
+		diff.append("--- a/").append(path).append('\n');
+		diff.append("+++ b/").append(path).append('\n');
+		diff.append("@@ -1,").append(actualLines.length).append(" +1,").append(formattedLines.length).append(" @@\n");
+
+		for (String line : actualLines) {
+			diff.append('-').append(line).append('\n');
+		}
+
+		for (String line : formattedLines) {
+			diff.append('+').append(line).append('\n');
+		}
+
+		return diff.toString();
+	}
+
+	/**
+	 * Formats a single lint issue as a JSON line.
+	 */
+	private static String formatLintAsJson(String path, Lint lint, String ruleCode) {
+		return String.format(
+				"{"
+						+ "\"source\":\"%s\","
+						+ "\"code\":\"%s\","
+						+ "\"level\":\"warning\","
+						+ "\"message\":\"%s\","
+						+ "\"path\":\"%s\","
+						+ "\"line\":%d,"
+						+ "\"column\":%d"
+						+ "}",
+				escapeJson(SOURCE),
+				escapeJson(ruleCode),
+				escapeJson(lint.getDetail()),
+				escapeJson(path),
+				lint.getLineStart(),
+				1);
+	}
+
+	/**
+	 * Escapes special characters in a string for JSON compatibility.
+	 */
+	private static String escapeJson(String str) {
+		if (str == null) {
+			return "";
+		}
+		return str
+				.replace("\\", "\\\\")
+				.replace("\"", "\\\"")
+				.replace("\n", "\\n")
+				.replace("\r", "\\r")
+				.replace("\t", "\\t")
+				.replace("\b", "\\b")
+				.replace("\f", "\\f");
+	}
+}

lib-extra/src/main/java/com/diffplug/spotless/extra/middleware/package-info.java

@@ -0,0 +1,7 @@
+@ParametersAreNonnullByDefault
+@ReturnValuesAreNonnullByDefault
+package com.diffplug.spotless.extra.middleware;
+
+import javax.annotation.ParametersAreNonnullByDefault;
+
+import com.diffplug.spotless.annotations.ReturnValuesAreNonnullByDefault;

lib-extra/src/test/java/com/diffplug/spotless/extra/middleware/ReviewDogGeneratorTest.java

@@ -0,0 +1,112 @@
+/*
+ * Copyright 2022-2025 DiffPlug
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package com.diffplug.spotless.extra.middleware;
+
+import java.io.File;
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.Collections;
+import java.util.List;
+
+import org.junit.jupiter.api.Test;
+
+import com.diffplug.selfie.Selfie;
+import com.diffplug.spotless.FormatterStep;
+import com.diffplug.spotless.Lint;
+
+public class ReviewDogGeneratorTest {
+
+	@Test
+	public void diffSingleLine() {
+		String result = ReviewDogGenerator.rdjsonlDiff("test.txt", "dirty", "clean");
+		Selfie.expectSelfie(result).toBe("{\"message\":{\"path\":\"test.txt\",\"message\":\"File requires formatting\",\"diff\":\"--- a/test.txt\\n+++ b/test.txt\\n@@ -1,1 +1,1 @@\\n-dirty\\n+clean\\n\"}}");
+	}
+
+	@Test
+	public void diffNoChange() {
+		String result = ReviewDogGenerator.rdjsonlDiff("test.txt", "same", "same");
+		Selfie.expectSelfie(result).toBe("");
+	}
+
+	@Test
+	public void diffMultipleLines() {
+		String actual = "Line 1\nLine 2\nDirty line\nLine 4";
+		String formatted = "Line 1\nLine 2\nClean line\nLine 4";
+
+		String result = ReviewDogGenerator.rdjsonlDiff("src/main.java", actual, formatted);
+		Selfie.expectSelfie(result).toBe("{\"message\":{\"path\":\"src/main.java\",\"message\":\"File requires formatting\",\"diff\":\"--- a/src/main.java\\n+++ b/src/main.java\\n@@ -1,4 +1,4 @@\\n-Line 1\\n-Line 2\\n-Dirty line\\n-Line 4\\n+Line 1\\n+Line 2\\n+Clean line\\n+Line 4\\n\"}}");
+	}
+
+	@Test
+	public void lintsEmpty() {
+		List<FormatterStep> steps = new ArrayList<>();
+		List<List<Lint>> lintsPerStep = new ArrayList<>();
+
+		String result = ReviewDogGenerator.rdjsonlLints("test.txt", steps, lintsPerStep);
+		Selfie.expectSelfie(result).toBe("");
+	}
+
+	@Test
+	public void lintsSingleIssue() {
+		FormatterStep step = FormatterStep.create(
+				"testStep",
+				"formatter-state",
+				state -> rawUnix -> rawUnix);
+		List<FormatterStep> steps = Collections.singletonList(step);
+
+		Lint lint = Lint.atLine(1, "TEST001", "Test lint message");
+		List<List<Lint>> lintsPerStep = Collections.singletonList(Collections.singletonList(lint));
+
+		String result = ReviewDogGenerator.rdjsonlLints("src/main.java", steps, lintsPerStep);
+		Selfie.expectSelfie(result).toBe("{\"source\":\"spotless\",\"code\":\"testStep\",\"level\":\"warning\",\"message\":\"Test lint message\",\"path\":\"src/main.java\",\"line\":1,\"column\":1}");
+	}
+
+	@Test
+	public void lintsMultipleIssues() {
+		FormatterStep step1 = new FormatterStep() {
+			@Override
+			public String getName() {
+				return "step1";
+			}
+
+			@Override
+			public String format(String rawUnix, File file) {
+				return rawUnix;
+			}
+
+			@Override
+			public void close() {}
+		};
+
+		FormatterStep step2 = FormatterStep.create(
+				"step2",
+				"formatter-state",
+				state -> rawUnix -> rawUnix);
+
+		List<FormatterStep> steps = Arrays.asList(step1, step2);
+
+		Lint lint1 = Lint.atLine(1, "RULE1", "First issue");
+		Lint lint2 = Lint.atLine(5, "RULE2", "Second issue");
+
+		List<List<Lint>> lintsPerStep = Arrays.asList(
+				Collections.singletonList(lint1),
+				Collections.singletonList(lint2));
+
+		String result = ReviewDogGenerator.rdjsonlLints("src/main.java", steps, lintsPerStep);
+		Selfie.expectSelfie(result).toBe("{\"source\":\"spotless\",\"code\":\"step1\",\"level\":\"warning\",\"message\":\"First issue\",\"path\":\"src/main.java\",\"line\":1,\"column\":1}",
+				"{\"source\":\"spotless\",\"code\":\"step2\",\"level\":\"warning\",\"message\":\"Second issue\",\"path\":\"src/main.java\",\"line\":5,\"column\":1}");
+	}
+}

lib/src/main/java/com/diffplug/spotless/LintState.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2024 DiffPlug
+ * Copyright 2024-2025 DiffPlug
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.

plugin-gradle/CHANGES.md

@@ -6,6 +6,8 @@ We adhere to the [keepachangelog](https://keepachangelog.com/en/1.0.0/) format (
 ### Changed
 * Bump default `eclipse` version to latest `4.34` -> `4.35`. ([#2458](https://github.com/diffplug/spotless/pull/2458))
 * Bump default `greclipse` version to latest `4.32` -> `4.35`. ([#2458](https://github.com/diffplug/spotless/pull/2458))
+### Added
+* Implement conversion of diff content to ReviewDog format ([#2478](https://github.com/diffplug/spotless/pull/2478))
 
 ## [7.0.3] - 2025-04-07
 ### Changed

plugin-gradle/README.md

@@ -88,6 +88,7 @@ Spotless supports all of Gradle's built-in performance features (incremental bui
   - [Multiple (or custom) language-specific blocks](#multiple-or-custom-language-specific-blocks)
   - [Inception (languages within languages within...)](#inception-languages-within-languages-within)
   - [Disabling warnings and error messages](#disabling-warnings-and-error-messages)
+  - [Reviewdog integration for CI](#reviewdog-integration-for-ci)
   - [Dependency resolution modes](#dependency-resolution-modes)
   - [How do I preview what `spotlessApply` will do?](#how-do-i-preview-what-spotlessapply-will-do)
   - [Example configurations (from real-world projects)](#example-configurations-from-real-world-projects)
@@ -1768,6 +1769,47 @@ spotless {
     ignoreErrorForPath('path/to/file.java') // ignore errors by all steps on this specific file
 ```
 
+<a name="reviewdog"></a>
+
+## Reviewdog integration for CI
+
+**CURRENTLY IN BETA** – bug reports are welcome! This is a challenging feature to test comprehensively, so we anticipate needing a few releases to get everything right.
+Spotless can generate reports compatible with [Reviewdog](https://github.com/reviewdog/reviewdog), a tool that automates code review tasks by posting formatting issues as comments on pull requests.
+
+### Enabling Reviewdog integration
+
+To enable Reviewdog output:
+
+```gradle 
+spotless {
+  reviewdogOutput file("${buildDir}/spotless-reviewdog.json")
+}
+```
+
+### Automatic report generation on check failure
+
+To generate reports when `spotlessCheck` fails:
+
+```gradle
+tasks.named('spotlessCheck').configure {
+  ignoreFailures = true
+  doLast {
+    if (state.failure != null) {
+      logger.lifecycle("Spotless check failed – generating Reviewdog report")
+
+      if (project.hasProperty('reviewdogOutput')) {
+        // Insert logic here to generate reviewdogOutput
+        // Example: file(project.reviewdogOutput).text = spotlessCheckOutput
+      }
+    }
+  }
+}
+
+```
+
+For Reviewdog setup instructions, visit: https://github.com/reviewdog/reviewdog#installation
+
+
 <a name="dependency-resolution-modes"></a>
 ## Dependency resolution modes