Skip execution in incremental (m2e) builds by default

plugin-maven/README.md

@@ -1696,6 +1696,28 @@ cmd> mvn spotless:apply -DspotlessFiles=my/file/pattern.java,more/generic/.*-pat
 
 The patterns are matched using `String#matches(String)` against the absolute file path.
 
+## Does Spotless support incremental builds in Eclipse?
+
+Spotless comes with [m2e](https://eclipse.dev/m2e/) support. However, by default its execution is skipped in incremental builds as most developers want to fix all issues in one go via explicit `mvn spotless:apply` prior to raising a PR and don't want to be bothered with Spotless issues during working on the source code in the IDE.
+To enable it use the following parameter
+
+```
+<configuration>
+    <enableForIncrementalBuild>true</enableForIncrementalBuild><!-- this is false by default -->
+</configuration>
+```
+
+In addition Eclipse problem markers are being emitted. By default they have the severity `WARNING`.
+You can adjust this with 
+
+```
+<configuration>
+    <incrementalBuildMessageSeverity>ERROR</incrementalBuildMessageSeverity><!-- WARNING or ERROR -->
+</configuration>
+```
+
+Note that for Incremental build support the goals have to be bound to a phase prior to `test`.
+
 <a name="examples"></a>
 
 ## Example configurations (from real-world projects)

plugin-maven/src/main/java/com/diffplug/spotless/maven/AbstractSpotlessMojo.java

@@ -201,6 +201,13 @@ public abstract class AbstractSpotlessMojo extends AbstractMojo {
 	@Parameter
 	private UpToDateChecking upToDateChecking = UpToDateChecking.enabled();
 
+	/**
+	 * If set to {@code true} will also run on incremental builds (i.e. within Eclipse with m2e).
+	 * Otherwise this goal is skipped in incremental builds and only runs on full builds.
+	 */
+	@Parameter(defaultValue = "false")
+	protected boolean enableForIncrementalBuild;
+
 	protected abstract void process(Iterable<File> files, Formatter formatter, UpToDateChecker upToDateChecker) throws MojoExecutionException;
 
 	private static final int MINIMUM_JRE = 11;
@@ -245,6 +252,10 @@ private boolean shouldSkip() {
 		if (skip) {
 			return true;
 		}
+		if (buildContext.isIncremental() && !enableForIncrementalBuild) {
+			getLog().debug("Skipping for incremental builds as parameter 'enableForIncrementalBuilds' is set to 'false'");
+			return true;
+		}
 
 		switch (goal) {
 		case GOAL_CHECK:

plugin-maven/src/main/java/com/diffplug/spotless/maven/SpotlessCheckMojo.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2016-2023 DiffPlug
+ * Copyright 2016-2024 DiffPlug
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -24,6 +24,7 @@
 import org.apache.maven.plugin.MojoExecutionException;
 import org.apache.maven.plugins.annotations.LifecyclePhase;
 import org.apache.maven.plugins.annotations.Mojo;
+import org.apache.maven.plugins.annotations.Parameter;
 import org.sonatype.plexus.build.incremental.BuildContext;
 
 import com.diffplug.spotless.Formatter;
@@ -38,6 +39,30 @@
 @Mojo(name = AbstractSpotlessMojo.GOAL_CHECK, defaultPhase = LifecyclePhase.VERIFY, threadSafe = true)
 public class SpotlessCheckMojo extends AbstractSpotlessMojo {
 
+	private static final String INCREMENTAL_MESSAGE_PREFIX = "Spotless Violation: ";
+
+	public enum MessageSeverity {
+		WARNING(BuildContext.SEVERITY_WARNING), ERROR(BuildContext.SEVERITY_ERROR);
+
+		private final int severity;
+
+		MessageSeverity(int severity) {
+			this.severity = severity;
+		}
+
+		public int getSeverity() {
+			return severity;
+		}
+	}
+
+	/**
+	 * The severity used to emit messages during incremental builds.
+	 * Either {@code WARNING} or {@code ERROR}.
+	 * @see AbstractSpotlessMojo#enableForIncrementalBuild
+	 */
+	@Parameter(defaultValue = "WARNING")
+	private MessageSeverity incrementalBuildMessageSeverity;
+
 	@Override
 	protected void process(Iterable<File> files, Formatter formatter, UpToDateChecker upToDateChecker) throws MojoExecutionException {
 		ImpactedFilesTracker counter = new ImpactedFilesTracker();
@@ -51,14 +76,14 @@ protected void process(Iterable<File> files, Formatter formatter, UpToDateChecke
 				}
 				continue;
 			}
-
+			buildContext.removeMessages(file);
 			try {
 				PaddedCell.DirtyState dirtyState = PaddedCell.calculateDirtyState(formatter, file);
 				if (!dirtyState.isClean() && !dirtyState.didNotConverge()) {
 					problemFiles.add(file);
 					if (buildContext.isIncremental()) {
 						Map.Entry<Integer, String> diffEntry = DiffMessageFormatter.diff(formatter, file);
-						buildContext.addMessage(file, diffEntry.getKey() + 1, 0, diffEntry.getValue(), BuildContext.SEVERITY_ERROR, null);
+						buildContext.addMessage(file, diffEntry.getKey() + 1, 0, INCREMENTAL_MESSAGE_PREFIX + diffEntry.getValue(), incrementalBuildMessageSeverity.getSeverity(), null);
 					}
 					counter.cleaned();
 				} else {