[POC] Parallel formatting for Maven plugin

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

@@ -0,0 +1,89 @@
+/*
+ * Copyright 2022 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.maven;
+
+import static java.util.concurrent.CompletableFuture.supplyAsync;
+
+import java.io.File;
+import java.io.IOException;
+import java.io.UncheckedIOException;
+import java.nio.file.Files;
+import java.util.concurrent.CompletableFuture;
+import java.util.concurrent.Executor;
+import java.util.concurrent.Executors;
+import java.util.concurrent.ThreadFactory;
+
+import com.diffplug.spotless.Formatter;
+import com.diffplug.spotless.PaddedCell;
+import com.diffplug.spotless.PaddedCell.DirtyState;
+
+public class FormattingParallelizer {
+
+	public static final FormattingParallelizer INSTANCE = new FormattingParallelizer();
+
+	private FormattingParallelizer() {}
+
+	private final Executor readerExecutor = Executors.newFixedThreadPool(2, daemonThreadFactory());
+
+	private final Executor formatterExecutor = Executors.newFixedThreadPool(
+			Runtime.getRuntime().availableProcessors() * 2, daemonThreadFactory());
+
+	private final Executor writerExecutor = Executors.newFixedThreadPool(2, daemonThreadFactory());
+
+	CompletableFuture<Void> format(File file, Formatter formatter) {
+		return readFileContent(file)
+				.thenApplyAsync(raw -> calculateDirtyState(file, formatter, raw), formatterExecutor)
+				.thenAcceptAsync(dirtyState -> writeFormatted(dirtyState, file), writerExecutor);
+	}
+
+	private CompletableFuture<byte[]> readFileContent(File file) {
+		return supplyAsync(
+				() -> {
+					try {
+						return Files.readAllBytes(file.toPath());
+					} catch (IOException e) {
+						throw new UncheckedIOException(e);
+					}
+				},
+				readerExecutor);
+	}
+
+	private DirtyState calculateDirtyState(File file, Formatter formatter, byte[] rawBytes) {
+		try {
+			return PaddedCell.calculateDirtyState(formatter, file, rawBytes);
+		} catch (IOException e) {
+			throw new UncheckedIOException(e);
+		}
+	}
+
+	private void writeFormatted(DirtyState dirtyState, File file) {
+		if (!dirtyState.isClean() && !dirtyState.didNotConverge()) {
+			try {
+				dirtyState.writeCanonicalTo(file);
+			} catch (IOException e) {
+				throw new UncheckedIOException(e);
+			}
+		}
+	}
+
+	private static ThreadFactory daemonThreadFactory() {
+		return runnable -> {
+			Thread thread = new Thread(runnable);
+			thread.setDaemon(true);
+			return thread;
+		};
+	}
+}

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2016-2021 DiffPlug
+ * Copyright 2016-2022 DiffPlug
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -16,13 +16,14 @@
 package com.diffplug.spotless.maven;
 
 import java.io.File;
-import java.io.IOException;
+import java.util.ArrayList;
+import java.util.List;
+import java.util.concurrent.CompletableFuture;
 
 import org.apache.maven.plugin.MojoExecutionException;
 import org.apache.maven.plugins.annotations.Mojo;
 
 import com.diffplug.spotless.Formatter;
-import com.diffplug.spotless.PaddedCell;
 import com.diffplug.spotless.maven.incremental.UpToDateChecker;
 
 /**
@@ -33,6 +34,8 @@ public class SpotlessApplyMojo extends AbstractSpotlessMojo {
 
 	@Override
 	protected void process(Iterable<File> files, Formatter formatter, UpToDateChecker upToDateChecker) throws MojoExecutionException {
+		List<CompletableFuture<Void>> futures = new ArrayList<>();
+
 		for (File file : files) {
 			if (upToDateChecker.isUpToDate(file.toPath())) {
 				if (getLog().isDebugEnabled()) {
@@ -41,15 +44,12 @@ protected void process(Iterable<File> files, Formatter formatter, UpToDateChecke
 				continue;
 			}
 
-			try {
-				PaddedCell.DirtyState dirtyState = PaddedCell.calculateDirtyState(formatter, file);
-				if (!dirtyState.isClean() && !dirtyState.didNotConverge()) {
-					dirtyState.writeCanonicalTo(file);
-				}
-			} catch (IOException e) {
-				throw new MojoExecutionException("Unable to format file " + file, e);
-			}
+			futures.add(FormattingParallelizer.INSTANCE.format(file, formatter));
+		}
+
+		CompletableFuture.allOf(futures.toArray(new CompletableFuture[0])).join();
 
+		for (File file : files) {
 			upToDateChecker.setUpToDate(file.toPath());
 		}
 	}