From 3ab7adb515327ac3b4d9ea34c65646c6ddc95e38 Mon Sep 17 00:00:00 2001
From: Ned Twigg <ned.twigg@diffplug.com>
Date: Thu, 4 Jun 2020 21:47:20 -0700
Subject: [PATCH 01/11] Refactor all gradle-specific stuff out of GitRatchet.

---
 .../diffplug/gradle/spotless/GitRatchet.java  | 26 +++++++++-----
 .../gradle/spotless/GradleGitRatchet.java     | 35 +++++++++++++++++++
 .../spotless/RegisterDependenciesTask.java    |  6 ++--
 .../gradle/spotless/SpotlessTaskBase.java     |  6 ++--
 4 files changed, 59 insertions(+), 14 deletions(-)
 create mode 100644 plugin-gradle/src/main/java/com/diffplug/gradle/spotless/GradleGitRatchet.java

diff --git a/plugin-gradle/src/main/java/com/diffplug/gradle/spotless/GitRatchet.java b/plugin-gradle/src/main/java/com/diffplug/gradle/spotless/GitRatchet.java
index b15daacf61..c04913b23c 100644
--- a/plugin-gradle/src/main/java/com/diffplug/gradle/spotless/GitRatchet.java
+++ b/plugin-gradle/src/main/java/com/diffplug/gradle/spotless/GitRatchet.java
@@ -40,14 +40,20 @@
 import org.eclipse.jgit.treewalk.filter.IndexDiffFilter;
 import org.eclipse.jgit.treewalk.filter.PathFilter;
 import org.eclipse.jgit.util.FS;
-import org.gradle.api.Project;
 
 import com.diffplug.common.base.Errors;
 import com.diffplug.common.collect.HashBasedTable;
 import com.diffplug.common.collect.Table;
 import com.diffplug.spotless.FileSignature;
 
-class GitRatchet implements AutoCloseable {
+/**
+ * How to use:
+ * - For best performance, you should have one instance of GitRatchet, shared by all projects.
+ * - Use {@link #rootTreeShaOf(Object, String)} to turn `origin/master` into the SHA of the tree object at that reference
+ * - Use {@link #isClean(Object, ObjectId, File)} to see if the given file is "git clean" relative to that tree
+ * - If you have up-to-date checking and want the best possible performance, use {@link #subtreeShaOf(Object, ObjectId)} to optimize up-to-date checks on a per-project basis.
+ */
+public abstract class GitRatchet<Project> implements AutoCloseable {
 	/**
 	 * This is the highest-level method, which all the others serve.  Given the sha
 	 * of a git tree (not a commit!), and the file in question, this method returns
@@ -130,17 +136,17 @@ private static boolean worktreeIsCleanCheckout(TreeWalk treeWalk) {
 	private Repository repositoryFor(Project project) throws IOException {
 		Repository repo = gitRoots.get(project);
 		if (repo == null) {
-			if (isGitRoot(project.getProjectDir())) {
-				repo = createRepo(project.getProjectDir());
+			if (isGitRoot(getDir(project))) {
+				repo = createRepo(getDir(project));
 			} else {
-				Project parentProj = project.getParent();
+				Project parentProj = getParent(project);
 				if (parentProj == null) {
-					repo = traverseParentsUntil(project.getProjectDir().getParentFile(), null);
+					repo = traverseParentsUntil(getDir(project).getParentFile(), null);
 					if (repo == null) {
 						throw new IllegalArgumentException("Cannot find git repository in any parent directory");
 					}
 				} else {
-					repo = traverseParentsUntil(project.getProjectDir().getParentFile(), parentProj.getProjectDir());
+					repo = traverseParentsUntil(getDir(project).getParentFile(), getDir(parentProj));
 					if (repo == null) {
 						repo = repositoryFor(parentProj);
 					}
@@ -151,6 +157,10 @@ private Repository repositoryFor(Project project) throws IOException {
 		return repo;
 	}
 
+	protected abstract File getDir(Project project);
+
+	protected abstract @Nullable Project getParent(Project project);
+
 	private static @Nullable Repository traverseParentsUntil(File startWith, File file) throws IOException {
 		do {
 			if (isGitRoot(startWith)) {
@@ -204,7 +214,7 @@ public synchronized ObjectId subtreeShaOf(Project project, ObjectId rootTreeSha)
 			ObjectId subtreeSha = subtreeShaCache.get(project);
 			if (subtreeSha == null) {
 				Repository repo = repositoryFor(project);
-				File directory = project.getProjectDir();
+				File directory = getDir(project);
 				if (repo.getWorkTree().equals(directory)) {
 					subtreeSha = rootTreeSha;
 				} else {
diff --git a/plugin-gradle/src/main/java/com/diffplug/gradle/spotless/GradleGitRatchet.java b/plugin-gradle/src/main/java/com/diffplug/gradle/spotless/GradleGitRatchet.java
new file mode 100644
index 0000000000..4972162816
--- /dev/null
+++ b/plugin-gradle/src/main/java/com/diffplug/gradle/spotless/GradleGitRatchet.java
@@ -0,0 +1,35 @@
+/*
+ * Copyright 2020 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.gradle.spotless;
+
+import java.io.File;
+
+import javax.annotation.Nullable;
+
+import org.gradle.api.Project;
+
+/** Gradle implementation of GitRatchet. */
+class GradleGitRatchet extends GitRatchet<Project> {
+	@Override
+	protected File getDir(Project project) {
+		return project.getProjectDir();
+	}
+
+	@Override
+	protected @Nullable Project getParent(Project project) {
+		return project.getParent();
+	}
+}
diff --git a/plugin-gradle/src/main/java/com/diffplug/gradle/spotless/RegisterDependenciesTask.java b/plugin-gradle/src/main/java/com/diffplug/gradle/spotless/RegisterDependenciesTask.java
index 4fd4c46c9a..c7f06f7805 100644
--- a/plugin-gradle/src/main/java/com/diffplug/gradle/spotless/RegisterDependenciesTask.java
+++ b/plugin-gradle/src/main/java/com/diffplug/gradle/spotless/RegisterDependenciesTask.java
@@ -1,5 +1,5 @@
 /*
- * Copyright 2016 DiffPlug
+ * Copyright 2016-2020 DiffPlug
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -106,10 +106,10 @@ public void trivialFunction() throws IOException {
 		Files.write(Integer.toString(getSteps().size()), unitOutput, StandardCharsets.UTF_8);
 	}
 
-	GitRatchet gitRatchet = new GitRatchet();
+	GradleGitRatchet gitRatchet = new GradleGitRatchet();
 
 	@Internal
-	GitRatchet getGitRatchet() {
+	GradleGitRatchet getGitRatchet() {
 		return gitRatchet;
 	}
 }
diff --git a/plugin-gradle/src/main/java/com/diffplug/gradle/spotless/SpotlessTaskBase.java b/plugin-gradle/src/main/java/com/diffplug/gradle/spotless/SpotlessTaskBase.java
index 4f768059c3..25cedac9e5 100644
--- a/plugin-gradle/src/main/java/com/diffplug/gradle/spotless/SpotlessTaskBase.java
+++ b/plugin-gradle/src/main/java/com/diffplug/gradle/spotless/SpotlessTaskBase.java
@@ -76,7 +76,7 @@ public void setLineEndingsPolicy(LineEnding.Policy lineEndingsPolicy) {
 
 	/*** API which performs git up-to-date tasks. */
 	@Nullable
-	GitRatchet ratchet;
+	GradleGitRatchet ratchet;
 	/** The sha of the tree at repository root, used for determining if an individual *file* is clean according to git. */
 	ObjectId rootTreeSha;
 	/**
@@ -86,14 +86,14 @@ public void setLineEndingsPolicy(LineEnding.Policy lineEndingsPolicy) {
 	 */
 	private ObjectId subtreeSha = ObjectId.zeroId();
 
-	public void setupRatchet(GitRatchet gitRatchet, String ratchetFrom) {
+	public void setupRatchet(GradleGitRatchet gitRatchet, String ratchetFrom) {
 		ratchet = gitRatchet;
 		rootTreeSha = gitRatchet.rootTreeShaOf(getProject(), ratchetFrom);
 		subtreeSha = gitRatchet.subtreeShaOf(getProject(), rootTreeSha);
 	}
 
 	@Internal
-	GitRatchet getRatchet() {
+	GradleGitRatchet getRatchet() {
 		return ratchet;
 	}
 

From 52988c84984f269ec254550da800966c43a2ef22 Mon Sep 17 00:00:00 2001
From: Ned Twigg <ned.twigg@diffplug.com>
Date: Thu, 4 Jun 2020 21:48:12 -0700
Subject: [PATCH 02/11] Move GitRatchet into lib-extra so it can be used by
 plugin-maven.

---
 .../src/main/java/com/diffplug/spotless/extra}/GitRatchet.java  | 2 +-
 .../java/com/diffplug/gradle/spotless/GradleGitRatchet.java     | 2 ++
 2 files changed, 3 insertions(+), 1 deletion(-)
 rename {plugin-gradle/src/main/java/com/diffplug/gradle/spotless => lib-extra/src/main/java/com/diffplug/spotless/extra}/GitRatchet.java (99%)

diff --git a/plugin-gradle/src/main/java/com/diffplug/gradle/spotless/GitRatchet.java b/lib-extra/src/main/java/com/diffplug/spotless/extra/GitRatchet.java
similarity index 99%
rename from plugin-gradle/src/main/java/com/diffplug/gradle/spotless/GitRatchet.java
rename to lib-extra/src/main/java/com/diffplug/spotless/extra/GitRatchet.java
index c04913b23c..df6211f4f6 100644
--- a/plugin-gradle/src/main/java/com/diffplug/gradle/spotless/GitRatchet.java
+++ b/lib-extra/src/main/java/com/diffplug/spotless/extra/GitRatchet.java
@@ -13,7 +13,7 @@
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
-package com.diffplug.gradle.spotless;
+package com.diffplug.spotless.extra;
 
 import java.io.File;
 import java.io.IOException;
diff --git a/plugin-gradle/src/main/java/com/diffplug/gradle/spotless/GradleGitRatchet.java b/plugin-gradle/src/main/java/com/diffplug/gradle/spotless/GradleGitRatchet.java
index 4972162816..fb67991847 100644
--- a/plugin-gradle/src/main/java/com/diffplug/gradle/spotless/GradleGitRatchet.java
+++ b/plugin-gradle/src/main/java/com/diffplug/gradle/spotless/GradleGitRatchet.java
@@ -21,6 +21,8 @@
 
 import org.gradle.api.Project;
 
+import com.diffplug.spotless.extra.GitRatchet;
+
 /** Gradle implementation of GitRatchet. */
 class GradleGitRatchet extends GitRatchet<Project> {
 	@Override

From e882b6fd460e62518193fc952ce9db7d67d484b5 Mon Sep 17 00:00:00 2001
From: Ned Twigg <ned.twigg@diffplug.com>
Date: Thu, 4 Jun 2020 21:59:00 -0700
Subject: [PATCH 03/11] Fix Spotbugs warning.

---
 .../src/main/java/com/diffplug/spotless/extra/GitRatchet.java | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/lib-extra/src/main/java/com/diffplug/spotless/extra/GitRatchet.java b/lib-extra/src/main/java/com/diffplug/spotless/extra/GitRatchet.java
index df6211f4f6..ca4b2406a4 100644
--- a/lib-extra/src/main/java/com/diffplug/spotless/extra/GitRatchet.java
+++ b/lib-extra/src/main/java/com/diffplug/spotless/extra/GitRatchet.java
@@ -200,7 +200,7 @@ public synchronized ObjectId rootTreeShaOf(Project project, String reference) {
 				rootTreeShaCache.put(repo, reference, treeSha);
 			}
 			return treeSha;
-		} catch (Exception e) {
+		} catch (IOException e) {
 			throw Errors.asRuntime(e);
 		}
 	}
@@ -225,7 +225,7 @@ public synchronized ObjectId subtreeShaOf(Project project, ObjectId rootTreeSha)
 				subtreeShaCache.put(project, subtreeSha);
 			}
 			return subtreeSha;
-		} catch (Exception e) {
+		} catch (IOException e) {
 			throw Errors.asRuntime(e);
 		}
 	}

From 464b841eeeded9f684c28fe1453e100c6781f4b6 Mon Sep 17 00:00:00 2001
From: Ned Twigg <ned.twigg@diffplug.com>
Date: Sat, 27 Jun 2020 22:07:25 -0700
Subject: [PATCH 04/11] Adapt GitRatchet for Maven.

---
 .../spotless/maven/AbstractSpotlessMojo.java  | 33 +++++++---
 .../spotless/maven/FormatterConfig.java       | 12 +++-
 .../spotless/maven/FormatterFactory.java      | 15 ++++-
 .../spotless/maven/MavenGitRatchet.java       | 62 +++++++++++++++++++
 .../spotless/maven/SpotlessApplyMojo.java     |  5 +-
 .../spotless/maven/SpotlessCheckMojo.java     |  4 +-
 6 files changed, 116 insertions(+), 15 deletions(-)
 create mode 100644 plugin-maven/src/main/java/com/diffplug/spotless/maven/MavenGitRatchet.java

diff --git a/plugin-maven/src/main/java/com/diffplug/spotless/maven/AbstractSpotlessMojo.java b/plugin-maven/src/main/java/com/diffplug/spotless/maven/AbstractSpotlessMojo.java
index ede8e58389..e12be9dd05 100644
--- a/plugin-maven/src/main/java/com/diffplug/spotless/maven/AbstractSpotlessMojo.java
+++ b/plugin-maven/src/main/java/com/diffplug/spotless/maven/AbstractSpotlessMojo.java
@@ -1,5 +1,5 @@
 /*
- * Copyright 2016 DiffPlug
+ * Copyright 2016-2020 DiffPlug
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -19,12 +19,19 @@
 
 import java.io.File;
 import java.io.IOException;
-import java.util.*;
+import java.util.Arrays;
+import java.util.Collections;
+import java.util.HashSet;
+import java.util.List;
+import java.util.Objects;
+import java.util.Set;
 import java.util.function.Predicate;
 import java.util.regex.Pattern;
 import java.util.stream.Collectors;
 import java.util.stream.Stream;
 
+import javax.annotation.Nullable;
+
 import org.apache.maven.plugin.AbstractMojo;
 import org.apache.maven.plugin.MojoExecutionException;
 import org.apache.maven.plugins.annotations.Component;
@@ -36,6 +43,7 @@
 import org.eclipse.aether.RepositorySystemSession;
 import org.eclipse.aether.repository.RemoteRepository;
 
+import com.diffplug.common.collect.Iterables;
 import com.diffplug.spotless.Formatter;
 import com.diffplug.spotless.LineEnding;
 import com.diffplug.spotless.Provisioner;
@@ -76,6 +84,9 @@ public abstract class AbstractSpotlessMojo extends AbstractMojo {
 	@Parameter(defaultValue = DEFAULT_LINE_ENDINGS)
 	private LineEnding lineEndings;
 
+	@Parameter
+	private String ratchetFrom;
+
 	@Parameter
 	private LicenseHeader licenseHeader;
 
@@ -110,12 +121,11 @@ public abstract class AbstractSpotlessMojo extends AbstractMojo {
 	@Parameter(property = "spotlessFiles")
 	private String filePatterns;
 
-	protected abstract void process(List<File> files, Formatter formatter) throws MojoExecutionException;
+	protected abstract void process(Iterable<File> files, Formatter formatter) throws MojoExecutionException;
 
 	@Override
 	public final void execute() throws MojoExecutionException {
 		List<FormatterFactory> formatterFactories = getFormatterFactories();
-
 		for (FormatterFactory formatterFactory : formatterFactories) {
 			execute(formatterFactory);
 		}
@@ -123,8 +133,17 @@ public final void execute() throws MojoExecutionException {
 
 	private void execute(FormatterFactory formatterFactory) throws MojoExecutionException {
 		List<File> files = collectFiles(formatterFactory);
-		try (Formatter formatter = formatterFactory.newFormatter(files, getFormatterConfig())) {
-			process(files, formatter);
+		FormatterConfig config = getFormatterConfig();
+		@Nullable
+		String ratchetFrom = formatterFactory.ratchetFrom(config);
+		Iterable<File> toFormat;
+		if (ratchetFrom == null) {
+			toFormat = files;
+		} else {
+			toFormat = Iterables.filter(files, MavenGitRatchet.instance().isGitDirty(baseDir, ratchetFrom));
+		}
+		try (Formatter formatter = formatterFactory.newFormatter(files, config)) {
+			process(toFormat, formatter);
 		}
 	}
 
@@ -172,7 +191,7 @@ private FormatterConfig getFormatterConfig() {
 		Provisioner provisioner = MavenProvisioner.create(resolver);
 		List<FormatterStepFactory> formatterStepFactories = getFormatterStepFactories();
 		FileLocator fileLocator = getFileLocator();
-		return new FormatterConfig(baseDir, encoding, lineEndings, provisioner, fileLocator, formatterStepFactories);
+		return new FormatterConfig(baseDir, encoding, lineEndings, ratchetFrom, provisioner, fileLocator, formatterStepFactories);
 	}
 
 	private FileLocator getFileLocator() {
diff --git a/plugin-maven/src/main/java/com/diffplug/spotless/maven/FormatterConfig.java b/plugin-maven/src/main/java/com/diffplug/spotless/maven/FormatterConfig.java
index bc2e5812a4..af52d1fa87 100644
--- a/plugin-maven/src/main/java/com/diffplug/spotless/maven/FormatterConfig.java
+++ b/plugin-maven/src/main/java/com/diffplug/spotless/maven/FormatterConfig.java
@@ -1,5 +1,5 @@
 /*
- * Copyright 2016 DiffPlug
+ * Copyright 2016-2020 DiffPlug
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -20,6 +20,8 @@
 import java.io.File;
 import java.util.List;
 
+import javax.annotation.Nullable;
+
 import com.diffplug.spotless.LineEnding;
 import com.diffplug.spotless.Provisioner;
 
@@ -27,14 +29,16 @@ public class FormatterConfig {
 
 	private final String encoding;
 	private final LineEnding lineEndings;
+	private final @Nullable String ratchetFrom;
 	private final Provisioner provisioner;
 	private final FileLocator fileLocator;
 	private final List<FormatterStepFactory> globalStepFactories;
 
-	public FormatterConfig(File baseDir, String encoding, LineEnding lineEndings, Provisioner provisioner,
+	public FormatterConfig(File baseDir, String encoding, LineEnding lineEndings, @Nullable String ratchetFrom, Provisioner provisioner,
 			FileLocator fileLocator, List<FormatterStepFactory> globalStepFactories) {
 		this.encoding = encoding;
 		this.lineEndings = lineEndings;
+		this.ratchetFrom = ratchetFrom;
 		this.provisioner = provisioner;
 		this.fileLocator = fileLocator;
 		this.globalStepFactories = globalStepFactories;
@@ -48,6 +52,10 @@ public LineEnding getLineEndings() {
 		return lineEndings;
 	}
 
+	public @Nullable String getRatchetFrom() {
+		return ratchetFrom;
+	}
+
 	public Provisioner getProvisioner() {
 		return provisioner;
 	}
diff --git a/plugin-maven/src/main/java/com/diffplug/spotless/maven/FormatterFactory.java b/plugin-maven/src/main/java/com/diffplug/spotless/maven/FormatterFactory.java
index d3ad7062ca..55cf468ba2 100644
--- a/plugin-maven/src/main/java/com/diffplug/spotless/maven/FormatterFactory.java
+++ b/plugin-maven/src/main/java/com/diffplug/spotless/maven/FormatterFactory.java
@@ -1,5 +1,5 @@
 /*
- * Copyright 2016 DiffPlug
+ * Copyright 2016-2020 DiffPlug
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -25,6 +25,8 @@
 import java.util.Objects;
 import java.util.Set;
 
+import javax.annotation.Nullable;
+
 import org.apache.maven.plugins.annotations.Parameter;
 
 import com.diffplug.common.collect.Sets;
@@ -41,6 +43,12 @@ public abstract class FormatterFactory {
 	@Parameter
 	private LineEnding lineEndings;
 
+	/** Sentinel to distinguish between "don't ratchet this format" and "use spotless parent format". */
+	private static final String RATCHETFROM_NOT_SET_AT_FORMAT_LEVEL = " not set at format level ";
+
+	@Parameter(defaultValue = RATCHETFROM_NOT_SET_AT_FORMAT_LEVEL)
+	private String ratchetFrom;
+
 	@Parameter
 	private String[] includes;
 
@@ -128,6 +136,11 @@ private LineEnding lineEndings(FormatterConfig config) {
 		return lineEndings == null ? config.getLineEndings() : lineEndings;
 	}
 
+	@Nullable
+	String ratchetFrom(FormatterConfig config) {
+		return ratchetFrom == RATCHETFROM_NOT_SET_AT_FORMAT_LEVEL ? config.getRatchetFrom() : ratchetFrom;
+	}
+
 	private FormatterStepConfig stepConfig(Charset encoding, FormatterConfig config) {
 		return new FormatterStepConfig(encoding, licenseHeaderDelimiter(), config.getProvisioner(), config.getFileLocator());
 	}
diff --git a/plugin-maven/src/main/java/com/diffplug/spotless/maven/MavenGitRatchet.java b/plugin-maven/src/main/java/com/diffplug/spotless/maven/MavenGitRatchet.java
new file mode 100644
index 0000000000..75c18a9bdc
--- /dev/null
+++ b/plugin-maven/src/main/java/com/diffplug/spotless/maven/MavenGitRatchet.java
@@ -0,0 +1,62 @@
+/*
+ * Copyright 2020 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 java.io.File;
+import java.io.IOException;
+import java.util.function.Predicate;
+
+import org.eclipse.jgit.lib.ObjectId;
+
+import com.diffplug.common.base.Errors;
+import com.diffplug.spotless.extra.GitRatchet;
+
+class MavenGitRatchet extends GitRatchet<File> {
+	@Override
+	protected File getDir(File project) {
+		return project;
+	}
+
+	@Override
+	protected File getParent(File project) {
+		return project.getParentFile();
+	}
+
+	private static MavenGitRatchet instance = new MavenGitRatchet();
+
+	static MavenGitRatchet instance() {
+		if (instance == null) {
+			synchronized (MavenGitRatchet.class) {
+				if (instance == null) {
+					instance = new MavenGitRatchet();
+				}
+			}
+		}
+		return instance;
+	}
+
+	/** A predicate which returns only the "git dirty" files. */
+	Predicate<File> isGitDirty(File baseDir, String ratchetFrom) {
+		ObjectId sha = rootTreeShaOf(baseDir, ratchetFrom);
+		return file -> {
+			try {
+				return !isClean(baseDir, sha, file);
+			} catch (IOException e) {
+				throw Errors.asRuntime(e);
+			}
+		};
+	}
+}
diff --git a/plugin-maven/src/main/java/com/diffplug/spotless/maven/SpotlessApplyMojo.java b/plugin-maven/src/main/java/com/diffplug/spotless/maven/SpotlessApplyMojo.java
index 3d85c27ab2..893a70f3f4 100644
--- a/plugin-maven/src/main/java/com/diffplug/spotless/maven/SpotlessApplyMojo.java
+++ b/plugin-maven/src/main/java/com/diffplug/spotless/maven/SpotlessApplyMojo.java
@@ -1,5 +1,5 @@
 /*
- * Copyright 2016 DiffPlug
+ * Copyright 2016-2020 DiffPlug
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -17,7 +17,6 @@
 
 import java.io.File;
 import java.io.IOException;
-import java.util.List;
 
 import org.apache.maven.plugin.MojoExecutionException;
 import org.apache.maven.plugins.annotations.Mojo;
@@ -35,7 +34,7 @@ public class SpotlessApplyMojo extends AbstractSpotlessMojo {
 	private boolean skip;
 
 	@Override
-	protected void process(List<File> files, Formatter formatter) throws MojoExecutionException {
+	protected void process(Iterable<File> files, Formatter formatter) throws MojoExecutionException {
 		if (skip) {
 			getLog().info("Spotless apply skipped");
 			return;
diff --git a/plugin-maven/src/main/java/com/diffplug/spotless/maven/SpotlessCheckMojo.java b/plugin-maven/src/main/java/com/diffplug/spotless/maven/SpotlessCheckMojo.java
index 4457ed5bc6..6f555890ca 100644
--- a/plugin-maven/src/main/java/com/diffplug/spotless/maven/SpotlessCheckMojo.java
+++ b/plugin-maven/src/main/java/com/diffplug/spotless/maven/SpotlessCheckMojo.java
@@ -1,5 +1,5 @@
 /*
- * Copyright 2016 DiffPlug
+ * Copyright 2016-2020 DiffPlug
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -40,7 +40,7 @@ public class SpotlessCheckMojo extends AbstractSpotlessMojo {
 	private boolean skip;
 
 	@Override
-	protected void process(List<File> files, Formatter formatter) throws MojoExecutionException {
+	protected void process(Iterable<File> files, Formatter formatter) throws MojoExecutionException {
 		if (skip) {
 			getLog().info("Spotless check skipped");
 			return;

From 6811ec3dc2dbea8157b02167fe8947a2b114704e Mon Sep 17 00:00:00 2001
From: Ned Twigg <ned.twigg@diffplug.com>
Date: Sat, 27 Jun 2020 22:29:44 -0700
Subject: [PATCH 05/11] Switch from @Nullable to Optional, because I can't get
 @Nullable to work with the build XD

---
 plugin-maven/build.gradle                            |  1 +
 .../spotless/maven/AbstractSpotlessMojo.java         | 12 +++++-------
 .../com/diffplug/spotless/maven/FormatterConfig.java |  9 ++++-----
 .../diffplug/spotless/maven/FormatterFactory.java    | 12 +++++++-----
 4 files changed, 17 insertions(+), 17 deletions(-)

diff --git a/plugin-maven/build.gradle b/plugin-maven/build.gradle
index a98948ff8b..62a945bd71 100644
--- a/plugin-maven/build.gradle
+++ b/plugin-maven/build.gradle
@@ -78,6 +78,7 @@ dependencies {
 
 	implementation "com.diffplug.durian:durian-core:${VER_DURIAN}"
 	implementation "com.diffplug.durian:durian-collect:${VER_DURIAN}"
+	implementation "org.eclipse.jgit:org.eclipse.jgit:${VER_JGIT}"
 
 	testImplementation project(":testlib")
 	testImplementation "junit:junit:${VER_JUNIT}"
diff --git a/plugin-maven/src/main/java/com/diffplug/spotless/maven/AbstractSpotlessMojo.java b/plugin-maven/src/main/java/com/diffplug/spotless/maven/AbstractSpotlessMojo.java
index e12be9dd05..03ab863e89 100644
--- a/plugin-maven/src/main/java/com/diffplug/spotless/maven/AbstractSpotlessMojo.java
+++ b/plugin-maven/src/main/java/com/diffplug/spotless/maven/AbstractSpotlessMojo.java
@@ -24,14 +24,13 @@
 import java.util.HashSet;
 import java.util.List;
 import java.util.Objects;
+import java.util.Optional;
 import java.util.Set;
 import java.util.function.Predicate;
 import java.util.regex.Pattern;
 import java.util.stream.Collectors;
 import java.util.stream.Stream;
 
-import javax.annotation.Nullable;
-
 import org.apache.maven.plugin.AbstractMojo;
 import org.apache.maven.plugin.MojoExecutionException;
 import org.apache.maven.plugins.annotations.Component;
@@ -134,13 +133,12 @@ public final void execute() throws MojoExecutionException {
 	private void execute(FormatterFactory formatterFactory) throws MojoExecutionException {
 		List<File> files = collectFiles(formatterFactory);
 		FormatterConfig config = getFormatterConfig();
-		@Nullable
-		String ratchetFrom = formatterFactory.ratchetFrom(config);
+		Optional<String> ratchetFrom = formatterFactory.ratchetFrom(config);
 		Iterable<File> toFormat;
-		if (ratchetFrom == null) {
+		if (!ratchetFrom.isPresent()) {
 			toFormat = files;
 		} else {
-			toFormat = Iterables.filter(files, MavenGitRatchet.instance().isGitDirty(baseDir, ratchetFrom));
+			toFormat = Iterables.filter(files, MavenGitRatchet.instance().isGitDirty(baseDir, ratchetFrom.get()));
 		}
 		try (Formatter formatter = formatterFactory.newFormatter(files, config)) {
 			process(toFormat, formatter);
@@ -191,7 +189,7 @@ private FormatterConfig getFormatterConfig() {
 		Provisioner provisioner = MavenProvisioner.create(resolver);
 		List<FormatterStepFactory> formatterStepFactories = getFormatterStepFactories();
 		FileLocator fileLocator = getFileLocator();
-		return new FormatterConfig(baseDir, encoding, lineEndings, ratchetFrom, provisioner, fileLocator, formatterStepFactories);
+		return new FormatterConfig(baseDir, encoding, lineEndings, Optional.ofNullable(ratchetFrom), provisioner, fileLocator, formatterStepFactories);
 	}
 
 	private FileLocator getFileLocator() {
diff --git a/plugin-maven/src/main/java/com/diffplug/spotless/maven/FormatterConfig.java b/plugin-maven/src/main/java/com/diffplug/spotless/maven/FormatterConfig.java
index af52d1fa87..6127d93d58 100644
--- a/plugin-maven/src/main/java/com/diffplug/spotless/maven/FormatterConfig.java
+++ b/plugin-maven/src/main/java/com/diffplug/spotless/maven/FormatterConfig.java
@@ -19,8 +19,7 @@
 
 import java.io.File;
 import java.util.List;
-
-import javax.annotation.Nullable;
+import java.util.Optional;
 
 import com.diffplug.spotless.LineEnding;
 import com.diffplug.spotless.Provisioner;
@@ -29,12 +28,12 @@ public class FormatterConfig {
 
 	private final String encoding;
 	private final LineEnding lineEndings;
-	private final @Nullable String ratchetFrom;
+	private final Optional<String> ratchetFrom;
 	private final Provisioner provisioner;
 	private final FileLocator fileLocator;
 	private final List<FormatterStepFactory> globalStepFactories;
 
-	public FormatterConfig(File baseDir, String encoding, LineEnding lineEndings, @Nullable String ratchetFrom, Provisioner provisioner,
+	public FormatterConfig(File baseDir, String encoding, LineEnding lineEndings, Optional<String> ratchetFrom, Provisioner provisioner,
 			FileLocator fileLocator, List<FormatterStepFactory> globalStepFactories) {
 		this.encoding = encoding;
 		this.lineEndings = lineEndings;
@@ -52,7 +51,7 @@ public LineEnding getLineEndings() {
 		return lineEndings;
 	}
 
-	public @Nullable String getRatchetFrom() {
+	public Optional<String> getRatchetFrom() {
 		return ratchetFrom;
 	}
 
diff --git a/plugin-maven/src/main/java/com/diffplug/spotless/maven/FormatterFactory.java b/plugin-maven/src/main/java/com/diffplug/spotless/maven/FormatterFactory.java
index 55cf468ba2..bbed6ad1a4 100644
--- a/plugin-maven/src/main/java/com/diffplug/spotless/maven/FormatterFactory.java
+++ b/plugin-maven/src/main/java/com/diffplug/spotless/maven/FormatterFactory.java
@@ -23,10 +23,9 @@
 import java.util.ArrayList;
 import java.util.List;
 import java.util.Objects;
+import java.util.Optional;
 import java.util.Set;
 
-import javax.annotation.Nullable;
-
 import org.apache.maven.plugins.annotations.Parameter;
 
 import com.diffplug.common.collect.Sets;
@@ -136,9 +135,12 @@ private LineEnding lineEndings(FormatterConfig config) {
 		return lineEndings == null ? config.getLineEndings() : lineEndings;
 	}
 
-	@Nullable
-	String ratchetFrom(FormatterConfig config) {
-		return ratchetFrom == RATCHETFROM_NOT_SET_AT_FORMAT_LEVEL ? config.getRatchetFrom() : ratchetFrom;
+	Optional<String> ratchetFrom(FormatterConfig config) {
+		if (ratchetFrom == RATCHETFROM_NOT_SET_AT_FORMAT_LEVEL) {
+			return config.getRatchetFrom();
+		} else {
+			return Optional.ofNullable(ratchetFrom);
+		}
 	}
 
 	private FormatterStepConfig stepConfig(Charset encoding, FormatterConfig config) {

From 89b4a270a98a9b3df9529500b6903c9836c87c52 Mon Sep 17 00:00:00 2001
From: Ned Twigg <ned.twigg@diffplug.com>
Date: Sat, 27 Jun 2020 22:30:15 -0700
Subject: [PATCH 06/11] Add test for `ratchetFrom`.

---
 .../spotless/maven/GitRatchetMavenTest.java   | 133 ++++++++++++++++++
 1 file changed, 133 insertions(+)
 create mode 100644 plugin-maven/src/test/java/com/diffplug/spotless/maven/GitRatchetMavenTest.java

diff --git a/plugin-maven/src/test/java/com/diffplug/spotless/maven/GitRatchetMavenTest.java b/plugin-maven/src/test/java/com/diffplug/spotless/maven/GitRatchetMavenTest.java
new file mode 100644
index 0000000000..ae8cac7669
--- /dev/null
+++ b/plugin-maven/src/test/java/com/diffplug/spotless/maven/GitRatchetMavenTest.java
@@ -0,0 +1,133 @@
+/*
+ * Copyright 2020 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 java.io.IOException;
+
+import org.eclipse.jgit.api.Git;
+import org.eclipse.jgit.api.errors.GitAPIException;
+import org.eclipse.jgit.lib.Constants;
+import org.eclipse.jgit.lib.ObjectId;
+import org.eclipse.jgit.lib.RefDatabase;
+import org.junit.Test;
+
+public class GitRatchetMavenTest extends MavenIntegrationHarness {
+	private static final String TEST_PATH = "src/markdown/test.md";
+
+	private Git initRepo() throws IllegalStateException, GitAPIException, IOException {
+		Git git = Git.init().setDirectory(rootFolder()).call();
+		RefDatabase refDB = git.getRepository().getRefDatabase();
+		refDB.newUpdate(Constants.R_HEADS + "main", false).setNewObjectId(ObjectId.zeroId());
+		refDB.newUpdate(Constants.HEAD, false).link(Constants.R_HEADS + "main");
+		refDB.newUpdate(Constants.R_HEADS + Constants.MASTER, false).delete();
+		return git;
+	}
+
+	@Test
+	public void singleProjectExhaustive() throws Exception {
+		try (Git git = initRepo()) {
+			writePom(
+					"<formats>",
+					"  <format>",
+					"    <ratchetFrom>baseline</ratchetFrom>",
+					"    <includes>",
+					"      <include>src/markdown/*.md</include>",
+					"    </includes>",
+					"    <replace>",
+					"      <name>Lowercase hello</name>",
+					"      <search>HELLO</search>",
+					"      <replacement>hello</replacement>",
+					"    </replace>",
+					"    <replace>",
+					"      <name>Lowercase world</name>",
+					"      <search>WORLD</search>",
+					"      <replacement>world</replacement>",
+					"    </replace>",
+					"    <replace>",
+					"      <name>Lowercase world</name>",
+					"      <search>MOM</search>",
+					"      <replacement>mom</replacement>",
+					"    </replace>",
+					"  </format>",
+					"</formats>");
+			setFile(TEST_PATH).toContent("HELLO");
+			git.add().addFilepattern(TEST_PATH).call();
+			git.commit().setMessage("Initial state").call();
+			// tag this initial state as the baseline for spotless to ratchet from
+			git.tag().setName("baseline").call();
+
+			// so at this point we have test.md, and it would normally be dirty,
+			// but because it is unchanged, spotless says it is clean
+			assertClean();
+
+			// but if we change it so that it is not clean, spotless will now say it is dirty
+			setFile(TEST_PATH).toContent("HELLO WORLD");
+			assertDirty();
+			mavenRunner().withArguments("spotless:apply").runNoError();
+			assertFile(TEST_PATH).hasContent("hello world");
+
+			// but if we make it unchanged again, it goes back to being clean
+			setFile(TEST_PATH).toContent("HELLO");
+			assertClean();
+
+			// and if we make the index dirty
+			setFile(TEST_PATH).toContent("HELLO WORLD");
+			git.add().addFilepattern(TEST_PATH).call();
+			{
+				// and the content dirty in the same way, then it's dirty
+				assertDirty();
+				// if we make the content something else dirty, then it's dirty
+				setFile(TEST_PATH).toContent("HELLO MOM");
+				assertDirty();
+				// if we make the content unchanged, even though index it and index are dirty, then it's clean
+				setFile(TEST_PATH).toContent("HELLO");
+				assertClean();
+				// if we delete the file, but it's still in the index, then it's clean
+				setFile(TEST_PATH).deleted();
+				assertClean();
+			}
+			// if we remove the file from the index
+			git.rm().addFilepattern(TEST_PATH).setCached(true).call();
+			{
+				// and it's gone in real life too, then it's clean
+				assertClean();
+				// if the content is there and unchanged, then it's clean
+				setFile(TEST_PATH).toContent("HELLO");
+				assertClean();
+				// if the content is dirty, then it's dirty
+				setFile(TEST_PATH).toContent("HELLO WORLD");
+				assertDirty();
+			}
+
+			// new files always get checked
+			setFile("new.md").toContent("HELLO");
+			{
+				assertDirty();
+				// even if they are added
+				git.add().addFilepattern("new.md").call();
+				assertDirty();
+			}
+		}
+	}
+
+	private void assertClean() throws Exception {
+		mavenRunner().withArguments("spotless:check").runNoError();
+	}
+
+	private void assertDirty() throws Exception {
+		mavenRunner().withArguments("spotless:check").runHasError();
+	}
+}

From f3fc82c453800df80f6689a63f080ca8dffebaeb Mon Sep 17 00:00:00 2001
From: Ned Twigg <ned.twigg@diffplug.com>
Date: Sat, 27 Jun 2020 22:43:04 -0700
Subject: [PATCH 07/11] Rename <Foo>GitRatchet to GitRatchet<Foo>, which is
 clearer.

---
 .../{GradleGitRatchet.java => GitRatchetGradle.java}   |  2 +-
 .../gradle/spotless/RegisterDependenciesTask.java      |  4 ++--
 .../com/diffplug/gradle/spotless/SpotlessTaskBase.java |  6 +++---
 ...{RatchetFromTest.java => GitRatchetGradleTest.java} |  2 +-
 .../diffplug/spotless/maven/AbstractSpotlessMojo.java  |  2 +-
 .../{MavenGitRatchet.java => GitRatchetMaven.java}     | 10 +++++-----
 6 files changed, 13 insertions(+), 13 deletions(-)
 rename plugin-gradle/src/main/java/com/diffplug/gradle/spotless/{GradleGitRatchet.java => GitRatchetGradle.java} (95%)
 rename plugin-gradle/src/test/java/com/diffplug/gradle/spotless/{RatchetFromTest.java => GitRatchetGradleTest.java} (99%)
 rename plugin-maven/src/main/java/com/diffplug/spotless/maven/{MavenGitRatchet.java => GitRatchetMaven.java} (86%)

diff --git a/plugin-gradle/src/main/java/com/diffplug/gradle/spotless/GradleGitRatchet.java b/plugin-gradle/src/main/java/com/diffplug/gradle/spotless/GitRatchetGradle.java
similarity index 95%
rename from plugin-gradle/src/main/java/com/diffplug/gradle/spotless/GradleGitRatchet.java
rename to plugin-gradle/src/main/java/com/diffplug/gradle/spotless/GitRatchetGradle.java
index fb67991847..0fd6b6572e 100644
--- a/plugin-gradle/src/main/java/com/diffplug/gradle/spotless/GradleGitRatchet.java
+++ b/plugin-gradle/src/main/java/com/diffplug/gradle/spotless/GitRatchetGradle.java
@@ -24,7 +24,7 @@
 import com.diffplug.spotless.extra.GitRatchet;
 
 /** Gradle implementation of GitRatchet. */
-class GradleGitRatchet extends GitRatchet<Project> {
+class GitRatchetGradle extends GitRatchet<Project> {
 	@Override
 	protected File getDir(Project project) {
 		return project.getProjectDir();
diff --git a/plugin-gradle/src/main/java/com/diffplug/gradle/spotless/RegisterDependenciesTask.java b/plugin-gradle/src/main/java/com/diffplug/gradle/spotless/RegisterDependenciesTask.java
index c7f06f7805..b770e66627 100644
--- a/plugin-gradle/src/main/java/com/diffplug/gradle/spotless/RegisterDependenciesTask.java
+++ b/plugin-gradle/src/main/java/com/diffplug/gradle/spotless/RegisterDependenciesTask.java
@@ -106,10 +106,10 @@ public void trivialFunction() throws IOException {
 		Files.write(Integer.toString(getSteps().size()), unitOutput, StandardCharsets.UTF_8);
 	}
 
-	GradleGitRatchet gitRatchet = new GradleGitRatchet();
+	GitRatchetGradle gitRatchet = new GitRatchetGradle();
 
 	@Internal
-	GradleGitRatchet getGitRatchet() {
+	GitRatchetGradle getGitRatchet() {
 		return gitRatchet;
 	}
 }
diff --git a/plugin-gradle/src/main/java/com/diffplug/gradle/spotless/SpotlessTaskBase.java b/plugin-gradle/src/main/java/com/diffplug/gradle/spotless/SpotlessTaskBase.java
index 25cedac9e5..2435f46d78 100644
--- a/plugin-gradle/src/main/java/com/diffplug/gradle/spotless/SpotlessTaskBase.java
+++ b/plugin-gradle/src/main/java/com/diffplug/gradle/spotless/SpotlessTaskBase.java
@@ -76,7 +76,7 @@ public void setLineEndingsPolicy(LineEnding.Policy lineEndingsPolicy) {
 
 	/*** API which performs git up-to-date tasks. */
 	@Nullable
-	GradleGitRatchet ratchet;
+	GitRatchetGradle ratchet;
 	/** The sha of the tree at repository root, used for determining if an individual *file* is clean according to git. */
 	ObjectId rootTreeSha;
 	/**
@@ -86,14 +86,14 @@ public void setLineEndingsPolicy(LineEnding.Policy lineEndingsPolicy) {
 	 */
 	private ObjectId subtreeSha = ObjectId.zeroId();
 
-	public void setupRatchet(GradleGitRatchet gitRatchet, String ratchetFrom) {
+	public void setupRatchet(GitRatchetGradle gitRatchet, String ratchetFrom) {
 		ratchet = gitRatchet;
 		rootTreeSha = gitRatchet.rootTreeShaOf(getProject(), ratchetFrom);
 		subtreeSha = gitRatchet.subtreeShaOf(getProject(), rootTreeSha);
 	}
 
 	@Internal
-	GradleGitRatchet getRatchet() {
+	GitRatchetGradle getRatchet() {
 		return ratchet;
 	}
 
diff --git a/plugin-gradle/src/test/java/com/diffplug/gradle/spotless/RatchetFromTest.java b/plugin-gradle/src/test/java/com/diffplug/gradle/spotless/GitRatchetGradleTest.java
similarity index 99%
rename from plugin-gradle/src/test/java/com/diffplug/gradle/spotless/RatchetFromTest.java
rename to plugin-gradle/src/test/java/com/diffplug/gradle/spotless/GitRatchetGradleTest.java
index 387df29dbe..010f8d2112 100644
--- a/plugin-gradle/src/test/java/com/diffplug/gradle/spotless/RatchetFromTest.java
+++ b/plugin-gradle/src/test/java/com/diffplug/gradle/spotless/GitRatchetGradleTest.java
@@ -34,7 +34,7 @@
 import org.gradle.testkit.runner.TaskOutcome;
 import org.junit.Test;
 
-public class RatchetFromTest extends GradleIntegrationHarness {
+public class GitRatchetGradleTest extends GradleIntegrationHarness {
 	private static final String TEST_PATH = "src/markdown/test.md";
 
 	private Git initRepo() throws IllegalStateException, GitAPIException, IOException {
diff --git a/plugin-maven/src/main/java/com/diffplug/spotless/maven/AbstractSpotlessMojo.java b/plugin-maven/src/main/java/com/diffplug/spotless/maven/AbstractSpotlessMojo.java
index 03ab863e89..d3f410a754 100644
--- a/plugin-maven/src/main/java/com/diffplug/spotless/maven/AbstractSpotlessMojo.java
+++ b/plugin-maven/src/main/java/com/diffplug/spotless/maven/AbstractSpotlessMojo.java
@@ -138,7 +138,7 @@ private void execute(FormatterFactory formatterFactory) throws MojoExecutionExce
 		if (!ratchetFrom.isPresent()) {
 			toFormat = files;
 		} else {
-			toFormat = Iterables.filter(files, MavenGitRatchet.instance().isGitDirty(baseDir, ratchetFrom.get()));
+			toFormat = Iterables.filter(files, GitRatchetMaven.instance().isGitDirty(baseDir, ratchetFrom.get()));
 		}
 		try (Formatter formatter = formatterFactory.newFormatter(files, config)) {
 			process(toFormat, formatter);
diff --git a/plugin-maven/src/main/java/com/diffplug/spotless/maven/MavenGitRatchet.java b/plugin-maven/src/main/java/com/diffplug/spotless/maven/GitRatchetMaven.java
similarity index 86%
rename from plugin-maven/src/main/java/com/diffplug/spotless/maven/MavenGitRatchet.java
rename to plugin-maven/src/main/java/com/diffplug/spotless/maven/GitRatchetMaven.java
index 75c18a9bdc..30cdd66361 100644
--- a/plugin-maven/src/main/java/com/diffplug/spotless/maven/MavenGitRatchet.java
+++ b/plugin-maven/src/main/java/com/diffplug/spotless/maven/GitRatchetMaven.java
@@ -24,7 +24,7 @@
 import com.diffplug.common.base.Errors;
 import com.diffplug.spotless.extra.GitRatchet;
 
-class MavenGitRatchet extends GitRatchet<File> {
+class GitRatchetMaven extends GitRatchet<File> {
 	@Override
 	protected File getDir(File project) {
 		return project;
@@ -35,13 +35,13 @@ protected File getParent(File project) {
 		return project.getParentFile();
 	}
 
-	private static MavenGitRatchet instance = new MavenGitRatchet();
+	private static GitRatchetMaven instance = new GitRatchetMaven();
 
-	static MavenGitRatchet instance() {
+	static GitRatchetMaven instance() {
 		if (instance == null) {
-			synchronized (MavenGitRatchet.class) {
+			synchronized (GitRatchetMaven.class) {
 				if (instance == null) {
-					instance = new MavenGitRatchet();
+					instance = new GitRatchetMaven();
 				}
 			}
 		}

From 6be4e303243dc379632d37b2707e9ba1695fcdf8 Mon Sep 17 00:00:00 2001
From: Ned Twigg <ned.twigg@diffplug.com>
Date: Sat, 27 Jun 2020 23:31:03 -0700
Subject: [PATCH 08/11] Fix spotbugs warning.

---
 .../main/java/com/diffplug/spotless/maven/GitRatchetMaven.java  | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/plugin-maven/src/main/java/com/diffplug/spotless/maven/GitRatchetMaven.java b/plugin-maven/src/main/java/com/diffplug/spotless/maven/GitRatchetMaven.java
index 30cdd66361..0f841c2901 100644
--- a/plugin-maven/src/main/java/com/diffplug/spotless/maven/GitRatchetMaven.java
+++ b/plugin-maven/src/main/java/com/diffplug/spotless/maven/GitRatchetMaven.java
@@ -35,7 +35,7 @@ protected File getParent(File project) {
 		return project.getParentFile();
 	}
 
-	private static GitRatchetMaven instance = new GitRatchetMaven();
+	private static volatile GitRatchetMaven instance = new GitRatchetMaven();
 
 	static GitRatchetMaven instance() {
 		if (instance == null) {

From 44c4fd04aa5ec2d7de8e2e6e5f7a03ccfb944083 Mon Sep 17 00:00:00 2001
From: Ned Twigg <ned.twigg@diffplug.com>
Date: Sat, 27 Jun 2020 23:05:34 -0700
Subject: [PATCH 09/11] Add ratchet docs to the readme.

---
 plugin-maven/README.md | 22 +++++++++++++++++++++-
 1 file changed, 21 insertions(+), 1 deletion(-)

diff --git a/plugin-maven/README.md b/plugin-maven/README.md
index 66c2155209..bcba6f6e3b 100644
--- a/plugin-maven/README.md
+++ b/plugin-maven/README.md
@@ -594,7 +594,25 @@ By default, `spotless:check` is bound to the `verify` phase.  You might want to
 - If you don't like what spotless did, `git reset --hard`
 - If you'd like to remove the "checkpoint" commit, `git reset --soft head~1` will make the checkpoint commit "disappear" from history, but keeps the changes in your working directory.
 
-<a name="examples"></a>
+<a name="ratchet"></a>
+
+## How can I enforce formatting gradually?
+
+If your project is not currently enforcing formatting, then it can be a noisy transition.  Having a giant commit where every single file gets changed makes the history harder to read.  To address this, you can use the `ratchet` feature:
+
+```xml
+<configuration>
+  <ratchetFrom>origin/main</ratchetFrom> <!-- only format files which have changed since origin/main -->
+  <!-- ... define formats ... -->
+</configuration>
+```
+
+In this mode, Spotless will apply only to files which have changed since `origin/main`.  You can ratchet from [any point you want](https://javadoc.io/static/org.eclipse.jgit/org.eclipse.jgit/5.6.1.202002131546-r/org/eclipse/jgit/lib/Repository.html#resolve-java.lang.String-), even `HEAD`.  You can also set `ratchetFrom` per-format if you prefer (e.g. `<configuration><java><ratchetFrom>...`).
+
+However, we strongly recommend that you use a non-local branch, such as a tag or `origin/main`.  The problem with `HEAD` or any local branch is that as soon as you commit a file, that is now the canonical formatting, even if it was formatted incorrectly.  By instead specifying `origin/main` or a tag, your CI server will fail unless every changed file is at least as good or better than it was before the change.
+
+This is especially helpful for injecting accurate copyright dates using the [license step](#license-header).
+
 
 ## Can I apply Spotless to specific files?
 
@@ -606,6 +624,8 @@ 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.
 
+<a name="examples"></a>
+
 ## Example configurations (from real-world projects)
 
 - [Apache Avro](https://github.com/apache/avro/blob/8026c8ffe4ef67ab419dba73910636bf2c1a691c/lang/java/pom.xml#L307-L334)

From dba5e9fdacb8c08a69875b2d2e31365a58c90f11 Mon Sep 17 00:00:00 2001
From: Ned Twigg <ned.twigg@diffplug.com>
Date: Sun, 28 Jun 2020 15:06:39 -0700
Subject: [PATCH 10/11] Better encapsulation, per feedback from @lutovich

---
 .../java/com/diffplug/gradle/spotless/GitRatchetGradle.java   | 2 +-
 .../java/com/diffplug/spotless/maven/GitRatchetMaven.java     | 4 +++-
 2 files changed, 4 insertions(+), 2 deletions(-)

diff --git a/plugin-gradle/src/main/java/com/diffplug/gradle/spotless/GitRatchetGradle.java b/plugin-gradle/src/main/java/com/diffplug/gradle/spotless/GitRatchetGradle.java
index 0fd6b6572e..bfa84258c1 100644
--- a/plugin-gradle/src/main/java/com/diffplug/gradle/spotless/GitRatchetGradle.java
+++ b/plugin-gradle/src/main/java/com/diffplug/gradle/spotless/GitRatchetGradle.java
@@ -24,7 +24,7 @@
 import com.diffplug.spotless.extra.GitRatchet;
 
 /** Gradle implementation of GitRatchet. */
-class GitRatchetGradle extends GitRatchet<Project> {
+final class GitRatchetGradle extends GitRatchet<Project> {
 	@Override
 	protected File getDir(Project project) {
 		return project.getProjectDir();
diff --git a/plugin-maven/src/main/java/com/diffplug/spotless/maven/GitRatchetMaven.java b/plugin-maven/src/main/java/com/diffplug/spotless/maven/GitRatchetMaven.java
index 0f841c2901..a759f05b06 100644
--- a/plugin-maven/src/main/java/com/diffplug/spotless/maven/GitRatchetMaven.java
+++ b/plugin-maven/src/main/java/com/diffplug/spotless/maven/GitRatchetMaven.java
@@ -24,7 +24,9 @@
 import com.diffplug.common.base.Errors;
 import com.diffplug.spotless.extra.GitRatchet;
 
-class GitRatchetMaven extends GitRatchet<File> {
+final class GitRatchetMaven extends GitRatchet<File> {
+	private GitRatchetMaven() {}
+
 	@Override
 	protected File getDir(File project) {
 		return project;

From a5d1453010efb62e73e77374950d4cbe9afdc6dd Mon Sep 17 00:00:00 2001
From: Ned Twigg <ned.twigg@diffplug.com>
Date: Mon, 29 Jun 2020 11:00:27 -0700
Subject: [PATCH 11/11] Update changelog.

---
 plugin-maven/CHANGES.md | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/plugin-maven/CHANGES.md b/plugin-maven/CHANGES.md
index 6aaebb196a..8e999a4897 100644
--- a/plugin-maven/CHANGES.md
+++ b/plugin-maven/CHANGES.md
@@ -4,8 +4,9 @@ We adhere to the [keepachangelog](https://keepachangelog.com/en/1.0.0/) format (
 
 ## [Unreleased]
 ### Added
-* `prettier` will now autodetect the parser (and formatter) to use based on the filename, unless you override this using `config` or `configFile` with the option `parser` or `filepath` ([#620](https://github.com/diffplug/spotless/pull/620)).
+* You can now ratchet a project's style by limiting Spotless only to files which have changed since a given [git reference](https://javadoc.io/static/org.eclipse.jgit/org.eclipse.jgit/5.6.1.202002131546-r/org/eclipse/jgit/lib/Repository.html#resolve-java.lang.String-), e.g. `ratchetFrom 'origin/main'`. ([#590](https://github.com/diffplug/spotless/pull/590))
 * Huge speed improvement for multi-module projects thanks to improved cross-project classloader caching ([#571](https://github.com/diffplug/spotless/pull/571), fixes [#559](https://github.com/diffplug/spotless/issues/559)).
+* `prettier` will now autodetect the parser (and formatter) to use based on the filename, unless you override this using `config` or `configFile` with the option `parser` or `filepath` ([#620](https://github.com/diffplug/spotless/pull/620)).
 
 ## [1.31.3] - 2020-06-17
 ### Changed