The new FormatterStep.create() and createLazy() utilized anonymous classes which captured their parameters in non-transient fields. Thanks to FindBugs for spotting the error! New implementation ensures that the *only state* is the serializable key.

Author: nedtwigg

src/main/java/com/diffplug/gradle/spotless/FormatExtensionStandardImpl.java

@@ -0,0 +1,86 @@
+/*
+ * Copyright 2016 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 java.io.Serializable;
+import java.util.Objects;
+import java.util.function.BiFunction;
+
+import com.diffplug.common.base.Throwing;
+import com.diffplug.gradle.spotless.FormatterStep.Strict;
+
+/**
+ * Standard implementation of FormatExtension which cleanly enforces
+ * separation of serializable configuration and a pure format function.
+ */
+abstract class FormatExtensionStandardImpl<Key extends Serializable> extends Strict<Key> {
+	private static final long serialVersionUID = 1L;
+
+	/** Transient because only the state matters. */
+	final transient String name;
+	/** Transient because only the state matters. */
+	final transient BiFunction<Key, String, String> formatter;
+
+	public FormatExtensionStandardImpl(String name, BiFunction<Key, String, String> formatter) {
+		this.name = Objects.requireNonNull(name);
+		this.formatter = Objects.requireNonNull(formatter);
+	}
+
+	@Override
+	public String getName() {
+		return name;
+	}
+
+	@Override
+	protected String format(Key key, String rawUnix, File file) {
+		return formatter.apply(key, rawUnix);
+	}
+
+	static class Eager<Key extends Serializable> extends FormatExtensionStandardImpl<Key> {
+		private static final long serialVersionUID = 1L;
+
+		/** Transient because the key is persisted by the superclass. */
+		final transient Key key;
+
+		public Eager(String name, Key key, BiFunction<Key, String, String> formatter) {
+			super(name, formatter);
+			this.key = Objects.requireNonNull(key);
+		}
+
+		@Override
+		protected Key calculateKey() throws Exception {
+			return key;
+		}
+	}
+
+	static class Lazy<Key extends Serializable> extends FormatExtensionStandardImpl<Key> {
+		private static final long serialVersionUID = 1L;
+
+		/** Transient because the key is persisted by the superclass. */
+		final transient Throwing.Specific.Supplier<Key, Exception> keySupplier;
+
+		public Lazy(String name, Throwing.Specific.Supplier<Key, Exception> keySupplier, BiFunction<Key, String, String> formatter) {
+			super(name, formatter);
+			this.keySupplier = Objects.requireNonNull(keySupplier);
+		}
+
+		@Override
+		protected Key calculateKey() throws Exception {
+			return keySupplier.get();
+		}
+	}
+}

src/main/java/com/diffplug/gradle/spotless/FormatterStep.java

@@ -39,7 +39,7 @@ public interface FormatterStep extends Serializable {
 	/**
 	 * Returns a formatted version of the given content.
 	 *
-	 * @param raw
+	 * @param rawUnix
 	 *            File's content, guaranteed to have unix-style newlines ('\n')
 	 * @param file
 	 *            the File which is being formatted
@@ -92,24 +92,7 @@ public static <Key extends Serializable> FormatterStep createLazy(
 			String name,
 			Throwing.Specific.Supplier<Key, Exception> keySupplier,
 			BiFunction<Key, String, String> formatter) {
-		return new Strict<Key>() {
-			private static final long serialVersionUID = 1L;
-
-			@Override
-			public String getName() {
-				return name;
-			}
-
-			@Override
-			protected Key calculateKey() throws Exception {
-				return keySupplier.get();
-			}
-
-			@Override
-			protected String format(Key key, String rawUnix, File file) {
-				return formatter.apply(key, rawUnix);
-			}
-		};
+		return new FormatExtensionStandardImpl.Lazy<>(name, keySupplier, formatter);
 	}
 
 	/**
@@ -126,24 +109,7 @@ public static <Key extends Serializable> FormatterStep create(
 			String name,
 			Key key,
 			BiFunction<Key, String, String> formatter) {
-		return new Strict<Key>() {
-			private static final long serialVersionUID = 1L;
-
-			@Override
-			public String getName() {
-				return name;
-			}
-
-			@Override
-			protected Key calculateKey() throws Exception {
-				return key;
-			}
-
-			@Override
-			protected String format(Key key, String rawUnix, File file) {
-				return formatter.apply(key, rawUnix);
-			}
-		};
+		return new FormatExtensionStandardImpl.Eager<>(name, key, formatter);
 	}
 
 	/** A FormatterStep which doesn't depend on the input file. */