Introduce a new FormatterStep base class for the serialized roundtrip world.

Author: nedtwigg

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

@@ -304,6 +304,8 @@ public void close() {
 		for (FormatterStep step : steps) {
 			if (step instanceof FormatterStepImpl.Standard) {
 				((FormatterStepImpl.Standard) step).cleanupFormatterFunc();
+			} else if (step instanceof FormatterStepEqualityOnStateSerialization) {
+				((FormatterStepEqualityOnStateSerialization) step).cleanupFormatterFunc();
 			}
 		}
 	}

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

@@ -0,0 +1,91 @@
+/*
+ * Copyright 2016-2023 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;
+
+import java.io.File;
+import java.io.Serializable;
+import java.util.Arrays;
+
+import edu.umd.cs.findbugs.annotations.SuppressFBWarnings;
+
+/**
+ * Standard implementation of FormatterStep which cleanly enforces
+ * separation of a lazily computed "state" object whose serialized form
+ * is used as the basis for equality and hashCode, which is separate
+ * from the serialized form of the step itself, which can include absolute paths
+ * and such without interfering with buildcache keys.
+ */
+@SuppressFBWarnings("SE_TRANSIENT_FIELD_NOT_RESTORED")
+public abstract class FormatterStepEqualityOnStateSerialization<State extends Serializable> implements FormatterStep, Serializable {
+	private static final long serialVersionUID = 1L;
+
+	protected abstract State stateSupplier() throws Exception;
+
+	protected abstract FormatterFunc stateToFormatter(State state) throws Exception;
+
+	private transient FormatterFunc formatter;
+	private transient State stateInternal;
+	private transient byte[] serializedStateInternal;
+
+	@Override
+	public String format(String rawUnix, File file) throws Exception {
+		if (formatter == null) {
+			formatter = stateToFormatter(state());
+		}
+		return formatter.apply(rawUnix, file);
+	}
+
+	@Override
+	public boolean equals(Object o) {
+		if (o == null) {
+			return false;
+		} else if (getClass() != o.getClass()) {
+			return false;
+		} else {
+			return Arrays.equals(serializedState(), ((FormatterStepEqualityOnStateSerialization<?>) o).serializedState());
+		}
+	}
+
+	@Override
+	public int hashCode() {
+		return Arrays.hashCode(serializedState());
+	}
+
+	void cleanupFormatterFunc() {
+		if (formatter instanceof FormatterFunc.Closeable) {
+			((FormatterFunc.Closeable) formatter).close();
+			formatter = null;
+		}
+	}
+
+	private State state() throws Exception {
+		if (stateInternal == null) {
+			stateInternal = stateSupplier();
+		}
+		return stateInternal;
+	}
+
+	private byte[] serializedState() {
+		if (serializedStateInternal == null) {
+			try {
+				serializedStateInternal = LazyForwardingEquality.toBytes(state());
+			} catch (Exception e) {
+				throw new RuntimeException(e);
+			}
+		}
+		return serializedStateInternal;
+	}
+}