Add a roundtrip-serializable method to FormatterStep.

Author: nedtwigg

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

@@ -104,6 +104,29 @@ public final String format(String rawUnix, File file) throws Exception {
 		}
 	}
 
+	/**
+	 * @param name
+	 *             The name of the formatter step.
+	 * @param roundtripInit
+	 *             If the step has any state, this supplier will calculate it lazily. The supplier doesn't
+	 *             have to be serializable, but the result it calculates needs to be serializable.
+	 * @param equalityFunc
+	 * 		       A pure serializable function (method reference recommended) which takes the result of `roundtripInit`,
+	 * 		       and returns a serializable object whose serialized representation will be used for `.equals` and
+	 * 		       `.hashCode` of the FormatterStep.
+	 * @param formatterFunc
+	 * 		       A pure serializable function (method reference recommended) which takes the result of `equalityFunc`,
+	 * 		       and returns a `FormatterFunc` which will be used for the actual formatting.
+	 * @return A FormatterStep which can be losslessly roundtripped through the java serialization machinery.
+	 */
+	static <RoundtripState extends Serializable, EqualityState extends Serializable> FormatterStep createLazy(
+			String name,
+			ThrowingEx.Supplier<RoundtripState> roundtripInit,
+			SerializedFunction<RoundtripState, EqualityState> equalityFunc,
+			SerializedFunction<EqualityState, FormatterFunc> formatterFunc) {
+		return new FormatterStepSerializationRoundtrip(name, roundtripInit, equalityFunc, formatterFunc);
+	}
+
 	/**
 	 * @param name
 	 *             The name of the formatter step

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

@@ -0,0 +1,71 @@
+/*
+ * Copyright 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.IOException;
+import java.io.ObjectStreamException;
+import java.io.Serializable;
+
+import edu.umd.cs.findbugs.annotations.Nullable;
+
+class FormatterStepSerializationRoundtrip<RoundtripState extends Serializable, EqualityState extends Serializable> extends FormatterStepEqualityOnStateSerialization<EqualityState> {
+	private final String name;
+	private final transient ThrowingEx.Supplier<RoundtripState> initializer;
+	private @Nullable RoundtripState roundtripStateInternal;
+	private final SerializedFunction<RoundtripState, EqualityState> equalityStateExtractor;
+	private final SerializedFunction<EqualityState, FormatterFunc> equalityStateToFormatter;
+
+	public FormatterStepSerializationRoundtrip(String name, ThrowingEx.Supplier<RoundtripState> initializer, SerializedFunction<RoundtripState, EqualityState> equalityStateExtractor, SerializedFunction<EqualityState, FormatterFunc> equalityStateToFormatter) {
+		this.name = name;
+		this.initializer = initializer;
+		this.equalityStateExtractor = equalityStateExtractor;
+		this.equalityStateToFormatter = equalityStateToFormatter;
+	}
+
+	@Override
+	public String getName() {
+		return name;
+	}
+
+	@Override
+	protected EqualityState stateSupplier() throws Exception {
+		if (roundtripStateInternal != null) {
+			roundtripStateInternal = initializer.get();
+		}
+		return equalityStateExtractor.apply(roundtripStateInternal);
+	}
+
+	@Override
+	protected FormatterFunc stateToFormatter(EqualityState equalityState) throws Exception {
+		return equalityStateToFormatter.apply(equalityState);
+	}
+
+	// override serialize output
+	private void writeObject(java.io.ObjectOutputStream out) throws IOException {
+		if (roundtripStateInternal == null) {
+			roundtripStateInternal = ThrowingEx.get(initializer::get);
+		}
+		out.defaultWriteObject();
+	}
+
+	private void readObject(java.io.ObjectInputStream in) throws IOException, ClassNotFoundException {
+		in.defaultReadObject();
+	}
+
+	private void readObjectNoData() throws ObjectStreamException {
+		throw new UnsupportedOperationException();
+	}
+}

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

@@ -0,0 +1,21 @@
+/*
+ * Copyright 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.Serializable;
+
+@FunctionalInterface
+interface SerializedFunction<T, R> extends Serializable, ThrowingEx.Function<T, R> {}