Add a sentinel step which triggers the "serialize to byte array hack"

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

@@ -16,11 +16,14 @@
 package com.diffplug.spotless;
 
 import java.io.IOException;
+import java.io.Serializable;
 import java.util.ArrayList;
 import java.util.Collection;
 import java.util.List;
 import java.util.Objects;
 
+import com.diffplug.spotless.yaml.SerializeToByteArrayHack;
+
 import edu.umd.cs.findbugs.annotations.SuppressFBWarnings;
 
 /**
@@ -50,27 +53,43 @@
  * to make Spotless work with all of Gradle's cache systems at once.
  */
 public class ConfigurationCacheHackList implements java.io.Serializable {
-	private static final long serialVersionUID = 1L;
+	private static final long serialVersionUID = 6914178791997323870L;
+
 	private boolean optimizeForEquality;
 	private ArrayList<Object> backingList = new ArrayList<>();
 
+	private boolean shouldWeSerializeToByteArrayFirst() {
+		return backingList.stream().anyMatch(step -> step instanceof SerializeToByteArrayHack);
+	}
+
 	private void writeObject(java.io.ObjectOutputStream out) throws IOException {
+		boolean serializeToByteArrayFirst = shouldWeSerializeToByteArrayFirst();
+		out.writeBoolean(serializeToByteArrayFirst);
 		out.writeBoolean(optimizeForEquality);
 		out.writeInt(backingList.size());
 		for (Object obj : backingList) {
 			// if write out the list on its own, we'll get java's non-deterministic object-graph serialization
 			// by writing each object to raw bytes independently, we avoid this
-			out.writeObject(obj);
+			if (serializeToByteArrayFirst) {
+				out.writeObject(LazyForwardingEquality.toBytes((Serializable) obj));
+			} else {
+				out.writeObject(obj);
+			}
 		}
 	}
 
 	@SuppressFBWarnings("MC_OVERRIDABLE_METHOD_CALL_IN_READ_OBJECT")
 	private void readObject(java.io.ObjectInputStream in) throws IOException, ClassNotFoundException {
+		boolean serializeToByteArrayFirst = in.readBoolean();
 		optimizeForEquality = in.readBoolean();
 		backingList = new ArrayList<>();
 		int size = in.readInt();
 		for (int i = 0; i < size; i++) {
-			backingList.add(in.readObject());
+			if (serializeToByteArrayFirst) {
+				backingList.add(LazyForwardingEquality.fromBytes((byte[]) in.readObject()));
+			} else {
+				backingList.add(in.readObject());
+			}
 		}
 	}
 

lib/src/main/java/com/diffplug/spotless/yaml/SerializeToByteArrayHack.java

@@ -0,0 +1,42 @@
+/*
+ * Copyright 2025 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.yaml;
+
+import java.io.File;
+
+import javax.annotation.Nullable;
+
+import com.diffplug.spotless.FormatterStep;
+
+public class SerializeToByteArrayHack implements FormatterStep {
+	private static final long serialVersionUID = 8071047581828362545L;
+
+	@Override
+	public String getName() {
+		return "hack to force serializing objects to byte array";
+	}
+
+	@Nullable
+	@Override
+	public String format(String rawUnix, File file) throws Exception {
+		return null;
+	}
+
+	@Override
+	public void close() throws Exception {
+
+	}
+}

testlib/src/test/java/com/diffplug/spotless/combined/CombinedJavaFormatStepTest.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2023-2024 DiffPlug
+ * Copyright 2023-2025 DiffPlug
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -31,6 +31,7 @@
 import com.diffplug.spotless.java.GoogleJavaFormatStep;
 import com.diffplug.spotless.java.ImportOrderStep;
 import com.diffplug.spotless.java.RemoveUnusedImportsStep;
+import com.diffplug.spotless.yaml.SerializeToByteArrayHack;
 
 public class CombinedJavaFormatStepTest extends ResourceHarness {
 
@@ -45,6 +46,7 @@ void checkIssue1679() {
 		FenceStep toggleOffOnPair = FenceStep.named(FenceStep.defaultToggleName()).openClose("formatting:off", "formatting:on");
 		try (StepHarness formatter = StepHarness.forSteps(
 				toggleOffOnPair.preserveWithin(List.of(
+						new SerializeToByteArrayHack(),
 						gjf,
 						indentWithSpaces,
 						importOrder,