Introduce FileSignature.RoundTrippable and JarState.Promised so that maven-based formatters can be safely roundtripped.

Author: nedtwigg

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2016-2021 DiffPlug
+ * 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.
@@ -33,6 +33,7 @@
 import java.util.Locale;
 import java.util.Map;
 
+import edu.umd.cs.findbugs.annotations.Nullable;
 import edu.umd.cs.findbugs.annotations.SuppressFBWarnings;
 
 /** Computes a signature for any needed files. */
@@ -43,6 +44,8 @@ public final class FileSignature implements Serializable {
 	 * Transient because not needed to uniquely identify a FileSignature instance, and also because
 	 * Gradle only needs this class to be Serializable so it can compare FileSignature instances for
 	 * incremental builds.
+	 *
+	 * We don't want these absolute paths to screw up buildcache keys.
 	 */
 	@SuppressFBWarnings("SE_TRANSIENT_FIELD_NOT_RESTORED")
 	private final transient List<File> files;
@@ -93,6 +96,45 @@ private FileSignature(final List<File> files) throws IOException {
 		}
 	}
 
+	/** A view of `FileSignature` which can be safely roundtripped. */
+	public static class RoundTrippable implements Serializable {
+		private final List<File> files;
+		private transient @Nullable FileSignature cached;
+
+		private RoundTrippable(List<File> files, FileSignature cached) {
+			this.files = files;
+			this.cached = cached;
+		}
+
+		public FileSignature stripAbsolutePaths() throws IOException {
+			if (cached == null) {
+				// null when restored via serialization
+				cached = new FileSignature(files);
+			}
+			return cached;
+		}
+	}
+
+	public RoundTrippable roundTrippable() {
+		return new RoundTrippable(files, this);
+	}
+
+	public static @Nullable RoundTrippable roundTrippableNullable(@Nullable FileSignature signature) {
+		if (signature != null) {
+			return signature.roundTrippable();
+		} else {
+			return null;
+		}
+	}
+
+	public static @Nullable FileSignature stripAbsolutePathsNullable(@Nullable RoundTrippable roundTrippable) throws IOException {
+		if (roundTrippable != null) {
+			return roundTrippable.stripAbsolutePaths();
+		} else {
+			return null;
+		}
+	}
+
 	/** Returns all of the files in this signature, throwing an exception if there are more or less than 1 file. */
 	public Collection<File> files() {
 		return Collections.unmodifiableList(files);

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

@@ -17,6 +17,7 @@
 
 import java.io.File;
 import java.io.IOException;
+import java.io.ObjectStreamException;
 import java.io.Serializable;
 import java.net.URI;
 import java.net.URL;
@@ -36,6 +37,48 @@
  * catch changes in a SNAPSHOT version.
  */
 public final class JarState implements Serializable {
+	/** A lazily evaluated JarState, which becomes a set of files when serialized. */
+	public static class Promised implements Serializable {
+		private final transient ThrowingEx.Supplier<JarState> supplier;
+		private FileSignature.RoundTrippable cached;
+
+		public Promised(ThrowingEx.Supplier<JarState> supplier) {
+			this.supplier = supplier;
+		}
+
+		public JarState get() {
+			try {
+				if (cached == null) {
+					JarState result = supplier.get();
+					cached = result.fileSignature.roundTrippable();
+					return result;
+				}
+				return new JarState(cached.stripAbsolutePaths());
+			} catch (Exception e) {
+				throw ThrowingEx.asRuntime(e);
+			}
+		}
+
+		// override serialize output
+		private void writeObject(java.io.ObjectOutputStream out)
+				throws IOException {
+			get();
+			out.defaultWriteObject();
+		}
+
+		private void readObject(java.io.ObjectInputStream in) throws IOException, ClassNotFoundException {
+			in.defaultReadObject();
+		}
+
+		private void readObjectNoData() throws ObjectStreamException {
+			throw new UnsupportedOperationException();
+		}
+	}
+
+	public static Promised promise(ThrowingEx.Supplier<JarState> supplier) {
+		return new Promised(supplier);
+	}
+
 	private static final long serialVersionUID = 1L;
 
 	private final FileSignature fileSignature;