Merge pull request #43586 from tmaciejewski

* pr/43586:
  Polish "Add support for symlinks in FileWatcher"
  Add support for symlinks in FileWatcher

Closes gh-43586

Author: snicoll

spring-boot-project/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/ssl/FileWatcher.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2023 the original author or authors.
+ * Copyright 2012-2024 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -86,14 +86,26 @@ void watch(Set<Path> paths, Runnable action) {
 					this.thread = new WatcherThread();
 					this.thread.start();
 				}
-				this.thread.register(new Registration(paths, action));
+				Set<Path> actualPaths = new HashSet<>();
+				for (Path path : paths) {
+					actualPaths.add(resolveSymlinkIfNecessary(path));
+				}
+				this.thread.register(new Registration(actualPaths, action));
 			}
 			catch (IOException ex) {
 				throw new UncheckedIOException("Failed to register paths for watching: " + paths, ex);
 			}
 		}
 	}
 
+	private static Path resolveSymlinkIfNecessary(Path path) throws IOException {
+		if (Files.isSymbolicLink(path)) {
+			Path target = Files.readSymbolicLink(path);
+			return resolveSymlinkIfNecessary(target);
+		}
+		return path;
+	}
+
 	@Override
 	public void close() throws IOException {
 		synchronized (this.lock) {

spring-boot-project/spring-boot-autoconfigure/src/test/java/org/springframework/boot/autoconfigure/ssl/FileWatcherTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2023 the original author or authors.
+ * Copyright 2012-2024 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -94,6 +94,32 @@ void shouldWatchFile(@TempDir Path tempDir) throws Exception {
 		callback.expectChanges();
 	}
 
+	@Test
+	void shouldFollowSymlink(@TempDir Path tempDir) throws Exception {
+		Path realFile = tempDir.resolve("realFile.txt");
+		Path symLink = tempDir.resolve("symlink.txt");
+		Files.createFile(realFile);
+		Files.createSymbolicLink(symLink, realFile);
+		WaitingCallback callback = new WaitingCallback();
+		this.fileWatcher.watch(Set.of(symLink), callback);
+		Files.writeString(realFile, "Some content");
+		callback.expectChanges();
+	}
+
+	@Test
+	void shouldFollowSymlinkRecursively(@TempDir Path tempDir) throws Exception {
+		Path realFile = tempDir.resolve("realFile.txt");
+		Path symLink = tempDir.resolve("symlink.txt");
+		Path symLink2 = tempDir.resolve("symlink2.txt");
+		Files.createFile(realFile);
+		Files.createSymbolicLink(symLink, symLink2);
+		Files.createSymbolicLink(symLink2, realFile);
+		WaitingCallback callback = new WaitingCallback();
+		this.fileWatcher.watch(Set.of(symLink), callback);
+		Files.writeString(realFile, "Some content");
+		callback.expectChanges();
+	}
+
 	@Test
 	void shouldIgnoreNotWatchedFiles(@TempDir Path tempDir) throws Exception {
 		Path watchedFile = tempDir.resolve("watched.txt");