Polish FileSystemWatcher and improve its thread safety

- Limit shared state between FileSystemWatcher and the watching thread
- Use a private monitor rather than synchronizing on this
- Use a Runnable implementation rather than subclassing Thread
- Synchronize consistently when reading and writing state

Closes gh-6039

spring-boot-devtools/src/main/java/org/springframework/boot/devtools/filewatch/FileSystemWatcher.java

@@ -21,6 +21,7 @@ import java.io.FileFilter;
 import java.util.ArrayList;
 import java.util.Collection;
 import java.util.Collections;
+import java.util.HashMap;
 import java.util.LinkedHashMap;
 import java.util.LinkedHashSet;
 import java.util.List;
@@ -44,7 +45,7 @@ public class FileSystemWatcher {
 
 	private static final long DEFAULT_QUIET_PERIOD = 400;
 
-	private List<FileChangeListener> listeners = new ArrayList<FileChangeListener>();
+	private final List<FileChangeListener> listeners = new ArrayList<FileChangeListener>();
 
 	private final boolean daemon;
 
@@ -52,14 +53,16 @@ public class FileSystemWatcher {
 
 	private final long quietPeriod;
 
-	private Thread watchThread;
+	private final AtomicInteger remainingScans = new AtomicInteger(-1);
 
-	private AtomicInteger remainingScans = new AtomicInteger(-1);
+	private final Map<File, FolderSnapshot> folders = new HashMap<File, FolderSnapshot>();
 
-	private Map<File, FolderSnapshot> folders = new LinkedHashMap<File, FolderSnapshot>();
+	private Thread watchThread;
 
 	private FileFilter triggerFilter;
 
+	private final Object monitor = new Object();
+
 	/**
 	 * Create a new {@link FileSystemWatcher} instance.
 	 */
@@ -89,11 +92,13 @@ public class FileSystemWatcher {
 	 * {@link #start() started}.
 	 * @param fileChangeListener the listener to add
 	 */
-	public synchronized void addListener(FileChangeListener fileChangeListener) {
+	public void addListener(FileChangeListener fileChangeListener) {
 		Assert.notNull(fileChangeListener, "FileChangeListener must not be null");
+		synchronized (this.monitor) {
 			checkNotStarted();
 			this.listeners.add(fileChangeListener);
 		}
+	}
 
 	/**
 	 * Add source folders to monitor. Cannot be called after the watcher has been
@@ -102,72 +107,143 @@ public class FileSystemWatcher {
 	 */
 	public void addSourceFolders(Iterable<File> folders) {
 		Assert.notNull(folders, "Folders must not be null");
+		synchronized (this.monitor) {
 			for (File folder : folders) {
 				addSourceFolder(folder);
 			}
 		}
+	}
 
 	/**
 	 * Add a source folder to monitor. Cannot be called after the watcher has been
 	 * {@link #start() started}.
 	 * @param folder the folder to monitor
 	 */
-	public synchronized void addSourceFolder(File folder) {
+	public void addSourceFolder(File folder) {
 		Assert.notNull(folder, "Folder must not be null");
 		Assert.isTrue(folder.isDirectory(),
 				"Folder '" + folder + "' must exist and must" + " be a directory");
+		synchronized (this.monitor) {
 			checkNotStarted();
 			this.folders.put(folder, null);
 		}
+	}
 
 	/**
 	 * Set an optional {@link FileFilter} used to limit the files that trigger a change.
 	 * @param triggerFilter a trigger filter or null
 	 */
-	public synchronized void setTriggerFilter(FileFilter triggerFilter) {
+	public void setTriggerFilter(FileFilter triggerFilter) {
+		synchronized (this.monitor) {
 			this.triggerFilter = triggerFilter;
 		}
+	}
 
 	private void checkNotStarted() {
+		synchronized (this.monitor) {
 			Assert.state(this.watchThread == null, "FileSystemWatcher already started");
 		}
+	}
 
 	/**
 	 * Start monitoring the source folder for changes.
 	 */
-	public synchronized void start() {
+	public void start() {
+		synchronized (this.monitor) {
 			saveInitialSnapshots();
 			if (this.watchThread == null) {
-			this.watchThread = new Thread() {
+				Map<File, FolderSnapshot> localFolders = new HashMap<File, FolderSnapshot>();
+				localFolders.putAll(this.folders);
+				this.watchThread = new Thread(new Watcher(this.remainingScans,
+						new ArrayList<FileChangeListener>(this.listeners),
+						this.triggerFilter, this.pollInterval, this.quietPeriod,
+						localFolders));
+				this.watchThread.setName("File Watcher");
+				this.watchThread.setDaemon(this.daemon);
+				this.watchThread.start();
+			}
+		}
+	}
+
+	private void saveInitialSnapshots() {
+		for (File folder : this.folders.keySet()) {
+			this.folders.put(folder, new FolderSnapshot(folder));
+		}
+	}
+
+	/**
+	 * Stop monitoring the source folders.
+	 */
+	public void stop() {
+		stopAfter(0);
+	}
+
+	/**
+	 * Stop monitoring the source folders.
+	 * @param remainingScans the number of remaining scans
+	 */
+	void stopAfter(int remainingScans) {
+		synchronized (this.monitor) {
+			Thread thread = this.watchThread;
+			if (thread != null) {
+				this.remainingScans.set(remainingScans);
+				if (remainingScans <= 0) {
+					thread.interrupt();
+				}
+				if (Thread.currentThread() != thread) {
+					try {
+						thread.join();
+					}
+					catch (InterruptedException ex) {
+						Thread.currentThread().interrupt();
+					}
+				}
+				this.watchThread = null;
+			}
+		}
+	}
+
+	private static final class Watcher implements Runnable {
+
+		private final AtomicInteger remainingScans;
+
+		private final List<FileChangeListener> listeners;
+
+		private final FileFilter triggerFilter;
+
+		private final long pollInterval;
+
+		private final long quietPeriod;
+
+		private Map<File, FolderSnapshot> folders;
+
+		private Watcher(AtomicInteger remainingScans, List<FileChangeListener> listeners,
+				FileFilter triggerFilter, long pollInterval, long quietPeriod,
+				Map<File, FolderSnapshot> folders) {
+			this.remainingScans = remainingScans;
+			this.listeners = listeners;
+			this.triggerFilter = triggerFilter;
+			this.pollInterval = pollInterval;
+			this.quietPeriod = quietPeriod;
+			this.folders = folders;
+		}
+
 		@Override
 		public void run() {
-					int remainingScans = FileSystemWatcher.this.remainingScans.get();
+			int remainingScans = this.remainingScans.get();
 			while (remainingScans > 0 || remainingScans == -1) {
 				try {
 					if (remainingScans > 0) {
-								FileSystemWatcher.this.remainingScans.decrementAndGet();
+						this.remainingScans.decrementAndGet();
 					}
 					scan();
 				}
 				catch (InterruptedException ex) {
 					// Ignore
 				}
-						remainingScans = FileSystemWatcher.this.remainingScans.get();
+				remainingScans = this.remainingScans.get();
 			}
 		};
-			};
-			this.watchThread.setName("File Watcher");
-			this.watchThread.setDaemon(this.daemon);
-			this.remainingScans = new AtomicInteger(-1);
-			this.watchThread.start();
-		}
-	}
-
-	private void saveInitialSnapshots() {
-		for (File folder : this.folders.keySet()) {
-			this.folders.put(folder, new FolderSnapshot(folder));
-		}
-	}
 
 		private void scan() throws InterruptedException {
 			Thread.sleep(this.pollInterval - this.quietPeriod);
@@ -231,34 +307,6 @@ public class FileSystemWatcher {
 			}
 		}
 
-	/**
-	 * Stop monitoring the source folders.
-	 */
-	public synchronized void stop() {
-		stopAfter(0);
-	}
-
-	/**
-	 * Stop monitoring the source folders.
-	 * @param remainingScans the number of remaining scans
-	 */
-	synchronized void stopAfter(int remainingScans) {
-		Thread thread = this.watchThread;
-		if (thread != null) {
-			this.remainingScans.set(remainingScans);
-			if (remainingScans <= 0) {
-				thread.interrupt();
-			}
-			if (Thread.currentThread() != thread) {
-				try {
-					thread.join();
-				}
-				catch (InterruptedException ex) {
-					Thread.currentThread().interrupt();
-				}
-			}
-			this.watchThread = null;
-		}
 	}
 
 }