diff --git a/spring-boot-devtools/src/main/java/org/springframework/boot/devtools/filewatch/FileSystemWatcher.java b/spring-boot-devtools/src/main/java/org/springframework/boot/devtools/filewatch/FileSystemWatcher.java
index 58d15551461f..13b9e39dc591 100644
--- a/spring-boot-devtools/src/main/java/org/springframework/boot/devtools/filewatch/FileSystemWatcher.java
+++ b/spring-boot-devtools/src/main/java/org/springframework/boot/devtools/filewatch/FileSystemWatcher.java
@@ -21,6 +21,7 @@
 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.
 	 */
@@ -89,10 +92,12 @@ public FileSystemWatcher(boolean daemon, long pollInterval, long quietPeriod) {
 	 * {@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");
-		checkNotStarted();
-		this.listeners.add(fileChangeListener);
+		synchronized (this.monitor) {
+			checkNotStarted();
+			this.listeners.add(fileChangeListener);
+		}
 	}
 
 	/**
@@ -102,8 +107,10 @@ public synchronized void addListener(FileChangeListener fileChangeListener) {
 	 */
 	public void addSourceFolders(Iterable<File> folders) {
 		Assert.notNull(folders, "Folders must not be null");
-		for (File folder : folders) {
-			addSourceFolder(folder);
+		synchronized (this.monitor) {
+			for (File folder : folders) {
+				addSourceFolder(folder);
+			}
 		}
 	}
 
@@ -112,54 +119,49 @@ public void addSourceFolders(Iterable<File> folders) {
 	 * {@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");
-		checkNotStarted();
-		this.folders.put(folder, null);
+		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) {
-		this.triggerFilter = triggerFilter;
+	public void setTriggerFilter(FileFilter triggerFilter) {
+		synchronized (this.monitor) {
+			this.triggerFilter = triggerFilter;
+		}
 	}
 
 	private void checkNotStarted() {
-		Assert.state(this.watchThread == null, "FileSystemWatcher already started");
+		synchronized (this.monitor) {
+			Assert.state(this.watchThread == null, "FileSystemWatcher already started");
+		}
 	}
 
 	/**
 	 * Start monitoring the source folder for changes.
 	 */
-	public synchronized void start() {
-		saveInitialSnapshots();
-		if (this.watchThread == null) {
-			this.watchThread = new Thread() {
-				@Override
-				public void run() {
-					int remaining = FileSystemWatcher.this.remainingScans.get();
-					while (remaining > 0 || remaining == -1) {
-						try {
-							if (remaining > 0) {
-								FileSystemWatcher.this.remainingScans.decrementAndGet();
-							}
-							scan();
-						}
-						catch (InterruptedException ex) {
-							// Ignore
-						}
-						remaining = FileSystemWatcher.this.remainingScans.get();
-					}
-				};
-			};
-			this.watchThread.setName("File Watcher");
-			this.watchThread.setDaemon(this.daemon);
-			this.remainingScans = new AtomicInteger(-1);
-			this.watchThread.start();
+	public void start() {
+		synchronized (this.monitor) {
+			saveInitialSnapshots();
+			if (this.watchThread == null) {
+				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();
+			}
 		}
 	}
 
@@ -169,72 +171,10 @@ private void saveInitialSnapshots() {
 		}
 	}
 
-	private void scan() throws InterruptedException {
-		Thread.sleep(this.pollInterval - this.quietPeriod);
-		Map<File, FolderSnapshot> previous;
-		Map<File, FolderSnapshot> current = this.folders;
-		do {
-			previous = current;
-			current = getCurrentSnapshots();
-			Thread.sleep(this.quietPeriod);
-		}
-		while (isDifferent(previous, current));
-		if (isDifferent(this.folders, current)) {
-			updateSnapshots(current.values());
-		}
-	}
-
-	private boolean isDifferent(Map<File, FolderSnapshot> previous,
-			Map<File, FolderSnapshot> current) {
-		if (!previous.keySet().equals(current.keySet())) {
-			return true;
-		}
-		for (Map.Entry<File, FolderSnapshot> entry : previous.entrySet()) {
-			FolderSnapshot previousFolder = entry.getValue();
-			FolderSnapshot currentFolder = current.get(entry.getKey());
-			if (!previousFolder.equals(currentFolder, this.triggerFilter)) {
-				return true;
-			}
-		}
-		return false;
-	}
-
-	private Map<File, FolderSnapshot> getCurrentSnapshots() {
-		Map<File, FolderSnapshot> snapshots = new LinkedHashMap<File, FolderSnapshot>();
-		for (File folder : this.folders.keySet()) {
-			snapshots.put(folder, new FolderSnapshot(folder));
-		}
-		return snapshots;
-	}
-
-	private void updateSnapshots(Collection<FolderSnapshot> snapshots) {
-		Map<File, FolderSnapshot> updated = new LinkedHashMap<File, FolderSnapshot>();
-		Set<ChangedFiles> changeSet = new LinkedHashSet<ChangedFiles>();
-		for (FolderSnapshot snapshot : snapshots) {
-			FolderSnapshot previous = this.folders.get(snapshot.getFolder());
-			updated.put(snapshot.getFolder(), snapshot);
-			ChangedFiles changedFiles = previous.getChangedFiles(snapshot,
-					this.triggerFilter);
-			if (!changedFiles.getFiles().isEmpty()) {
-				changeSet.add(changedFiles);
-			}
-		}
-		if (!changeSet.isEmpty()) {
-			fireListeners(Collections.unmodifiableSet(changeSet));
-		}
-		this.folders = updated;
-	}
-
-	private void fireListeners(Set<ChangedFiles> changeSet) {
-		for (FileChangeListener listener : this.listeners) {
-			listener.onChange(changeSet);
-		}
-	}
-
 	/**
 	 * Stop monitoring the source folders.
 	 */
-	public synchronized void stop() {
+	public void stop() {
 		stopAfter(0);
 	}
 
@@ -242,23 +182,131 @@ public synchronized void stop() {
 	 * 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();
+	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;
 			}
-			if (Thread.currentThread() != thread) {
+		}
+	}
+
+	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 = this.remainingScans.get();
+			while (remainingScans > 0 || remainingScans == -1) {
 				try {
-					thread.join();
+					if (remainingScans > 0) {
+						this.remainingScans.decrementAndGet();
+					}
+					scan();
 				}
 				catch (InterruptedException ex) {
-					Thread.currentThread().interrupt();
+					// Ignore
 				}
+				remainingScans = this.remainingScans.get();
+			}
+		};
+
+		private void scan() throws InterruptedException {
+			Thread.sleep(this.pollInterval - this.quietPeriod);
+			Map<File, FolderSnapshot> previous;
+			Map<File, FolderSnapshot> current = this.folders;
+			do {
+				previous = current;
+				current = getCurrentSnapshots();
+				Thread.sleep(this.quietPeriod);
+			}
+			while (isDifferent(previous, current));
+			if (isDifferent(this.folders, current)) {
+				updateSnapshots(current.values());
 			}
-			this.watchThread = null;
 		}
+
+		private boolean isDifferent(Map<File, FolderSnapshot> previous,
+				Map<File, FolderSnapshot> current) {
+			if (!previous.keySet().equals(current.keySet())) {
+				return true;
+			}
+			for (Map.Entry<File, FolderSnapshot> entry : previous.entrySet()) {
+				FolderSnapshot previousFolder = entry.getValue();
+				FolderSnapshot currentFolder = current.get(entry.getKey());
+				if (!previousFolder.equals(currentFolder, this.triggerFilter)) {
+					return true;
+				}
+			}
+			return false;
+		}
+
+		private Map<File, FolderSnapshot> getCurrentSnapshots() {
+			Map<File, FolderSnapshot> snapshots = new LinkedHashMap<File, FolderSnapshot>();
+			for (File folder : this.folders.keySet()) {
+				snapshots.put(folder, new FolderSnapshot(folder));
+			}
+			return snapshots;
+		}
+
+		private void updateSnapshots(Collection<FolderSnapshot> snapshots) {
+			Map<File, FolderSnapshot> updated = new LinkedHashMap<File, FolderSnapshot>();
+			Set<ChangedFiles> changeSet = new LinkedHashSet<ChangedFiles>();
+			for (FolderSnapshot snapshot : snapshots) {
+				FolderSnapshot previous = this.folders.get(snapshot.getFolder());
+				updated.put(snapshot.getFolder(), snapshot);
+				ChangedFiles changedFiles = previous.getChangedFiles(snapshot,
+						this.triggerFilter);
+				if (!changedFiles.getFiles().isEmpty()) {
+					changeSet.add(changedFiles);
+				}
+			}
+			if (!changeSet.isEmpty()) {
+				fireListeners(Collections.unmodifiableSet(changeSet));
+			}
+			this.folders = updated;
+		}
+
+		private void fireListeners(Set<ChangedFiles> changeSet) {
+			for (FileChangeListener listener : this.listeners) {
+				listener.onChange(changeSet);
+			}
+		}
+
 	}
 
 }