This closes #1384

artemis-server/src/main/java/org/apache/activemq/artemis/core/server/files/FileStoreMonitor.java

@@ -44,6 +44,7 @@ public class FileStoreMonitor extends ActiveMQScheduledComponent {
    private final Set<Callback> callbackList = new HashSet<>();
    private final Set<FileStore> stores = new HashSet<>();
    private double maxUsage;
+   private final Object monitorLock = new Object();
 
    public FileStoreMonitor(ScheduledExecutorService scheduledExecutorService,
                            Executor executor,
@@ -54,30 +55,37 @@ public class FileStoreMonitor extends ActiveMQScheduledComponent {
       this.maxUsage = maxUsage;
    }
 
-   public synchronized FileStoreMonitor addCallback(Callback callback) {
+   public FileStoreMonitor addCallback(Callback callback) {
+      synchronized (monitorLock) {
          callbackList.add(callback);
          return this;
       }
+   }
 
-   public synchronized FileStoreMonitor addStore(File file) throws IOException {
+   public FileStoreMonitor addStore(File file) throws IOException {
+      synchronized (monitorLock) {
          // JDBC storage may return this as null, and we may need to ignore it
          if (file != null && file.exists()) {
             addStore(Files.getFileStore(file.toPath()));
          }
          return this;
       }
+   }
 
-   public synchronized FileStoreMonitor addStore(FileStore store) {
+   public FileStoreMonitor addStore(FileStore store) {
+      synchronized (monitorLock) {
          stores.add(store);
          return this;
       }
+   }
 
    @Override
    public void run() {
       tick();
    }
 
-   public synchronized void tick() {
+   public void tick() {
+      synchronized (monitorLock) {
          boolean over = false;
 
          FileStore lastStore = null;
@@ -106,6 +114,7 @@ public class FileStoreMonitor extends ActiveMQScheduledComponent {
             }
          }
       }
+   }
 
    public double getMaxUsage() {
       return maxUsage;