perf(FileWatcher): close WatchService synchronously

media-streaming-spring-boot-autoconfigure/src/main/java/com/github/johnnymillergh/boot/mediastreamingspringbootautoconfigure/configuration/MediaStreamingBootstrap.java

@@ -13,6 +13,7 @@
 import javax.annotation.PostConstruct;
 import javax.annotation.PreDestroy;
 import java.nio.file.Path;
+import java.util.Optional;
 
 /**
  * Description: MediaStreamingBootstrap, change description here.
@@ -85,6 +86,6 @@ public void run(String... args) {
     @PreDestroy
     private void preDestroy() {
         log.debug("Destroying {}, fileWatcher: {}", this.getClass().getSimpleName(), fileWatcher);
-        fileWatcher.destroy();
+        Optional.ofNullable(fileWatcher).ifPresent(FileWatcher::destroy);
     }
 }

media-streaming-spring-boot-autoconfigure/src/main/java/com/github/johnnymillergh/boot/mediastreamingspringbootautoconfigure/filewatch/FileWatcher.java

@@ -90,6 +90,7 @@ private void monitor() {
 
     @SneakyThrows
     public void destroy() {
+        WatchServiceSingleton.close();
         THREAD_POOL.awaitTermination(5, TimeUnit.SECONDS);
         log.debug("THREAD_POOL for FileWatcher was terminated.");
     }

media-streaming-spring-boot-autoconfigure/src/main/java/com/github/johnnymillergh/boot/mediastreamingspringbootautoconfigure/filewatch/WatchServiceSingleton.java

@@ -1,21 +1,38 @@
 package com.github.johnnymillergh.boot.mediastreamingspringbootautoconfigure.filewatch;
 
 import lombok.SneakyThrows;
+import lombok.extern.slf4j.Slf4j;
 
+import java.io.IOException;
 import java.nio.file.FileSystems;
 import java.nio.file.WatchService;
+import java.util.Optional;
 
 /**
  * Description: WatchServiceSingleton, change description here.
  *
  * @author Johnny Miller (锺俊), email: [email protected], date: 10/20/2020 4:52 PM
  **/
+@Slf4j
 public class WatchServiceSingleton {
     private static volatile WatchService singletonInstance;
 
+    /**
+     * Private constructors are required for Singleton classes so that objects of such classes cannot be instantiated
+     * from outside the class. If Singleton classes do not have private constructors, objects of such classes can be
+     * created from outside too and hence the class will no longer be Singleton class.
+     *
+     * @author Johnny Miller (锺俊), email: [email protected], date: 10/26/2020 1:57 PM
+     */
     private WatchServiceSingleton() {
     }
 
+    /**
+     * Gets WatchService instance. Minimize synchronized range, with double null check.
+     *
+     * @return the WatchService instance
+     * @author Johnny Miller (锺俊), email: [email protected], date: 10/26/2020 1:51 PM
+     */
     @SneakyThrows
     public static WatchService getInstance() {
         if (singletonInstance == null) {
@@ -27,4 +44,19 @@ public static WatchService getInstance() {
         }
         return singletonInstance;
     }
+
+    /**
+     * Close WatchService synchronously.
+     *
+     * @author Johnny Miller (锺俊), email: [email protected], date: 10/26/2020 1:49 PM
+     */
+    public static synchronized void close() {
+        Optional.ofNullable(singletonInstance).ifPresent(watchService -> {
+            try {
+                watchService.close();
+            } catch (IOException e) {
+                log.error("Exception occurred when closing WatchService.", e);
+            }
+        });
+    }
 }