use extra scheduler to handle health check.

src/main/java/fastdfs/client/FastdfsClient.java

@@ -7,6 +7,7 @@
 import java.io.Closeable;
 import java.io.File;
 import java.io.IOException;
+import java.util.HashSet;
 import java.util.LinkedList;
 import java.util.List;
 import java.util.Objects;
@@ -32,6 +33,8 @@ public final class FastdfsClient implements Closeable {
     public static final long DEFAULT_CHECK_INTERVAL = 10000;
 
     private final FastdfsExecutor executor;
+    private final FastdfsScheduler scheduler;
+
     private final TrackerClient trackerClient;
     private final StorageClient storageClient;
 
@@ -46,7 +49,8 @@ private FastdfsClient(Builder builder) {
         );
 
         this.executor = new FastdfsExecutor(settings);
-        this.trackerClient = new TrackerClient(executor, builder.selector, builder.trackers, builder.fall, builder.rise, builder.checkTimeout, builder.checkInterval);
+        this.scheduler = new FastdfsScheduler(new HashSet<>(builder.trackers).size());
+        this.trackerClient = new TrackerClient(executor, scheduler, builder.selector, builder.trackers, builder.fall, builder.rise, builder.checkTimeout, builder.checkInterval);
         this.storageClient = new StorageClient(executor);
     }
 
@@ -711,7 +715,18 @@ public CompletableFuture<FileInfo> infoGet(FileId fileId) {
 
     @Override
     public void close() throws IOException {
-        executor.close();
+        try {
+            executor.close();
+        } catch (Exception e) {
+            // do nothing;
+        }
+
+        try {
+            scheduler.close();
+        } catch (Exception e) {
+            // do nothing;
+        }
+
         trackerClient.close();
     }
 

src/main/java/fastdfs/client/FastdfsScheduler.java

@@ -0,0 +1,47 @@
+package fastdfs.client;
+
+import java.io.Closeable;
+import java.io.IOException;
+import java.util.concurrent.*;
+import java.util.concurrent.atomic.AtomicInteger;
+
+class FastdfsScheduler implements Closeable {
+    private final ScheduledExecutorService loop;
+
+    FastdfsScheduler(int threads) {
+        this.loop = Executors.newScheduledThreadPool(threads, new ThreadFactory() {
+            final String threadPrefix = "fastdfs-scheduler-";
+            final AtomicInteger threadNumber = new AtomicInteger(1);
+
+            @Override
+            public Thread newThread(Runnable r) {
+                return new Thread(null, r, threadPrefix + threadNumber.getAndIncrement());
+            }
+        });
+    }
+
+    ScheduledFuture<?> schedule(Runnable command, long delay, TimeUnit unit) {
+        return loop.schedule(command, delay, unit);
+    }
+
+    <V> ScheduledFuture<V> schedule(Callable<V> callable, long delay, TimeUnit unit) {
+        return loop.schedule(callable, delay, unit);
+    }
+
+    ScheduledFuture<?> scheduleAtFixedRate(Runnable command, long initialDelay, long period, TimeUnit unit) {
+        return loop.scheduleAtFixedRate(command, initialDelay, period, unit);
+    }
+
+    ScheduledFuture<?> scheduleWithFixedDelay(Runnable command, long initialDelay, long delay, TimeUnit unit) {
+        return loop.scheduleWithFixedDelay(command, initialDelay, delay, unit);
+    }
+
+    @Override
+    public void close() throws IOException {
+        try {
+            loop.shutdown();
+        } catch (Exception e) {
+            // do nothing.
+        }
+    }
+}

src/main/java/fastdfs/client/TrackerClient.java

@@ -18,10 +18,10 @@ final class TrackerClient implements Closeable {
     private final TrackerSelector selector;
     private final TrackerMonitor monitor;
 
-    TrackerClient(FastdfsExecutor executor, TrackerSelector selector, List<TrackerServer> servers, int fall, int rise, long checkTimeout, long checkInterval) {
+    TrackerClient(FastdfsExecutor executor, FastdfsScheduler scheduler, TrackerSelector selector, List<TrackerServer> servers, int fall, int rise, long checkTimeout, long checkInterval) {
         this.executor = executor;
         this.selector = servers.size() == 1 ? TrackerSelector.FIRST : selector;
-        this.monitor = new TrackerMonitor(executor, servers, fall, rise, checkTimeout, checkInterval);
+        this.monitor = new TrackerMonitor(executor, scheduler, servers, fall, rise, checkTimeout, checkInterval);
     }
 
     private CompletableFuture<InetSocketAddress> trackerSelect() {

src/main/java/fastdfs/client/TrackerMonitor.java

@@ -28,12 +28,12 @@ class TrackerMonitor implements Closeable {
     private final ReentrantReadWriteLock lock = new ReentrantReadWriteLock();
     private final Logger logger = LoggerFactory.getLogger(getClass());
 
-    TrackerMonitor(FastdfsExecutor executor, List<TrackerServer> servers, int fall, int rise, long checkTimeout, long checkInterval) {
+    TrackerMonitor(FastdfsExecutor executor, FastdfsScheduler scheduler, List<TrackerServer> servers, int fall, int rise, long checkTimeout, long checkInterval) {
         this.fall = fall;
         this.rise = rise;
 
         this.aliveServers = new LinkedList<>(servers);
-        this.aliveTasks = new HashSet<>(servers).stream().map(server -> executor.scheduleAtFixedRate(() -> {
+        this.aliveTasks = new HashSet<>(servers).stream().map(server -> scheduler.scheduleAtFixedRate(() -> {
             try {
                 CompletableFuture<Boolean> promise = executor.execute(server.toInetAddress(), new ActiveTestRequestor(), new ActiveTestReplier());
                 if (promise.get(checkTimeout, TimeUnit.MILLISECONDS)) {