fix singleton with volatile (#818)

escheduler-alert/src/main/java/cn/escheduler/alert/AlertServer.java

@@ -39,7 +39,7 @@ public class AlertServer {
 
     private AlertSender alertSender;
 
-    private static AlertServer instance;
+    private static volatile AlertServer instance;
 
     private AlertServer() {
 

escheduler-common/src/main/java/cn/escheduler/common/thread/ThreadPoolExecutors.java

@@ -40,7 +40,7 @@ public class ThreadPoolExecutors {
 
     private static final Logger logger = LoggerFactory.getLogger(ThreadPoolExecutors.class);
     private static Executor executor;
-    private static ThreadPoolExecutors threadPoolExecutors;
+    private static volatile ThreadPoolExecutors threadPoolExecutors;
 
     private ThreadPoolExecutors(){}