Update JetCacheExecutor.java and Update AbstractExternalCacheTest.java (

#811)

* Update JetCacheExecutor.java

做了以下两点改动：
1.使用了AtomicInteger以确保threadCount的线程安全性
2.在创建新的线程时使用了getAndIncrement()方法来递增计数

* Update AbstractExternalCacheTest.java

修改assertNull()使用错误

* Update AbstractExternalCacheTest.java

jetcache-core/src/main/java/com/alicp/jetcache/support/JetCacheExecutor.java

@@ -4,6 +4,7 @@
 import java.util.concurrent.ScheduledThreadPoolExecutor;
 import java.util.concurrent.ThreadFactory;
 import java.util.concurrent.ThreadPoolExecutor;
+import java.util.concurrent.atomic.AtomicInteger;
 
 /**
  * Created on 2017/5/3.
@@ -14,7 +15,7 @@ public class JetCacheExecutor {
     protected volatile static ScheduledExecutorService defaultExecutor;
     protected volatile static ScheduledExecutorService heavyIOExecutor;
 
-    private static int threadCount;
+    private static AtomicInteger threadCount = new AtomicInteger(0);
 
     static {
         Runtime.getRuntime().addShutdownHook(new Thread() {
@@ -55,7 +56,7 @@ public static ScheduledExecutorService heavyIOExecutor() {
         synchronized (JetCacheExecutor.class) {
             if (heavyIOExecutor == null) {
                 ThreadFactory tf = r -> {
-                    Thread t = new Thread(r, "JetCacheHeavyIOExecutor" + threadCount++);
+                    Thread t = new Thread(r, "JetCacheHeavyIOExecutor" + threadCount.getAndIncrement());
                     t.setDaemon(true);
                     return t;
                 };

jetcache-test/src/main/java/com/alicp/jetcache/test/external/AbstractExternalCacheTest.java

@@ -38,7 +38,7 @@ protected void nullKeyConvertorTest() {
             d2.setName("HL2");
 
             cache.put(d1, "V2");
-            Assert.assertNull("V2", cache.get(d2));
+            Assert.assertNull(cache.get(d2));
             Assert.assertNull(cache.get(d3));
         }
     }