Add a class cast check to avoid ClassCastException (#505)

FIX #504

Signed-off-by: Christophe TARDELLA <christophe.tardella@gmail.com>

Author: kebuu

simpleclient_hotspot/src/main/java/io/prometheus/client/hotspot/MemoryAllocationExports.java

@@ -25,8 +25,10 @@ public class MemoryAllocationExports extends Collector {
 
   public MemoryAllocationExports() {
     AllocationCountingNotificationListener listener = new AllocationCountingNotificationListener(allocatedCounter);
-    for (GarbageCollectorMXBean garbageCollectorMXBean : ManagementFactory.getGarbageCollectorMXBeans()) {
-      ((NotificationEmitter) garbageCollectorMXBean).addNotificationListener(listener, null, null);
+    for (GarbageCollectorMXBean garbageCollectorMXBean : getGarbageCollectorMXBeans()) {
+      if (garbageCollectorMXBean instanceof NotificationEmitter) {
+        ((NotificationEmitter) garbageCollectorMXBean).addNotificationListener(listener, null, null);
+      }
     }
   }
 
@@ -97,4 +99,8 @@ private static long getAndSet(Map<String, Long> map, String key, long value) {
       return last == null ? 0 : last;
     }
   }
+
+  protected List<GarbageCollectorMXBean> getGarbageCollectorMXBeans() {
+    return ManagementFactory.getGarbageCollectorMXBeans();
+  }
 }

simpleclient_hotspot/src/test/java/io/prometheus/client/hotspot/MemoryAllocationExportsTest.java

@@ -2,6 +2,13 @@
 
 import io.prometheus.client.Counter;
 import org.junit.Test;
+import org.mockito.Mockito;
+
+import javax.management.NotificationEmitter;
+import javax.management.NotificationFilter;
+import java.lang.management.GarbageCollectorMXBean;
+import java.util.Arrays;
+import java.util.List;
 
 import static org.junit.Assert.assertEquals;
 
@@ -42,4 +49,20 @@ public void testListenerLogic() {
     listener.handleMemoryPool("TestPool", 17, 20);
     assertEquals(153, child.get(), 0.001);
   }
+
+  @Test
+  public void testConstructorShouldIgnoreNotNotificationEmitterClasses() {
+    final GarbageCollectorMXBean notificationEmitterMXBean = Mockito.mock(GarbageCollectorMXBean.class, Mockito.withSettings().extraInterfaces(NotificationEmitter.class));
+    final GarbageCollectorMXBean notNotificationEmitterMXBean = Mockito.mock(GarbageCollectorMXBean.class);
+
+    new MemoryAllocationExports() {
+      @Override
+      protected List<GarbageCollectorMXBean> getGarbageCollectorMXBeans() {
+          return Arrays.asList(notificationEmitterMXBean, notNotificationEmitterMXBean);
+      }
+    };
+
+    Mockito.verify((NotificationEmitter) notificationEmitterMXBean).addNotificationListener(Mockito.any(MemoryAllocationExports.AllocationCountingNotificationListener.class), Mockito.isNull(NotificationFilter.class), Mockito.isNull());
+    Mockito.verifyZeroInteractions(notNotificationEmitterMXBean);
+  }
 }