HADOOP-13290. Appropriate use of generics in FairCallQueue. Contributed by Jonathan Hung.

(cherry picked from commit 728bf7f6988e13f17f03d2e3a4037b7238c4fd45)

hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/ipc/FairCallQueue.java

@@ -300,7 +300,7 @@ public class FairCallQueue<E extends Schedulable> extends AbstractQueue<E>
   @Override
   public int size() {
     int size = 0;
-    for (BlockingQueue q : this.queues) {
+    for (BlockingQueue<E> q : this.queues) {
       size += q.size();
     }
     return size;
@@ -346,7 +346,7 @@ public class FairCallQueue<E extends Schedulable> extends AbstractQueue<E>
   @Override
   public int remainingCapacity() {
     int sum = 0;
-    for (BlockingQueue q : this.queues) {
+    for (BlockingQueue<E> q : this.queues) {
       sum += q.remainingCapacity();
     }
     return sum;
@@ -362,7 +362,7 @@ public class FairCallQueue<E extends Schedulable> extends AbstractQueue<E>
       new HashMap<String, MetricsProxy>();
 
     // Weakref for delegate, so we don't retain it forever if it can be GC'd
-    private WeakReference<FairCallQueue> delegate;
+    private WeakReference<FairCallQueue<? extends Schedulable>> delegate;
 
     // Keep track of how many objects we registered
     private int revisionNumber = 0;
@@ -381,14 +381,15 @@ public class FairCallQueue<E extends Schedulable> extends AbstractQueue<E>
       return mp;
     }
 
-    public void setDelegate(FairCallQueue obj) {
-      this.delegate = new WeakReference<FairCallQueue>(obj);
+    public void setDelegate(FairCallQueue<? extends Schedulable> obj) {
+      this.delegate
+          = new WeakReference<FairCallQueue<? extends Schedulable>>(obj);
       this.revisionNumber++;
     }
 
     @Override
     public int[] getQueueSizes() {
-      FairCallQueue obj = this.delegate.get();
+      FairCallQueue<? extends Schedulable> obj = this.delegate.get();
       if (obj == null) {
         return new int[]{};
       }
@@ -398,7 +399,7 @@ public class FairCallQueue<E extends Schedulable> extends AbstractQueue<E>
 
     @Override
     public long[] getOverflowedCalls() {
-      FairCallQueue obj = this.delegate.get();
+      FairCallQueue<? extends Schedulable> obj = this.delegate.get();
       if (obj == null) {
         return new long[]{};
       }

hadoop-common-project/hadoop-common/src/test/java/org/apache/hadoop/ipc/TestFairCallQueue.java

@@ -23,6 +23,9 @@ import static org.mockito.Mockito.when;
 
 import junit.framework.TestCase;
 
+import javax.management.MBeanServer;
+import javax.management.ObjectName;
+import java.lang.management.ManagementFactory;
 import java.util.concurrent.CountDownLatch;
 import java.util.concurrent.TimeUnit;
 import java.util.concurrent.BlockingQueue;
@@ -392,4 +395,20 @@ public class TestFairCallQueue extends TestCase {
     assertEquals(call, fcq.take());
     assertEquals(0, fcq.size());
   }
-}
+
+  public void testFairCallQueueMXBean() throws Exception {
+    MBeanServer mbs = ManagementFactory.getPlatformMBeanServer();
+    ObjectName mxbeanName = new ObjectName(
+        "Hadoop:service=ns,name=FairCallQueue");
+
+    Schedulable call = mockCall("c");
+    fcq.put(call);
+    int[] queueSizes = (int[]) mbs.getAttribute(mxbeanName, "QueueSizes");
+    assertEquals(1, queueSizes[0]);
+    assertEquals(0, queueSizes[1]);
+    fcq.take();
+    queueSizes = (int[]) mbs.getAttribute(mxbeanName, "QueueSizes");
+    assertEquals(0, queueSizes[0]);
+    assertEquals(0, queueSizes[1]);
+  }
+}