[ 
https://issues.apache.org/jira/browse/HADOOP-15121?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16333014#comment-16333014
 ] 

Hanisha Koneru commented on HADOOP-15121:
-----------------------------------------

We can check that the {{metricsProxy#delegate}} object is not the same as the 
current {{DecayRpcScheduler}} object before setting it again. We can do this in 
\{{MetricsProxy#getInstance()}}.
{code:java}
if (mp == null) {
  // We must create one
  mp = new MetricsProxy(namespace, numLevels, drs);
  INSTANCES.put(namespace, mp);
} else {
  if (mp.delegate.get() != drs) {
    mp.setDelegate(drs);
  }
}{code}

> Encounter NullPointerException when using DecayRpcScheduler
> -----------------------------------------------------------
>
>                 Key: HADOOP-15121
>                 URL: https://issues.apache.org/jira/browse/HADOOP-15121
>             Project: Hadoop Common
>          Issue Type: Bug
>    Affects Versions: 2.8.2
>            Reporter: Tao Jie
>            Assignee: Tao Jie
>            Priority: Major
>         Attachments: HADOOP-15121.001.patch, HADOOP-15121.002.patch, 
> HADOOP-15121.003.patch, HADOOP-15121.004.patch, HADOOP-15121.005.patch, 
> HADOOP-15121.006.patch, HADOOP-15121.007.patch
>
>
> I set ipc.8020.scheduler.impl to org.apache.hadoop.ipc.DecayRpcScheduler, but 
> got excetion in namenode:
> {code}
> 2017-12-15 15:26:34,662 ERROR impl.MetricsSourceAdapter 
> (MetricsSourceAdapter.java:getMetrics(202)) - Error getting metrics from 
> source DecayRpcSchedulerMetrics2.ipc.8020
> java.lang.NullPointerException
>         at 
> org.apache.hadoop.ipc.DecayRpcScheduler$MetricsProxy.getMetrics(DecayRpcScheduler.java:781)
>         at 
> org.apache.hadoop.metrics2.impl.MetricsSourceAdapter.getMetrics(MetricsSourceAdapter.java:199)
>         at 
> org.apache.hadoop.metrics2.impl.MetricsSourceAdapter.updateJmxCache(MetricsSourceAdapter.java:182)
>         at 
> org.apache.hadoop.metrics2.impl.MetricsSourceAdapter.getMBeanInfo(MetricsSourceAdapter.java:155)
>         at 
> com.sun.jmx.interceptor.DefaultMBeanServerInterceptor.getNewMBeanClassName(DefaultMBeanServerInterceptor.java:333)
>         at 
> com.sun.jmx.interceptor.DefaultMBeanServerInterceptor.registerMBean(DefaultMBeanServerInterceptor.java:319)
>         at 
> com.sun.jmx.mbeanserver.JmxMBeanServer.registerMBean(JmxMBeanServer.java:522)
>         at org.apache.hadoop.metrics2.util.MBeans.register(MBeans.java:66)
>         at 
> org.apache.hadoop.metrics2.impl.MetricsSourceAdapter.startMBeans(MetricsSourceAdapter.java:222)
>         at 
> org.apache.hadoop.metrics2.impl.MetricsSourceAdapter.start(MetricsSourceAdapter.java:100)
>         at 
> org.apache.hadoop.metrics2.impl.MetricsSystemImpl.registerSource(MetricsSystemImpl.java:268)
>         at 
> org.apache.hadoop.metrics2.impl.MetricsSystemImpl.register(MetricsSystemImpl.java:233)
>         at 
> org.apache.hadoop.ipc.DecayRpcScheduler$MetricsProxy.registerMetrics2Source(DecayRpcScheduler.java:709)
>         at 
> org.apache.hadoop.ipc.DecayRpcScheduler$MetricsProxy.<init>(DecayRpcScheduler.java:685)
>         at 
> org.apache.hadoop.ipc.DecayRpcScheduler$MetricsProxy.getInstance(DecayRpcScheduler.java:693)
>         at 
> org.apache.hadoop.ipc.DecayRpcScheduler.<init>(DecayRpcScheduler.java:236)
>         at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native 
> Method)
>         at 
> sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:62)
>         at 
> sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
>         at java.lang.reflect.Constructor.newInstance(Constructor.java:423)
>         at 
> org.apache.hadoop.ipc.CallQueueManager.createScheduler(CallQueueManager.java:102)
>         at 
> org.apache.hadoop.ipc.CallQueueManager.<init>(CallQueueManager.java:76)
>         at org.apache.hadoop.ipc.Server.<init>(Server.java:2612)
>         at org.apache.hadoop.ipc.RPC$Server.<init>(RPC.java:958)
>         at 
> org.apache.hadoop.ipc.ProtobufRpcEngine$Server.<init>(ProtobufRpcEngine.java:374)
>         at 
> org.apache.hadoop.ipc.ProtobufRpcEngine.getServer(ProtobufRpcEngine.java:349)
>         at org.apache.hadoop.ipc.RPC$Builder.build(RPC.java:800)
> at 
> org.apache.hadoop.hdfs.server.namenode.NameNodeRpcServer.<init>(NameNodeRpcServer.java:415)
>         at 
> org.apache.hadoop.hdfs.server.namenode.NameNode.createRpcServer(NameNode.java:755)
>         at 
> org.apache.hadoop.hdfs.server.namenode.NameNode.initialize(NameNode.java:697)
>         at 
> org.apache.hadoop.hdfs.server.namenode.NameNode.<init>(NameNode.java:905)
>         at 
> org.apache.hadoop.hdfs.server.namenode.NameNode.<init>(NameNode.java:884)
>         at 
> org.apache.hadoop.hdfs.server.namenode.NameNode.createNameNode(NameNode.java:1610)
>         at 
> org.apache.hadoop.hdfs.server.namenode.NameNode.main(NameNode.java:1678)
> {code}
> It seems that {{metricsProxy}} in DecayRpcScheduler should initiate its 
> {{delegate}} field in its Initialization method



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