Sort fields in ReflectionUtils.java

The tests in org.apache.hadoop.metrics2.impl.TestMetricsSystemImpl#testInitFirstVerifyCallBacks and org.apache.hadoop.metrics2.impl.TestMetricsSystemImpl#testInitFirstVerifyStopInvokedImmediately can fail.

java.lang.AssertionError:

Element 0 for metrics expected:<MetricCounterLong{info=MetricsInfoImpl

{name=C1, description=C1 desc}

, value=1}>

but was:<MetricGaugeLong{info=MetricsInfoImpl

{name=G1, description=G1 desc}

, value=2}>

at org.junit.Assert.fail(Assert.java:88)

at org.junit.Assert.failNotEquals(Assert.java:834)

at org.junit.Assert.assertEquals(Assert.java:118)

at org.apache.hadoop.test.MoreAsserts.assertEquals(MoreAsserts.java:60)

at org.apache.hadoop.metrics2.impl.TestMetricsSystemImpl.checkMetricsRecords(TestMetricsSystemImpl.java:439)

at org.apache.hadoop.metrics2.impl.TestMetricsSystemImpl.testInitFirstVerifyCallBacks(TestMetricsSystemImpl.java:178)

 

The root cause of this failure can be analyzed in the following stack trace:

java.lang.Class.getDeclaredFields(Class.java:1916)
org.apache.hadoop.util.ReflectionUtils.getDeclaredFieldsIncludingInherited(ReflectionUtils.java:353)
org.apache.hadoop.metrics2.lib.MetricsSourceBuilder.<init>(MetricsSourceBuilder.java:68)
org.apache.hadoop.metrics2.lib.MetricsAnnotations.newSourceBuilder(MetricsAnnotations.java:43)
org.apache.hadoop.metrics2.impl.MetricsSystemImpl.register(MetricsSystemImpl.java:223)
org.apache.hadoop.metrics2.impl.TestMetricsSystemImpl.testInitFirstVerifyCallBacks(TestMetricsSystemImpl.java:156)

The specification about getDeclaredFields() says that "the elements in the returned array are not sorted and are not in any particular order". The documentation is here for your reference: https://docs.oracle.com/javase/8/docs/api/java/lang/Class.html#getDeclaredFields--
And the behaviour might be different for different JVM versions or vendors

 

The fix is to sort the fields returned by getDeclaredFields() so that the non-deterministic behaviour can be eliminated completely. In this way, the test becomes more stable and it will not suffer from the failure above any more.