HADOOP-14954. MetricsSystemImpl#init should increment refCount when already initialized. Contributed by John Zhuge.

hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/metrics2/impl/MetricsSystemImpl.java

@@ -140,6 +140,16 @@ public MetricsSystemImpl() {
     this(null);
   }
 
+  @VisibleForTesting
+  boolean isMonitoring() {
+    return monitoring;
+  }
+
+  @VisibleForTesting
+  int getRefCount() {
+    return refCount;
+  }
+
   /**
    * Initialized the metrics system with a prefix.
    * @param prefix  the system will look for configs with the prefix
@@ -147,12 +157,12 @@ public MetricsSystemImpl() {
    */
   @Override
   public synchronized MetricsSystem init(String prefix) {
+    ++refCount;
     if (monitoring && !DefaultMetricsSystem.inMiniClusterMode()) {
       LOG.warn(this.prefix +" metrics system already initialized!");
       return this;
     }
     this.prefix = checkNotNull(prefix, "prefix");
-    ++refCount;
     if (monitoring) {
       // in mini cluster mode
       LOG.info(this.prefix +" metrics system started (again)");

hadoop-common-project/hadoop-common/src/test/java/org/apache/hadoop/metrics2/impl/TestMetricsSystemImpl.java

@@ -26,6 +26,8 @@
 
 import javax.annotation.Nullable;
 
+import org.hamcrest.CoreMatchers;
+import org.junit.Assert;
 import org.junit.Test;
 import org.junit.runner.RunWith;
 
@@ -559,6 +561,50 @@ public void testRegisterSourceJmxCacheTTL() {
     ms.shutdown();
   }
 
+  @Test
+  public void testInitShutdown() {
+    boolean oldMode = DefaultMetricsSystem.inMiniClusterMode();
+    try {
+      DefaultMetricsSystem.setMiniClusterMode(true);
+      runInitShutdownTests();
+
+      DefaultMetricsSystem.setMiniClusterMode(false);
+      runInitShutdownTests();
+    } finally {
+      DefaultMetricsSystem.setMiniClusterMode(oldMode);
+    }
+  }
+
+  private void runInitShutdownTests() {
+    MetricsSystemImpl ms = new MetricsSystemImpl();
+    Assert.assertThat(ms.isMonitoring(), CoreMatchers.is(false));
+    Assert.assertThat(ms.getRefCount(), CoreMatchers.is(0));
+
+    ms.init("TestMetricsSystem1");
+    Assert.assertThat(ms.isMonitoring(), CoreMatchers.is(true));
+    Assert.assertThat(ms.getRefCount(), CoreMatchers.is(1));
+
+    ms.shutdown();
+    Assert.assertThat(ms.isMonitoring(), CoreMatchers.is(false));
+    Assert.assertThat(ms.getRefCount(), CoreMatchers.is(0));
+
+    ms.init("TestMetricsSystem2");
+    Assert.assertThat(ms.isMonitoring(), CoreMatchers.is(true));
+    Assert.assertThat(ms.getRefCount(), CoreMatchers.is(1));
+
+    ms.init("TestMetricsSystem3");
+    Assert.assertThat(ms.isMonitoring(), CoreMatchers.is(true));
+    Assert.assertThat(ms.getRefCount(), CoreMatchers.is(2));
+
+    ms.shutdown();
+    Assert.assertThat(ms.isMonitoring(), CoreMatchers.is(true));
+    Assert.assertThat(ms.getRefCount(), CoreMatchers.is(1));
+
+    ms.shutdown();
+    Assert.assertThat(ms.isMonitoring(), CoreMatchers.is(false));
+    Assert.assertThat(ms.getRefCount(), CoreMatchers.is(0));
+  }
+
   @Metrics(context="test")
   private static class TestSource {
     @Metric("C1 desc") MutableCounterLong c1;