HADOOP-16925. MetricsConfig incorrectly loads the configuration whose value is String list in the properties file (#1896)

Contributed by Jiayi Liu

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

@@ -37,6 +37,7 @@ import com.google.common.collect.Maps;
 import org.apache.commons.configuration2.Configuration;
 import org.apache.commons.configuration2.PropertiesConfiguration;
 import org.apache.commons.configuration2.SubsetConfiguration;
+import org.apache.commons.configuration2.convert.DefaultListDelimiterHandler;
 import org.apache.commons.configuration2.ex.ConfigurationException;
 import org.apache.commons.configuration2.io.FileHandler;
 import org.apache.hadoop.metrics2.MetricsFilter;
@@ -111,6 +112,7 @@ class MetricsConfig extends SubsetConfiguration {
     for (String fname : fileNames) {
       try {
         PropertiesConfiguration pcf = new PropertiesConfiguration();
+        pcf.setListDelimiterHandler(new DefaultListDelimiterHandler(','));
         FileHandler fh = new FileHandler(pcf);
         fh.setFileName(fname);
         fh.load();

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

@@ -133,6 +133,22 @@ public class TestMetricsConfig {
     assertEq(expected, mc2);
   }
 
+  /**
+   * Test the config value separated by delimiter
+   */
+  @Test public void testDelimiterConf() {
+    String filename = getTestFilename("test-metrics2-delimiter");
+    new ConfigBuilder().add("p1.foo", "p1foo1,p1foo2,p1foo3").save(filename);
+
+    MetricsConfig mc = MetricsConfig.create("p1", filename);
+    Configuration expected = new ConfigBuilder()
+        .add("foo", "p1foo1")
+        .add("foo", "p1foo2")
+        .add("foo", "p1foo3")
+        .config;
+    assertEq(expected, mc);
+  }
+
   /**
    * Return a test filename in the class path
    * @param basename