HADOOP-16062. Add ability to disable Configuration reload registry

hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/conf/Configuration.java

@@ -320,7 +320,7 @@ private static boolean getRestrictParserDefault(Object resource) {
   /**
    * Configuration objects.
    */
-  private static final WeakHashMap<Configuration,Object> REGISTRY = 
+  private static WeakHashMap<Configuration, Object> REGISTRY =
     new WeakHashMap<Configuration,Object>();
 
   /**
@@ -828,9 +828,7 @@ public Configuration() {
   public Configuration(boolean loadDefaults) {
     this.loadDefaults = loadDefaults;
 
-    synchronized(Configuration.class) {
-      REGISTRY.put(this, null);
-    }
+    registerConfiguration(this);
   }
 
   /** 
@@ -864,18 +862,43 @@ public Configuration(Configuration other) {
       this.propertyTagsMap.putAll(other.propertyTagsMap);
     }
 
-    synchronized(Configuration.class) {
-      REGISTRY.put(this, null);
-    }
+    registerConfiguration(this);
+
     this.classLoader = other.classLoader;
     this.loadDefaults = other.loadDefaults;
     setQuietMode(other.getQuietMode());
   }
 
+  /**
+   * Registers the configuration to enable reloading.
+   */
+  private static void registerConfiguration(Configuration conf) {
+    WeakHashMap<Configuration, Object> registry = REGISTRY;
+    if (registry != null) {
+      synchronized (Configuration.class) {
+        registry.put(conf, null);
+      }
+    }
+  }
+
+  public static synchronized void setConfigurationRegistryEnabled(boolean enable) {
+    if (enable) {
+      if (REGISTRY == null) {
+        REGISTRY = new WeakHashMap<Configuration, Object>();
+      }
+    } else {
+      REGISTRY = null;
+    }
+  }
+
   /**
    * Reload existing configuration instances.
    */
   public static synchronized void reloadExistingConfigurations() {
+    if (REGISTRY == null) {
+      LOG.debug("Configuration registry is disabled!");
+      return;
+    }
     if (LOG.isDebugEnabled()) {
       LOG.debug("Reloading " + REGISTRY.keySet().size()
           + " existing configurations");
@@ -891,11 +914,14 @@ public static synchronized void reloadExistingConfigurations() {
    * @param name file name. File should be present in the classpath.
    */
   public static synchronized void addDefaultResource(String name) {
+    WeakHashMap<Configuration, Object> registry = REGISTRY;
     if(!defaultResources.contains(name)) {
       defaultResources.add(name);
-      for(Configuration conf : REGISTRY.keySet()) {
-        if(conf.loadDefaults) {
-          conf.reloadConfiguration();
+      if (registry != null) {
+        for (Configuration conf : registry.keySet()) {
+          if (conf.loadDefaults) {
+            conf.reloadConfiguration();
+          }
         }
       }
     }

hadoop-common-project/hadoop-common/src/test/java/org/apache/hadoop/conf/TestConfiguration.java

@@ -46,6 +46,9 @@
 import java.util.Properties;
 import java.util.Random;
 import java.util.Set;
+import java.util.concurrent.Semaphore;
+import java.util.concurrent.atomic.AtomicBoolean;
+import java.util.concurrent.atomic.AtomicInteger;
 import java.util.concurrent.atomic.AtomicBoolean;
 import java.util.regex.Pattern;
 import static java.util.concurrent.TimeUnit.*;
@@ -65,6 +68,7 @@
 import static org.junit.Assert.*;
 
 import org.apache.commons.lang3.StringUtils;
+import org.apache.commons.lang3.concurrent.AtomicInitializer;
 import org.apache.hadoop.conf.Configuration.IntegerRanges;
 import org.apache.hadoop.fs.FileUtil;
 import org.apache.hadoop.fs.Path;
@@ -2716,4 +2720,27 @@ public void testConcurrentModificationDuringIteration() throws InterruptedExcept
 
     assertFalse("ConcurrentModificationException occurred", exceptionOccurred.get());
   }
+
+  @Test
+  public void testReloadingCanBeDisabled() {
+    try {
+      AtomicInteger reloadCount = new AtomicInteger(0);
+      Configuration config = new Configuration() {
+        @Override
+        public synchronized void reloadConfiguration() {
+          reloadCount.incrementAndGet();
+        }
+      };
+      assertThat(reloadCount.get(), is(0));
+      Configuration.reloadExistingConfigurations();
+      assertThat(reloadCount.get(), is(1));
+      Configuration.setConfigurationRegistryEnabled(false);
+      assertThat(reloadCount.get(), is(1));
+      Configuration.reloadExistingConfigurations();
+      assertThat(reloadCount.get(), is(1));
+
+    } finally {
+      Configuration.setConfigurationRegistryEnabled(true);
+    }
+  }
 }