[GitHub] [zookeeper] maoling commented on a change in pull request #1276: ZOOKEEPER-3497:Change MBeanRegistry to a standard singleton

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maoling commented on a change in pull request #1276:
URL: https://github.com/apache/zookeeper/pull/1276#discussion_r777801548



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File path: zookeeper-server/src/main/java/org/apache/zookeeper/jmx/MBeanRegistry.java
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@@ -43,28 +43,26 @@
     public static final String DOMAIN = "org.apache.ZooKeeperService";
 
     private static final Logger LOG = LoggerFactory.getLogger(MBeanRegistry.class);
-    private static volatile MBeanRegistry instance = new MBeanRegistry();
 
     private final Object LOCK = new Object();
 
     private Map<ZKMBeanInfo, String> mapBean2Path = new ConcurrentHashMap<ZKMBeanInfo, String>();
 
     private MBeanServer mBeanServer;
 
-    /**
-     * Useful for unit tests. Change the MBeanRegistry instance
-     *
-     * @param instance new instance
-     */
-    public static void setInstance(MBeanRegistry instance) {
-        MBeanRegistry.instance = instance;
+    private static class MBeanRegistryHolder {

Review comment:
       @eolivelli This is just a singleton style a programmer prefers. Look at this:
   [1] https://en.wikipedia.org/wiki/Initialization-on-demand_holder_idiom#Example_Java_Implementation
   [2] https://stackoverflow.com/questions/17799976/why-is-static-inner-class-singleton-thread-safe




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