Comment the locking mechanisms in ContextInfoManager

[lttng-ust.git] / liblttng-ust-java-agent / java / lttng-ust-agent-common / org / lttng / ust / agent / context / ContextInfoManager.java

diff --git a/liblttng-ust-java-agent/java/lttng-ust-agent-common/org/lttng/ust/agent/context/ContextInfoManager.java b/liblttng-ust-java-agent/java/lttng-ust-agent-common/org/lttng/ust/agent/context/ContextInfoManager.java
index 86ded59b346ab6984ca4594826d4d9f5716cb5d4..7a8c80e9a83d6040f1ed630b2932870e65f4eb66 100644 (file)
--- a/liblttng-ust-java-agent/java/lttng-ust-agent-common/org/lttng/ust/agent/context/ContextInfoManager.java
+++ b/liblttng-ust-java-agent/java/lttng-ust-agent-common/org/lttng/ust/agent/context/ContextInfoManager.java
@@ -40,6 +40,10 @@ public final class ContextInfoManager {
        private final Map<String, IContextInfoRetriever> contextInfoRetrievers = new ConcurrentHashMap<String, IContextInfoRetriever>();
        private final Map<String, Long> contextInforRetrieverRefs = new HashMap<String, Long>();
 
+       /**
+        * Lock used to keep the two maps above in sync when retrievers are
+        * registered or unregistered.
+        */
        private final Object retrieverLock = new Object();
 
        /** Singleton class, constructor should not be accessed directly */
@@ -163,6 +167,13 @@ public final class ContextInfoManager {
         *         was none
         */
        public IContextInfoRetriever getContextInfoRetriever(String retrieverName) {
+               /*
+                * Note that this method does not take the retrieverLock, it lets
+                * concurrent threads access the ConcurrentHashMap directly.
+                *
+                * It's fine for a get() to happen during a registration or
+                * unregistration, it's first-come-first-serve.
+                */
                return contextInfoRetrievers.get(retrieverName);
        }