ivankelly commented on a change in pull request #832: Issue 620: Close the 
fileChannels for read when they are idle
URL: https://github.com/apache/bookkeeper/pull/832#discussion_r171810861
 
 

 ##########
 File path: 
bookkeeper-server/src/main/java/org/apache/bookkeeper/bookie/EntryLogger.java
 ##########
 @@ -330,54 +350,47 @@ private int readFromLogChannel(long entryLogId, 
BufferedReadChannel channel, Byt
      * A thread-local variable that wraps a mapping of log ids to 
bufferedchannels
      * These channels should be used only for reading. logChannel is the one
      * that is used for writes.
+     * We use this Guava cache to store the BufferedReadChannel.
+     * When the BufferedReadChannel is removed, the underlying fileChannel's 
refCnt decrease 1,
+     * temporally use 1h to relax replace after reading.
      */
-    private final ThreadLocal<Map<Long, BufferedReadChannel>> logid2Channel =
-            new ThreadLocal<Map<Long, BufferedReadChannel>>() {
+    private final ThreadLocal<Cache<Long, EntryLogBufferedReadChannel>> 
logid2ReadChannel =
+            new ThreadLocal<Cache<Long, EntryLogBufferedReadChannel>>() {
         @Override
-        public Map<Long, BufferedReadChannel> initialValue() {
+        public Cache<Long, EntryLogBufferedReadChannel> initialValue() {
             // Since this is thread local there only one modifier
             // We dont really need the concurrency, but we need to use
             // the weak values. Therefore using the concurrency level of 1
-            return new MapMaker().concurrencyLevel(1)
-                .weakValues()
-                .makeMap();
+            return CacheBuilder.newBuilder().concurrencyLevel(1)
+                    .expireAfterAccess(readChannelCacheExpireTimeMs, 
TimeUnit.MILLISECONDS)
+                    //decrease the refCnt
+                    .removalListener(removal -> ((EntryLogBufferedReadChannel) 
removal.getValue()).release())
 
 Review comment:
   >  ```.removalListener((RemovalListener<String, 
EntryLogBufferedReadChannel>) notification -> notification.getValue.release)```
   
   Another option would be to define the parameter type in the lambda.
   ```
   .removalListener((RemovalNotification<String, EntryLogBufferedReadChannel> 
notification) -> notification.getValue().release())
   ```
   
   I'm fine with any of these, I only commented originally because I thought 
nothing was needed at all.
   
   > because it is not pure RemovalListener<K, V>.
   
   @sijie What is because it's not pure? I'm not sure of the context.
   

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