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https://issues.apache.org/jira/browse/SYNAPSE-595?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Asankha C. Perera resolved SYNAPSE-595.
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Resolution: Invalid
Assignee: Asankha C. Perera
Supun
The reason why double checked locking typically fails for the singleton pattern
implementation is because the Java memory model allows a thread to observe a
lazily constructed object in a partially initialized state on possibly multi
CPU/core systems. This is well described by Joshua Bloch in Effective Java -
item # 48. It is not the case here, and connMap is a Synchronized Map, so the
"List connections = (List) connMap.get(key);" will *always* get the latest
valid object.
So as far as I see, this JIRA should not be categorized as a "major" "bug" and
thus your code patch does not "fix" anything, except for always re-entering the
same object twice for every single connection return.
- List connections = (List) connMap.get(key);
- if (connections == null) {
- synchronized(connMap) {
- // use double locking to make sure
- connections = (List) connMap.get(key);
- if (connections == null) {
- connections = Collections.synchronizedList(new
LinkedList());
- connMap.put(key, connections);
- }
+ List connections;
+ synchronized(connMap) {
+ connections = (List) connMap.get(key);
+ if (connections == null) {
+ connections = Collections.synchronizedList(new LinkedList());
+ connMap.put(key, connections);
Ofcourse this class could still be improved, esp with some new features of the
JDK, but its better done as an enhancement and including other overall
improvements.
asankha
> Possible race condition in ConnectionPool implementation of nhhtp transport
> ---------------------------------------------------------------------------
>
> Key: SYNAPSE-595
> URL: https://issues.apache.org/jira/browse/SYNAPSE-595
> Project: Synapse
> Issue Type: Bug
> Components: Transports
> Reporter: Supun Kamburugamuva
> Assignee: Asankha C. Perera
> Attachments: SYNAPSE-595.patch
>
>
> The nhttp transport connection pool uses double checked locking when it
> releases a connection. But double checked locking is not a recommended
> practice in Java and its use is avoided. Simply because it is not working.
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