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https://issues.apache.org/jira/browse/XMLRPC-167?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12704376#action_12704376
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Alan Burlison commented on XMLRPC-167:
--------------------------------------

I couldn't replicate the problem using those test classes even before the fix 
for XMLRPC-168, but my real app showed it within a few seconds.  I'll upgrade 
my XML-RPC libs to include the fix for XMLRPC-168 and start poking around 
tomorrow - I started looking at it today while you were very kindly looking at 
XMLRPC-168, but other things got in the way.  I'll let you know what I find.  
Thanks!

> XmlRpcClient is supposed to be thread-safe but it isn't
> -------------------------------------------------------
>
>                 Key: XMLRPC-167
>                 URL: https://issues.apache.org/jira/browse/XMLRPC-167
>             Project: XML-RPC
>          Issue Type: Bug
>          Components: Source
>    Affects Versions: 3.1.2
>         Environment: Solaris
>            Reporter: Alan Burlison
>
> http://ws.apache.org/xmlrpc/apidocs/org/apache/xmlrpc/client/XmlRpcClient.html
>  says:
> ----------
> A configured XmlRpcClient object is thread safe: In other words, the 
> suggested use is, that you configure the client using 
> setTransportFactory(XmlRpcTransportFactory) and similar methods, store it in 
> a field and never modify it again. Without modifications, the client may be 
> used for an arbitrary number of concurrent requests.
> ----------
> I have a simple test case that creates two threads that share a single 
> XmlRpcClient instance, and if I run the test I see the following errors:
> ----------
> Fatal Error] :1:1: Content is not allowed in prolog.
> operation failed, Failed to parse server's response: Content is not allowed 
> in prolog.
> [Fatal Error] :1:10: Element type "xlvrsion" must be followed by either 
> attribute specifications, ">" or "/>".
> operation failed, Failed to parse server's response: Element type "xlvrsion" 
> must be followed by either attribute specifications, ">" or "/>".
> ----------
> ----------
> [Fatal Error] :1:1: Content is not allowed in prolog.
> [Fatal Error] :1:5: Element type "xlv" must be followed by either attribute 
> specifications, ">" or "/>".
> operation failed, Failed to parse server's response: Element type "xlv" must 
> be followed by either attribute specifications, ">" or "/>".
> operation failed, Failed to parse server's response: Content is not allowed 
> in prolog.
> ----------
> If I wrap the call to XmlRpcClient.execute with a synchronized block that 
> locks on the XmlRpcClient object, the code works fine.
> It looks like the execute code is not locking around the read/write to/from 
> the server, so the threads are competing to read the response from the 
> server, and are tripping over each other.

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