[Py4j-users] Multiprocessing error

[Eleftherios A. <ele...@df...>]

Dear all,

I would like your help with a particular issue which occurs when I am 
trying to use python multiprocessing, along with Py4j Java objects.


def singlethread(gateway):
     print "Thread starting"

     #create a new view for the jvm
     meteor_view = gateway.new_jvm_view()
     #import required packages
     java_import(meteor_view, 'edu.cmu.meteor.scorer.*')
     #initialize the java object
     scorer = meteor_view.MeteorScorer()
     print "object initialized"
     #run object function
     stats = scorer.getMeteorStats("Test sentence", "Test sentence !");
     print stats.score

if __name__ == '__main__':
     socket_no = 25336
     gatewayclient = GatewayClient('localhost', socket_no)
     print "Gclient started"
     gateway = JavaGateway(gatewayclient, auto_convert=True, 
auto_field=True)
     print "Gateway started"

     p = multiprocessing.Pool(2)
     print "Multipool initialized"
     p.map(singlethread, [gateway, gateway, gateway])



output:

Gclient started
Gateway started
Multipool initialized
Exception in thread Thread-2:
Traceback (most recent call last):
   File "/usr/local/lib/python2.7/threading.py", line 552, in 
__bootstrap_inner
     self.run()
   File "/usr/local/lib/python2.7/threading.py", line 505, in run
     self.__target(*self.__args, **self.__kwargs)
   File "/usr/local/lib/python2.7/multiprocessing/pool.py", line 313, in 
_handle_tasks
     put(task)
   File "/usr/local/lib/python2.7/site-packages/py4j/java_gateway.py", 
line 432, in __call__
     self.target_id, self.name)
   File "/usr/local/lib/python2.7/site-packages/py4j/protocol.py", line 
271, in get_return_value
     raise Py4JError('An error occurred while calling %s%s%s. 
Trace:\n%s\n' % (target_id, '.', name, value))
Py4JError: An error occurred while calling t.__getnewargs__. Trace:
java.lang.StringIndexOutOfBoundsException: String index out of range: -1
     at java.lang.String.substring(String.java:1949)
     at java.lang.String.substring(String.java:1916)
     at py4j.Gateway.invoke(Gateway.java:250)
     at py4j.commands.AbstractCommand.invokeMethod(AbstractCommand.java:124)
     at py4j.commands.CallCommand.execute(CallCommand.java:81)
     at py4j.GatewayConnection.run(GatewayConnection.java:175)
     at java.lang.Thread.run(Thread.java:636)


I would be happy if somebody can help me find out what exactly the error 
may be, or if you can indicate where I should look for the problem.

Interestingly enough, this problem doesn't occur when running without 
multiprocessing. It also runs fine with other Java programs. But how can 
I specify which aspect of the Java program causes the error?

best
Eleftherios

-- 
MSc. Inf. Eleftherios Avramidis
DFKI GmbH, Alt-Moabit 91c, 10559 Berlin
Tel. +49-30 238 95-1806

Fax. +49-30 238 95-1810

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