In Jython, the _del_ method of objects that inherit from 'object' is never called: http://bugs.jython.org/issue1057

Starting in PyMongo 2.2, Pool inherits from object, and relies on its _del_ being called eventually in order to close its idle sockets, of which there are up to max_pool_size (default 10). So a loop like:

from pymongo import Connection
import threading

def run():
    for i in range(200):
        c.test.test.find_one()

for i in range(3):
    c = Connection()
    threads = [threading.Thread(target=run) for i in range(20)]
    for t in threads: t.start()
    for t in threads: t.join()

... will leave max_pool_size sockets open in each run. No amount of calling disconnect() or gc.collect() will close them. One would expect the JVM garbage collector to close the sockets some time after they are dereferenced, but in tests it does not. The solution is not to explicitly inherit Pool from object.