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I'm noticing some strange behavior in my app that smells like a lack of thread-safety. I'm working on reproducing it, but in the meantime I wanted to ensure I'm making the right assumptions about how the class that contains my endpoint handlers is used from a threading perspective. Most of what happens is opaque to me, because I'm not the one instantiating the class in the first place. To state the obvious, it must be some black magic in Endpoints.

MY ASSUMPTION

An instance of the class that holds my endpoint handlers is created for every single request that comes into my app. Based upon that assumption, it's ok for that class to have non-thread-safe objects that get used by my handlers.

MY FEAR

The instances of Endpoint handler classes are reused across requests.

So, which is it? Regardless of the answer, I think it would make sense for me to remove the ambiguity in my app and assume the worst, because I don't think I have any control over how Endpoints behaves. In my case, I'm creating a JDO/DataNucleus PersistenceManager (not thread-safe) when constructing the class housing my endpoint handlers. I should probably just create it in each handler as a local, or use a ThreadLocal.

I can probably also fashion a test to prove one or the other. I'll post back an answer to my own question if I do.

John Michelau
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  • Why would you assume a new instance for each request? And, out of curiosity is your application defined as thread safe in `appengine-web.xml`? – Dan Holevoet Dec 20 '13 at 21:20
  • Clearly because I didn't think it through. As I was writing this, I came to the internal conclusion that my assumption was dumb, but I posted it anyway for confirmation and since others might benefit. – John Michelau Dec 20 '13 at 22:20
  • Yes, the app is defined as threadsafe in appengine-web.xml. I do want my instances to be reused. – John Michelau Dec 20 '13 at 22:22
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    I can't speak for GAE, but in normal servlet environments servlet instances (= endpoint classes) are reused: http://stackoverflow.com/questions/2183974/difference-between-each-instance-of-servlet-and-each-thread-of-servlet-in-servle – Peter Knego Dec 22 '13 at 08:45
  • @PeterKnego, I think your answer is the correct one. If you care to post it as an answer, I'd accept it. Endpoint containers are probably just Servlet containers. There's not a lot of documentation on Endpoints (being new, this makes sense), and I'm new to both, hence the novice mistake. The thread you posted is very helpful. – John Michelau Dec 22 '13 at 21:56
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    Just thought I'd mention, if you have a filter before your endpoint and check threads in filter and endpoint (by using Thread.currentThread().getName() ) , you see 2 different threads. – jkb016 Jun 02 '15 at 19:55

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