Play Framework: Block the bootstrap thread onStart

Question

When you override onStart in Global, would it block the thread running the bootstrap process? In other words I want to make sure that Play does not start until call returns from onStart.

If this is not the case, what are good solution to do critical init process which need to happen before Play start accepting requests?

Answer 1

Yes, Global.onStart will block before the first request.

The GlobalSettings object in Play is a plugin. When the application is started, the routes are loaded first, then the plugins are loaded serially by calling the onStart method of each. The GlobalSettings plugin comes last in this initialization, which ensures that all other plugins are loaded first so that all of your db connections, cache, etc are available there.

The plugin initialization has to block a single thread to guarantee that:

1. All plugins are initialized in the exact order that is desired to prevent initialization order errors (if one plugin depends on another initializing first--for example the database plugin must always initialize before the evolutions plugin).

2. All plugins are initialized before the application can start processing requests. (Otherwise, you wouldn't be able to guarantee the the DB plugin was ready on the first request, for example).