I am new to Guice, and to DI libraries (I've never used Spring, etc.),
although I've used similar patterns myself (always take dependencies in
ctor; try to restrict new to "factories"). I'm trying to see how I can
use Guice on a totally new project, but I'm trying to understand lifecycles.
As an example application to learn, I wanted to use some JDBC via a SQL
DataSource object; I figured Guice to inject this is a great idea. I
implemented a @Provides method in my module to actually initialize the
DataSource and return it (I hope that is OK), and I marked it as
Singleton to restrict to one DataSource per injector (application).
But, how do I shutdown that DataSource, or any other resource that I
acquire? Right now my application is
Guice.createInjector.getInstance(TheMainClass).go() and then after that
(go returns after the app is done), I do getInstance for the DataSource
to call close on it. That really doesn't seem right to me; what if no
one ever asked for the DataSource?
I realize that there are pooling data sources that can timeout idle
connections, and you can make a resource in which close does nothing if
no one ever opened/used it. But what if the resource doesn't work that
way? What method does one use to say the module is "done"? Or is the way
I did it the best way?
This problem comes more apparent if I have a Guice app that wants to do
a task; let's say a button "backup DB to FTP" that might never be used.
But if it does I want a bunch of "session" state that is "singleton" to
that case, and then all cleaned up at the end? How do I do that? Scopes?
Child injectors? I thought Scopes would be it but don't seem to make
sense; the SessionScope/RequestScope I learned rely on "global" state,
and I'm not in a servlet anyway.
Thanks,
Jason
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