Document Singleton Scope Requirements

[dclements]

I don't think this is a bug in your system, but it would be helpful to have some documentation around where singleton needs to be used to prevent memory problems.

Our situation was as follows:

We migrated to using dropwizard-guicier from creating the instances for the API resource classes. Basically this allowed us to go from:

environment.jersey().register(injector.getInstance(Foo.class));

to:

environment.jersey().register(Foo.class);

Everything looked like it worked well, but then it turned out that every request was creating a new instance of Foo and then jersey was holding on to that reference indefinitely, leading to eventual out of memory errors. This does not appear to be a problem for, e.g., Filters but it definitely is for API resources (I haven't had a chance to dig into it yet and find a root cause).

Changing the binding to scope it as a singleton removed this problem. My understanding is that HK2 bypasses this problem by considering certain things Singleton as opposed to PerLookup by default, though I am not as well versed on the details here.

For this project it would be helpful from a documentation standpoint to indicate which objects need to be marked as Singletons to prevent memory issues.

[dclements]

Relevant details:

• Dropwizard 1.3.14
• dropwizard-guicier: 1.3.5.2

[jhaber]

Hmm, do you have a small app that reproduces the issue? You could maybe build on the existing example app if it's more convenient:
https://github.com/jhaber/dropwizard-guicier-example

Internally we do binder.bind(Foo.class); which will get picked up and registered as a Jersey resource by DropwizardModule here

However, I would expect that to be roughly equivalent to environment.jersey().register(Foo.class) so I'm a bit stumped

[jhaber]

Also none of our Jersey resource classes are singletons

[dclements]

Weird, let me see if I can build a reliable reproduction.