Ensuring a valid auth token is always available

Question

I've been trying to figure out how to authenticate users for my android app. It is based on a website which already has a developed api, using JWT to authenticate.

I have come against the problem of refreshing tokens. Let's say I want to fetch something from the API and I need the auth token for that. I check my current auth token. If it is expired, I need to get a new one using some sort of refresh token.

However, it seems like almost no matter how I think of trying to implement it, I run into a few problems:

1. I don't want the UI thread to wait while I get a new token
2. I would prefer that I don't have to explicitly check whether the token is there (and then refresh it) before making any API call

I've come up with one solution that solves #1 and at least minimizes the pain of #2. I can have some sort of getToken method. As an example, using JS style promises because they're easier for me to understand:

function getToken() {
  return new Promise((resolve) => {
    // Check for token, and return if valid.
    // Otherwise, go to the server and get a new one
    ...
    resolve(token)
  }
}

// When making an API call
getToken().then((token) => {
  // Call API
})

I think I can work this out so that the request will never be running on the UI thread, which solves #1, and as far as #2, it's at least bearable.

My question is this: is there a better way to do this? It kind of seems like AccountManager might be able to handle this sort of thing for me, but the documentation for it is subpar at best, so I'm not sure how I would even implement it. If AccountManager can do it and you know of a good tutorial for it, please comment with that.

Answer 1

What I ended up doing was creating a method getToken which either returns the current token or gets a new one (blocking). With this strategy, I need to make sure that it never gets called from the UI thread. I created a Retrofit2 interceptor which calls getToken. The benefit of this method is that I can just call my Retrofit methods without worrying about the token at all, and it checks for expiration and gets a new one as necessary.