You can send them directly without using FCM and there are several libraries available, including a Java one.
Unfortunately iOS currently has no support for Web Push. Subscriptions to your service are on a device level rather than by user, so if I sign up on my desktop you cannot send notifications to my mobile unless I sign up again in my mobile browser.
Notifications pushed to Android will be displayed if an instance of the browser is running, in the real world (for me anyway) Chrome always seems to be running in the background somewhere so I get notifications through in pretty much real time. The downside is web push notifications go straight into the notification shade, they do not pop up on screen first.
The rough workflow goes like this:
User visits your page, you load service worker and check for web push
capability, if satisfied you can request permission to send
notifications.
If user grants permission you pass your public key to your service
worker to create a subscription for that user, this returns an
endpoint and two keys which you need to push a notification to them.
- Your webpush library runs as a server instance and takes care of all
the encryption and token handling, you configure it however you like
to dispatch messages, usually in response to HTTP POST requests but
it's up to you.
- Within your service worker you define an event handler for receipt of
a push message. This is where you create and display the notification
to the user, again how you do this is up to you.
You can pass parameters in the payload of the notification and use them as variables within the notification you display or you can hard code values, you can specify different behaviours depending upon whether the user has your page in focus or not, you can add buttons and set different actions for them, trigger events upon dismissal, customise the vibrate pattern, replace or stack the notifications, access the data in existing notifications etc etc. All this is handled by your service worker, receiving the notification alone does nothing at all.
Your service worker is just a script written in javascript which you link from your page. It is loaded and installed by the browser the first time a user visits and then runs independently when invoked.
Service workers are very powerful. You can also use them to implement complex caching rules, serve content while offline, push data between different browser windows etc. A service worker can spawn more service workers and as they run outside of the main thread of your browser they are ideal for offloading cpu intensive tasks to without delaying the rendering of your page.
Final point to note, your site must be served over SSL to be able to deploy a service worker.
