Add GET endpoint for one-shot pv value reads

[rerpha]

Closes #8

[georgweiss]

Thanks for the PR.

Some remarks...

Retrying a read read operation is not the preferred pattern. Instead you could consider something like

 CountDownLatch countDownLatch = new CountDownLatch(1);
        AtomicReference<VType> value = new AtomicReference<>(null);
        try {
            PV pv = PVPool.getPV(name);
            pv.onValueEvent().subscribe(vtype -> {
                if (!VTypeHelper.isDisconnected(vtype)) {
                    value.set(pv.read());
                }
               countDownLatch.countDown();
            });
            countDownLatch.await(5000, TimeUnit.MILLISECONDS);
            PVPool.releasePV(pv);
            if(value.get() == null){
                return null;
            }
            return Vtype2Json.toJson(name, value.get(), null, true, true);
        } catch (Exception e) {
            // Do something sensible here to avoid HTTP 500 status
        }

The above connects to the PV and reads the value when connected. If the PV is offline - or does not exist - the code waits at most 5000 ms and then proceeds.
Also, returning a non-JSON formatted string forces the client to inspect the response. So returning null (=empty body) may be the preferred option.

[rerpha]

Thanks for the PR.

Some remarks...

Retrying a read read operation is not the preferred pattern. Instead you could consider something like

 CountDownLatch countDownLatch = new CountDownLatch(1);
        AtomicReference<VType> value = new AtomicReference<>(null);
        try {
            PV pv = PVPool.getPV(name);
            pv.onValueEvent().subscribe(vtype -> {
                if (!VTypeHelper.isDisconnected(vtype)) {
                    value.set(pv.read());
                }
               countDownLatch.countDown();
            });
            countDownLatch.await(5000, TimeUnit.MILLISECONDS);
            PVPool.releasePV(pv);
            if(value.get() == null){
                return null;
            }
            return Vtype2Json.toJson(name, value.get(), null, true, true);
        } catch (Exception e) {
            // Do something sensible here to avoid HTTP 500 status
        }

The above connects to the PV and reads the value when connected. If the PV is offline - or does not exist - the code waits at most 5000 ms and then proceeds. Also, returning a non-JSON formatted string forces the client to inspect the response. So returning null (=empty body) may be the preferred option.

Thanks - will implement. I wasn't sure if retrying was the best approach, seemed messy!

[rerpha]

ok - all done I think @georgweiss - let me know what you think