(Disclaimer: I work at Twilio)

Twilio offers a Java SDK for sending SMS via the Twilio REST API.

if all you want is simple notifications, many carriers support SMS via email; see SMS through E-Mail

There is an API called SMSLib, it's really awesome.

Now you have a lot of Saas providers that can give you this service using their APIs

Ex: mailchimp, esendex, Twilio, ...

The best SMS API I've seen in Java is JSMPP. It is powerful, easy to use, and I used it myself for an enterprise-level application (sending over 20K SMS messages daily).

This API created to reduce the verbosity of the existing SMPP API. It's very simple and easy to use because it hides the complexity of the low level protocol communication such as automatically enquire link request-response.

I've tried some other APIs such as Ozeki, but most of them either is commercial or has limitation in its throughput (i.e can't send more than 3 SMS messages in a second, for example).

You Can Do this With A GSM Modem and Java Communications Api [Tried And Tested]

1. First You Need TO Set Java Comm Api

   This Article Describes In Detail How to Set Up Communication Api

2. Next You Need A GSM Modem (preferably sim900 Module )

3. Java JDK latest version preferable

4. AT Command Guide