(1)I spent my 4th year Computing project on implementing time series forecasting for Java heap usage prediction using ARIMA, Holt Winters etc, so I might be in a good position to advise you on this.

Your best option by far is using the R language, you can call on the forecasting libraries provided by R, through Java by using the JRI library found here. R is well documented, free and open source. You can even run R on a server and then make calls to it via command line using Rserve, which then returns forecasts over HTTP but JRI is the local equivalent if memory serves me correctly.

survey on Time Series Analysis Lib