Java: what library is used to create diagrams like below?

I came across the following,

what Java library allows me to create those lines connecting boxes, i don't know what it's called.

enter image description here

Check out JGraph - graph visualization library. It is pretty powerful and versatile, open source, Java version is Swing based. Java version is free. See their sample diagrams on the front page.

You may want to check out GraphViz, but their site is currently down. Its a general purpose data visualization library. It's not a Java library internally but it has a Java wrapper called Grappa also on their site's download page. In the mean time, you can google it up and take a look at this wiki page.