FrontEnd:

Include jquery.Jcrop.css, jquery.Jcrop.js and jquery.Jcrop.min.js

Open an image in a div with a form. Take 4 hidden input fields:

<input type="hidden" id="x" name="x" /> // x coordinate of the image
<input type="hidden" id="y" name="y" /> // y coordinate of the image
<input type="hidden" id="w" name="w" /> // width of the specified rectangular region
<input type="hidden" id="h" name="h" /> // height of the specified rectangular region

Call the function on defining the cropped image

$('.cropbox').Jcrop({
        trueSize: [width, height],
        onSelect :  function setCoordinates(c) {
                  $('#x').val(frm.x);
                  $('#y').val(frm.y);
                  $('#w').val(frm.w);
                  $('#xh).val(frm.h);
            }
        });

Here trueSize: [width, height] will help in maintaining the original height, width of the image (since on UI we may need to show the image in an small size)

BackEnd:

BufferedImage originalImgage = ImageIO.read(new File("path of original image"));
BufferedImage SubImage = originalImgage.getSubimage(x, y, w, h);

Save subimage to DB.