Opencv provides Hough circle Detection algorithm which can be used to detect circles. Some information about how algorithm works and its example using Opencv in cpp can be found in below link
We will see how to use cvHoughCircle using Javacv. first we will have to process image to get grayscale or binary image. Using cvSmooth() helps most of the time for good detection however it depending upon kind of object and background more image processing may be required.
First we will load image and then convert it to grayscale. Then use cvSmooth() to smooth the edges. cvHoughCircle() is used to detect circles and are stored in CvSeq. cvGetSeqElem() is used to extract each circle. We have to use each element in CvPoint3D32f. Center of circle is obtained in CvPoint type using cvPointFrom32f(). Obtained center and radius is used to draw circle on input image using cvCircle.
Following code is a demonstration of all the above processes.