In Ghost4J library (http://ghost4j.sourceforge.net), since version 0.4.0 you can use a SimpleRenderer to do the job with few lines of code:
Load PDF or PS file (use PSDocument class for that):
PDFDocument document = new PDFDocument();
document.load(new File("input.pdf"));
Create the renderer
SimpleRenderer renderer = new SimpleRenderer();
// set resolution (in DPI)
renderer.setResolution(300);
Render
List images = renderer.render(document);
Then you can do what you want with your image objects, for example, you can write them as PNG like this:
for (int i = 0; i < images.size(); i++) {
ImageIO.write((RenderedImage) images.get(i), "png", new File((i + 1) + ".png"));
}
Note: Ghost4J uses the native Ghostscript C API so you need to have a Ghostscript installed on your box.
I hope it will help you :)