How to Display a JavaScript-like Alert in Java

Just put the following in your Java code:

javax.swing.JOptionPane.showMessageDialog(null, "Your message here");

Note:
Similar to JavaScript, your code will pause executing until you close the message dialog. So be careful when deploying your code for the client.

How to Access Image Dimensions from MultipartFile in Java

Here's how to access image dimensions from MultipartFile:

java.awt.image.BufferedImage image = javax.imageio.ImageIO.read(new BufferedInputStream(multipartFile.getInputStream()));
System.out.println("Image height: " + image.getHeight());
System.out.println("Image width: " + image.getWidth());