Use JMagick (docs). Read the documentation. It provides all the functionality of ImageMagick. You may also look into another ImageMagick Java wrapper, im4java.
There is a good starters document for im4java here
Here is an example, I've worked out.
/** Typical scaling implementation using JMagick **/
ImageInfo origInfo = new ImageInfo(absPath); //load image info
MagickImage image = new MagickImage(origInfo); //load image
image = image.scaleImage(finalWidth, finalHeight); //to Scale image
image.setFileName(absNewFilePath); //give new location
image.writeImage(origInfo); //save
Edit #1:
If you are wondering for the Jar file of JMagick. Download jMagick tarball, untar it.
$ tar xvzf jmagick-linux-6.4.0-Q32.tar.gz
./jmagick-6.4.0.jar
./jmagick.jar
./libJMagick-6.4.0.so
./libJMagick.so
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