Setting jpg compression level with ImageIO in Java

A more succinct way is to get the ImageWriter directly from ImageIO:

ImageWriter jpgWriter = ImageIO.getImageWritersByFormatName("jpg").next();
ImageWriteParam jpgWriteParam = jpgWriter.getDefaultWriteParam();
jpgWriteParam.setCompressionMode(ImageWriteParam.MODE_EXPLICIT);
jpgWriteParam.setCompressionQuality(0.7f);

ImageOutputStream outputStream = createOutputStream(); // For example implementations see below
jpgWriter.setOutput(outputStream);
IIOImage outputImage = new IIOImage(image, null, null);
jpgWriter.write(null, outputImage, jpgWriteParam);
jpgWriter.dispose();

The call to ImageWriteParam.setCompressionMode(ImageWriteParam.MODE_EXPLICIT) is needed in order to explicitly set the compression’s level (quality).

In ImageWriteParam.setCompressionQuality() 1.0f is maximum quality, minimum compression, while 0.0f is minimum quality, maximum compression.

ImageWriter.setOutput should be passed an ImageOutputStream. While the method accepts Object, according to documentation it’s usually not supported:

Use of a general Object other than an ImageOutputStream is intended for writers that interact directly with an output device or imaging protocol. The set of legal classes is advertised by the writer’s service provider’s getOutputTypes method; most writers will return a single-element array containing only ImageOutputStream.class to indicate that they accept only an ImageOutputStream.

Most cases should be handled by these two classes:

• FileImageOutputStream – an implementation of ImageOutputStream that writes its output directly to a File or RandomAccessFile.
• MemoryCacheImageOutputStream – an implementation of ImageOutputStream that writes its output to a regular OutputStream. Usually used with ByteArrayOutputStream (thanks for the tip, @lmiguelmh!).