dependencies {
  compile fileTree(dir:'src/main/resources/libs',include:['*.jar'])
}

<dependency> 
  <groupId>com.github.nintha</groupId> 
  <artifactId>webp-imageio-core</artifactId> 
  <version>{versoin}</version> 
  <scope>system</scope> 
  <systemPath>${project.basedir}/libs/webp-imageio-core-{version}.jar</systemPath> 
</dependency>

public static void main(String args[]) throws IOException {
  String inputPngPath = "test_pic/test.png";
  String inputJpgPath = "test_pic/test.jpg";
  String outputWebpPath = "test_pic/test_.webp";

  // Obtain an image to encode from somewhere
  BufferedImage image = ImageIO.read(new File(inputJpgPath));

  // Obtain a WebP ImageWriter instance
  ImageWriter writer = ImageIO.getImageWritersByMIMEType("image/webp").next();

  // Configure encoding parameters
  WebPWriteParam writeParam = new WebPWriteParam(writer.getLocale());
  writeParam.setCompressionMode(WebPWriteParam.MODE_DEFAULT);

  // Configure the output on the ImageWriter
  writer.setOutput(new FileImageOutputStream(new File(outputWebpPath)));

  // Encode
  writer.write(null, new IIOImage(image, null, null), writeParam);
}

public static void main(String args[]) throws IOException {
  String inputWebpPath = "test_pic/test.webp";
  String outputJpgPath = "test_pic/test_.jpg";
  String outputJpegPath = "test_pic/test_.jpeg";
  String outputPngPath = "test_pic/test_.png";

  // Obtain a WebP ImageReader instance
  ImageReader reader = ImageIO.getImageReadersByMIMEType("image/webp").next();

  // Configure decoding parameters
  WebPReadParam readParam = new WebPReadParam();
  readParam.setBypassFiltering(true);

  // Configure the input on the ImageReader
  reader.setInput(new FileImageInputStream(new File(inputWebpPath)));

  // Decode the image
  BufferedImage image = reader.read(0, readParam);

  ImageIO.write(image, "png", new File(outputPngPath));
  ImageIO.write(image, "jpg", new File(outputJpgPath));
  ImageIO.write(image, "jpeg", new File(outputJpegPath));

}