Thread Executor, Thread Pool simple example.

ThumbNailGen.java

import java.io.File;
import java.io.FilenameFilter;
import java.io.IOException;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.logging.Level;
import java.util.logging.Logger;
import net.coobird.thumbnailator.Thumbnails;

/**
 *
 * @author Vikesh
 */
public class ThumbNailGen {

  private static final String INPUT_DIR = "G:\\Images\\2013-12-19";
  private static final String OUTPUT_DIR = "G:\\Images\\2013-12-19\\thumb";
  private static final double DEFAULT_SCALE = 0.30;
  /**
   * @param args the command line arguments
   */
  public static void main(String[] args) throws IOException {
    File inputDir = null;
    File outputDir = null;
    double scale = DEFAULT_SCALE;
    if (args.length == 2) {
      inputDir = new File(args[0]);
      outputDir = new File(args[1]);
    } else {
      inputDir = new File(INPUT_DIR);
      outputDir = new File(OUTPUT_DIR);
    }
    if (args.length == 3) {
      scale = Double.parseDouble(args[2]);
    }
    if (!outputDir.exists()) {
      outputDir.mkdir();
    }
    if (inputDir.isDirectory()) {
      String[] list = inputDir.list(new FilenameFilter() {
      @Override
      public boolean accept(File dir, String name) {
        String fName = name.toLowerCase();
        return fName.endsWith("jpg") || fName.endsWith("jpeg");
      }
    });
    ExecutorService executorService = Executors.newFixedThreadPool(4);
    for (String file : list) {
      final File img = new File(inputDir, file);
      final File parent = outputDir;
      final double tempScale = scale;
      final String outName = file;
      executorService.submit(new Runnable() {
        @Override
        public void run() {
          try {
            Thumbnails.of(img).scale(tempScale).toFile(new File(parent, outName));
          } catch (IOException ex) {
            Logger.getLogger(ThumbNailGen.class.getName()).log(Level.SEVERE, null, ex);
          }
        }
      });
    }
    executorService.shutdown();
    }
  }
}