方法1:

Image src = ToolKit.getToolKit().createImage("文件路徑");
然后先生成一個BufferedImage bi作為畫布.
BufferedImage bi = new BufferedImage（目標寬, 目標高,BufferedImage.TYPE_INT_RGB);
得到它的Graphics對象:
Graphics g = bi.getGraphics();
然后往這個畫而上畫原圖就行了:
g.grawImage(src,0,0,目標寬,目標高,null);
找一個編碼類,如JPEGEncoder,GIFEncoder
把bi編碼輸出就行了.

方法2:
import javax.imageio.ImageIO;
import javax.imageio.IIOException;
import java.awt.image.BufferedImage;
import java.awt.Image;
import java.io.File;
import java.awt.image.AffineTransformOp;
import java.awt.geom.AffineTransform;

public class Test {

??? public static void main (String argv[]) {
??????? try {
??????????? File fi = new File("C:/1.jpg"); //大圖文件
??????????? File fo = new File("C:/2.jpg"); //將要轉換出的小圖文件

??????????? AffineTransform transform = new AffineTransform();
??????????? BufferedImage bis = ImageIO.read(fi);

??????????? int w = bis.getWidth();
??????????? int h = bis.getHeight();
??????????? double scale = (double)w/h;

??????????? int nw = 120;
??????????? int nh = (nw * h) / w;
??????????? if(nh>120) {
??????????????? nh = 120;
??????????????? nw = (nh * w) / h;
??????????? }

??????????? double sx = (double)nw / w;
??????????? double sy = (double)nh / h;

??????????? transform.setToScale(sx,sy);

??????????? AffineTransformOp ato = new AffineTransformOp(transform, null);
??????????? BufferedImage bid = new BufferedImage(nw, nh, BufferedImage.TYPE_3BYTE_BGR);
??????????? ato.filter(bis,bid);
??????????? ImageIO.write(bid, "jpeg", fo);
??????? } catch(Exception e) {
??????????? e.printStackTrace();
??????? }
??? }
}