用于图片缩放的工具类

网上找到的，略微地改动了一番

大致能满足这样的需求：

当图片长度或宽度超过一个正方形边长时，等比缩小，使图片能够放入正方形内。缩放后尽可能保持图片原先的质量。

import com.sun.image.codec.jpeg.JPEGImageEncoder;
import com.sun.image.codec.jpeg.JPEGCodec;
import com.sun.image.codec.jpeg.JPEGEncodeParam;
import java.io.File;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.awt.Color;
import java.awt.Graphics;
import java.awt.Image;
import java.awt.image.BufferedImage;
import java.awt.image.Kernel;
import java.awt.image.ConvolveOp;
import javax.swing.ImageIcon;

public class ImageUtil {

    public static void resize(File originalFile, File resizedFile, int newWidth, float quality) throws IOException {

        if (quality > 1) {
            throw new IllegalArgumentException("Quality has to be between 0 and 1");
        }

        ImageIcon ii = new ImageIcon(originalFile.getCanonicalPath());
        Image i = ii.getImage();
        Image resizedImage = null;

        // No necessary to resized small image, just copy it. --Deolin
        if (i.getWidth(null) <= newWidth && i.getHeight(null) <= newWidth) {
            int byteread = 0;
            @SuppressWarnings("resource")
            InputStream in = new FileInputStream(originalFile);
            @SuppressWarnings("resource")
            OutputStream out = new FileOutputStream(resizedFile);
            byte[] buffer = new byte[1024];
            while ((byteread = in.read(buffer)) != -1) {
                out.write(buffer, 0, byteread);
            }
            return;
        }

        int iWidth = i.getWidth(null);
        int iHeight = i.getHeight(null);

        if (iWidth > iHeight) {
            resizedImage = i.getScaledInstance(newWidth, (newWidth * iHeight) / iWidth, Image.SCALE_SMOOTH);
        } else {
            resizedImage = i.getScaledInstance((newWidth * iWidth) / iHeight, newWidth, Image.SCALE_SMOOTH);
        }

        // This code ensures that all the pixels in the image are loaded.
        Image temp = new ImageIcon(resizedImage).getImage();

        // Create the buffered image.
        BufferedImage bufferedImage = new BufferedImage(temp.getWidth(null), temp.getHeight(null),
                BufferedImage.TYPE_INT_RGB);

        // Copy image to buffered image.
        Graphics g = bufferedImage.createGraphics();

        // Clear background and paint the image.
        g.setColor(Color.WHITE);
        g.fillRect(0, 0, temp.getWidth(null), temp.getHeight(null));
        g.drawImage(temp, 0, 0, null);
        g.dispose();

        // Soften.
        float softenFactor = 0.05f;
        float[] softenArray = { 0, softenFactor, 0, softenFactor, 1 - (softenFactor * 4), softenFactor, 0, softenFactor,
                0 };
        Kernel kernel = new Kernel(3, 3, softenArray);
        ConvolveOp cOp = new ConvolveOp(kernel, ConvolveOp.EDGE_NO_OP, null);
        bufferedImage = cOp.filter(bufferedImage, null);

        // Write the jpeg to a file.
        FileOutputStream out = new FileOutputStream(resizedFile);

        // Encodes image as a JPEG data stream
        JPEGImageEncoder encoder = JPEGCodec.createJPEGEncoder(out);

        JPEGEncodeParam param = encoder.getDefaultJPEGEncodeParam(bufferedImage);

        param.setQuality(quality, true);

        encoder.setJPEGEncodeParam(param);
        encoder.encode(bufferedImage);
    }

    public static void main(String[] args) throws IOException {
        File originalImage = new File("C:\\1.bmp");
        resize(originalImage, new File("C:\\thumb_1.bmp"), 250, 1f);
    }

}

示例中关键参数是newWidth 250，代表了将图片宽或高较长的那边缩放到250，另一边等比例缩放，达到能够放入250*250的正方形中的效果；

如果图片宽高相等，则会缩放到250*250的大小。

如果图片宽高都小于250，那么它原本就能够放入正方形，则会复制自身

测试效果