在Java中,我們可以使用javax.imageio。ImageIO類來讀取和寫入一個圖像。

1. 讀一個圖像

從文件讀取圖像。

  BufferedImage image = ImageIO.read(new File("c:\\test\\image.png"));

讀一個圖像從一個URL。

  BufferedImage image = ImageIO.read(new URL("https://example.com/image.png"));

2.寫入或保存圖像

以不同的圖像格式寫入或保存圖像。

  ImageIO.write(bufferedImage , "jpg", new File("c:\\test\\image.jpg"));
  ImageIO.write(bufferedImage , "gif", new File("c:\\test\\image.gif"));
  ImageIO.write(bufferedImage , "png", new File("c:\\test\\image.png"));

下面的代碼片段列出了所有支持的格式。

  String writerNames[] = ImageIO.getWriterFormatNames();
  Arrays.stream(writerNames).forEach(System.out::println);

輸出

PG
jpg
tiff
bmp
BMP
gif
GIF
WBMP
png
PNG
JPEG
tif
TIF
TIFF
wbmp
jpeg

3. 讀取、調整大小和保存圖像。

一個完整的 Java 示例，ImageIO用于從 URL(Google 徽標)讀取圖像、調整大小并將其300x150保存到文件中。

package com.mkyong.io.image;
import javax.imageio.ImageIO;
import java.awt.*;
import java.awt.image.BufferedImage;
import java.io.File;
import java.io.IOException;
import java.net.MalformedURLException;
import java.net.URL;
public class ReadWriteImage {
    // Google logo url
    private static final String GOOGLE_LOGO =
            "https://www.google.com/images/branding/googlelogo/2x/googlelogo_color_272x92dp.png";
    public static void main(String[] args) {
        try {
            URL url = new URL(GOOGLE_LOGO);
            // read an image from url
            BufferedImage image = ImageIO.read(url);
            // resize image to 300x150
            Image scaledImage = image.getScaledInstance(300, 150, Image.SCALE_DEFAULT);
            // save the resize image aka thumbnail
            ImageIO.write(
                    convertToBufferedImage(scaledImage),
                    "png",
                    new File("C:\\test\\google.png"));
        } catch (MalformedURLException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }
        System.out.println("Done");
    }
    // convert Image to BufferedImage
    public static BufferedImage convertToBufferedImage(Image img) {
        if (img instanceof BufferedImage) {
            return (BufferedImage) img;
        }
        // Create a buffered image with transparency
        BufferedImage bi = new BufferedImage(
                img.getWidth(null), img.getHeight(null),
                BufferedImage.TYPE_INT_ARGB);
        Graphics2D graphics2D = bi.createGraphics();
        graphics2D.drawImage(img, 0, 0, null);
        graphics2D.dispose();
        return bi;
    }
}

下載的圖像被調整為 300×150。