注：代码在JPG和PNG格式图片下测试通过，其他图片格式请自行测试和修改

import java.awt.AlphaComposite;import java.awt.Color;import java.awt.Font;import java.awt.FontMetrics;import java.awt.Graphics2D;import java.awt.RenderingHints;import java.awt.geom.AffineTransform;import java.awt.image.BufferedImage;import java.io.File;import java.util.Iterator;import javax.imageio.IIOImage;import javax.imageio.ImageIO;import javax.imageio.ImageReader;import javax.imageio.ImageWriteParam;import javax.imageio.ImageWriter;import javax.imageio.stream.FileImageInputStream;import javax.imageio.stream.FileImageOutputStream;import org.junit.Test;public class Wjt { @Test public void t3() throws Exception {  File rootDir = new File("C:/Users/Administrator/Desktop/tem1");  String[] fa = { "1.png", "2.jpg" };  for (String s : fa) {   String f = s;   {    FileImageInputStream fileImageInputStream = new FileImageInputStream(new File(rootDir, f));    Iterator imageReaders = ImageIO.getImageReaders(fileImageInputStream); //根据图片文件获取读取器    ImageReader imageReader = imageReaders.hasNext() ? imageReaders.next() : null; //检测读取器是否存在    imageReader.setInput(fileImageInputStream); //设置读取器输入流    BufferedImage bufferedImage = imageReader.read(0); //从读取器取BufferedImage，这里只有一个，所以可以直接写0    { //开始加水印     Graphics2D graphics2d = bufferedImage.createGraphics(); //取画笔     graphics2d.setRenderingHint(RenderingHints.KEY_TEXT_ANTIALIASING, RenderingHints.VALUE_TEXT_ANTIALIAS_ON); //设置画笔文字抗锯齿     graphics2d.transform(AffineTransform.getTranslateInstance(bufferedImage.getWidth() / 2, bufferedImage.getHeight() / 2)); //将画笔移动到图片中央     graphics2d.transform(AffineTransform.getRotateInstance(Math.toRadians(45))); //将画笔旋转一定弧度     float radius = Double.valueOf(Math.sqrt(bufferedImage.getWidth() * bufferedImage.getWidth() + bufferedImage.getHeight() * bufferedImage.getHeight()) / 2).floatValue(); //计算图片外切圆半径     Font waterMarkerFont = new Font("微软雅黑", Font.PLAIN, bufferedImage.getHeight() / 25); //设置画笔要用的字体，这里可以设置字体样式和大小     graphics2d.setFont(waterMarkerFont); //设置画笔字体     graphics2d.setColor(Color.WHITE); //设置画笔颜色     graphics2d.setComposite(AlphaComposite.getInstance(AlphaComposite.SRC_ATOP, 0.3F)); //设置画笔操作透明度     FontMetrics fontMetrics = graphics2d.getFontMetrics(); //取画笔字体信息     String waterMarker = "THIS IS WATERMARK."; //水印文字     int stringWidth = fontMetrics.stringWidth(waterMarker); //计算水印文字宽度，很多人卡在了这一步     for (float i = -radius; i <= radius; i += (stringWidth + bufferedImage.getHeight() / 25)) { //循环添加水印，这里可以设置水印水平距离，单位：px      for (float j = -radius; j <= radius; j += (fontMetrics.getHeight() + bufferedImage.getHeight() / 25)) { //这里可以设置水印垂直距离，单位：px       graphics2d.drawString(waterMarker, i, j); //画水印      }     }     graphics2d.dispose(); //释放画笔资源    }    ImageWriter imageWriter = ImageIO.getImageWriter(imageReader); //根据图片文件读取器获取图片文件写入器    String[] split = f.split("\\.");    FileImageOutputStream fileOutputStream = new FileImageOutputStream(new File(rootDir, split[0] + "r." + split[1])); //生成目标图片文件路径    imageWriter.setOutput(fileOutputStream); //设置写入器输出流    IIOImage iioImage = new IIOImage(bufferedImage, null, null);    ImageWriteParam imageWriteParam = imageWriter.getDefaultWriteParam();    if (imageReader.getFormatName().equals("JPEG")) { //jpg格式图片可设置输出图片文件的质量     imageWriteParam.setCompressionMode(ImageWriteParam.MODE_EXPLICIT); //这一行固定这样写，跟下一行搭配     imageWriteParam.setCompressionQuality(0.947F); //跟上一行搭配，设置输出图片文件的质量         }    imageWriter.write(null, iioImage, imageWriteParam); //写文件    imageWriter.dispose(); //关闭写入器    imageReader.dispose(); //关闭读取器   }  } }}