在这里介绍两种方法，分别是添加图片水印和文字水印，水印图片的格式可以是GIF,PNG的透明文件，我们一般采用的是PNG格式的图片，因为它的质量和GIF相比要高一些，源码如下：

import java.awt.Color;
import java.awt.Font;
import java.awt.Graphics;
import java.awt.Image;
import java.awt.image.BufferedImage;
import java.io.File;
import java.io.FileOutputStream;

import javax.imageio.ImageIO;

import com.sun.image.codec.jpeg.JPEGCodec;
import com.sun.image.codec.jpeg.JPEGImageEncoder;

public final class TestCenter {
    public TestCenter() {

    }

    /**//*
     * public final static String getPRessImgPath() { return applicationContext
     * .getRealPath("/template/data/util/shuiyin.gif"); }
     */

    /** *//**
     * 把图片印刷到图片上
     *
     * @param pressImg --
     *            水印文件
     * @param targetImg --
     *            目标文件
     * @param x
     *            --x坐标
     * @param y
     *            --y坐标
     */
    public final static void pressImage(String pressImg, String targetImg,
            int x, int y) {
        try {
            //目标文件
            File _file = new File(targetImg);
            System.out.println("file: "+_file.toString()+">>>>"+targetImg);
            Image src = ImageIO.read(_file);
            int wideth = src.getWidth(null);
            int height = src.getHeight(null);
            BufferedImage image = new BufferedImage(wideth, height,
                    BufferedImage.TYPE_INT_RGB);
            Graphics g = image.createGraphics();
            g.drawImage(src, 0, 0, wideth, height, null);

            //水印文件
            File _filebiao = new File(pressImg);
            Image src_biao = ImageIO.read(_filebiao);
            int wideth_biao = src_biao.getWidth(null);
            int height_biao = src_biao.getHeight(null);
            g.drawImage(src_biao, (wideth - wideth_biao) / 2,
                    (height - height_biao) / 2, wideth_biao, height_biao, null);
            //水印文件结束
            g.dispose();
            FileOutputStream out = new FileOutputStream(targetImg);
            JPEGImageEncoder encoder = JPEGCodec.createJPEGEncoder(out);
            encoder.encode(image);
            out.close();
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

    /** *//**
     * 打印文字水印图片
     *
     * @param pressText
     *            --文字
     * @param targetImg --
     *            目标图片
     * @param fontName --
     *            字体名
     * @param fontStyle --
     *            字体样式
     * @param color --
     *            字体颜色
     * @param fontSize --
     *            字体大小
     * @param x --
     *            偏移量
     * @param y
     */

    public static void pressText(String pressText, String targetImg,
            String fontName, int fontStyle, int color, int fontSize, int x,
            int y) {
        try {
            File _file = new File(targetImg);
            Image src = ImageIO.read(_file);
            int wideth = src.getWidth(null);
            int height = src.getHeight(null);
            BufferedImage image = new BufferedImage(wideth, height,
                    BufferedImage.TYPE_INT_RGB);
            Graphics g = image.createGraphics();
            g.drawImage(src, 0, 0, wideth, height, null);
            // String s="www.jzxue.com";
            g.setColor(Color.RED);
            g.setFont(new Font(fontName, fontStyle, fontSize));

            g.drawString(pressText, wideth - fontSize - x, height - fontSize
                    / 2 - y);
            g.dispose();
            FileOutputStream out = new FileOutputStream(targetImg);
            JPEGImageEncoder encoder = JPEGCodec.createJPEGEncoder(out);
            encoder.encode(image);
            out.close();
        } catch (Exception e) {
            System.out.println(e);
        }
    }

    public static void main(String[] args) {
        pressImage("D:\\123.gif","D:\\aa.gif", 0, 0);
        //pressText("刘若英,我很好,后来...","D:\\aa.gif", "宋体", 16, 10, 16, 200, 300);
    }
}

第二种是添加文字水印：

import java.awt.*;
import java.awt.image.*;
import java.io.*;
import javax.swing.*;
import com.sun.image.codec.jpeg.*;

public class TestCenter {
    /** *//**
     * 给图片添加水印
     *
     * @param filePath
     *            需要添加水印的图片的路径
     * @param markContent
     *            水印的文字
     * @param markContentColor
     *            水印文字的颜色
     * @param qualNum
     *            图片质量
     * @return
     */
    public boolean createMark(String filePath, String markContent,
            Color markContentColor, float qualNum) {
        ImageIcon imgIcon = new ImageIcon(filePath);
        Image theImg = imgIcon.getImage();
        int width = theImg.getWidth(null);
        int height = theImg.getHeight(null);
        BufferedImage bimage = new BufferedImage(width, height,
                BufferedImage.TYPE_INT_RGB);
        Graphics2D g = bimage.createGraphics();
        g.setColor(markContentColor);
        g.setBackground(Color.white);
        g.drawImage(theImg, 0, 0, null);
        g.drawString(markContent, width / 5, height / 5); // 添加水印的文字和设置水印文字出现的内容
        g.dispose();
        try {
            FileOutputStream out = new FileOutputStream(filePath);
            JPEGImageEncoder encoder = JPEGCodec.createJPEGEncoder(out);
            JPEGEncodeParam param = encoder.getDefaultJPEGEncodeParam(bimage);
            param.setQuality(qualNum, true);
            encoder.encode(bimage, param);
            out.close();
        } catch (Exception e) {
            return false;
        }
        return true;
    }
   
    public static void main(String[] args)
{
     TestCenter tc = new TestCenter();
     tc.createMark("D:\\123.gif", "水印的文字", Color.red, 1);
}

}