1. 生成png图片
 
int width = 400;
 
int height = 300;
 
// 创建BufferedImage对象
 
BufferedImage image = new BufferedImage(width, height,     BufferedImage.TYPE_INT_RGB);
 
// 获取Graphics2D
 
Graphics2D g2d = image.createGraphics();
 
// 画图
 
g2d.setColor(new Color(255,0,0));
 
g2d.setStroke(new BasicStroke(1));
 
g2d.draw
 
//释放对象
 
g2d.dispose();
 
// 保存文件   
 
ImageIO.write(image, "png", new File("c:/test.png"));
 
int width = 400;
 
int height = 300;
 
// 创建BufferedImage对象
 
BufferedImage image = new BufferedImage(width, height,     BufferedImage.TYPE_INT_RGB);
 
// 获取Graphics2D
 
Graphics2D g2d = image.createGraphics();
 
// 画图
 
g2d.setColor(new Color(255,0,0));
 
g2d.setStroke(new BasicStroke(1));
 
g2d.draw
 
//释放对象
 
g2d.dispose();
 
// 保存文件   
 
ImageIO.write(image, "png", new File("c:/test.png"));
 
这只是绘制图形的代码，其背景是黑色的，如何才能背景透明呢？继续搜索，没有得到结果，不过搜出以下代码，它只是把自己绘制的图形设置为透明或半透明，背景并不透明，如下：
 
2. 绘制半透明图形
 
int width = 400;
 
int height = 300;
 
// 创建BufferedImage对象
 
BufferedImage image = new BufferedImage(width, height,     BufferedImage.TYPE_INT_RGB);
 
// 获取Graphics2D
 
Graphics2D g2d = image.createGraphics();
 
// 设置透明度
 
g2d.setComposite(AlphaComposite.getInstance(AlphaComposite.SRC_ATOP, 1.0f)); // 1.0f为透明度 ，值从0-1.0，依次变得不透明
 
// 画图
 
g2d.setColor(new Color(255,0,0));
 
g2d.setStroke(new BasicStroke(1));
 
g2d.draw
 
//释放对象
 
//透明度设置 结束
 
g2d.setComposite(AlphaComposite.getInstance(AlphaComposite.SRC_OVER));
 
g2d.dispose();
 
// 保存文件   
 
ImageIO.write(image, "png", new File("c:/test.png"));
 
这样绘制的图形应该说是前景透明的，背景依然是黑色，:(
 
网上没有看到有益的代码，在csdn上一位说自己实现了，但却没有说怎么实现的，没办法只能自己摸索了，耗了半个多小时，几乎查看了BufferedImage 和Graphics2D 所有方法和属性，终于找到了解决方案，只不过是增加两行代码而已，如下：
 
int width = 400;
 
int height = 300;
 
// 创建BufferedImage对象
 
BufferedImage image = new BufferedImage(width, height,     BufferedImage.TYPE_INT_RGB);
 
// 获取Graphics2D
 
Graphics2D g2d = image.createGraphics();
 
// ---------- 增加下面的代码使得背景透明 -----------------
 
image = g2d.getDeviceConfiguration().createCompatibleImage(width, height, Transparency.TRANSLUCENT);
 
g2d.dispose();
 
g2d = image.createGraphics();
 
// ---------- 背景透明代码结束 -----------------
 
// 画图
 
g2d.setColor(new Color(255,0,0));
 
g2d.setStroke(new BasicStroke(1));
 
g2d.draw
 
//释放对象
 
g2d.dispose();
 
// 保存文件   
 
ImageIO.write(image, "png", new File("c:/test.png"));
 
================================================
 
白背景变透明
 
package com.goldgrid.socket.client;
 
import java.awt.Graphics2D;
import java.awt.Image;
import java.awt.image.BufferedImage;
import java.io.ByteArrayOutputStream;
import java.io.File;
 
import javax.imageio.ImageIO;
import javax.swing.ImageIcon;
 
import org.apache.log4j.Logger;
 
public class Alpha {
 
    protected static Logger logger = Logger.getLogger(ClientSocket.class);
 
public byte[] transferAlpha(Image image) {
 
   ByteArrayOutputStream byteArrayOutputStream = new ByteArrayOutputStream();
 
   try {
    ImageIcon imageIcon = new ImageIcon(image);
    BufferedImage bufferedImage = new BufferedImage(imageIcon
      .getIconWidth(), imageIcon.getIconHeight(),
      BufferedImage.TYPE_4BYTE_ABGR);
    Graphics2D g2D = (Graphics2D) bufferedImage.getGraphics();
    g2D.drawImage(imageIcon.getImage(), 0, 0, imageIcon
      .getImageObserver());
    int alpha = 0;
    for (int j1 = bufferedImage.getMinY(); j1 < bufferedImage
      .getHeight(); j1++) {
     for (int j2 = bufferedImage.getMinX(); j2 < bufferedImage
       .getWidth(); j2++) {
      int rgb = bufferedImage.getRGB(j2, j1);
    
      int R =(rgb & 0xff0000 ) >> 16 ;
      int G= (rgb & 0xff00 ) >> 8 ;
      int B= (rgb & 0xff );
      if(((255-R)<30) && ((255-G)<30) && ((255-B)<30)){
       rgb = ((alpha + 1) << 24) | (rgb & 0x00ffffff);
      }
 
      bufferedImage.setRGB(j2, j1, rgb);
 
     }
    }
 
    g2D.drawImage(bufferedImage, 0, 0, imageIcon.getImageObserver());
    //ImageIO.write(bufferedImage, "png", new File("d:/test.png.png"));
 
    ImageIO.write(bufferedImage, "png", byteArrayOutputStream);
   } catch (Exception e) {
    logger.error(e.toString());
   }finally{
   
   }
 
   return byteArrayOutputStream.toByteArray();
 
}
}