jsp做了一个简单的图形验证码，产生四个随机字符
<%@page contentType="image/jpeg" import="java.awt.*,java.awt.image.*,java.util.*,javax.imageio.*"%><%! Color getRandColor(int fc, int bc) { Random random = new Random(); if (fc > 255) fc = 255; if (bc > 255) bc = 255; int r = fc + random.nextInt(bc - fc); int g = fc + random.nextInt(bc - fc); int b = fc + random.nextInt(bc - fc); return new Color(r, g, b);}%><%response.setHeader("Pragma", "No-cache");response.setHeader("Cache-Control", "no-cache");response.setDateHeader("Expires", 0);int width = 60, height = 20;BufferedImage image = new BufferedImage(width, height, BufferedImage.TYPE_INT_RGB);Graphics g = image.getGraphics();Random random = new Random();g.setColor(getRandColor(200, 250));g.fillRect(0, 0, width, height);g.setFont(new Font("Verdana Arial Helvetica sans-serif", Font.PLAIN, 20));g.setColor(getRandColor(160, 255));for (int i = 0; i < 200; i++) { int x = random.nextInt(width); int y = random.nextInt(height); int xl = random.nextInt(10); int yl = random.nextInt(10); g.drawLine(x, y, x + xl, y + yl);}char c[] = new char[62];for (int i = 97, j = 0; i < 123; i++, j++) { c[j] = (char) i;}for (int o = 65, p = 26; o < 91; o++, p++) { c[p] = (char) o;}for (int m = 48, n = 52; m < 58; m++, n++) { c[n] = (char) m;}String sRand = "";for (int i = 0; i < 4; i++) { int x = random.nextInt(62); String rand = String.valueOf(c[x]); sRand += rand; g.setColor(new Color(20 + random.nextInt(110), 20 + random.nextInt(110), 20 + random.nextInt(110))); g.drawString(rand, 13 * i + 6, 16);}g.dispose();ImageIO.setUseCache(true);ImageIO.write(image, "JPEG", response.getOutputStream());session.setAttribute("rand", sRand);%> 异常日志如下： org.apache.catalina.core.StandardWrapperValve invoke 严重: Servlet.service() for servlet jsp threw exception java.lang.IllegalStateException: getOutputStream() has already been called for this response at org.apache.catalina.connector.Response.getWriter(Response.java:596) at org.apache.catalina.connector.ResponseFacade.getWriter(ResponseFacade.java:186) at org.apache.jasper.runtime.JspWriterImpl.initOut(JspWriterImpl.java:124) at org.apache.jasper.runtime.JspWriterImpl.flushBuffer(JspWriterImpl.java:117) at org.apache.jasper.runtime.PageContextImpl.release(PageContextImpl.java:191) at org.apache.jasper.runtime.JspFactoryImpl.internalReleasePageContext(JspFactoryImpl.java:115) at org.apache.jasper.runtime.JspFactoryImpl.releasePageContext(JspFactoryImpl.java:75) 意思就是在输出前已经有out流了，查了整个代码，也没输出啊。灵异事件？狗了一下，解决了这个问题，修改jsp源码中<%%>和<%%>之间的空格和换行。原来代码：<%@page contentType="image/jpeg"%> <% // code 11 %> <% // code 22 %> 修改后：<%@page contentType="image/jpeg"%><% // code 11 %><% // code 22 %>原因应该是page声明那一行会默认一个输出空行，而Content-type是image就会出错啦！ http://blog.csdn.net/smallfish_xy/archive/2009/09/10/4538884.aspx汉字验证：<%@page contentType="image/jpeg" pageEncoding="UTF-8" import="java.awt.*,java.awt.image.*,java.util.*,javax.imageio.*"%><%! //生成随机颜色 Color getRandColor(Random random, int fc, int bc) { if (fc > 255) fc = 255; if (bc > 255) bc = 255; int r = fc + random.nextInt(bc - fc); int g = fc + random.nextInt(bc - fc); int b = fc + random.nextInt(bc - fc); return new Color(r, g, b); }%><% //设置页面不缓存 response.setHeader("Pragma", "No-cache"); response.setHeader("Cache-Control", "no-cache"); response.setDateHeader("Expires", 0); // 设置图片的长宽 int width = 176, height = 30; //设置备选汉字，剔除一些不雅的汉字 String base = "\u7684\u4e00\u4e86\u662f\u6211\u4e0d\u5728\u4eba\u4eec\u6709\u6765\u4ed6\u8fd9\u4e0a\u7740\u4e2a\u5730\u5230\u5927\u91cc\u8bf4\u5c31\u53bb\u5b50\u5f97\u4e5f\u548c\u90a3\u8981\u4e0b\u770b\u5929\u65f6\u8fc7\u51fa\u5c0f\u4e48\u8d77\u4f60\u90fd\u628a\u597d\u8fd8\u591a\u6ca1\u4e3a\u53c8\u53ef\u5bb6\u5b66\u53ea\u4ee5\u4e3b\u4f1a\u6837\u5e74\u60f3\u751f\u540c\u8001\u4e2d\u5341\u4ece\u81ea\u9762\u524d\u5934\u9053\u5b83\u540e\u7136\u8d70\u5f88\u50cf\u89c1\u4e24\u7528\u5979\u56fd\u52a8\u8fdb\u6210\u56de\u4ec0\u8fb9\u4f5c\u5bf9\u5f00\u800c\u5df1\u4e9b\u73b0\u5c71\u6c11\u5019\u7ecf\u53d1\u5de5\u5411\u4e8b\u547d\u7ed9\u957f\u6c34\u51e0\u4e49\u4e09\u58f0\u4e8e\u9ad8\u624b\u77e5\u7406\u773c\u5fd7\u70b9\u5fc3\u6218\u4e8c\u95ee\u4f46\u8eab\u65b9\u5b9e\u5403\u505a\u53eb\u5f53\u4f4f\u542c\u9769\u6253\u5462\u771f\u5168\u624d\u56db\u5df2\u6240\u654c\u4e4b\u6700\u5149\u4ea7\u60c5\u8def\u5206\u603b\u6761\u767d\u8bdd\u4e1c\u5e2d\u6b21\u4eb2\u5982\u88ab\u82b1\u53e3\u653e\u513f\u5e38\u6c14\u4e94\u7b2c\u4f7f\u5199\u519b\u5427\u6587\u8fd0\u518d\u679c\u600e\u5b9a\u8bb8\u5feb\u660e\u884c\u56e0\u522b\u98de\u5916\u6811\u7269\u6d3b\u90e8\u95e8\u65e0\u5f80\u8239\u671b\u65b0\u5e26\u961f\u5148\u529b\u5b8c\u5374\u7ad9\u4ee3\u5458\u673a\u66f4\u4e5d\u60a8\u6bcf\u98ce\u7ea7\u8ddf\u7b11\u554a\u5b69\u4e07\u5c11\u76f4\u610f\u591c\u6bd4\u9636\u8fde\u8f66\u91cd\u4fbf\u6597\u9a6c\u54ea\u5316\u592a\u6307\u53d8\u793e\u4f3c\u58eb\u8005\u5e72\u77f3\u6ee1\u65e5\u51b3\u767e\u539f\u62ff\u7fa4\u7a76\u5404\u516d\u672c\u601d\u89e3\u7acb\u6cb3\u6751\u516b\u96be\u65e9\u8bba\u5417\u6839\u5171\u8ba9\u76f8\u7814\u4eca\u5176\u4e66\u5750\u63a5\u5e94\u5173\u4fe1\u89c9\u6b65\u53cd\u5904\u8bb0\u5c06\u5343\u627e\u4e89\u9886\u6216\u5e08\u7ed3\u5757\u8dd1\u8c01\u8349\u8d8a\u5b57\u52a0\u811a\u7d27\u7231\u7b49\u4e60\u9635\u6015\u6708\u9752\u534a\u706b\u6cd5\u9898\u5efa\u8d76\u4f4d\u5531\u6d77\u4e03\u5973\u4efb\u4ef6\u611f\u51c6\u5f20\u56e2\u5c4b\u79bb\u8272\u8138\u7247\u79d1\u5012\u775b\u5229\u4e16\u521a\u4e14\u7531\u9001\u5207\u661f\u5bfc\u665a\u8868\u591f\u6574\u8ba4\u54cd\u96ea\u6d41\u672a\u573a\u8be5\u5e76\u5e95\u6df1\u523b\u5e73\u4f1f\u5fd9\u63d0\u786e\u8fd1\u4eae\u8f7b\u8bb2\u519c\u53e4\u9ed1\u544a\u754c\u62c9\u540d\u5440\u571f\u6e05\u9633\u7167\u529e\u53f2\u6539\u5386\u8f6c\u753b\u9020\u5634\u6b64\u6cbb\u5317\u5fc5\u670d\u96e8\u7a7f\u5185\u8bc6\u9a8c\u4f20\u4e1a\u83dc\u722c\u7761\u5174\u5f62\u91cf\u54b1\u89c2\u82e6\u4f53\u4f17\u901a\u51b2\u5408\u7834\u53cb\u5ea6\u672f\u996d\u516c\u65c1\u623f\u6781\u5357\u67aa\u8bfb\u6c99\u5c81\u7ebf\u91ce\u575a\u7a7a\u6536\u7b97\u81f3\u653f\u57ce\u52b3\u843d\u94b1\u7279\u56f4\u5f1f\u80dc\u6559\u70ed\u5c55\u5305\u6b4c\u7c7b\u6e10\u5f3a\u6570\u4e61\u547c\u6027\u97f3\u7b54\u54e5\u9645\u65e7\u795e\u5ea7\u7ae0\u5e2e\u5566\u53d7\u7cfb\u4ee4\u8df3\u975e\u4f55\u725b\u53d6\u5165\u5cb8\u6562\u6389\u5ffd\u79cd\u88c5\u9876\u6025\u6797\u505c\u606f\u53e5\u533a\u8863\u822c\u62a5\u53f6\u538b\u6162\u53d4\u80cc\u7ec6"; //备选汉字的长度 int length = base.length(); //创建内存图像 BufferedImage image = new BufferedImage(width, height, BufferedImage.TYPE_INT_RGB); // 获取图形上下文 Graphics g = image.getGraphics(); //创建随机类的实例 Random random = new Random(); // 设定图像背景色(因为是做背景，所以偏淡) g.setColor(getRandColor(random, 200, 250)); g.fillRect(0, 0, width, height); //备选字体 String[] fontTypes = { "\u5b8b\u4f53", "\u65b0\u5b8b\u4f53", "\u9ed1\u4f53", "\u6977\u4f53", "\u96b6\u4e66"}; int fontTypesLength = fontTypes.length; //在图片背景上增加噪点 g.setColor(getRandColor(random, 160, 200)); g.setFont(new Font("Times New Roman", Font.PLAIN, 14)); for (int i = 0; i < 6; i++) { g.drawString("*********************************************", 0, 5 * (i + 2)); } //取随机产生的认证码(6个汉字) //保存生成的汉字字符串 String sRand = ""; for (int i = 0; i < 6; i++) { int start = random.nextInt(length); String rand = base.substring(start, start + 1); sRand += rand; //设置字体的颜色 g.setColor(getRandColor(random, 10, 150)); //设置字体 g.setFont(new Font(fontTypes[random.nextInt(fontTypesLength)], Font.BOLD, 18 + random.nextInt(6))); //将此汉字画到图片上 g.drawString(rand, 24 * i + 10 + random.nextInt(8), 24); } //将认证码存入session session.setAttribute("rand", sRand); g.dispose(); //输出图象到页面 ImageIO.write(image, "JPEG", response.getOutputStream());%> ?