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此代码生成一个4位的随机数，但是快速刷时报错org.apache.catalina.core.StandardWrapperValve invoke
求解
详细的代码和错误提示如下：package com.xjdnw.utility;import java.awt.Color;
import java.awt.Font;
import java.awt.Graphics;
import java.awt.image.BufferedImage;
import java.io.ByteArrayInputStream;
import java.io.ByteArrayOutputStream;
import java.io.IOException;
import java.util.Random;
import javax.imageio.ImageIO;
import javax.imageio.stream.ImageOutputStream;
import org.springframework.stereotype.Component;import com.opensymphony.xwork2.ActionContext;
import com.opensymphony.xwork2.ActionSupport;@Component("Validatecode")
public class Validatecode extends ActionSupport {    private static final long serialVersionUID = 7190523148768780483L;
    private ByteArrayInputStream image;// 图像
    private String str;// 验证码    private Validatecode() {
        init();// 初始化属性
    }    /*
     * 取得RandomNumUtil实例
     */
    public static Validatecode Instance() {
        return new Validatecode();
    }    /*
     * 取得验证码图片
     */
    public ByteArrayInputStream getImage() {
        return this.image;
    }    /*
     * 取得图片的验证码
     */
    public String getString() {
        return this.str;
    }    private void init() {
        // 在内存中创建图象
        int width = 60, height = 20;
        BufferedImage image = new BufferedImage(width, height, BufferedImage.TYPE_INT_RGB);
        // 获取图形上下文
        Graphics g = image.getGraphics();
        // 生成随机类
        Random random = new Random();
        // 设定背景色
        g.setColor(getRandColor(200, 250));
        g.fillRect(0, 0, width, height);
        // 设定字体
        g.setFont(new Font("Times New Roman", Font.PLAIN, 18));
        // 随机产生155条干扰线，使图象中的认证码不易被其它程序探测到
        g.setColor(getRandColor(160, 200));
        for (int i = 0; i < 155; i++) {
            int x = random.nextInt(width);
            int y = random.nextInt(height);
            int xl = random.nextInt(12);
            int yl = random.nextInt(12);
            g.drawLine(x, y, x + xl, y + yl);
        }
        // 取随机产生的认证码(6位数字)
        String sRand = "";
        for (int i = 0; i < 4; i++) {
            String rand = String.valueOf(random.nextInt(10));
            sRand += rand;
            // 将认证码显示到图象中
            g.setColor(new Color(20 + random.nextInt(110), 20 + random.nextInt(110), 20 + random.nextInt(110)));
            // 调用函数出来的颜色相同，可能是因为种子太接近，所以只能直接生成
            g.drawString(rand, 13 * i + 6, 16);
        }
        // 赋值验证码
        this.str = sRand;        // 图象生效
        g.dispose();
        ByteArrayInputStream input = null;
        ByteArrayOutputStream output = new ByteArrayOutputStream();
        
        try {
            ImageOutputStream imageOut = ImageIO.createImageOutputStream(output);
            ImageIO.write(image, "JPEG", imageOut);
            imageOut.close();
            input = new ByteArrayInputStream(output.toByteArray());
            this.image = input;/* 赋值图像 */
            
        } catch (Exception e) {
            System.out.println("Validatecode Error：" + e.toString());
        }finally{
            if(input!=null){
                try {
                    input.close();
                } catch (IOException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                }
            }
            if(output!=null){
                try {
                    output.close();
                } catch (IOException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                }
            }
                
        }        
    }    /*
     * 给定范围获得随机颜色
     */
    private 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);
    }    private ByteArrayInputStream inputStream;    public String execute() throws Exception {
        Validatecode rdnu = Instance();
        setInputStream(rdnu.getImage());
        ActionContext.getContext().getSession().put("Validatecode", getString());
        return SUCCESS;
    }    public void setInputStream(ByteArrayInputStream inputStream) {
        this.inputStream = inputStream;
    }    public ByteArrayInputStream getInputStream() {
        return inputStream;
    }}2010-7-24 16:54:35 org.apache.catalina.core.StandardWrapperValve invoke
严重: Servlet.service() for servlet default threw exception
java.lang.IllegalStateException
at org.apache.catalina.connector.ResponseFacade.sendError(ResponseFacade.java:407)
at org.apache.struts2.dispatcher.Dispatcher.sendError(Dispatcher.java:752)
at org.apache.struts2.dispatcher.Dispatcher.serviceAction(Dispatcher.java:505)
at org.apache.struts2.dispatcher.ng.ExecuteOperations.executeAction(ExecuteOperations.java:77)
at org.apache.struts2.dispatcher.ng.filter.StrutsPrepareAndExecuteFilter.doFilter(StrutsPrepareAndExecuteFilter.java:91)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:235)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
at org.springframework.orm.hibernate3.support.OpenSessionInViewFilter.doFilterInternal(OpenSessionInViewFilter.java:198)
at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:76)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:235)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:230)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:175)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:128)
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:104)
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:109)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:261)
at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:844)
at org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.process(Http11Protocol.java:581)
at org.apache.tomcat.util.net.JIoEndpoint$Worker.run(JIoEndpoint.java:447)
at java.lang.Thread.run(Thread.java:619)