Captcha with java

A simple CAPTCHA generating java class


import java.awt.Color;
import java.awt.Font;
import java.awt.FontMetrics;
import java.awt.Graphics2D;
import java.awt.geom.AffineTransform;
import java.awt.image.BufferedImage;
import java.io.IOException;
import java.util.Iterator;

import javax.imageio.IIOImage;
import javax.imageio.ImageIO;
import javax.imageio.ImageWriteParam;
import javax.imageio.ImageWriter;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;


public class Captcha {
public static Color TEXT_COLOR = Color.white;
public static Color CIRCLE_COLOR = new Color(160,160,160);
public static int NUMBER_OF_CHARACTERS = 6;
public static Color BACKGROUND_COLOR = Color.red;
public static String SESSION_NAME = "captchaText";

public void buildCapcha(HttpServletRequest request, HttpServletResponse response){
response.setContentType("image/jpg");

try {
Color textColor = TEXT_COLOR;
Color circleColor = CIRCLE_COLOR;
Font textFont = new Font("Arial", Font.PLAIN, 24);
int charsToPrint = NUMBER_OF_CHARACTERS;
int width = request.getParameter("width") != null ? Integer.parseInt(request.getParameter("width")) : 150;
int height = request.getParameter("height") != null ? Integer.parseInt(request.getParameter("height")) : 80;
int circlesToDraw = 4;
float horizMargin = 20.0f;
float imageQuality = 0.95f; // max is 1.0 (this is for jpeg)
double rotationRange = 0.7; // this is radians
BufferedImage bufferedImage = new BufferedImage(width, height, BufferedImage.TYPE_INT_RGB);

Graphics2D g = (Graphics2D) bufferedImage.getGraphics();

//Draw an oval
g.setColor(BACKGROUND_COLOR);
g.fillRect(0, 0, width, height);

// lets make some noisey circles
g.setColor(circleColor);
for ( int i = 0; i < circlesToDraw; i++ ) {
int circleRadius = (int) (Math.random() * height / 2.0);
int circleX = (int) (Math.random() * width - circleRadius);
int circleY = (int) (Math.random() * height - circleRadius);
g.drawOval(circleX, circleY, circleRadius * 2, circleRadius * 2);
}

g.setColor(textColor);
g.setFont(textFont);

FontMetrics fontMetrics = g.getFontMetrics();
int maxAdvance = fontMetrics.getMaxAdvance();
int fontHeight = fontMetrics.getHeight();

// i removed 1 and l and i because there are confusing to users...
// Z, z, and N also get confusing when rotated
// 0, O, and o are also confusing...
// lowercase G looks a lot like a 9 so i killed it
// this should ideally be done for every language...
// i like controlling the characters though because it helps prevent confusion
String elegibleChars = "ABCDEFGHJKLMPQRSTUVWXYabcdefhjkmnpqrstuvwxy23456789";
char[] chars = elegibleChars.toCharArray();

float spaceForLetters = -horizMargin * 2 + width;
float spacePerChar = spaceForLetters / (charsToPrint - 1.0f);

StringBuffer finalString = new StringBuffer();

for ( int i = 0; i < charsToPrint; i++ ) {
double randomValue = Math.random();
int randomIndex = (int) Math.round(randomValue * (chars.length - 1));
char characterToShow = chars[randomIndex];
System.out.println(characterToShow);
finalString.append(characterToShow);

// this is a separate canvas used for the character so that
// we can rotate it independently

int charWidth = fontMetrics.charWidth(characterToShow);
int charDim = Math.max(maxAdvance, fontHeight);
int halfCharDim = (int) (charDim / 2);

BufferedImage charImage = new BufferedImage(charDim, charDim, BufferedImage.TYPE_INT_ARGB);
Graphics2D charGraphics = charImage.createGraphics();
charGraphics.translate(halfCharDim, halfCharDim);
double angle = (Math.random() - 0.5) * rotationRange;
charGraphics.transform(AffineTransform.getRotateInstance(angle));
charGraphics.translate(-halfCharDim,-halfCharDim);
charGraphics.setColor(textColor);
charGraphics.setFont(textFont);

int charX = (int) (0.5 * charDim - 0.5 * charWidth);
charGraphics.drawString("" + characterToShow, charX,
(int) ((charDim - fontMetrics.getAscent())
/ 2 + fontMetrics.getAscent()));

float x = horizMargin + spacePerChar * (i) - charDim / 2.0f;
int y = (int) ((height - charDim) / 2);
//System.out.println("x=" + x + " height=" + height + " charDim=" + charDim + " y=" + y + " advance=" + maxAdvance + " fontHeight=" + fontHeight + " ascent=" + fontMetrics.getAscent());
g.drawImage(charImage, (int) x, y, charDim, charDim, null, null);

charGraphics.dispose();
}

//Write the image as a jpg
Iterator iter = ImageIO.getImageWritersByFormatName("JPG");
if( iter.hasNext() ) {
ImageWriter writer = (ImageWriter)iter.next();
ImageWriteParam iwp = writer.getDefaultWriteParam();
iwp.setCompressionMode(ImageWriteParam.MODE_EXPLICIT);
iwp.setCompressionQuality(imageQuality);
writer.setOutput(ImageIO.createImageOutputStream(response.getOutputStream()));
IIOImage imageIO = new IIOImage(bufferedImage, null, null);
writer.write(null, imageIO, iwp);
} else {
throw new RuntimeException("no encoder found for jsp");
}

// let's stick the final string in the session
request.getSession().setAttribute(SESSION_NAME, finalString.toString());

g.dispose();
} catch (IOException ioe) {
throw new RuntimeException("Unable to build image" , ioe);
}
}

}