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Images from Drawings
The previous example creates an image from a GIF file. You can also create images from drawing and send the image to the browser. The following example creates a servlet that displays a clock to show the current time, as shown in Figure 2.
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package servletdemo;
import javax.servlet.*;
import javax.servlet.http.*;
import java.io.*;
import java.util.*;
import java.text.*;
import java.awt.*;
import java.awt.image.BufferedImage;
import javax.swing.*;
import Acme.JPM.Encoders.GifEncoder;
public class ImageContentWithDrawing extends HttpServlet {
/**Initialize global variables*/
private final static int width = 300;
private final static int height = 300;
/**Process the HTTP Get request*/
public void doGet(HttpServletRequest request, HttpServletResponse
response) throws ServletException, IOException {
response.setContentType("image/gif");
OutputStream out = response.getOutputStream();
// Create image
Image image = new BufferedImage(width, height,
BufferedImage.TYPE_INT_ARGB);
// Get Graphics context of the image
Graphics g = image.getGraphics();
drawClock(g); // Draw a clock on graphics
// Encode the image and send to the output stream
new GifEncoder(image, out, true).encode();
out.close(); // Close stream
}
private void drawClock(Graphics g) {
// Initialize clock parameters
int clockRadius =
(int)(Math.min(width, height)*0.7*0.5);
int xCenter = (width)/2;
int yCenter = (height)/2;
// Draw circle
g.setColor(Color.black);
g.drawOval(xCenter - clockRadius,yCenter - clockRadius,
2*clockRadius, 2*clockRadius);
g.drawString("12", xCenter-5, yCenter-clockRadius+12);
g.drawString("9", xCenter-clockRadius+3, yCenter+5);
g.drawString("3", xCenter+clockRadius-10, yCenter+3);
g.drawString("6", xCenter-3, yCenter+clockRadius-3);
// Get current time using GregorianCalendar
TimeZone timeZone = TimeZone.getDefault();
GregorianCalendar cal = new GregorianCalendar(timeZone);
// Draw second hand
int second = (int)cal.get(GregorianCalendar.SECOND);
int sLength = (int)(clockRadius*0.9);
int xSecond =
(int)(xCenter + sLength*Math.sin(second*(2*Math.PI/60)));
int ySecond =
(int)(yCenter - sLength*Math.cos(second*(2*Math.PI/60)));
g.setColor(Color.red);
g.drawLine(xCenter, yCenter, xSecond, ySecond);
// Draw minute hand
int minute = (int)cal.get(GregorianCalendar.MINUTE);
int mLength = (int)(clockRadius*0.75);
int xMinute =
(int)(xCenter + mLength*Math.sin(minute*(2*Math.PI/60)));
int yMinute =
(int)(yCenter - mLength*Math.cos(minute*(2*Math.PI/60)));
g.setColor(Color.blue);
g.drawLine(xCenter, yCenter, xMinute, yMinute);
// Draw hour hand
int hour = (int)cal.get(GregorianCalendar.HOUR_OF_DAY);
int hLength = (int)(clockRadius*0.6);
int xHour = (int)(xCenter +
hLength*Math.sin((hour+minute/60.0)*(2*Math.PI/12)));
int yHour = (int)(yCenter -
hLength*Math.cos((hour+minute/60.0)*(2*Math.PI/12)));
g.setColor(Color.green);
g.drawLine(xCenter, yCenter, xHour, yHour);
// Set display format in specified style, locale, and timezone
DateFormat formatter = DateFormat.getDateTimeInstance
(DateFormat.MEDIUM, DateFormat.LONG);
// Display current date
g.setColor(Color.red);
String today = formatter.format(cal.getTime());
FontMetrics fm = g.getFontMetrics();
g.drawString(today, (width -
fm.stringWidth(today))/2, yCenter+clockRadius+30);
}
}
Figure 2 The servlet returns a clock that displays the current time.
The program creates an image with the specified width, height, and image type using the BufferedImage class, as follows:
Image image = new BufferedImage(width, height, BufferedImage.TYPE_INT_ARGB);
To draw things on the image, you need to get its graphics context using the getGraphics method, like this:
Graphics g = image.getGraphics();
You can use various drawing methods in the Graphics class to draw a simple shape, or you can use Java 2D to draw more sophisticated graphics. This example uses simple drawing methods to draw a clock to display current time.
The servlets in the previous two examples return images. Often images are mixed with text. In this case, you have to set the content type to image/gif before sending images, and you must set the content type to text/html before sending the text. However, the content type cannot be changed in one request. To circumvent this restriction, you may embed a get request for displaying the image in the HTML content. When the HTML content is displayed, a separate get request for retrieving the image is then sent to the server. So, the text and the image are obtained through two separate get requests. For more details, please refer to my book Rapid Java Application Development Using JBuilder 4/5/6 (Prentice Hall PTR, 2001).