Put a NASA Photo into the Background of a Text Area #7
Chapter 1, Basic JComponents
Put a NASA Photo into the Background
of a Text Area Hack #7
This hack will repurpose an existing web page, one of NASA’s photo sites, by
pulling their “Astronomy Picture of the Day” into the background of a text
You’ve already learned how to draw a watermark image in the background
of a text area
[Hack #6] using a ScrollPaneWatermark. This hack will pull a
photo down from the Web and reuse that class to put the photo in the back-
ground. The photo itself comes from NASA’s “Astronomy Picture of the
Day” page: http://antwrp.gsfc.nasa.gov/apod/. The URL to the image changes
each day, but the page itself does not. To pull the image down you will load
the page, find the image URL, then load the image itself and put it into the
ScrollPaneWatermark. Depending on the day, it may look something like
Figure 1-22.
Figure 1-21. Finished ScrollPane hack
Figure 1-22. Text area with a background image
Chapter 1, Basic JComponents
#7 Put a NASA Photo into the Background of a Text Area
The code in Example 1-14 defines a class called BackgroundLoader, which
Runnable so it can be placed on its own thread. The constructor
takes as an argument the
, which the loader will put the
image into. The
run( )
method contains a loop that will run every two hours,
loading the page, finding the
URL, then loading the image into the
First, you open the web page and load it into a
page_buffer by looping
through an
InputStreamReader, copying the page data into a string buffer.
The Reader will take care of encoding issues so you don’t
need to worry about reading HTTP headers or converting to
Example 1-14. A thread to load a background image
public class BackgroundLoader implements Runnable {
private ScrollPaneWatermark watermark;
public BackgroundLoader(ScrollPaneWatermark watermark) {
this.watermark = watermark;
public void run( ) {
while(true) {
try {
String base_url = "http://antwrp.gsfc.nasa.gov/apod/";
URL url = new URL(base_url);
Reader input = new InputStreamReader(url.openStream( ));
char buf[] = new char[1024];
StringBuffer page_buffer = new StringBuffer( );
while(true) {
int n = input.read(buf);
if(n < 0) { break; }
// Locate the Image URL (see next section)
} catch (Exception ex) {
System.out.println("exception: " + ex);
ex.printStackTrace( );
Put a NASA Photo into the Background of a Text Area #7
Chapter 1, Basic JComponents
Page in hand, now you need to find the URL for the image itself. If you load
the page into a web browser and view the page source, you will see that
there is only a single
tag in the entire page. This makes the image very
easy to find:
Pattern pattern = Pattern.compile("<IMG SRC=\"(.*)\"");
Matcher matcher = pattern.matcher(page_buffer);
matcher.find( );
String img_url = base_url + matcher.group(1);
watermark.setBackgroundTexture(new URL(img_url));
watermark.repaint( );
Thread.currentThread( ).sleep(1000*60*60*2);
First, you must create a Pattern object with a regex (regular expression) that
matches the URL. You will notice in the previous code that there are paren-
theses around the
.*. This is called a grouping. The matcher will store any-
thing that matches the parentheses in a series of groups that you can query
later. This lets you define exactly which part of the pattern you want to pull
out. After creating a
Matcher and calling find( ), you can pull out the image
URL with the line
matcher.group(1), prepending it with the base URL for
the page.
With the final image URL ready, just set the background texture for the
watermark, refresh the screen, and then sleep for two hours. The calcula-
tion in the code (
1000*60*60*2) evaluates to the number of milliseconds in a
two-hour block of time. The page only changes once a day, but this way the
change will be picked up sooner if the program is running overnight.
With all of the parts assembled, you can now create a main method,
shown in Example 1-15, that builds a simple text editor with the
ScrollPaneWatermark to show the space image in the background.
Example 1-15. Testing the space image background
public static void main(String[] args) throws IOException {
JFrame frame = new JFrame("Blocking Window");
JTextArea jta = new JTextArea(10,40);
ScrollPaneWatermark viewport = new ScrollPaneWatermark( );
JScrollPane scroll = new JScrollPane( );
Container comp = frame.getContentPane( );

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