Swing Hacks

Put a NASA Photo into the Background of a Text Area #7

Chapter 1, Basic JComponents

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HACK

H A C K

#7

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

area.

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

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Chapter 1, Basic JComponents

#7 Put a NASA Photo into the Background of a Text Area

HACK

The code in Example 1-14 defines a class called BackgroundLoader, which

implements

Runnable so it can be placed on its own thread. The constructor

takes as an argument the

ScrollPaneWatermark

, 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

SRC

URL, then loading the image into the

watermark.

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

Unicode.

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; }

page_buffer.append(buf,0,n);

}

// 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

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31

HACK

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

IMG

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);

jta.setForeground(Color.white);

ScrollPaneWatermark viewport = new ScrollPaneWatermark( );

viewport.setView(jta);

viewport.setOpaque(false);

JScrollPane scroll = new JScrollPane( );

scroll.setViewport(viewport);

Container comp = frame.getContentPane( );

comp.add("Center",scroll);