Blackberry device hang when downloading image online - multithreading

Question

Every time when my apps went to the layout that download image online, the device will hang and need to wait the download finish only can movable.
I did some researched. They recommend download it in another Thread. However, I not understand how to implement the download function in another Thread.
Here is my code to call the download image function.
Main.getUiApplication().invokeLater(new Runnable() {
public void run() {
for (j = 0; j < imagepath.length; j++) {
if (!imagepath[j].toString().equals("no picture")
&& Config_GlobalFunction.isConnected()) {
loader = new Util_LazyLoader(imagepath[j],
new Util_BitmapDowloadListener() {
public void ImageDownloadCompleted(
Bitmap bmp) {
imagebitmap[j] = bmp;
invalidate();
}
});
loader.run();
}
}
}
}, 500, false);
And the lazyloader
public class Util_LazyLoader implements Runnable {
String url = null;
Util_BitmapDowloadListener listener = null;
public Util_LazyLoader(String url, Util_BitmapDowloadListener listener) {
this.url = url;
this.listener = listener;
}
public void run() {
Bitmap bmpImage = getImageFromWeb(url);
listener.ImageDownloadCompleted(bmpImage);
}
private Bitmap getImageFromWeb(String url) {
HttpConnection connection = null;
InputStream inputStream = null;
EncodedImage bitmap;
byte[] dataArray = null;
try {
connection = (HttpConnection) (new ConnectionFactory())
.getConnection(url + Database_Webservice.ht_params)
.getConnection();
int responseCode = connection.getResponseCode();
if (responseCode == HttpConnection.HTTP_OK) {
inputStream = connection.openDataInputStream();
dataArray = IOUtilities.streamToBytes(inputStream);
}
} catch (Exception ex) {
} finally {
try {
inputStream.close();
connection.close();
} catch (Exception e) {
}
}
if (dataArray != null) {
bitmap = EncodedImage.createEncodedImage(dataArray, 0,
dataArray.length);
return bitmap.getBitmap();
} else {
return null;
}
}
}
I need help on it as I not familiar in networking.

Answer 1

So, the Util_LazyLoader is already well written to support background image downloads, because it implements the Runnable interface. You can start the download like this:
Util_LazyLoader loader =
new Util_LazyLoader(imagepath[j],
new Util_BitmapDowloadListener() {
public void ImageDownloadCompleted(final Bitmap bmp) {
UiApplication.getUiApplication().invokeLater(new Runnable() {
public void run() {
imagebitmap[j] = bmp;
invalidate();
}
});
}
});
Thread backgroundWorker = new Thread(loader);
backgroundWorker.start();
instead of directly calling the loader.run() method yourself.
A Runnable class is just one that has a run() method. You give your Runnable loader object to a new Thread and tell it to start(). This will cause that Thread to execute the run() method in another thread, instead of the UI thread. As long as you don't run network operations on the UI thread, your app should not appear to the user to be frozen.
Note: in your original code, you have this:
Main.getUiApplication().invokeLater(new Runnable() {
public void run() {
You probably don't need that at all. If that code is being run from the main (UI) thread, then all that's doing is telling the app to invoke that locally-defined run() method, also on the UI thread. You do pass a 500 millisecond delay as well. Maybe you need that (?). If you just want it to run right away, though, get rid of the code above (invokeLater(new Runnable() { public void run() { ...). Just use the code I posted (at the top of this answer) to create the backgroundWorker and then call its start() method.
Also, take note of two things in my implementation:
1. I used the UiApplication.invokeLater() method once the bitmap has been received. After the network operation completes, the UI must be updated. But that should not be done on the background thread. So, you create a Runnable to run on the background thread, and then once the download is complete, you create another Runnable to update the UI:
public void run() {
imagebitmap[j] = bmp;
invalidate();
}
2. Because I create another Runnable, and use the bmp variable inside that Runnable, I must declare it as a final parameter. The compiler requires you to do that. Another option would be to use the event lock directly, instead of invokeLater():
public void ImageDownloadCompleted(Bitmap bmp) {
synchronized(UiApplication.getEventLock()) {
imagebitmap[j] = bmp;
invalidate();
}
}
Either should work for you.