Read all text from a file
Java 11 added the readString() method to read small files as a String
, preserving line terminators:
String content = Files.readString(path, StandardCharsets.US_ASCII);
For versions between Java 7 and 11, here's a compact, robust idiom, wrapped up in a utility method:
static String readFile(String path, Charset encoding)
throws IOException
{
byte[] encoded = Files.readAllBytes(Paths.get(path));
return new String(encoded, encoding);
}
Read lines of text from a file
Java 7 added a convenience method to read a file as lines of text, represented as a List<String>
. This approach is "lossy" because the line separators are stripped from the end of each line.
List<String> lines = Files.readAllLines(Paths.get(path), encoding);
Java 8 added the Files.lines()
method to produce a Stream<String>
. Again, this method is lossy because line separators are stripped. If an IOException
is encountered while reading the file, it is wrapped in an UncheckedIOException
, since Stream
doesn't accept lambdas that throw checked exceptions.
try (Stream<String> lines = Files.lines(path, encoding)) {
lines.forEach(System.out::println);
}
This Stream
does need a close()
call; this is poorly documented on the API, and I suspect many people don't even notice Stream
has a close()
method. Be sure to use an ARM-block as shown.
If you are working with a source other than a file, you can use the lines()
method in BufferedReader
instead.
Memory utilization
The first method, that preserves line breaks, can temporarily require memory several times the size of the file, because for a short time the raw file contents (a byte array), and the decoded characters (each of which is 16 bits even if encoded as 8 bits in the file) reside in memory at once. It is safest to apply to files that you know to be small relative to the available memory.
The second method, reading lines, is usually more memory efficient, because the input byte buffer for decoding doesn't need to contain the entire file. However, it's still not suitable for files that are very large relative to available memory.
For reading large files, you need a different design for your program, one that reads a chunk of text from a stream, processes it, and then moves on to the next, reusing the same fixed-sized memory block. Here, "large" depends on the computer specs. Nowadays, this threshold might be many gigabytes of RAM. The third method, using a Stream<String>
is one way to do this, if your input "records" happen to be individual lines. (Using the readLine()
method of BufferedReader
is the procedural equivalent to this approach.)
Character encoding
One thing that is missing from the sample in the original post is the character encoding. There are some special cases where the platform default is what you want, but they are rare, and you should be able justify your choice.
The StandardCharsets
class defines some constants for the encodings required of all Java runtimes:
String content = readFile("test.txt", StandardCharsets.UTF_8);
The platform default is available from the Charset
class itself:
String content = readFile("test.txt", Charset.defaultCharset());
Note: This answer largely replaces my Java 6 version. The utility of Java 7 safely simplifies the code, and the old answer, which used a mapped byte buffer, prevented the file that was read from being deleted until the mapped buffer was garbage collected. You can view the old version via the "edited" link on this answer.
There are many ways to download files. Following I will post most common ways; it is up to you to decide which method is better for your app.
1. Use AsyncTask
and show the download progress in a dialog
This method will allow you to execute some background processes and update the UI at the same time (in this case, we'll update a progress bar).
Imports:
import android.os.PowerManager;
import java.io.InputStream;
import java.io.OutputStream;
import java.io.FileOutputStream;
import java.net.HttpURLConnection;
This is an example code:
// declare the dialog as a member field of your activity
ProgressDialog mProgressDialog;
// instantiate it within the onCreate method
mProgressDialog = new ProgressDialog(YourActivity.this);
mProgressDialog.setMessage("A message");
mProgressDialog.setIndeterminate(true);
mProgressDialog.setProgressStyle(ProgressDialog.STYLE_HORIZONTAL);
mProgressDialog.setCancelable(true);
// execute this when the downloader must be fired
final DownloadTask downloadTask = new DownloadTask(YourActivity.this);
downloadTask.execute("the url to the file you want to download");
mProgressDialog.setOnCancelListener(new DialogInterface.OnCancelListener() {
@Override
public void onCancel(DialogInterface dialog) {
downloadTask.cancel(true); //cancel the task
}
});
The AsyncTask
will look like this:
// usually, subclasses of AsyncTask are declared inside the activity class.
// that way, you can easily modify the UI thread from here
private class DownloadTask extends AsyncTask<String, Integer, String> {
private Context context;
private PowerManager.WakeLock mWakeLock;
public DownloadTask(Context context) {
this.context = context;
}
@Override
protected String doInBackground(String... sUrl) {
InputStream input = null;
OutputStream output = null;
HttpURLConnection connection = null;
try {
URL url = new URL(sUrl[0]);
connection = (HttpURLConnection) url.openConnection();
connection.connect();
// expect HTTP 200 OK, so we don't mistakenly save error report
// instead of the file
if (connection.getResponseCode() != HttpURLConnection.HTTP_OK) {
return "Server returned HTTP " + connection.getResponseCode()
+ " " + connection.getResponseMessage();
}
// this will be useful to display download percentage
// might be -1: server did not report the length
int fileLength = connection.getContentLength();
// download the file
input = connection.getInputStream();
output = new FileOutputStream("/sdcard/file_name.extension");
byte data[] = new byte[4096];
long total = 0;
int count;
while ((count = input.read(data)) != -1) {
// allow canceling with back button
if (isCancelled()) {
input.close();
return null;
}
total += count;
// publishing the progress....
if (fileLength > 0) // only if total length is known
publishProgress((int) (total * 100 / fileLength));
output.write(data, 0, count);
}
} catch (Exception e) {
return e.toString();
} finally {
try {
if (output != null)
output.close();
if (input != null)
input.close();
} catch (IOException ignored) {
}
if (connection != null)
connection.disconnect();
}
return null;
}
The method above (doInBackground
) runs always on a background thread. You shouldn't do any UI tasks there. On the other hand, the onProgressUpdate
and onPreExecute
run on the UI thread, so there you can change the progress bar:
@Override
protected void onPreExecute() {
super.onPreExecute();
// take CPU lock to prevent CPU from going off if the user
// presses the power button during download
PowerManager pm = (PowerManager) context.getSystemService(Context.POWER_SERVICE);
mWakeLock = pm.newWakeLock(PowerManager.PARTIAL_WAKE_LOCK,
getClass().getName());
mWakeLock.acquire();
mProgressDialog.show();
}
@Override
protected void onProgressUpdate(Integer... progress) {
super.onProgressUpdate(progress);
// if we get here, length is known, now set indeterminate to false
mProgressDialog.setIndeterminate(false);
mProgressDialog.setMax(100);
mProgressDialog.setProgress(progress[0]);
}
@Override
protected void onPostExecute(String result) {
mWakeLock.release();
mProgressDialog.dismiss();
if (result != null)
Toast.makeText(context,"Download error: "+result, Toast.LENGTH_LONG).show();
else
Toast.makeText(context,"File downloaded", Toast.LENGTH_SHORT).show();
}
For this to run, you need the WAKE_LOCK permission.
<uses-permission android:name="android.permission.WAKE_LOCK" />
2. Download from Service
The big question here is: how do I update my activity from a service?. In the next example we are going to use two classes you may not be aware of: ResultReceiver
and IntentService
. ResultReceiver
is the one that will allow us to update our thread from a service; IntentService
is a subclass of Service
which spawns a thread to do background work from there (you should know that a Service
runs actually in the same thread of your app; when you extends Service
, you must manually spawn new threads to run CPU blocking operations).
Download service can look like this:
public class DownloadService extends IntentService {
public static final int UPDATE_PROGRESS = 8344;
public DownloadService() {
super("DownloadService");
}
@Override
protected void onHandleIntent(Intent intent) {
String urlToDownload = intent.getStringExtra("url");
ResultReceiver receiver = (ResultReceiver) intent.getParcelableExtra("receiver");
try {
//create url and connect
URL url = new URL(urlToDownload);
URLConnection connection = url.openConnection();
connection.connect();
// this will be useful so that you can show a typical 0-100% progress bar
int fileLength = connection.getContentLength();
// download the file
InputStream input = new BufferedInputStream(connection.getInputStream());
String path = "/sdcard/BarcodeScanner-debug.apk" ;
OutputStream output = new FileOutputStream(path);
byte data[] = new byte[1024];
long total = 0;
int count;
while ((count = input.read(data)) != -1) {
total += count;
// publishing the progress....
Bundle resultData = new Bundle();
resultData.putInt("progress" ,(int) (total * 100 / fileLength));
receiver.send(UPDATE_PROGRESS, resultData);
output.write(data, 0, count);
}
// close streams
output.flush();
output.close();
input.close();
} catch (IOException e) {
e.printStackTrace();
}
Bundle resultData = new Bundle();
resultData.putInt("progress" ,100);
receiver.send(UPDATE_PROGRESS, resultData);
}
}
Add the service to your manifest:
<service android:name=".DownloadService"/>
And the activity will look like this:
// initialize the progress dialog like in the first example
// this is how you fire the downloader
mProgressDialog.show();
Intent intent = new Intent(this, DownloadService.class);
intent.putExtra("url", "url of the file to download");
intent.putExtra("receiver", new DownloadReceiver(new Handler()));
startService(intent);
Here is were ResultReceiver
comes to play:
private class DownloadReceiver extends ResultReceiver{
public DownloadReceiver(Handler handler) {
super(handler);
}
@Override
protected void onReceiveResult(int resultCode, Bundle resultData) {
super.onReceiveResult(resultCode, resultData);
if (resultCode == DownloadService.UPDATE_PROGRESS) {
int progress = resultData.getInt("progress"); //get the progress
dialog.setProgress(progress);
if (progress == 100) {
dialog.dismiss();
}
}
}
}
2.1 Use Groundy library
Groundy is a library that basically helps you run pieces of code in a background service, and it is based on the ResultReceiver
concept shown above. This library is deprecated at the moment. This is how the whole code would look like:
The activity where you are showing the dialog...
public class MainActivity extends Activity {
private ProgressDialog mProgressDialog;
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.main);
findViewById(R.id.btn_download).setOnClickListener(new View.OnClickListener() {
public void onClick(View view) {
String url = ((EditText) findViewById(R.id.edit_url)).getText().toString().trim();
Bundle extras = new Bundler().add(DownloadTask.PARAM_URL, url).build();
Groundy.create(DownloadExample.this, DownloadTask.class)
.receiver(mReceiver)
.params(extras)
.queue();
mProgressDialog = new ProgressDialog(MainActivity.this);
mProgressDialog.setProgressStyle(ProgressDialog.STYLE_HORIZONTAL);
mProgressDialog.setCancelable(false);
mProgressDialog.show();
}
});
}
private ResultReceiver mReceiver = new ResultReceiver(new Handler()) {
@Override
protected void onReceiveResult(int resultCode, Bundle resultData) {
super.onReceiveResult(resultCode, resultData);
switch (resultCode) {
case Groundy.STATUS_PROGRESS:
mProgressDialog.setProgress(resultData.getInt(Groundy.KEY_PROGRESS));
break;
case Groundy.STATUS_FINISHED:
Toast.makeText(DownloadExample.this, R.string.file_downloaded, Toast.LENGTH_LONG);
mProgressDialog.dismiss();
break;
case Groundy.STATUS_ERROR:
Toast.makeText(DownloadExample.this, resultData.getString(Groundy.KEY_ERROR), Toast.LENGTH_LONG).show();
mProgressDialog.dismiss();
break;
}
}
};
}
A GroundyTask
implementation used by Groundy to download the file and show the progress:
public class DownloadTask extends GroundyTask {
public static final String PARAM_URL = "com.groundy.sample.param.url";
@Override
protected boolean doInBackground() {
try {
String url = getParameters().getString(PARAM_URL);
File dest = new File(getContext().getFilesDir(), new File(url).getName());
DownloadUtils.downloadFile(getContext(), url, dest, DownloadUtils.getDownloadListenerForTask(this));
return true;
} catch (Exception pokemon) {
return false;
}
}
}
And just add this to the manifest:
<service android:name="com.codeslap.groundy.GroundyService"/>
It couldn't be easier I think. Just grab the latest jar from Github and you are ready to go. Keep in mind that Groundy's main purpose is to make calls to external REST apis in a background service and post results to the UI with easily. If you are doing something like that in your app, it could be really useful.
3. Use DownloadManager
class (GingerBread
and newer only)
GingerBread brought a new feature, DownloadManager
, which allows you to download files easily and delegate the hard work of handling threads, streams, etc. to the system.
First, let's see a utility method:
/**
* @param context used to check the device version and DownloadManager information
* @return true if the download manager is available
*/
public static boolean isDownloadManagerAvailable(Context context) {
if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.GINGERBREAD) {
return true;
}
return false;
}
Method's name explains it all. Once you are sure DownloadManager
is available, you can do something like this:
String url = "url you want to download";
DownloadManager.Request request = new DownloadManager.Request(Uri.parse(url));
request.setDescription("Some descrition");
request.setTitle("Some title");
// in order for this if to run, you must use the android 3.2 to compile your app
if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.HONEYCOMB) {
request.allowScanningByMediaScanner();
request.setNotificationVisibility(DownloadManager.Request.VISIBILITY_VISIBLE_NOTIFY_COMPLETED);
}
request.setDestinationInExternalPublicDir(Environment.DIRECTORY_DOWNLOADS, "name-of-the-file.ext");
// get download service and enqueue file
DownloadManager manager = (DownloadManager) getSystemService(Context.DOWNLOAD_SERVICE);
manager.enqueue(request);
Download progress will be showing in the notification bar.
Final thoughts
First and second methods are just the tip of the iceberg. There are lots of things you have to keep in mind if you want your app to be robust. Here is a brief list:
- You must check whether user has an internet connection available
- Make sure you have the right permissions (
INTERNET
and WRITE_EXTERNAL_STORAGE
); also ACCESS_NETWORK_STATE
if you want to check internet availability.
- Make sure the directory were you are going to download files exist and has write permissions.
- If download is too big you may want to implement a way to resume the download if previous attempts failed.
- Users will be grateful if you allow them to interrupt the download.
Unless you need detailed control of the download process, then consider using DownloadManager
(3) because it already handles most of the items listed above.
But also consider that your needs may change. For example, DownloadManager
does no response caching. It will blindly download the same big file multiple times. There's no easy way to fix it after the fact. Where if you start with a basic HttpURLConnection
(1, 2), then all you need is to add an HttpResponseCache
. So the initial effort of learning the basic, standard tools can be a good investment.
This class was deprecated in API level 26. ProgressDialog is a modal
dialog, which prevents the user from interacting with the app. Instead
of using this class, you should use a progress indicator like
ProgressBar, which can be embedded in your app's UI. Alternatively,
you can use a notification to inform the user of the task's progress. For more details Link
Best Answer
Add a main eclipse update site to the target. Take care of the version. Then from this site choose the RCP target and Equinox target features. Make sure to check "Include all environments".
Example of update site: http://download.eclipse.org/eclipse/updates/4.2
Addition
Here is the full answer to this question.
Preferred way of using the "DeltaPack"
The DeltaPack is used to get platform specific artifacts so that you can export your RCP product for multiple platforms. See the next section for more information on the DeltaPack
Adding platform specific artifacts
The preferred way of getting the platform specific artifacts is to just add them to the target platform. There is no need to look-up and download the "DeltaPack" if you follow these instructions.
Open your product file and select the "Export" option. You will see that the "Export for multiple platforms" checkbox is available.
Old way of using the "DeltaPack"
What is the DeltaPack
The DeltaPack is a distribution from Eclipse that contains platform specific files for all supported platforms and widget sets. It contains:
SWT Platform Layers
Various plugins to handle connection to the native OS widgets.
Various launchers (exe, cmd, etc..)
Files to launch Eclipse from the command line
Other platform specific plugins
Hooks into secure storage, native networking and native Eclipse filesystem hooks
In total there are some 70 plugins to support the complete range of supported operating systems (around a dozen).
When do you need it
You need it if you want to export your RCP application to multiple targets (a combination of OS/ARCH/WIDGETSET: e.g. linux/ppc64/gtk ). This is typically done from the product configuration file.
How to get it
You can get it from: http://download.eclipse.org/eclipse/downloads/
Click on any of the builds and look for DeltaPack. Please note that there are discussions on removing the DeltaPack and getting it the preferred way.
How to add it to your target platform
Open your product file and select the "Export" option. You will see that the "Export for multiple platforms" checkbox is available.