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When an app wants to access a file shared by another app, the requesting app (the client)
usually sends a request to the app sharing the files (the server). In most cases, the request
starts an Activity
in the server app that displays the files it can share.
The user picks a file, after which the server app returns the file's content URI to the
client app.
This lesson shows you how a client app requests a file from a server app, receives the file's content URI from the server app, and opens the file using the content URI.
Send a Request for the File
To request a file from the server app, the client app calls
startActivityForResult
with an
Intent
containing the action such as
ACTION_PICK
and a MIME type that the client app
can handle.
For example, the following code snippet demonstrates how to send an
Intent
to a server app in order to start the
Activity
described in Sharing a File:
public class MainActivity extends Activity { private Intent mRequestFileIntent; private ParcelFileDescriptor mInputPFD; ... @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_main); mRequestFileIntent = new Intent(Intent.ACTION_PICK); mRequestFileIntent.setType("image/jpg"); ... } ... protected void requestFile() { /** * When the user requests a file, send an Intent to the * server app. * files. */ startActivityForResult(mRequestFileIntent, 0); ... } ... }
Access the Requested File
The server app sends the file's content URI back to the client app in an
Intent
. This Intent
is passed to the client
app in its override of onActivityResult()
. Once
the client app has the file's content URI, it can access the file by getting its
FileDescriptor
.
File security is preserved in this process because the content URI is the only piece of data that the client app receives. Since this URI doesn't contain a directory path, the client app can't discover and open any other files in the server app. Only the client app gets access to the file, and only for the permissions granted by the server app. The permissions are temporary, so once the client app's task stack is finished, the file is no longer accessible outside the server app.
The next snippet demonstrates how the client app handles the
Intent
sent from the server app, and how the client app gets the
FileDescriptor
using the content URI:
/* * When the Activity of the app that hosts files sets a result and calls * finish(), this method is invoked. The returned Intent contains the * content URI of a selected file. The result code indicates if the * selection worked or not. */ @Override public void onActivityResult(int requestCode, int resultCode, Intent returnIntent) { // If the selection didn't work if (resultCode != RESULT_OK) { // Exit without doing anything else return; } else { // Get the file's content URI from the incoming Intent Uri returnUri = returnIntent.getData(); /* * Try to open the file for "read" access using the * returned URI. If the file isn't found, write to the * error log and return. */ try { /* * Get the content resolver instance for this context, and use it * to get a ParcelFileDescriptor for the file. */ mInputPFD = getContentResolver().openFileDescriptor(returnUri, "r"); } catch (FileNotFoundException e) { e.printStackTrace(); Log.e("MainActivity", "File not found."); return; } // Get a regular file descriptor for the file FileDescriptor fd = mInputPFD.getFileDescriptor(); ... } }
The method openFileDescriptor()
returns a ParcelFileDescriptor
for the file. From this object, the client
app gets a FileDescriptor
object, which it can then use to read the file.