FileProvider android 7 文件共享

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FileProvider is a special subclass of 

ContentProvider

 that facilitates secure
sharing of files associated with an app by creating a 

content://

 

Uri

 for
a file instead of a 

file:///

 

Uri

.

A content URI allows you to grant read and write access using temporary access permissions. When you create an 

Intent

 containing
a content URI, in order to send the content URI to a client app, you can also call 

Intent.setFlags()

 to
add permissions. These permissions are available to the client app for as long as the stack for a receiving 

Activity

 is
active. For an 

Intent

 going to a 

Service

,
the permissions are available as long as the 

Service

 is running.

In comparison, to control access to a 

file:///

 

Uri

 you have to modify the
file system permissions of the underlying file. The permissions you provide become available to any app, and remain in effect until you change them. This level of access is fundamentally insecure.

The increased level of file access security offered by a content URI makes FileProvider a key part of Android's security infrastructure.

Defining a FileProvider

Since the default functionality of FileProvider includes content URI generation for files, you don't need to define a subclass in code. Instead, you can include a FileProvider in your app by specifying it entirely in XML. To specify the FileProvider component
itself, add a 

<provider>

 element to your app manifest. Set the 

android:name

 attribute
to 

android.support.v4.content.FileProvider

. Set the 

android:authorities

 attribute to a URI authority based on a domain you control; for example, if you control the domain 

mydomain.com

 you should
use the authority 

com.mydomain.fileprovider

. Set the

android:exported

 attribute to 

false

; the FileProvider does not need to be public. Set the android:grantUriPermissions attribute
to 

true

, to allow you to grant temporary access to files. For example:

<manifest>
    ...
    <application>
        ...
        <provider
            android:name="android.support.v4.content.FileProvider"
            android:authorities="com.mydomain.fileprovider"
            android:exported="false"
            android:grantUriPermissions="true">
            ...
        </provider>
        ...
    </application>
</manifest>

If you want to override any of the default behavior of FileProvider methods, extend the FileProvider class and use the fully-qualified class name in the

android:name

 attribute of the 

<provider>

 element.

Specifying Available Files

A FileProvider can only generate a content URI for files in directories that you specify beforehand. To specify a directory, specify the its storage area and path in XML, using child elements of the 

<paths>

 element.
For example, the following 

paths

 element tells FileProvider that you intend to request content URIs for the 

images/

 subdirectory
of your private file area.

<paths xmlns:android="http://schemas.android.com/apk/res/android">
    <files-path name="my_images" path="images/"/>
    ...
</paths>

Put the 

<paths>

 element and its children in an XML file in your project. For example, you can add them to a new file called 

res/xml/file_paths.xml

. To link this file to the FileProvider, add a <meta-data> element
as a child of the 

<provider>

 element that defines the FileProvider. Set the 

<meta-data>

element's "android:name" attribute to 

android.support.FILE_PROVIDER_PATHS

. Set the element's "android:resource"
attribute to 

@xml/file_paths

(notice that you don't specify the 

.xml

 extension). For example:

<provider
    android:name="android.support.v4.content.FileProvider"
    android:authorities="com.mydomain.fileprovider"
    android:exported="false"
    android:grantUriPermissions="true">
    <meta-data
        android:name="android.support.FILE_PROVIDER_PATHS"
        android:resource="@xml/file_paths" />
</provider>

Generating the Content URI for a File

To share a file with another app using a content URI, your app has to generate the content URI. To generate the content URI, create a new 

File

 for
the file, then pass the 

File

 to 

getUriForFile()

.
You can send the content URI returned by 

getUriForFile()

 to
another app in an 

Intent

. The client app that receives the content URI can open the file
and access its contents by calling 

ContentResolver.openFileDescriptor

 to
get a 

ParcelFileDescriptor

.

For example, suppose your app is offering files to other apps with a FileProvider that has the authority 

com.mydomain.fileprovider

. To get a content URI for the file 

default_image.jpg

 in the 

images/

 subdirectory
of your internal storage add the following code:

File imagePath = new File(Context.getFilesDir(), "images");
File newFile = new File(imagePath, "default_image.jpg");
Uri contentUri = getUriForFile(getContext(), "com.mydomain.fileprovider", newFile);

As a result of the previous snippet, 

getUriForFile()

 returns
the content URI 

content://com.mydomain.fileprovider/my_images/default_image.jpg

.

Granting Temporary Permissions to a URI

To grant an access permission to a content URI returned from 

getUriForFile()

,
do one of the following:
Call the method 

Context.grantUriPermission(package,
Uri, mode_flags)

 for the 

content://

 

Uri

, using the desired
mode flags. This grants temporary access permission for the content URI to the specified package, according to the value of the the 

mode_flags

 parameter, which you can set to 

FLAG_GRANT_READ_URI_PERMISSION

, 

FLAG_GRANT_WRITE_URI_PERMISSION

 or
both. The permission remains in effect until you revoke it by calling

revokeUriPermission()

 or
until the device reboots.
Put the content URI in an 

Intent

 by calling 

setData()

.
Next, call the method 

Intent.setFlags()

 with
either 

FLAG_GRANT_READ_URI_PERMISSION

 or 

FLAG_GRANT_WRITE_URI_PERMISSION

 or
both.
Finally, send the 

Intent

 to another app. Most often, you
do this by calling 

setResult()

.
Permissions granted in an 

Intent

 remain in
effect while the stack of the receiving 

Activity

 is active. When the stack finishes, the
permissions are automatically removed. Permissions granted to one 

Activity

 in a client
app are automatically extended to other components of that app.

Serving a Content URI to Another App

There are a variety of ways to serve the content URI for a file to a client app. One common way is for the client app to start your app by calling

startActivityResult()

,
which sends an 

Intent

 to your app to start an 

Activity

 in
your app. In response, your app can immediately return a content URI to the client app or present a user interface that allows the user to pick a file. In the latter case, once the user picks the file your app can return its content URI. In both cases, your
app returns the content URI in an 

Intent

 sent via 

setResult()

.

You can also put the content URI in a 

ClipData

 object and then add the object to an 

Intent

 you
send to a client app. To do this, call

Intent.setClipData()

.
When you use this approach, you can add multiple 

ClipData

 objects to the 

Intent

,
each with its own content URI. When you call

Intent.setFlags()

 on the 

Intent

 to
set temporary access permissions, the same permissions are applied to all of the content URIs.

Note: The 

Intent.setClipData()

 method
is only available in platform version 16 (Android 4.1) and later. If you want to maintain compatibility with previous versions, you should send one content URI at a time in the 

Intent

.
Set the action to 

ACTION_SEND

 and put the URI in data by calling

setData()

.