在本教程中，我们将开发一个应用程序，该应用程序显示使用FileProvider从相机或者图库捕获的图像。
过去我们已经开发了类似的应用程序。
但是随着Android Nougat的引入，它给出了FileUriExposedException这样的运行时崩溃信息。

FileProvider

FileProvider是ContentProvider的特殊子类，它允许通过内容URI(而不是file：//URI)在应用程序之间共享文件。

使用file：//URI并不是最好的主意。
授予存储权限后，它将为所有应用程序授予访问文件的权限。

我们需要以某种方式进行限制，以使用户知道与之共享文件的应用程序。

为此，我们使用FileProviders，它们允许对文件的临时访问权限。
否则，我们只需从Uri.parse()获取文件的URI，便能够访问其他应用程序的文件。

通过在您的应用程序中使用FileProvider，您无需每次都要求用户授予WRITE_EXTERNAL_STORAGE权限。

定义FileProvider

要在我们的android应用程序中定义FileProvider，我们需要做以下事情：

• 在您的AndroidManifest文件中定义FileProvider
• 创建一个XML文件，其中包含FileProvider将与其他应用程序共享的所有路径

<provider
          android:name="android.support.v4.content.FileProvider"
          android:authorities="${applicationId}.provider"
          android:exported="false"
          android:grantUriPermissions="true">
          <meta-data
              android:name="android.support.FILE_PROVIDER_PATHS"
              android:resource="@xml/provider_paths"
      </provider>

在res目录中创建一个xml文件夹。
其中添加provider_paths.xml文件：

<?xml version="1.0" encoding="utf-8"?>
<paths xmlns:android="https://schemas.android.com/apk/res/android">
  <external-path name="external_files" path="."
</paths>

根据我们需要访问的存储，我们在外部路径中传递值。
可以传递的其他值的示例–sdcard

现在，让我们编写适用于Android Nougat及更高版本的从Camera and Gallery捕获图像的2.0版应用程序。

项目结构

具有所有权限的AndroidManifest.xml如下所示：

代码

activity_main.xml布局的代码为：

<?xml version="1.0" encoding="utf-8"?>
<android.support.design.widget.CoordinatorLayout xmlns:android="https://schemas.android.com/apk/res/android"
  xmlns:app="https://schemas.android.com/apk/res-auto"
  xmlns:tools="https://schemas.android.com/tools"
  android:layout_width="match_parent"
  android:layout_height="match_parent"
  android:fitsSystemWindows="true">

  <RelativeLayout
      android:id="@+id/content_main"
      android:layout_width="match_parent"
      android:layout_height="match_parent"
      android:padding="16dp"
      app:layout_behavior="@string/appbar_scrolling_view_behavior">

      <ImageView
          android:id="@+id/imageView"
          android:layout_width="250dp"
          android:layout_height="250dp"
          android:layout_centerInParent="true"
          android:adjustViewBounds="true"
          android:scaleType="centerCrop" 

  </RelativeLayout>

  <android.support.design.widget.FloatingActionButton
      android:id="@+id/fab"
      android:layout_width="wrap_content"
      android:layout_height="wrap_content"
      android:layout_gravity="bottom|end"
      android:layout_margin="16dp"
      app:srcCompat="@android:drawable/ic_menu_camera" 

</android.support.design.widget.CoordinatorLayout>

MainActivity.java的代码如下：

package com.theitroad.androidfileprovidercameragallery;

import android.annotation.TargetApi;
import android.app.Activity;
import android.content.ComponentName;
import android.content.DialogInterface;
import android.content.Intent;
import android.content.pm.PackageManager;
import android.content.pm.ResolveInfo;
import android.database.Cursor;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.net.Uri;
import android.os.Build;
import android.os.Bundle;
import android.os.Parcelable;
import android.provider.MediaStore;
import android.support.design.widget.FloatingActionButton;
import android.support.v7.app.AlertDialog;
import android.support.v7.app.AppCompatActivity;
import android.view.View;
import android.widget.ImageView;

import java.io.File;
import java.util.ArrayList;
import java.util.List;

import static android.Manifest.permission.CAMERA;
import static android.Manifest.permission.READ_EXTERNAL_STORAGE;
import static android.Manifest.permission.WRITE_EXTERNAL_STORAGE;

public class MainActivity extends AppCompatActivity {

  Uri picUri;

  private ArrayList<String> permissionsToRequest;
  private ArrayList<String> permissionsRejected = new ArrayList<>();
  private ArrayList<String> permissions = new ArrayList<>();

  private final static int ALL_PERMISSIONS_RESULT = 107;
  private final static int IMAGE_RESULT = 200;

  @Override
  protected void onCreate(Bundle savedInstanceState) {
      super.onCreate(savedInstanceState);
      setContentView(R.layout.activity_main);

      FloatingActionButton fab = findViewById(R.id.fab);
      fab.setOnClickListener(new View.OnClickListener() {
          @Override
          public void onClick(View view) {
              startActivityForResult(getPickImageChooserIntent(), IMAGE_RESULT);
          }
      });

      permissions.add(CAMERA);
      permissions.add(WRITE_EXTERNAL_STORAGE);
      permissions.add(READ_EXTERNAL_STORAGE);
      permissionsToRequest = findUnAskedPermissions(permissions);

      if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.M) {

          if (permissionsToRequest.size() > 0)
              requestPermissions(permissionsToRequest.toArray(new String[permissionsToRequest.size()]), ALL_PERMISSIONS_RESULT);
      }

  }

  public Intent getPickImageChooserIntent() {

      Uri outputFileUri = getCaptureImageOutputUri();

      List<Intent> allIntents = new ArrayList<>();
      PackageManager packageManager = getPackageManager();

      Intent captureIntent = new Intent(android.provider.MediaStore.ACTION_IMAGE_CAPTURE);
      List<ResolveInfo> listCam = packageManager.queryIntentActivities(captureIntent, 0);
      for (ResolveInfo res : listCam) {
          Intent intent = new Intent(captureIntent);
          intent.setComponent(new ComponentName(res.activityInfo.packageName, res.activityInfo.name));
          intent.setPackage(res.activityInfo.packageName);
          if (outputFileUri != null) {
              intent.putExtra(MediaStore.EXTRA_OUTPUT, outputFileUri);
          }
          allIntents.add(intent);
      }

      Intent galleryIntent = new Intent(Intent.ACTION_GET_CONTENT);
      galleryIntent.setType("image/*");
      List<ResolveInfo> listGallery = packageManager.queryIntentActivities(galleryIntent, 0);
      for (ResolveInfo res : listGallery) {
          Intent intent = new Intent(galleryIntent);
          intent.setComponent(new ComponentName(res.activityInfo.packageName, res.activityInfo.name));
          intent.setPackage(res.activityInfo.packageName);
          allIntents.add(intent);
      }

      Intent mainIntent = allIntents.get(allIntents.size() - 1);
      for (Intent intent : allIntents) {
          if (intent.getComponent().getClassName().equals("com.android.documentsui.DocumentsActivity")) {
              mainIntent = intent;
              break;
          }
      }
      allIntents.remove(mainIntent);

      Intent chooserIntent = Intent.createChooser(mainIntent, "Select source");
      chooserIntent.putExtra(Intent.EXTRA_INITIAL_INTENTS, allIntents.toArray(new Parcelable[allIntents.size()]));

      return chooserIntent;
  }

  private Uri getCaptureImageOutputUri() {
      Uri outputFileUri = null;
      File getImage = getExternalFilesDir("");
      if (getImage != null) {
          outputFileUri = Uri.fromFile(new File(getImage.getPath(), "profile.png"));
      }
      return outputFileUri;
  }

  @Override
  protected void onActivityResult(int requestCode, int resultCode, Intent data) {

      if (resultCode == Activity.RESULT_OK) {

          ImageView imageView = findViewById(R.id.imageView);

          if (requestCode == IMAGE_RESULT) {

              String filePath = getImageFilePath(data);
              if (filePath != null) {
                  Bitmap selectedImage = BitmapFactory.decodeFile(filePath);
                  imageView.setImageBitmap(selectedImage);
              }
          }

      }

  }

  private String getImageFromFilePath(Intent data) {
      boolean isCamera = data == null || data.getData() == null;

      if (isCamera) return getCaptureImageOutputUri().getPath();
      else return getPathFromURI(data.getData());

  }

  public String getImageFilePath(Intent data) {
      return getImageFromFilePath(data);
  }

  private String getPathFromURI(Uri contentUri) {
      String[] proj = {MediaStore.Audio.Media.DATA};
      Cursor cursor = getContentResolver().query(contentUri, proj, null, null, null);
      int column_index = cursor.getColumnIndexOrThrow(MediaStore.Audio.Media.DATA);
      cursor.moveToFirst();
      return cursor.getString(column_index);
  }

  @Override
  protected void onSaveInstanceState(Bundle outState) {
      super.onSaveInstanceState(outState);

      outState.putParcelable("pic_uri", picUri);
  }

  @Override
  protected void onRestoreInstanceState(Bundle savedInstanceState) {
      super.onRestoreInstanceState(savedInstanceState);

      //get the file url
      picUri = savedInstanceState.getParcelable("pic_uri");
  }

  private ArrayList<String> findUnAskedPermissions(ArrayList<String> wanted) {
      ArrayList<String> result = new ArrayList<String>();

      for (String perm : wanted) {
          if (!hasPermission(perm)) {
              result.add(perm);
          }
      }

      return result;
  }

  private boolean hasPermission(String permission) {
      if (canMakeSmores()) {
          if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.M) {
              return (checkSelfPermission(permission) == PackageManager.PERMISSION_GRANTED);
          }
      }
      return true;
  }

  private void showMessageOKCancel(String message, DialogInterface.OnClickListener okListener) {
      new AlertDialog.Builder(this)
              .setMessage(message)
              .setPositiveButton("OK", okListener)
              .setNegativeButton("Cancel", null)
              .create()
              .show();
  }

  private boolean canMakeSmores() {
      return (Build.VERSION.SDK_INT > Build.VERSION_CODES.LOLLIPOP_MR1);
  }

  @TargetApi(Build.VERSION_CODES.M)
  @Override
  public void onRequestPermissionsResult(int requestCode, String[] permissions, int[] grantResults) {

      switch (requestCode) {

          case ALL_PERMISSIONS_RESULT:
              for (String perms : permissionsToRequest) {
                  if (!hasPermission(perms)) {
                      permissionsRejected.add(perms);
                  }
              }

              if (permissionsRejected.size() > 0) {

                  if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.M) {
                      if (shouldShowRequestPermissionRationale(permissionsRejected.get(0))) {
                          showMessageOKCancel("These permissions are mandatory for the application. Please allow access.",
                                  new DialogInterface.OnClickListener() {
                                      @Override
                                      public void onClick(DialogInterface dialog, int which) {
                                          if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.M) {

                                              requestPermissions(permissionsRejected.toArray(new String[permissionsRejected.size()]), ALL_PERMISSIONS_RESULT);
                                          }
                                      }
                                  });
                          return;
                      }
                  }

              }

              break;
      }

  }
}

以下是在FileProvider授予适当权限的情况下从文件路径获取内容uri的方法：