Environment.getExternalStorageDir（）。list（）返回null

Question

我的MainActivity中有以下代码。

Log.d("FILES", Environment.getExternalStorageDirectory().getAbsolutePath());
Log.d("FILES", Environment.getRootDirectory().getAbsolutePath());
Log.d("FILES", Arrays.toString(Environment.getRootDirectory().list()));
Log.d("FILES", Arrays.toString(Environment.getExternalStorageDirectory().list()));
Log.d("FILES", Environment.getExternalStorageState());
Log.d("FILES", String.valueOf(Environment.getExternalStorageDirectory().listFiles() != null));

上面的代码在logcat中显示了这样的输出：

12-28 08:38:51.344 23429-23429/com.tahsinalsayeed.codeviewer D/FILES: /storage/emulated/0
12-28 08:38:51.344 23429-23429/com.tahsinalsayeed.codeviewer D/FILES: /system
12-28 08:38:51.344 23429-23429/com.tahsinalsayeed.codeviewer D/FILES: [lost+found, app, bin, build.prop, data, etc, fonts, framework, lib, media, plugin, preloadres, priv-app, recovery-from-boot.p, res, tts, usr, vendor, xbin]
12-28 08:38:51.347 23429-23429/com.tahsinalsayeed.codeviewer D/FILES: null
12-28 08:38:51.353 23429-23429/com.tahsinalsayeed.codeviewer D/FILES: mounted
12-28 08:38:51.356 23429-23429/com.tahsinalsayeed.codeviewer D/FILES: false

为什么我不能在getExternalStorageDir下列出文件，我无法做出正面或反面。根据对this问题的接受答案，这应该有效。我的清单中有以下权限。

<uses-permission android:name="android.permission.INTERNET" />
<uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" />
<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />
<uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE" />

我已经使用API​​ 23在物理设备和仿真器上进行了测试。

Answer 1

在Android 6.0（API级别23）中，用户在应用程序运行时向应用程序授予权限，而不是在安装应用程序时。

注意：清单中的提及权限在API 23中不起作用，您需要在运行时询问权限。

请关注此官方链接here

askForPermission(Manifest.permission.READ_EXTERNAL_STORAGE,100);

询问用户许可的方法

private void askForPermission(String permission, Integer requestCode) {
    if (ContextCompat.checkSelfPermission(MainActivity.this, permission) != PackageManager.PERMISSION_GRANTED) {

        // Should we show an explanation?
        if (ActivityCompat.shouldShowRequestPermissionRationale(MainActivity.this, permission)) {

            //This is called if user has denied the permission before
            //In this case I am just asking the permission again
            ActivityCompat.requestPermissions(MainActivity.this, new String[]{permission}, requestCode);

        } else {

            ActivityCompat.requestPermissions(MainActivity.this, new String[]{permission}, requestCode);
        }
    } else {
        Toast.makeText(this, "" + permission + " is already granted.", Toast.LENGTH_SHORT).show();
    }
}

要处理权限请求的结果，将调用onRequestPermissionsResult方法。它的代码如下：

注意：返回结果时，它会检查是否已授予权限。如果是，则传递requestCode。

@Override
public void onRequestPermissionsResult(int requestCode, String[] permissions, int[] grantResults) {
    super.onRequestPermissionsResult(requestCode, permissions, grantResults);
    if (ActivityCompat.checkSelfPermission(this, permissions[0]) == PackageManager.PERMISSION_GRANTED) {

        //permission granted
    } else {
        //Permission Denied
    }
}