Android读取外部SD不起作用

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

if (ContextCompat.checkSelfPermission(this, Manifest.permission.READ_EXTERNAL_STORAGE) != PackageManager.PERMISSION_GRANTED) {
    ActivityCompat.requestPermissions(this, new String[]{Manifest.permission.READ_EXTERNAL_STORAGE}, REQUEST_CODE);
}

String state = Environment.getExternalStorageState();
if (Environment.MEDIA_MOUNTED.equals(state)) {
    // SD卡已挂载并可读
} else {
    // SD卡不可用
}

File sdCard = Environment.getExternalStorageDirectory();
String filePath = sdCard.getAbsolutePath() + "/path/to/file";