我无法在android的外部存储中保存图像

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

if (Environment.MEDIA_MOUNTED.equals(Environment.getExternalStorageState())) {
    // 外部存储可用
} else {
    // 外部存储不可用
}

File directory = new File(Environment.getExternalStoragePublicDirectory(
        Environment.DIRECTORY_PICTURES), "MyApp");
if (!directory.exists()) {
    directory.mkdirs();
}

String fileName = "image_" + System.currentTimeMillis() + ".jpg";
File imageFile = new File(directory, fileName);

try {
    FileOutputStream fos = new FileOutputStream(imageFile);
    bitmap.compress(Bitmap.CompressFormat.JPEG, 100, fos);
    fos.close();
} catch (IOException e) {
    e.printStackTrace();
}