Java Android-如何从SD卡获取文件字节

我正试图从SD卡中获取文档的一个字节。我所做的就是：

1-）我选择文件

   fun openFile() {
    val intent = Intent(Intent.ACTION_OPEN_DOCUMENT).apply {
        addCategory(Intent.CATEGORY_OPENABLE)
        type = "application/pdf"
    }
    startActivityForResult(intent, PICK_PDF_FILE)
}

2-）我现在有了一个URI。我想把它转换成一个文件。然后我尝试获取字节

override fun onActivityResult(requestCode: Int, resultCode: Int, resultData: Intent?) {
    super.onActivityResult(requestCode, resultCode, resultData)

    if (requestCode == PICK_PDF_FILE && resultCode == Activity.RESULT_OK) {
        resultData?.data?.also { documentUri ->
            contentResolver.takePersistableUriPermission(
                documentUri,
                Intent.FLAG_GRANT_READ_URI_PERMISSION
            )
            var file = documentUri.toFile(); // I am trying to convert URI to FILE type. //ERROR LINE
            Log.d("test" , file.readBytes().toString()) // I'm trying to read the bytes.
        }
    }
}

但这个错误：

 Caused by: java.lang.IllegalArgumentException: Uri lacks 'file' scheme: 
 content://com.android.externalstorage.documents/document/primary%3ADCIM%2Ftest.pdf

---

考虑将文件加载为使用TaskCompletionSource异步加载的输入流：

TaskCompletionSource<Stream> tcs = new TaskCompletionSource<>();

在onActivityResult中：

if (requestCode == PICK_PDF_FILE && resultCode == Activity.RESULT_OK) {
   Uri uri = data.getData();
   tcs.setResult().getContentResolver().openInputStream(uri); 
}
else {
   tcs.setResult(null);
}

获得InputStream后，可以将InputStream转换为字节数组：

但是，根据您需要对字节执行的操作，您可以直接使用流。例如，如果您希望转换PDF，可以使用LEADTOOLS CloudServices库：

资料来源：

if (requestCode == PICK_PDF_FILE && resultCode == Activity.RESULT_OK) {
   Uri uri = data.getData();
   tcs.setResult().getContentResolver().openInputStream(uri); 
}
else {
   tcs.setResult(null);
}

File file = new File(path);
int size = (int) file.length();
byte[] bytes = new byte[size];
try {
    BufferedInputStream buf = new BufferedInputStream(new FileInputStream(file));
    buf.read(bytes, 0, bytes.length);
    buf.close();
} catch (FileNotFoundException e) {
    // TODO Auto-generated catch block
    e.printStackTrace();
} catch (IOException e) {
    // TODO Auto-generated catch block
    e.printStackTrace();
}