如何在运行KITKAT的设备上使用新的Lollipop SD卡访问API？

Kit-Kat问题无法写入外部SD卡，

如Goolge Document To simplify your code on devices running KITKAT or earlier, you can use fromFile(File) which emulates the behavior of a DocumentsProvider 中所述 - 下面的代码（新API）适用于Lollipop，但如何使用kitkat的新API？

另见Kit-Kat issue (New API)

public class MyActivity extends Activity {

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_my);
    Intent intent = new Intent(Intent.ACTION_OPEN_DOCUMENT_TREE);
    startActivityForResult(intent, 42);
}

public void onActivityResult(int requestCode, int resultCode, Intent resultData) {
    if (resultCode == RESULT_OK) {
        Uri treeUri = resultData.getData();
        DocumentFile pickedDir = DocumentFile.fromFile(new File("/mnt/extSdCard/Test"));

        // List all existing files inside picked directory
        for (DocumentFile file : pickedDir.listFiles()) {
            Log.d("Tag", "Found file " + file.getName() + " with size " + file.length());
        }

        // Create a new file and write into it
        DocumentFile newFile = pickedDir.createFile("text/plain", "My Novel");
        OutputStream out = null;
        try {
            out = getContentResolver().openOutputStream(newFile.getUri());
            out.write("A long time ago...".getBytes());
            out.close();
        } catch (Exception e) {
            e.printStackTrace();
        }

    }
}

@Override
public boolean onCreateOptionsMenu(Menu menu) {
    // Inflate the menu; this adds items to the action bar if it is present.
    getMenuInflater().inflate(R.menu.my, menu);
    return true;
}

@Override
public boolean onOptionsItemSelected(MenuItem item) {
    // Handle action bar item clicks here. The action bar will
    // automatically handle clicks on the Home/Up button, so long
    // as you specify a parent activity in AndroidManifest.xml.
    int id = item.getItemId();
    if (id == R.id.action_settings) {
        return true;
    }
    return super.onOptionsItemSelected(item);
}
}

How to use fromFile(File) ?

我试过但是说 Failed to createFile: java.io.IOException: open failed: EACCES (Permission denied)

即使在添加权限uses-permission android：name =“android.permission.WRITE_EXTERNAL_STORAGE”之后

Android版本4.4.2

protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_my);

    DocumentFile pickedDir = DocumentFile.fromFile(new File("/mnt/extSdCard/Test"));

    // List all existing files inside picked directory
    for (DocumentFile file : pickedDir.listFiles()) {
        Log.d("Tag", "Found file " + file.getName() + " with size " + file.length());
    }

    // Create a new file and write into it
    DocumentFile newFile = pickedDir.createFile("text/plain", "My Novel");
    OutputStream out = null;
    try {

        //Says NullPointerException
        out = getContentResolver().openOutputStream(newFile.getUri());                             out.write("A long time ago...".getBytes());
        out.close();
    } catch (Exception e) {
        e.printStackTrace();
    }        
}