Android/Java，API级别12。
我正在尝试检查Downloads文件夹中是否存在.zip文件。如果不存在，则使用DownloadManager从Internet下载。出于测试目的，我在DownloadManager的onReceive方法之后立即运行checkIfFileExists，前提是下载成功。我的问题是checkIfFileExists每次都返回false，即使我已经下载了这个文件，并且手动检查了它是否存在。相关代码如下。

DownloadManager dm;
    long queueid;
    String filename = "myfile.zip", url = "http://myurl/", uriString = "";

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        [...]
        BroadcastReceiver receiver = new BroadcastReceiver() {
            @Override
            public void onReceive(Context context, Intent intent) {
                String action = intent.getAction();
                if (DownloadManager.ACTION_DOWNLOAD_COMPLETE.equals(action))
                {
                    DownloadManager.Query req_query = new DownloadManager.Query();
                    req_query.setFilterById(queueid);

                    Cursor c = dm.query(req_query);
                    if (c==null)
                    {
                        Toast.makeText(context, "Download not found!", Toast.LENGTH_SHORT).show();
                    }
                    else
                    {
                        if (c.moveToFirst())
                        {
                            int columnIndex = c.getColumnIndex(DownloadManager.COLUMN_STATUS);
                            if (DownloadManager.STATUS_SUCCESSFUL==c.getInt(columnIndex))
                            {
                                uriString = c.getString(c.getColumnIndex(DownloadManager.COLUMN_LOCAL_URI));
                                Toast.makeText(context, "Download finished.", Toast.LENGTH_SHORT).show();
                                Toast.makeText(context, uriString, Toast.LENGTH_LONG).show();
                                checkIfFileExists();
                            }
                        }
                    }
                }
            }
        };

        registerReceiver(receiver, new IntentFilter(DownloadManager.ACTION_DOWNLOAD_COMPLETE));
    }

    public void onClickDownload(View v) // this method seems to be working with no issues.
    {
        dm = (DownloadManager) getSystemService(DOWNLOAD_SERVICE);
        DownloadManager.Request request = new DownloadManager.Request(Uri.parse(url));
        request.setDestinationInExternalFilesDir(this, Environment.DIRECTORY_DOWNLOADS, filename);
        queueid = dm.enqueue(request);
    }


    private boolean checkIfFileExists()
    {
        File file = new File(uriString);
        if(file.exists())
        {
            Toast.makeText(this, "Exists", Toast.LENGTH_SHORT).show();
            return true;
        }
        else
        {
            Toast.makeText(this, "Doesn't exist", Toast.LENGTH_SHORT).show();
            return false;
        }
    }

我希望if(file.exists())的计算结果为true，因为该文件确实存在。
实际发生的是if(file.exists())总是计算为false。
有人知道我哪里做错了吗？