NanoHttpd：尝试提供文件时找不到文件

Question

我一直在尝试在Android上创建一个服务器文件，如MP3。 我的挑战是，当提供MP3时，我得到File NOT Found Exception

以下是服务器类。

public class Server extends NanoHTTPD {

    private static final String MIME_AUDIO   = "audio/mpeg";
    private String URI;
    public Server(String uri,String type) throws IOException {
        super(8081);
        this.URI            = uri;
        start(NanoHTTPD.SOCKET_READ_TIMEOUT, false);
    }

    @Override
    public Response serve(IHTTPSession session) {
            FileInputStream fis = null;
            try {
                fis = new FileInputStream(Environment.getExternalStorageDirectory() + "/"+URI);
            } catch (FileNotFoundException e) {
                e.printStackTrace();
            }
            try {
                int bytes   = 0;
                if(fis != null){
                    bytes   = fis.available();
                }
                return newFixedLengthResponse(Response.Status.OK, MIME_AUDIO, fis, bytes);
            } catch (IOException e) {
                e.printStackTrace();
            }
        return newFixedLengthResponse(Response.Status.OK, MIME_HTML, "Unknown Request");
    }
}

以下是选择文件后启动服务器的方法。

public void onActivityResult(int i, int resultCode, Intent intent) {
        super.onActivityResult(i, resultCode, intent);
        switch (i) {
            case 1013:
                if (resultCode == RESULT_OK) {
                    Uri uri = intent.getData();
                    try {
                        new Server(uri.toString(),"audio").start();
                    } catch (IOException e) {
                        e.printStackTrace();
                    }
                }
        }
    }

我错过了什么？