HttpURLConnection java.io.FileNotFoundException

Question

如果我连接到看起来像是 Apache 服务器的东西，下面的代码效果很好，但是当我尝试连接到我的 .Net 服务器时，它会引发错误。我猜这是一个标头要求，但无论我尝试什么似乎都无法获得成功的响应。

public String Download(String Url)
{
 String filepath=null;
 try {
  //set the download URL, a url that points to a file on the internet
  //this is the file to be downloaded
  URL url = new URL(Url);
  //create the new connection
  HttpURLConnection urlConnection = (HttpURLConnection) url.openConnection();

  //set up some things on the connection
  urlConnection.setRequestMethod("GET");
  urlConnection.setDoOutput(true); 
   //and connect!
  urlConnection.connect();
  //set the path where we want to save the file
  //in this case, going to save it on the root directory of the sd card.
  File SDCardRoot = Environment.getExternalStorageDirectory();
  //create a new file, specifying the path, and the filename
  //which we want to save the file as.

  String filename= "effortback.png";   // you can download to any type of file ex:.jpeg (image) ,.txt(text file),.mp3 (audio file)
  Log.i("Local filename:",""+filename);
  File file = new File(SDCardRoot + "/",filename);

  //=====================================
  if(file.createNewFile())
  {
   file.createNewFile();
  }
  //=====================================

  //this will be used to write the downloaded data into the file we created
  FileOutputStream fileOutput = new FileOutputStream(file);

  //this will be used in reading the data from the internet
  InputStream inputStream = urlConnection.getInputStream();

  //=====================================
  //this is the total size of the file
  int totalSize = urlConnection.getContentLength();
  //variable to store total downloaded bytes
  int downloadedSize = 0;
  //=====================================

  //create a buffer...
  byte[] buffer = new byte[2048];
  int bufferLength = 0; //used to store a temporary size of the buffer

  //now, read through the input buffer and write the contents to the file
  while ( (bufferLength = inputStream.read(buffer)) > 0 ) {
   //add the data in the buffer to the file in the file output stream (the file on the sd card
   fileOutput.write(buffer, 0, bufferLength);
   //add up the size so we know how much is downloaded
   downloadedSize += bufferLength;
   //this is where you would do something to report the prgress, like this maybe
   Log.i("Progress:","downloadedSize:"+downloadedSize+"totalSize:"+ totalSize) ;

  }
  //close the output stream when done
  fileOutput.close();
  if(downloadedSize==totalSize)   filepath=file.getPath();

 //catch some possible errors...
 } catch (MalformedURLException e) {
  e.printStackTrace();
  Log.i("URL-ERROR:",e.toString());
 } catch (IOException e) {
  filepath=null;
  e.printStackTrace();
  Log.i("IO-ERROR:",e.toString());
 }
 Log.i("filepath:"," "+filepath) ;

 return filepath;

}

错误范围：

java.io.FileNotFoundException  //I always get this with either one of the below
   org.apache.harmony.luni.internal.net.www.protocol.http.HttpURLConnection.getInputStream(HttpURLConnection.java:1061)

//or this one below
libcore.net.http.HttpURLConnectionImpl.getInputStream(HttpURLConnectionTmpl.java:186)

似乎无论我尝试什么，我都无法让它工作。同样，如果我尝试从 Google 或其他一些网站下载图像，它会起作用，但不是全部，但绝对不是我的 (.Net)。我在这里想念什么？请帮忙。

Answer 1

如果您的服务器返回>= HTTPStatus.BAD_REQUEST (400)，您可以从HttpUrlConnection（和OkHttpClient）获得FileNotFoundException。你应该先检查状态码，看看你需要读什么流。

int status = connection.getResponseCode();

if(status >= HttpStatus.SC_BAD_REQUEST)
    in = connection.getErrorStream();
else
    in = connection.getInputStream();

HttpStatus 已弃用。最新的语法似乎是：

InputStream inputStream;
int status = urlConnection.getResponseCode();

if (status != HttpURLConnection.HTTP_OK)  {
    inputStream = urlConnection.getErrorStream();
}
else  {
    inputStream = urlConnection.getInputStream();
}

Answer 2

遇到了同样的问题，按照你说的解决了：

urlConnection.setDoOutput(false);

请注意，您必须将其设置为 false，因为默认情况下为 true。
请注意，您必须将其设置为 false，因为 Volley HurlStack 将其设置为 true。
保重 ;)

编辑： 我刚刚检查了代码，默认情况下它是错误的，正如@Abraham Philip 所说。但我必须将其设置为 false，因为如果我调用 getDoOutput()，它会返回 true。我发现 Volley HurlStack 将其设置为 true。所以，我的结论是，setDoOutput 为 false，它会起作用。

package java.net;
public abstract class URLConnection {
...

    /**
     * Specifies whether this {@code URLConnection} allows sending data.
     */
    protected boolean doOutput;

...
    public boolean getDoOutput() {
        return doOutput;
    }

    public void setDoOutput(boolean newValue) {
        checkNotConnected();
        this.doOutput = newValue;
    }

Answer 3

我遇到了一个问题，执行 HEAD-Request 时会抛出 FileNotFoundException。
原因是，对HEAD 的响应没有正文，因此对getInputStream 的调用会抛出FileNotFoundException。

因此，如果您正在执行HEAD，则不应尝试获取InputStream。