HttpURLConnection.getRequestProperties() throws IllegalStateException: "Already connected"

Question

After executing a request I would like to check the request headers, but it doesn't work.

I call getRequestProperties() on an instance of sun.net.www.protocol.http.HttpURLConnection and I always get an IllegalStateException with the message "Already connected". As if I wanted to set request properties. But I only want to read them.

The responsible code for this behaviour is in the HttpUrlConnection:
http://grepcode.com/file/repository.grepcode.com/java/root/jdk/openjdk/7u40-b43/sun/net/www/protocol/http/HttpURLConnection.java#HttpURLConnection.getRequestProperties%28%29

public synchronized Map<String, List<String>> getRequestProperties() {
    if (connected)
        throw new IllegalStateException("Already connected");
    // ...
}

Ok so maybe I should only read the request properties after disconnecting. But it turns out, disconnect() doesn't set connected to false. Although it should: http://grepcode.com/file/repository.grepcode.com/java/root/jdk/openjdk/7u40-b43/sun/net/www/protocol/http/HttpURLConnection.java#HttpURLConnection.disconnect%28%29

It also doesn't seem to make a difference if I read the stream to the end or not. Closing the InputStream before or after calling disconnect doesn't make a difference either.

I'm confused. Can you help me?

1. Why doesn't disconnect() set connected to false?
2. Why can't I read request properties while the urlConnection is connected?
3. How do you properly read request headers after the request?

The code to reproduce this is a Unit test for Android (I use Robolectric), but I think you can use it in a Java project as well and call it from main() after removing the test annotation:

/**
 * Test if HttpUrlConnection works as expected, because in some cases it seems it doesn't
 *
 * @throws Exception
 */
@Test
public void testHttpUrlConnection() throws Exception
{
    final URL url = new URL("http://www.stackoverflow.com");
    final HttpURLConnection urlConnection = ( HttpURLConnection ) url.openConnection( );
    urlConnection.setRequestMethod("GET");
    InputStream is = null;
    try
    {
        is = urlConnection.getInputStream();
        assertEquals(200, urlConnection.getResponseCode());
    }
    catch (IOException ex)
    {
        is = urlConnection.getErrorStream( );
    }
    final String result = copyStreamToString(is); // some html response
    // Streams must be closed before disconnecting (according to http://stackoverflow.com/a/11056207/3596676)
    is.close();
    assertTrue((Boolean) getFieldViaRecursiveReflection(urlConnection, "connected"));
    // urlConnection should always be disconnected (according to http://developer.android.com/reference/java/net/HttpURLConnection.html)
    urlConnection.disconnect();
    assertFalse((Boolean) getFieldViaRecursiveReflection(urlConnection, "connected")); // AssertionError
    // getRequestProperties throws IllegalStateException ("already connected")
    Map<String, List<String>> requestProperties = urlConnection.getRequestProperties();
    // do stuff with the properties
    // return the result
}

private static String copyStreamToString( final InputStream is ) throws IOException
{
    if ( is == null )
    {
        return "";
    }
    BufferedReader reader = new BufferedReader( new InputStreamReader( is ) );
    String result = copyBufferedReaderToString( reader );
    reader.close( );
    return result;
}

private static String copyBufferedReaderToString( final BufferedReader bufferedReader ) throws IOException
{
    StringBuffer sb = new StringBuffer( );
    String line;
    while ( ( line = bufferedReader.readLine( ) ) != null )
    {
        sb.append( line );
    }
    return sb.toString( );
}

private static Object getFieldViaRecursiveReflection(final Object object, final String attribute) throws Exception
{
    return getFieldViaRecursiveReflection(object, object.getClass(), attribute);
}

private static Object getFieldViaRecursiveReflection(final Object object, final Class<?> c, final String attribute) throws Exception
{
    try
    {
        final Field field = c.getDeclaredField(attribute);
        field.setAccessible(true);
        return field.get(object);
    }
    catch (NoSuchFieldException ex)
    {
        /* end of type hierarchy? */
        Class<?> superClass = c.getSuperclass();
        if (superClass == null)
        {
            throw ex;
        }
        else
        {
            return getFieldViaRecursiveReflection(object, superClass, attribute);
        }
    }
}

Answer 1

As no one posted an answer in the 2 months since I asked the question, but today I had to deal with the problem again and found a solution, I will answer my own question.

I can't answer all the questions in the answer (e.g. "why doesn't urlConnection.disconnect() set the connected attribute of urlConnection to false?"), but I found the solution for the main problem, which was that reading the headers of a request didn't work when the urlConnection was connected.

For some reason, which I can't remember, I wanted/needed to check the request headers after the request was done and the response was there. But I looked at the implementation of getRequestProperties() in sun.net.www.protocol.http.HttpURLConnection again (see the code here) and noticed that a method called filterAndAddHeaders gets called. So it seems that headers not only get read in that method, but set. I'm not sure why this is done in a getter method (the getRequestProperties() one), but it makes sense that when the request is already done, you should warn the user when he tries to add request headers - in this case with the IllegalStateException that bothered me so much.

To come to the solution:

I just moved the call to getRequestProperties() to before the request gets sent. And now everything works fine.

P.S.:

Please note that this is not all there is to it. One of my unit tests ran successfully even though I called getRequestProperties() after the request. In that case the urlConnection internal attribute connected was set to false. I haven't figured it all out, but it may have been related to the response status code 304 (not modified). Maybe this helps as a hint if you have to deal with this problem and for some reason can't move the getRequestProperties() call to before sending the request.