Why do you have to call URLConnection#getInputStream to be able to write out to URLConnection#getOutputStream?

Question

I\'m trying to write out to URLConnection#getOutputStream, however, no data is actually sent until I call URLConnection#getInputStream. Even if I set URLConnnection#doInput

Answer 1

As my experiments have shown (java 1.7.0_01) the code:

osw = new OutputStreamWriter(urlCon.getOutputStream());
osw.write("HELLO WORLD");
osw.flush();

Doesn't send anything to the server. It just saves what's written there to the memory buffer. Thus in case you're going to upload a large file via POST - you need to be sure that you have enough memory. On desktop/server it may not be such a big problem, but on android that may result in out of memory error. Here's the example of how the stack trace looks when trying to write to output stream, and memory runs out.

Exception in thread "Thread-488" java.lang.OutOfMemoryError: GC overhead limit exceeded
    at java.util.Arrays.copyOf(Arrays.java:2271)
    at java.io.ByteArrayOutputStream.grow(ByteArrayOutputStream.java:113)
    at java.io.ByteArrayOutputStream.ensureCapacity(ByteArrayOutputStream.java:93)
    at java.io.ByteArrayOutputStream.write(ByteArrayOutputStream.java:140)
    at sun.net.www.http.PosterOutputStream.write(PosterOutputStream.java:78)
    at sun.nio.cs.StreamEncoder.writeBytes(StreamEncoder.java:221)
    at sun.nio.cs.StreamEncoder.implWrite(StreamEncoder.java:282)
    at sun.nio.cs.StreamEncoder.write(StreamEncoder.java:125)
    at sun.nio.cs.StreamEncoder.write(StreamEncoder.java:135)
    at java.io.OutputStreamWriter.write(OutputStreamWriter.java:220)
    at java.io.Writer.write(Writer.java:157)
    at maxela.tables.weboperations.POSTRequest.makePOST(POSTRequest.java:138)

On the bottom of the trace you can see the makePOST() method which does the following:

     writer = new OutputStreamWriter(conn.getOutputStream());                      
    for (int j = 0 ; j < 3000 * 100 ; j++)
    {
      writer.write("&var" + j + "=garbagegarbagegarbage_"+ j);
    }
   writer.flush();

And writer.write() throws the exception. Also my experiments have shown that any exception related to the actual connection/IO with the server is thrown only after urlCon.getOutputStream() is called. Even urlCon.connect() seems to be "dummy" method which doesn't do any physical connection. However if you call urlCon.getContentLengthLong() which returns Content-Length: header field from the server response-headers - then URLConnection.getOutputStream() will be called automatically and in case there's exception - it will be thrown.

The exceptions thrown by urlCon.getOutputStream() are all IOException, and I have met the follwing ones:

                try
                {
                    urlCon.getOutputStream();
                }
                catch (UnknownServiceException ex)
                {
                    System.out.println("UnkownServiceException():" + ex.getMessage());
                }

                catch (ConnectException ex)
                {
                    System.out.println("ConnectException()");
                    Logger.getLogger(POSTRequest.class.getName()).log(Level.SEVERE, null, ex);
                }

                catch (IOException ex) {
                    System.out.println("IOException():" + ex.getMessage());
                    Logger.getLogger(POSTRequest.class.getName()).log(Level.SEVERE, null, ex);
                }

Hopefully my little research helps to people, as URLConnection class is a bit counter-intuitive in some cases thus, when implementing it - one needs to know what's it deals with.

Second reason is: when working with servers - the work with server may fail because of many reasons (connection, dns, firewall, httpresponses, server not being able to accept connection, server not being able to process request timely). Thus it is important to understand how exceptions raised can explain about what's actually happening with the connection.