为什么你必须调用URLConnection#getInputStream才能写出到URLConnection#getOutputStream？

我正在尝试写出URLConnection#getOutputStream，但是，在我调用URLConnection#getInputStream之前，实际上没有发送任何数据。即使我将URLConnnection#doInput设置为false，它仍然不会发送。有谁知道这是为什么？API 文档中没有任何内容对此进行了描述。

JAVA API Documentation on URLConnection： http://download.oracle.com/javase/6/docs/api/java/net/URLConnection.html

Java's Tutorial on Read from and Write to a URLConnection： http://download.oracle.com/javase/tutorial/networking/urls/readingWriting.html

import java.io.IOException;
import java.io.OutputStreamWriter;
import java.net.URL;
import java.net.URLConnection;

public class UrlConnectionTest {

    private static final String TEST_URL = "http://localhost:3000/test/hitme";

    public static void main(String[] args) throws IOException  {

        URLConnection urlCon = null;
        URL url = null;
        OutputStreamWriter osw = null;

        try {
            url = new URL(TEST_URL);
            urlCon = url.openConnection();
            urlCon.setDoOutput(true);
            urlCon.setRequestProperty("Content-Type", "text/plain");            

            ////////////////////////////////////////
            // SETTING THIS TO FALSE DOES NOTHING //
            ////////////////////////////////////////
            // urlCon.setDoInput(false);

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

            /////////////////////////////////////////////////
            // MUST CALL THIS OTHERWISE WILL NOT WRITE OUT //
            /////////////////////////////////////////////////
            urlCon.getInputStream();

            /////////////////////////////////////////////////////////////////////////////////////////////////////////
            // If getInputStream is called while doInput=false, the following exception is thrown:                 //
            // java.net.ProtocolException: Cannot read from URLConnection if doInput=false (call setDoInput(true)) //
            /////////////////////////////////////////////////////////////////////////////////////////////////////////

        } catch (Exception e) {
            e.printStackTrace();                
        } finally {
            if (osw != null) {
                osw.close();
            }
        }

    }

}