HttpUrlConnection

public synchronized OutputStream getOutputStream() throws IOException {

        try {
            if (!doOutput) {
                throw new ProtocolException("cannot write to a URLConnection"
                               + " if doOutput=false - call setDoOutput(true)");
            }

            if (method.equals("GET")) {
                method = "POST"; // Backward compatibility
            }
            if (!"POST".equals(method) && !"PUT".equals(method) &&
                "http".equals(url.getProtocol())) {
                throw new ProtocolException("HTTP method " + method +
                                            " doesn't support output");
            }
}

大坑。。。。