/**
 * Signals that the target server failed to respond with a valid HTTP response.
 *
 * @since 4.0
 */
public class NoHttpResponseException extends IOException {
    private static final long serialVersionUID = -7658940387386078766L;
    /**
     * Creates a new NoHttpResponseException with the specified detail message.
     *
     * @param message exception message
     */
    public NoHttpResponseException(final String message) {
        super(HttpException.clean(message));
    }
}

public class DefaultHttpResponseParser extends AbstractMessageParser<HttpResponse> {
    private final Log log = LogFactory.getLog(getClass());
    private final HttpResponseFactory responseFactory;
    private final CharArrayBuffer lineBuf;
    //......
    @Override
    protected HttpResponse parseHead(
            final SessionInputBuffer sessionBuffer) throws IOException, HttpException {
        //read out the HTTP status string
        int count = 0;
        ParserCursor cursor = null;
        do {
            // clear the buffer
            this.lineBuf.clear();
            final int i = sessionBuffer.readLine(this.lineBuf);
            if (i == -1 && count == 0) {
                // The server just dropped connection on us
                throw new NoHttpResponseException("The target server failed to respond");
            }
            cursor = new ParserCursor(0, this.lineBuf.length());
            if (lineParser.hasProtocolVersion(this.lineBuf, cursor)) {
                // Got one
                break;
            } else if (i == -1 || reject(this.lineBuf, count)) {
                // Giving up
                throw new ProtocolException("The server failed to respond with a " +
                        "valid HTTP response");
            }
            if (this.log.isDebugEnabled()) {
                this.log.debug("Garbage in response: " + this.lineBuf.toString());
            }
            count++;
        } while(true);
        //create the status line from the status string
        final StatusLine statusline = lineParser.parseStatusLine(this.lineBuf, cursor);
        return this.responseFactory.newHttpResponse(statusline, null);
    }
    protected boolean reject(final CharArrayBuffer line, final int count) {
        return false;
    }
}