OfficeDev/ews-java-api

@prvigneshkumar92
Check the documentation for InputStream#available():
"Returns an estimate of the number of bytes that can be read (or skipped over) from this input stream without blocking by the next invocation of a method for this input stream."
"The available method for class InputStream always returns 0."

Actually, available() says how many bytes can be read until read() call will block the execution.

So, just read the text from input stream.

import org.apache.commons.io.IOUtils;

import javax.servlet.ServletOutputStream;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;

public class Frontend extends HttpServlet {
    @Override
    public void doGet(HttpServletRequest request, HttpServletResponse response) {
        // TODO
    }

    @Override
    public void doPost(HttpServletRequest request, HttpServletResponse response) throws IOException {
        //String xml = IOUtils.toString(new BufferedReader(new InputStreamReader(request.getInputStream())));
        // or
        String xml = IOUtils.toString(request.getReader());

        String responseXml = "<?xml version=\"1.0\" encoding=\"UTF-8\"?>\n" +
                "<soap:Envelope xmlns:soap=\"http://schemas.xmlsoap.org/soap/envelope/\" xmlns:m=\"http://schemas.microsoft.com/exchange/services/2006/messages\" xmlns:t=\"http://schemas.microsoft.com/exchange/services/2006/types\" xmlns:xsi=\"http://www.w3.org/2001/XMLSchema-instance\">\n" +
                "   <soap:Body>\n" +
                "      <m:SendNotificationResult>\n" +
                "         <m:SubscriptionStatus>OK</m:SubscriptionStatus>\n" +
                "      </m:SendNotificationResult>\n" +
                "   </soap:Body>\n" +
                "</soap:Envelope>";

        response.setContentType("text/xml");
        byte[] bytes = responseXml.getBytes("UTF-8");
        response.setContentLength(bytes.length);
        ServletOutputStream out = response.getOutputStream();
        out.write(bytes, 0, bytes.length);
        out.flush();
        out.close();
    }
}

It's better to implement the push notification listener as RESTful Web Service (JAX-RS) or SOAP Web Service (JAX-WS).
Look at Jersey, Apache CXF.