Shibboleth 3 SP separated from Tomcat java web application in 2 different machines?

Question

I've a java web application running on Tomcat8 on machine A (https://machineA:8080/app)

and a machine B installed with Apache2 and Shibboleth 3 SP (https://machineB/Shibboleth.sso/)

which is configured to use https://samltest.id/ as Shibboleth IdP.

What I want to achieve is:

• When I access https://machineA:8080/app

  -> It invokes a java code: httpServletResponse.sendRedirect("https://machineB/Shibboleth.sso/Login")

  -> It redirects to Shibboleth IdP on https://samltest.id

  -> However, after I logged in with the test user here, it stopped on https://machineB but it doesn't redirect to https://machineA:8080/app?

  I wanted to have the subject-id attribute returned from https://samltest.id in my web application.

• There is another way around, which is Apache2 on machine B works as proxy protecting machine A

  with Tomcat8 as mentioned (Apache2 redirects to Tomcat8 via AJP 1.3) here https://shibboleth.atlassian.net/wiki/spaces/SHIB2/pages/2577072431/NativeSPJavaInstall.

  But it is not what I wanted.