IBM MobileFirst 7.1 war builder 'Ignoring contents of server/java folder'

I am trying to build my IBM MobileFirst 7.1 application war file using ant war-builder tasks. The ant console says the build was successful, but there is a warning that states:

Ignoring contents of server/java folder. To include this folder , set the 'compileJava' property to true.

As soon as I add this property to the ant task in build.xml, the result turns to error and I get the following log:

[war-builder] Copied 8 empty directories to 7 empty directories under ./bin/classes

[war-builder] Compiling 9 source files to ./bin/classes

[war-builder] CustomAuthenticator.java:7: error: package javax.servlet does not exist

[war-builder] import javax.servlet.ServletException;

[war-builder] CustomAuthenticator.java:8: error: package javax.servlet.http does not exist

[war-builder] import javax.servlet.http.HttpServletRequest;

[war-builder] CustomAuthenticator.java:9: error: package javax.servlet.http does not exist

[war-builder] import javax.servlet.http.HttpServletResponse;

[war-builder] CustomAuthenticator.java:12: error: package com.worklight.server.auth.api does not exist

[war-builder] import com.worklight.server.auth.api.AuthenticationResult;

[war-builder] CustomAuthenticator.java:13: error: package com.worklight.server.auth.api does not exist

[war-builder] import com.worklight.server.auth.api.AuthenticationStatus;

Of course, I am importing all this packages into my custom Authenticator, but the war-builder is failing to add them to the war. Can you help me to understand this situation and how to fix it?