This bug happens with Tomcat 9.0.6 and later.

9.0.5 and earlier (including the 8.5 releases) are NOT affected.

Assume the following in web.xml:

<error-page>
    <error-code>404</error-code>
    <location>/errorRedirect</location>
</error-page>

<servlet>
    <servlet-name>Error Redirect Servlet</servlet-name>
    <servlet-class>org.duckdns.owly.tomcat9test.ErrorRedirectServlet</servlet-class>
    <load-on-startup>1</load-on-startup>
</servlet>
<servlet-mapping>
    <servlet-name>Error Redirect Servlet</servlet-name>
    <url-pattern>/errorRedirect</url-pattern>
</servlet-mapping>

Tomcat 9.0.6 and later will always return a Response with status 404 if a non existing page is accessed - no matter what I do in ErrorRedirectServlet.doGet(Request, Response).

This is especially bad if one wants to send a redirect, as the location header is set, yet due to the status code not being 302 it is ignored by the browser.

I tested response.sendRedirect("error.html"), both with and without a preceding response.setStauts(302).

A small sample project is available at https://github.com/j-be/tomcat-9-redirect-test.