Using Filter Interview Questions & Answers

Q.1 Which elements are allowed in the <filter-mapping> element of the deployment descriptor? (Select three)
a. <servlet-name>
b. <filter-class>
c. <dispatcher>
d. <url-pattern>
e. <filter-chain>

Answers: a, c, and d Explanation
Answer a is correct because you can map filters to named servlets, as well as URL patterns. Answer c will control under which dispatching mechanism the filter is invoked.Answer d allows you to map the filter to an URL pattern.Answer b is a legitimate element, but it belongs in the <filter> element.Answer e is a nonexistent element.

Q.2 What is wrong with the following code?

public void doFilter(ServletRequest req, ServletResponse, res, FilterChain chain) throws ServletException, IOException {
    chain.doFilter(req, res);
    HttpServletRequest request = (HttpServletRequest)req;
    HttpSession session = request.getSession();
    if (session.getAttribute(“login”) == null) {
        session.setAttribute(“login””, new Login());
    }
}

a. The doFilter() method signature is incorrect; it should take HttpServlet-Request and HttpServletResponse.
b. The doFilter() method should also throw FilterException.
c. The call to chain.doFilter(req, res) should be this.doFilter(req, res, chain).
d. Accessing the request after chain.doFilter() results in an IllegalState-Exception.
e. Nothing is wrong with this filter.

Answer: e Explanation
Answers a and b are wrong; the doFilter() method’s signature and thrown exceptions are correct. Answer c, calling this.doFilter(req, res, chain), would result in unwanted recursion. Answer d is incorrect; no code here will throw an IllegalStateException.

Q.3 Given these filter mapping declarations:
<filter-mapping>
    <filter-name>FilterOne</filter-name>
    <url-pattern>/admin/*</url-pattern>
    <dispatcher>FORWARD</dispatcher>
</filter-mapping>

<filter-mapping>
    <filter-name>FilterTwo</filter-name>
    <url-pattern>/users/*</url-pattern>
</filter-mapping>

<filter-mapping>
    <filter-name>FilterThree</filter-name>
    <url-pattern>/admin/*</url-pattern>
</filter-mapping>

<filter-mapping>
    <filter-name>FilterTwo</filter-name>
    <url-pattern>/*</url-pattern>
</filter-mapping>

in what order are the filters invoked for the following browser request? /admin/index.jsp
a. FilterOne, FilterThree
b. FilterOne, FilterTwo, FilterThree
c. FilterThree, FilterTwo
d. FilterThree, FilterTwo
e. FilterThree
f. None of these filters are invoked.

Answer: d Explanation
FilterOne is cannot be invoked by calling /admin/index.jsp from the browser but only through a request dispatcher forward, so answers a and b are incorrect. Answer c lists the correct filters but in the wrong order. Answer e names only the first filter that is invoked. Answer d names the correct filters, in the correct order.