What is the difference between a servlet and filter? What do you recommend to use authorization to pages?
Filters are best suited for authorization. This is because they can be configured to run for all pages of a site. So you only need one filter to protect all your pages.
Use a Filter
when you want to filter and/or modify requests based on specific conditions. Use a Servlet
when you want to control, preprocess and/or postprocess requests.
The Sun Java EE tutorial mentions the following about filters:
A filter is an object that can transform the header and content (or both) of a request or response. Filters differ from web components in that filters usually do not themselves create a response. Instead, a filter provides functionality that can be “attached” to any kind of web resource. Consequently, a filter should not have any dependencies on a web resource for which it is acting as a filter; this way it can be composed with more than one type of web resource.
The main tasks that a filter can perform are as follows:
- Query the request and act accordingly.
- Block the request-and-response pair from passing any further.
- Modify the request headers and data. You do this by providing a customized version of the request.
- Modify the response headers and data. You do this by providing a customized version of the response.
- Interact with external resources.
For authorization, a Filter
is the best suited. Here's a basic kickoff example of how a filter checks requests for the logged-in user:
public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws ServletException, IOException {
if (((HttpServletRequest) request).getSession().getAttribute("user") == null) {
// User is not logged in. Redirect to login page.
((HttpServletResponse) response).sendRedirect("login");
} else {
// User is logged in. Just continue with request.
chain.doFilter(request, response);
}
}