How to add filters to servlet without modifying web.xml

Question

I'd like the ability to modify/configure filters in a different way than web.xml. Here is a static configuration of 2 filters. I'd like the ability to have one filter statically configured and allow that filter to load additional filters. I just wanted to know if anyone knows of lib that already has this.

Using Servlet API 2.5

<web-app>
  ...
  <filter>
    <filter-name>MyFilter1</filter-name>
    <filter-class>com.me.MyFilter1</filter-class>
  </filter>
  <filter-mapping>
    <filter-name>MyFilter1</filter-name>
    <url-pattern>/*</url-pattern>
  </filter-mapping>
  ...
  <filter>
    <filter-name>MyFilter2</filter-name>
    <filter-class>com.me.MyFilter2</filter-class>
  </filter>
  <filter-mapping>
    <filter-name>MyFilter2</filter-name>
    <url-pattern>/*</url-pattern>
  </filter-mapping>
  ...
</web-app>

I've seen this done in Guice with GuiceFilter where the Filters are configured at runtime.

Answer 1

Just do the same job as the container already does. I.e. reinvent the wheel of the chain of responsibility design pattern as is under the covers been used by servlet filters.

public class GodFilter implements Filter {

    private Map<Pattern, Filter> filters = new LinkedHashMap<Pattern, Filter>();

    @Override
    public void init(FilterConfig config) throws ServletException {
        Filter1 filter1 = new Filter1();
        filter1.init(config);
        filters.put(new Pattern("/foo/*"), filter1);

        Filter2 filter2 = new Filter2();
        filter2.init(config);
        filters.put(new Pattern("*.bar"), filter2);

        // ...
    }

    @Override
    public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws ServletException, IOException {
        HttpServletRequest hsr = (HttpServletRequest) request;
        String path = hsr.getRequestURI().substring(hsr.getContextPath().length());
        GodFilterChain godChain = new GodFilterChain(chain);

        for (Entry<Pattern, Filter> entry : filters.entrySet()) {
            if (entry.getKey().matches(path)) {
                godChain.addFilter(entry.getValue());
            }
        }

        godChain.doFilter(request, response);
    }

    @Override
    public void destroy() {
        for (Filter filter : filters.values()) {
            filter.destroy();
        }
    }

}

with those little helper classes (which can if necessary be made private static nested classes of the above GodFilter):

public class Pattern {

    private int position;
    private String url;

    public Pattern(String url) {
        this.position = url.startsWith("*") ? 1
                      : url.endsWith("*") ? -1
                      : 0;
        this.url = url.replaceAll("/?\\*", "");
    }

    public boolean matches(String path) {
        return (position == -1) ? path.startsWith(url)
             : (position == 1) ? path.endsWith(url)
             : path.equals(url);
    }

}

and

public class GodFilterChain implements FilterChain {

    private FilterChain chain;
    private List<Filter> filters = new ArrayList<Filter>();
    private Iterator<Filter> iterator;

    public GodFilterChain(FilterChain chain) {
        this.chain = chain;
    }

    @Override
    public void doFilter(ServletRequest request, ServletResponse response) throws IOException, ServletException {
        if (iterator == null) {
            iterator = filters.iterator();
        }

        if (iterator.hasNext()) {
            iterator.next().doFilter(request, response, this);
        } else {
            chain.doFilter(request, response);
        }
    }

    public void addFilter(Filter filter) {
        if (iterator != null) {
            throw new IllegalStateException();
        }

        filters.add(filter);
    }

}

You could if necessary also feed a XML config file with all possible filters so that you end up with easier configuration. You could use reflection to create filters in init() of your GodFilter.

Oh nevermind, that's what the web.xml and the container already is doing...

Answer 2

Servlet 3.0 has the @WebFilter annotation to define a filter. No need to declare it in web.xml anymore.

But loading a filter from a filter is not supported. You could implement it yourself: it's "just" the chain of responsibility pattern, but why would you?

Answer 3

It can be achieved in easy steps, even for pre-3.0 Servlet spec:

1. Add a filter containing a static & ordered collection of Classes (chain).
2. Map the filter to intercept every traffic.
3. Manipulate the order & existence of your helper classes (those will be called privately by your filter upon interception of traffic) in the chain.

Ref: Xstream uses same kind of pattern for Serializer, well not with Servlet/Filter though. :)

Answer 4

I personally like the @WebFilter annotation to register servlet filters.
But another solution is to add the filter at runtime using the ServletContext's addFilter function.

Implement ServletContextListener; something like:

public class MyContextListener implements ServletContextListener {

    @Override
    public void contextInitialized(ServletContextEvent ce) {
        ServletContext servletContext = ce.getServletContext();
    
        // you can even conditionally add this
        servletContext.addFilter("My filter 1", MyFilter1.class)
                .addMappingForUrlPatterns(allOf(DispatcherType.class), false, "/*");
    }
}

Register listener:

<listener>
    <listener-class>com.me.MyContextListener</listener-class>
</listener>   

And of course you need to implement a Filter. But in your question you already refer to an example filter 'MyFilter1'.