Filters in Servlet

Filters typically do not themselves create responses, but instead provide universal functions that can be "attached" to any type of the servlet or JSP page.

Importance of Filters in Servlet

Filters are the important for a number of reasons.

First, they provide ability to encapsulate recurring tasks in reusable units. Organized developers are constantly on the lookout for ways to modularize their code. Modular code is more manageable and documentable, is easier to debug, and if done well, can be reused in the another setting.

Second, filters can be used to transform response from a servlet or a JSP page. A common task for web application is to format data sent back to the client. Increasingly the clients require format (for example, WML) other than just HTML. To accommodate these clients, there is usually a strong component of transformation or filtering in a fully featured the web application. Many servlets and JSP containers have introduced proprietary filter mechanisms, resulting in a gain for the developer that deploys on that container, but reducing the reusability of such code. With the introduction of filters as part of Java Servlet specification, developers now have the opportunity to write reusable transformation components that are portable across containers.

Function of Filters

Filters can perform many different types of function:

Authentication-Blocking requests based on user identity.
Image conversion-Scaling maps, and so on.
Logging and auditing-Tracking users of a web application.
Data compression-Making downloads smaller.
Localization-Targeting the request and response to a particular locale.
XSL/T transformations of XML content-Targeting web application responses to more that one type of client.

Filters Method

The most important method in Filter interface is the doFilter method, which is the heart of the filter. This method usually performs some of the following actions:

Examines the request headers
Customizes the request object if it wishes to modify request headers or data or block the request entirely
Invokes the next entity in the filter chain.
Customizes the response object if it wishes to modify response headers or data

If the current filter is the last filter in the chain that end with the target servlet, the next entity is the resource at the end of the chain; otherwise, it is the next filter that was configured in the WAR. It invokes the next entity by calling doFilter method on the chain object (passing in the request and response it was called with, or the wrapped versions it may have created). Alternatively, it can choose to block request by not making the call to invoke the next entity. In the latter case, the filter is responsible for the filling out the response. Examines response headers after it has invoked the next filter in the chain Throws an exception to indicate an error in processing In addition to doFilter, you must implement init and destroy methods. The init method is called by the container when the filter is instantiated. If you wish to pass initialization parameters to the filter you retrieve them from the FilterConfig object passed to the init.

Example of using Filters in Servlet

The following example called HitCounter specifies the number of hits done in a paricular website

public final class HitCounterFilter implements Filter
{
private FilterConfig filterConfig = null;
public void init(FilterConfig filterConfig)
throws ServletException
{

this.filterConfig = filterConfig;
}
public void destroy()
{
this.filterConfig = null;
}
public void doFilter(ServletRequest request,
ServletResponse response, FilterChain chain)
throws IOException, ServletException
{
if (filterConfig == null)
return;
StringWriter sw = new StringWriter();
PrintWriter writer = new PrintWriter(sw);
Counter counter = (Counter)filterConfig.
getServletContext().
getAttribute("hitCounter");
writer.println();
writer.println("===============");
writer.println("The number of hits is: " +
counter.incCounter());
writer.println("===============");

// Log the resulting string
writer.flush();
filterConfig.getServletContext().
log(sw.getBuffer().toString());

...
chain.doFilter(request, wrapper);
...
}
}

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