Programming Filters

The filtering API is defined by the Filter, FilterChain, and FilterConfig interfaces in the javax.servlet package. You define a filter by implementing the Filter interface.

The most important method in this interface is doFilter, which is passed request, response, and filter chain objects. This method can perform the following actions:

• Examine the request headers.

• Customize the request object if the filter wishes to modify request headers or data.

• Customize the response object if the filter wishes to modify response headers or data.

• Invoke the next entity in the filter chain. If the current filter is the last filter in the chain that ends with the target web component or static resource, the next entity is the resource at the end of the chain; otherwise, it is the next filter that was configured in the WAR. The filter invokes the next entity by calling the 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 the request by not making the call to invoke the next entity. In the latter case, the filter is responsible for filling out the response.

• Examine response headers after it has invoked the next filter in the chain.

• Throw an exception to indicate an error in processing.

In addition to doFilter, you must implement the 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 init.