What Is the Use of Filter and Chain in Servlet

How filter chain invocation works?

Each filter implements the javax.servlet.Filter interface, which includes a doFilter() method that takes as input a request and response pair along with a filter chain, which is an instance of a class (provided by the servlet container) that implements the javax.servlet.FilterChain interface. The filter chain reflects the order of the filters. The servlet container, based on the configuration order in the web.xml file, constructs the chain of filters for any servlet or other resource that has filters mapped to it. For each filter in the chain, the filter chain object passed to it represents the remaining filters to be called, in order, followed by the target servlet.

If there are two filters, for example, the key steps of this mechanism would be as follows:

Sample Image

1.The target servlet is requested. The container detects that there are two filters and creates the filter chain.

2.The first filter in the chain is invoked by its doFilter() method.

3.The first filter completes any preprocessing, then calls the doFilter() method of the filter chain. This results in the second filter being invoked by its doFilter() method.

4.The second filter completes any preprocessing, then calls the doFilter() method of the filter chain. This results in the target servlet being invoked by its service() method.

5.When the target servlet is finished, the chain doFilter() call in the second filter returns, and the second filter can do any postprocessing.

6.When the second filter is finished, the chain doFilter() call in the first filter returns, and the first filter can do any postprocessing.

7.When the first filter is finished, execution is complete.

Filters can be interposed between servlets and the servlet container to wrap and preprocess requests or to wrap and postprocess responses. None of the filters are aware of their order. Ordering is handled entirely through the filter chain, according to the order in which filters are configured in web.xml

What is the purpose of javax.servlet.FilterChain?

My coworker (who's not registered on SO) explained that it's for applying global functionality to an app that you don't want to do in every single controller, such as checking if the user is logged in.

What is chain.doFilter doing in Filter.doFilter method?

Servlet Filters are an implementation of the Chain of responsibility design pattern.

All filters are chained (in the order of their definition in web.xml). The chain.doFilter() is proceeding to the next element in the chain. The last element of the chain is the target resource/servlet.

servlet chain filter

The FilterChain#doFilter() continues the request processing and only returns when the target controllers have finished their job and the response is been rendered and committed.

You should not invoke it if your intent is to change the request to a different server side destination by RequestDispatcher#forward() (or when you want to let the client side send a new request by HttpServletResponse#sendRedirect()). You should have noticed this by seeing IllegalStateException: response already committed in the server logs.

So, either remove it so that you only end up with the forward,

request.getRequestDispatcher("/Translate").forward(request, response);

or make it a conditional

if (someCondition) {
    chain.doFilter(request, response);
} else {
    request.getRequestDispatcher("/Translate").forward(request, response); 
}

Unrelated to the concrete question, if I understand/guess your actual functional requirement right, you're more looking for the RequestDispatcher#include() in the /process servlet. See also How do I execute multiple servlets in sequence?

Who will take the roll of service method in filters in java?

The doFilter() method is the one that is called whenever a Filter processes a request.

A simple example is as follows:

public void doFilter(ServletRequest request,
  ServletResponse response, FilterChain chain) 
  throws IOException, ServletException {
  // .. pre filter logic
  chain.doFilter(request, response);
  // .. post filter logic
}

The filter allows you to decide whether to continue processing the request i.e. whether subsequent filters will process this request and finally the servlet at the end. You can choose not to call chain.doFilter (good example of this would be if you're using the filter for authentication). See this guide for more information.

filterChain.doFilter is not moving to next filter

The doFilter() is called each time when a request/response is passed through the chain. The FilterChain.doFilter() method invokes the next filter in the chain and calls resource at the end of the chain if the filter is the last filter in the chain.

This is just a normal method and after it's execution the control will come to the next line. That's the reason you're getting the log after the if condition.

If you want to ignore the execution of the next filter for certain URIs, I recommend you use the else block.

if (ignorePath(path)) {
    chain.doFilter(request, response);
    LOGGER.log(Level.INFO, "Called doFilter for this URI");
} else {
    LOGGER.log(Level.INFO, "doFilter is not called for this URI");
}