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This article will explain in detail for you how to understand the chain.doFilter () function in Java filter doFilter. The content of the article is of high quality, so the editor shares it for you to do a reference. I hope you will have a certain understanding of the relevant knowledge after reading this article.

Understanding of chain.doFilter () function in filter doFilter

The chain.doFilter () function is not well understood in recent use, but considering that it is a filter. A filter, as its name implies, is before the execution of something begins.

What begins to be processed is called filtering. A method and a class acts as a filter. It is after a container (similar to Tomcat) starts, opens a website, and it will filter according to the configuration.

The doFilter () method is the main method of the filter, and the init () method is executed when the container starts. The chain.doFilter (request,response) method in the doFilter () method is not quite understood at first. Now I understand one thing.

The method of doFilter (request,response) is that when you open a page and the filter condition is met, it will continue to do what you did when you opened the page. At the same time, if you use httpservletrequest and httpservletResponse, getting the value of servletPath is the address of the file you want to open.

Examples are as follows:

Public void doFilter (ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException {HttpServletRequest req= (HttpServletRequest) request; HttpServletResponse resp= (HttpServletResponse) response; String servletPath= req.getServletPath (); HttpSession session=req.getSession (); String flag= (String) session.getAttribute ("flag") If (servletPathopathology empty empty & (servletPath.equals ("/ 03/index.jsp") | | servletPath.equals ("/ 03/login.jsp") | | servletPath.equals ("/ loginServlet17") {chain.doFilter (request, response);} else {if (invalid pathways nullable empty samples. Equals ("login_success")) {chain.doFilter (request, response) } else if (failed login, please log in again. Equals ("login_error")) {req.setAttribute ("msg", "login failed"); req.setAttribute ("return_uri", servletPath); RequestDispatcher rd=req.getRequestDispatcher ("/ 03/login.jsp"); rd.forward (req, resp) } else {req.setAttribute ("msg", "you are not logged in"); req.setAttribute ("return_uri", servletPath); RequestDispatcher rd=request.getRequestDispatcher ("/ 03/login.jsp"); rd.forward (req, resp);}} System.out.println (servletPath) } filter Filter&&chain.doFilter ()

When the container loads, it reads from web.xml and instantiates the filter according to the specified classpath.

So what exactly is the filter used for?

As the name suggests, filters must filter things we don't want. So execution must be before we visit a page. So it spreads out, and it can also perform some of the actions we want before visiting a page, or some pages.

So:

a. What is a filter? What's the difference between a filter and a normal class?

b. What is the role of chain.doFilter () in the filter?

So chain.doFilter (request,response) is a necessary operation for a filter to pass the request to the next filter if there are multiple filters that need to process the same url.

c. What is the order in which the filter is processed?

MyServlet / foo* servletFilter myServlet myFilter / foo* anotherFilter / foo/bar

If the URL of a normal request for access is / foo/bar, it will match all three filters. The filter chain will consist of three filters, myFilter, anotherFilter, and then servletFilter. MyFilter will be executed before anotherFilter because this is the order in which they appear in the deployment descriptor. They will all be executed before servletFilter, because URL mapping is always performed before Servlet name mapping.

This is the end of the understanding of the chain.doFilter () function in the Java filter doFilter. I hope the above can be helpful to you and learn more. If you think the article is good, you can share it for more people to see.

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