What is a filter?

A filter is a program that runs on the server before the servlet or JSP page associated with it. Filters can be attached to one or more servlets or JSP pages and can inspect request information entering these resources.

After this, the filter can make the following choices:

① Call the resource in a conventional way (that is, call a servlet or JSP page).

②Use the modified request information to call resources.

③Call the resource, but modify it before sending the response to the client.

④Block the resource call, instead go to other resources, return a specific status code or generate replacement output.

How the filter works

When a Servlet is used as a filter, it can process customer requests. After the processing is completed, it will be handed over to the next filter for processing, so that the customer's request is processed one by one in the filtering chain until the request is sent to the target. For example, there is a web page for submitting "modified registration information" on a website. After the user fills in the modified information and submits it, the server needs to do two tasks during processing: determine whether the client's session is valid; and unify the submitted data. coding. Both tasks can be processed in a filter chain consisting of two filters. When the filter processing is successful, the submitted data is sent to the final target; if the filter processing is unsuccessful, the view will be dispatched to the specified error page.

Filter life cycle

public void init(FilterConfig filterConfig) throws ServletException;//初始化

Like the Servlet program we wrote, the creation and destruction of Filter is the responsibility of the WEB server. When the web application starts, the web server will create an instance object of Filter, call its init method, read the web.xml configuration, and complete the initialization function of the object, thereby preparing for interception of subsequent user requests (the filter object only will be created once, and the init method will only be executed once). Developers can obtain the FilterConfig object representing the current filter configuration information through the parameters of the init method.

public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException;//拦截请求

This method completes the actual filtering operation. When the client requests access to the URL associated with the filter, the Servlet filter will first execute the doFilter method. The FilterChain parameter is used to access subsequent filters.

public void destroy();//销毁

Filter objects will reside in memory after creation and will be destroyed when the web application is removed or the server is stopped. Called before the web container unloads the Filter object. This method is only executed once in the Filter's life cycle. In this method, resources used by the filter can be released.

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