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This article introduces the knowledge of "how to use Filter filters in JavaWeb". In the operation of actual cases, many people will encounter such a dilemma, so let the editor lead you to learn how to deal with these situations. I hope you can read it carefully and be able to achieve something!

What is a filter

What 1.Filter filters do: intercept requests

two。 Common scenarios for intercepting requests:

(1) permission check (2) Journal operation (3) transaction management

1.1 use steps

Steps for using Filter filter: 1. Write a class to implement Filter interface 2, implement filtering method doFilter () 3, and configure the intercepting path of Filter in web.xml.

Second, the first experience

Create a new admin directory under the web project as a directory that requires permissions to access, with two files in it

2.1 mynav.html navigation Baidu Google Bing 2.2 FilterServlet program package com.filter.filter;import javax.servlet.*;import javax.servlet.http.HttpServletRequest;import javax.servlet.http.HttpSession;import java.io.IOException / * @ author ningqian * @ create-05-16 20:17 * / / Note the guide package is javaxpublic class FilterServlet implements Filter {@ Override public void init (FilterConfig filterConfig) throws ServletException {} @ Override public void doFilter (ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain) throws IOException, ServletException {System.out.println ("come to Filter filter"); HttpServletRequest httpServletRequest = (HttpServletRequest) servletRequest; HttpSession session = httpServletRequest.getSession () / / if the attribute user of session is empty, forward to the home page if (session.getAttribute ("user") = = null) {servletRequest.getRequestDispatcher ("/ index.jsp") .forward (servletRequest,servletResponse);} else {/ / if the attribute user of session is not empty, release filterChain.doFilter (servletRequest,servletResponse) } @ Override public void destroy () {}} 2.3 HelloServlet program package com.filter.filter;import java.io.*;import javax.servlet.http.*;import javax.servlet.annotation.*;@WebServlet (name = "helloServlet", value = "/ hello-servlet") public class HelloServlet extends HttpServlet {private String message; public void init () {message = "Hello World!" } public void doGet (HttpServletRequest request, HttpServletResponse response) throws IOException {response.setContentType ("text/html"); / / Hello PrintWriter out = response.getWriter (); out.println ("); out.println (" + message + "); out.println ("); HttpSession session = request.getSession (); session.setAttribute (" user "," ningqian ") } public void destroy () {} 2.4web.xml FilterServlet com.filter.filter.FilterServlet FilterServlet / admin/* HelloServlet com.filter.filter.HelloServlet HelloServlet / hello 2.5index.jsp FilterServlet com.filter.filter.FilterServlet FilterServlet / admin/* HelloServlet com.filter.filter.HelloServlet HelloServlet / hello III. test

1. Start the server

two。 Enter: http:localhost:8080/filter/admin/1.jpg in the browser address bar to directly access the pictures under the admin directory because the files under this directory are configured as restricted in web.xml, so the request is received by FilterServlet at this time. Use the doFilter method to filter the request and find that there is no user attribute in the session session, so forward the request to index.jsp.

3. Enter: http:localhost:8080/filter/ homepage is not under the restricted directory in the browser address bar, so you can access it directly. Click the link HelloServlet on the page. At this point, the request is sent to the server, and the HelloServlet program receives it. Through the doGet () method, the user of the session session is assigned.

4. Type: http:localhost:8080/filter/admin/1.jpg in the browser again, and you can access it normally.

IV. The life cycle of Filter

The lifecycle of Filter consists of several methods

1. Constructor method

2. Init initialization method

Step 1, 2, executed when the web project starts (Filter has been created)

3. DoFilter filtering method

Step 3, each time a request is intercepted, it will be executed

4. Destroy destruction

Step 4, when you stop the web project, it will be executed (stop the web project, and also destroy the Filter filter)

Fifth, FilterConfig class

See the name of the FilterConfig class, which is the configuration file class for the Filter filter. Every time Tomcat creates a Filter, it also creates a FilterConfig class that contains the configuration information for the Filter configuration file. The purpose of the FilterConfig class is to get the configuration content of the filter filter

1. Get the name of Filter, the content of filter-name: filterConfig.getFilterName ()

2. Get the init-param initialization parameters configured in Filter (configured in web.xml): filterConfig.getInitParameter ("username")

3. Get the ServletContext object: filterConfig.getServletContext ()

AdminFiltercom.atguigu.filter.AdminFilterusernamerooturljdbc:mysql://localhost3306/ test VI, FilterChain filter chain

Multiple filter

7. Intercept path of Filter

Filter filter only cares about whether the requested address matches, not whether the requested resource exists!

Exact matching 8.1 directory matching / admin/*8.2 suffix name matching * .html "how to use Filter filters in JavaWeb" is introduced here, thank you for reading. If you want to know more about the industry, you can follow the website, the editor will output more high-quality practical articles for you!

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