Filter and listener

filter

What is a filter

The filter is actually right web Resource advance ⾏ Intercept , do ⼀ After some processing, I'll give it to ⼀ A filter or servlet Handle , Is usually ⽤ To intercept request Into the ⾏ To deal with the , It can also be used for the returned response Into the ⾏ Intercept processing

Filter syntax format

• 1. Create a class to implement the interface

public class SignInFilter implements Filter{}

• 2. Override methods in the interface

/* initialization ⽅ Law   receive ⼀ individual FilterConfig Parameters of type   This parameter is right Filter Of ⼀ Some configuration */
 public void init(FilterConfig filterConfig) throws ServletException {
    
        System.out.println(" Filter initialization ");
    }

/* Filter ⽅ Law   It's mainly about request and response Into the ⾏⼀ Some treatment , Then give it to ⼀ A filter or Servlet Handle */
public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain) throws IOException, ServletException {
    
        System.out.println(" The filter performs processing operations ");
    }

/* Destruction method */
 public void destroy() {
    
        System.out.println(" The filter has been destroyed ");
    }

• 3. stay web.xml Configuration in file

    <filter>
        <filter-name> Filter name </filter-name>
        <filter-class>
             The path of the filter 
        </filter-class>
    </filter>

    <filter-mapping>
        <filter-name> Filter name </filter-name>
        <url-pattern> Resources that need to be filtered </url-pattern>
    </filter-mapping>

Filter to prevent users from entering the post login page without logging in

Here, we use annotation instead of web.xml Configuration in file

@WebFilter("*")
public class SignInFilter implements Filter {
    
    @Override
    public void init(FilterConfig filterConfig) throws ServletException {
    
        System.out.println(" Filter initialization ");
    }

    @Override
    public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain) throws IOException, ServletException {
    
        System.out.println(" The filter starts to operate ");
        HttpServletRequest request =(HttpServletRequest) servletRequest;
        HttpServletResponse response=(HttpServletResponse) servletResponse;
        request.setCharacterEncoding("utf-8");

       String requestURI =request.getRequestURI();
        HttpSession session = request.getSession();
        Object username=session.getAttribute("username");
        if (requestURI.endsWith("SignUp.jsp")&& username==null){
    
            response.sendRedirect("SignIn.html");
        }
        filterChain.doFilter(servletRequest,servletResponse);
        System.out.println(" End of filter ");
    }

    @Override
    public void destroy() {
    
        System.out.println(" The filter has been destroyed ");
    }
}

Monitor

What is a monitor

A listener is a component that listens for changes in the state of a domain object

The concept of the listener ：

• Event source ： Monitored objects ( Three domain objects request、session、servletContext)
• Monitor ： Listen to the event source object. Changes in the state of the event source object will trigger the listener
• Register listener ： Connect the listener to the event source ⾏ binding
• Respond to ⾏ by ： The function code involved when the listener listens to the state change of the event source （ Programmers write code ）

Listener classification

The first dimension

According to the monitored objects ：ServletRequest Domain 、HttpSession Domain 、ServletContext Domain

The second dimension

According to the monitoring content ： Listen for the creation and destruction of domain objects 、 Listen for changes in the properties of domain objects

\	ServletContext Domain	HttpSession Domain	ServletRequest Domain
Creation and destruction of domain objects	ServletContextListener	HttpSessionListener	ServletRequestListener
Changes in attributes within domain objects	ServletContextAttributeListener	HttpSessionAttributeListener	ServletRequestAttributeListener

A listener that monitors the creation and destruction of three domain objects

The writing steps of the listener

1. To write ⼀ A listener class to implement the listener connection ⼝
2. Cover the of the listener ⽅ Law
3. Need to be in web.xml Chin in ⾏ To configure — register

Example

monitor session Creation and destruction of
Here, we use annotation instead of web.xml Configuration in file

@WebListener("*")
public class SessionListener implements HttpSessionListener {
    
    @Override
    public void sessionCreated(HttpSessionEvent httpSessionEvent) {
    
        System.out.println("session establish ");
    }

    @Override
    public void sessionDestroyed(HttpSessionEvent httpSessionEvent) {
    
        System.out.println("session remove ");
    }
}

appendix

ServletContextListener
monitor ServletContext A listener for creating and destroying domains

Servlet Domain ⽣ Life cycle
When to create ： Server start creation
When to destroy ： The server is shut down and destroyed

ServletContextListener The main work of the listener ⽤
The initialization of the ⼯ do ： Initialize object 、 Initialization data （ Load database driver 、 Initialization of connection pool ）、 load ⼀ Some initial configurations ⽂ Pieces of (spring Configuration of ⽂ Pieces of ）、 Task scheduling ( Timer —Timer/TimerTask）

HttpSessionListener
monitor Httpsession Listener for domain creation and destruction

HttpSession Object's ⽣ Life cycle
When to create ： The first ⼀ Secondary modulation ⽤request.getSession Created on
When to destroy ： The server is shut down and destroyed 、session Be overdue （ Default 30 minute , Modify the default 30 Minutes is at Tomcat Of web.xml, Modify the current item ⽬ The expiration date of is ⾃⼰ term ⽬ Of web.xml in ）、⼿ Move to destroy

HttpSessionListener The main work of the listener ⽤：
Because every visit ⽹ Stations will be created by default session object （jsp⻚⾯ in page Directive session Property defaults to true, I.e. created when accessed session）, Sure ⽤ On count ⽹ Station visited ⼈

ServletRequestListener
monitor ServletRequest Listeners for domain creation and destruction