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Filter and listener listeners

2022-04-21 12:52:00 【InfoQ】

init() Method ： Create after the server starts Filter object , call init Method , Only once , Used to load resources

doFilter() Method ： Called every time a request is intercepted and a resource is requested , Multiple executions

destory() Method ： Called when the server shuts down normally distory Method destruction Filter object , Only once , Used to free resources

@WebFilter("/*") // This filter is executed before accessing all resources

public class myFilter implements Filter {

Create after the server starts Filter object , call init Method , Only once , Used to load resources

public void init(FilterConfig filterConfig) throws ServletException {

System.out.println(" initialization ");

}

Called every time a request is intercepted and a resource is requested , Multiple executions

public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain) throws IOException, ServletException {

System.out.println("oneStar");

// release

filterChain.doFilter(servletRequest,servletResponse);

System.out.println("BackoneStar");

}

Called when the server shuts down normally distory Method destruction Filter object , Only once , Used to free resources

public void destroy() {

System.out.println(" The destruction ");

}

5、 Filter configuration problem

【1】 Intercept path configuration

Specific resource paths ：/index.jsp ==> Only visit index.jsp Resource time , The filter will be executed

Intercept Directory ：/user/* ==> visit /user All resources under , Filters are executed

Suffix blocking ：*.jsp ==> Access all suffixes and ask jsp Resource time , Filters are executed

Intercept all resources ：/* ==> Access any resource , Filters are executed

【2】 Configuration of interception mode

Annotation configuration （ Set the name of the configuration file dispatcherTypes attribute ）

REQUEST： The default value is , Browser requests resources directly

FORWARD： Forward access to resources

INCLUDE： Contains access to resources

ERROR： Wrong jump resource

ASYNC： Asynchronous access to resources

web.xml To configure

To configure <dispatcher></dispatcher> Labels can be

// The browser directly requests index.jsp Resource time , The filter will be executed

//@Web

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Filter(value = "/index.jsp",dispatcherTypes = DispatcherType.REQUEST)

// Forward only index.jsp Resource time , The filter will be executed

//@WebFilter(value = "/index.jsp",dispatcherTypes = DispatcherType.FORWARD)

// The browser directly requests index.jsp resources , forward index.jsp Resource time , The filter will be executed

@WebFilter(value = "/index.jsp",dispatcherTypes = {DispatcherType.REQUEST,DispatcherType.FORWARD})

public class myFilter implements Filter {

6、 Filter chain （ Configure multiple filters ）

【1】 Execution order

filter 1

filter 2

Resource execution

filter 2

filter 1

【2】 Filter order

Annotation configuration ： Compare according to the string comparison rules of class names , If it's worth less, execute it first

eg：AFilter and BFilter,AFilter Execute first

web.xml：<filter-mapping> Whoever is defined above will execute first

Two 、Listener Monitor

==============

A listener is an object specially used to monitor and process events or state changes that occur on other objects , When something happens to the monitored object , state

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That is, take corresponding action . A listener is actually a common implementation of a specific interface java Program , This program is dedicated to listening to another java Object's method call or property change , When the monitored object occurs the above event , Listener a method is executed immediately . For example, monitor the number of online users .

1、ServletContextListener Interface