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Filter、Listener、Servlet Collectively referred to as web Three components . This article only covers the first two , about servlet We will open another article to introduce in detail . about Listener Just to understand .

Filter

Concept

web filter ： When the browser accesses resources , The filter intercepts the browser's request , Used to complete some special functions .
effect ： It is generally used for some general operations . for example ： validate logon 、 Unified coding processing, etc .

Basic use

Filter Three steps for using ：

1. Create a class implementation Filter Interface
2. Replication method
3. Configure interception path （ Two configurations xml And annotation configuration ）

Related configuration

Configure interception path ：
The way 1：xml To configure
Written before servlet When , use xml The configuration has such a paragraph xml To configure ：

   <servlet>
        <servlet-name> name </servlet-name>
        <servlet-class> Full class name </servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name> name </servlet-name>
        <url-pattern> Access directory </url-pattern>
    </servlet-mapping>

and filter Of xml To configure , Exactly the same as this ：

<filter>
    <filter-name> name </filter-name>
    <filter-class> Full class name </filter-class>
</filter>
<filter-mapping>
    <filter-name> name </filter-name>
    <url-pattern>/*</url-pattern> <!--  The path here is not an access path , But the setting of the intercepted path ,/*  Means intercept all --> </filter-mapping> 

The way 2： Annotation configuration

@webFilter（" Intercept path "）

Interception process

filter During the intercept , Order as follows ：

1. Execute interceptor
2. Resources after implementation of release
3. Execute the code below the release statement

Life cycle

1. init： After the server starts , Will create filter object , call init Method . Only once , Used to load resources .
2. doFilter： Every time a blocked resource is requested , This method will be implemented .
3. destory： After the server is shut down ,filter Object destroyed . The server is shut down normally , This method executes once . Used to free resources .

Filter chain

If there are multiple filters , The order of execution is ：
for example ： There are three filters , The order of execution is as follows ：

1. filter 1
2. filter 2
3. filter 3
4. Resource execution
5. filter 3
6. filter 2
7. filter 1

For the filter priority problem ：

1. Annotation configuration ：
   Compare according to the string of class name , Small first .
2. xml To configure ：
   <filter-mapping></fliter-mapping> Sequential execution （ From top to bottom ）

Listener

Concept

Three elements of event monitoring mechanism ： event , Event source , Monitor
In a nutshell , Is to bind the three elements together , When the event triggers , Execute the contents of the listener .

Basic use

1. Define a class , Realization ServletContextListener Interface
2. Replication method
3. To configure
   3.1 web.xml：

 		<listener>
            <listener-class> Full class name </listener-class>
       </listener>

3.2 Annotation configuration @WebListener

filter and Listeber That's it , If there are any questions , Commentable , It will be corrected immediately .