Filters, interceptors, listeners

filter 、 Interceptor 、 The monitor is tomcat Position in container

 

 

Concept
1、servlet：servlet It's a kind of running on the server side java Applications , Platform - and protocol-independent , It can generate web It works in the middle layer between the client request and the server response ;

2、filter：filter It's a reusable piece of code , It can be used to convert HTTP request , Response and header information . It cannot generate a request or response , It just modifies the request or response to a resource ;

3、listener： Monitor , adopt listener Can be strong web An execution action in the server , And make corresponding response according to its requirements . Is in the application,session,request Three objects are functional components that automatically execute code when creating, dying, or adding, modifying, or deleting properties ;

4、interceptor： Interceptors are more refined applications of filters , It can be applied not only to service Before and after the method can also be applied to before and after other methods Interceptor ;

The difference between filter and interceptor
1、 The interceptor is based on java The reflection mechanism of , And filters are based on function callbacks .

2、 Interceptors don't rely on servlet Containers , The filter depends on servlet Containers .

3、 Interceptors can only be used to action The request works , Filters work on almost all requests .

4、 Interceptors can access action Context 、 Value the objects in the stack , And the filter can't access .

5、 stay action In the life cycle of , Interceptors can be called many times , The filter can only be called once when the container is initialized .

6、 Interceptors can get IOC Each of the containers bean, And filters don't work , That's important , Inject a... Into the interceptor service, You can call the business logic

filter 、 Interceptor 、 The life cycle of the listener
1.servlet

servle The life cycle of starts with being loaded web In the memory of the server , And in web Service termination or reload servlet It's over ;
servlet Once loaded web The server , Generally not from web Delete... From server memory , until web Server down ;
Load ： Load when starting the server servlet Example ;
initialization ：web When the server receives the request , Or start at some point in between , call init（）;
call ： From the first visit to many subsequent visits , Only call doGet（） or dopost）（） Method ;
The destruction ; Called when the server is stopped destroy（） Method , Destroy instance ;

2.Filter
Need to achieve javax.servlet Bag Filter Three methods of interface init（）,doFilter（）,destroy（）;
load ： Load an instance of the filter when starting the server , And call init（） Method ;
call ： Only methods are called each time a request is made doFilter（） To deal with ;
The destruction ： Call before the server shuts down destroy（） Method , Destroy instance ;

3.Listener
web.xml The loading order of is ：context-param->listener->filter->servlet

4.Interceptor
Initialize the interceptor after loading the configuration file , When someone is right action At the request of , call interceptor Method , Finally, it is destroyed according to the stop of the server

duty
1、Servlet

Create and return a complete... Containing dynamic content based on the nature of the customer request html page
Create a that can be embedded into an existing html Part of the page html page （html fragment ）
Read the hidden data sent by the client
Read the display data sent by the client
With other server resources （ Including databases and java Applications for ） communicate
2.Filter

filter Can arrive at a request servlet Pre process user requests , You can also leave servlet Time processing http Respond to ：
In execution servlet Before , First, execute filter Program , And do some pretreatment for it ;
stay servlet Intercept after being called servlet Implementation
4.Listener
Listening container 、 application 、 Creation and destruction of requests , And set the attribute value during
servlet2.4 The specification provides 8 individual listener Interface , It can be divided into three categories , They are as follows ;

The first category ： And HttpContext Relevant listener Interface , Include ：ServletContextListener、ServletContextAttributeListener

The second category ： And HttpSession Relevant listner Interface . Include ：HttpSessionListener、HttpSessionAttributeListener、 HttpSessionBindingListener、 HttpSessionActivationListener

The third category ： And ServletRequest Relevant Listener Interface , Include ：ServletRequestListener、ServletRequestAttributeListener

5.Interceptor
Similar to filters , Through layers of interception , Handle user requests and responses ;
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