How to use Filter and Listener in Java

Filter: filter

Concept

Filter in life: water purifier, air purifier, bandit, web Filters: When accessing server resources, filters can intercept requests and complete some special functions. The function of filter:

is generally used to complete common operations. Such as: login verification, unified encoding processing, sensitive character filtering...

Quick Start

Steps

1. Define a class and implement the interface Filter

2. Copy method

3. Configure interception path

• web.xml

• Annotation

Code

@WebFilter("/*")//访问所有资源之前，都会执行该过滤器
public class FilterDemo1 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("filterDemo1被执行了....");


		//放行
		filterChain.doFilter(servletRequest,servletResponse);

	}

	@Override
	public void destroy() {

	}
}

Filter details

web.xml configuration

<filter>
	<filter-name>demo1</filter-name>
	<filter-class>cn.zjq.web.filter.FilterDemo1</filter-class>
</filter>
<filter-mapping>
	<filter-name>demo1</filter-name>
	<!-- 拦截路径 -->
	<url-pattern>/*</url-pattern>
</filter-mapping>

Filter execution process

• Execution filter

• Resources after execution and release

• Come back and execute the code below the filter release code

Filter life cycle method

• init: After the server starts, the Filter object will be created and then the init method will be called. Only executed once. Used to load resources

• doFilter: Will be executed every time a request is intercepted for a resource. Executed multiple times

• destroy: After the server is shut down, the Filter object is destroyed. If the server is shut down normally, the destroy method will be executed. Only executed once. Used to release resources

Filter configuration details

Interception path configuration:

• Specific resources Path: /index.jsp The filter will be executed only when accessing the index.jsp resource

• Interception directory: /user/* When accessing all resources under /user, the filter will be executed Will be executed

• Suffix name interception: *.jsp When accessing all suffix name jsp resources, the filter will be executed

• Intercept all Resources: /* When accessing all resources, the filter will be executed

Interception method configuration: The way the resource is accessed

Annotation configuration:

• Set dispatcherTypes attribute

• REQUEST: Default value. The browser directly requests the resource

• FORWARD: forwards the access resource

• INCLUDE: contains the access resource

• ERROR: Error jumping resource

• ASYNC: Asynchronous access to resource

web.xml configuration setting label

Filter chain (configure multiple filters)

Execution order: If there are two filters: Filter 1 and Filter 2

• Filter1

• Filter2

• Resource Execution

• Filter2

• Filter 1

Filter order problem:

• Annotation configuration: according to the class name String comparison rules compare, the smaller value is executed first

Between AFilter and BFilter, AFilter is executed first. 2. web.xml configuration: Whoever defines it above will execute it first

Listener: listener

Concept: one of the three major components of the web. Event listening mechanism

• Event: an event

• Event source: where the event occurred

• Listener: an object

• Register listener: bind events, event sources, and listeners together. When an event occurs on the event source, execute the listener code

ServletContextListener: listen to the creation and destruction methods of the ServletContext object: void contextDestroyed(ServletContextEvent sce) : This method will be called before the ServletContext object is destroyed void contextInitialized(ServletContextEvent sce): This method will be called after the ServletContext object is created. Steps:

• Define a class to implement ServletContextListener Interface

• Copy method

• Configuration

web.xml

<listener>
  <listener-class>cn.zjq.web.listener.ContextLoaderListener</listener-class>
</listener>

Specify initialization parameters

Note:

@WebListener

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