JavaWeb三大组件：Servlet、Filter、Listener

1.Filter (filter)

1.1. The concept

Web中的过滤器: When the resource access server, the filter can intercept the request to complete some special features

1.2. Role filter

一般用于完成通用的操作Such as login authentication, unified coding process, sensitive character filtering

1.3. Filter Quick Start

step:

1.定义一个类，实现接口Filter

A new or directly the Filter

the Filter can modify the template, and a method similar modifications Servlet template:

2.覆写方法
3.配置拦截路径

1.注解的配置

2.web.xml配置

Quick start code is as follows:

package xpu.edu.web.Filter;

import javax.servlet.*;
import javax.servlet.annotation.WebFilter;
import java.io.IOException;

@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...");

        //放行，不然访问不到资源
        filterChain.doFilter(servletRequest,servletResponse);
    }

    @Override
    public void destroy() {

    }
}

1.4. Filter details

1.4.1. Filter execution flow

执行过滤器
执行放行后的资源
回来执行过滤器放行代码下面的代码

package xpu.edu.web.Filter;

import javax.servlet.*;
import javax.servlet.annotation.WebFilter;
import java.io.IOException;

@WebFilter("/*")
public class FilterDemo2 implements Filter {
    public void destroy() {
    }

    public void doFilter(ServletRequest req, ServletResponse resp, FilterChain chain) throws ServletException, IOException {
        //对request对象请求参数增强
        System.out.println("FilterDemo2执行了");
        //放行
        chain.doFilter(req, resp);
        //对response对象的响应消息增强
        System.out.println("FilterDemo2回来了");
    }

    public void init(FilterConfig config) throws ServletException {

    }
}

1.4.2 Lifecycle filter method

init: After the server is started, it will create a Filter object, and then call the init method is executed only once, usually used to load resources
doFilter: Every request will be executed when intercepted resources, perform multiple
destroy: After the server is shut down, Filter object is destroyed, if the server is shut down properly, it will perform the destroy method is performed only once, typically used for releasing resources

1.4.3. Filter Configuration Explanation

拦截路径的配置

1. Specific resource path: such as "/index.jsp", when the only access index.jsp resources, the filter will be executed

2. Directory interception: such as "/ user / *", all resource filters will be executed under the user access

3. extension interception: such as "* .jsp", resource access all jsp suffix filters will be executed

4. intercept all the resources: "/ *", the filter will have access to all the resources to perform

拦截方式的配置

Interception way: how resources are being accessed, such as the browser to request access to or forwarding

Interception way configuration

1. Notes Configuration: dispatcherTypes property, there are five values (actually enumeration):

REQUEST: default browser directly request resources

FORWARD: Forwarding access to resources

INCLUDE: includes access to resources

ERROR: error Jump Resources

ASYNC: asynchronous access resources

Configuring multiple values:

2.web.xml
Settings <dispatcher> </ dispatcher> tag to

1.4.4. Filter chain (a plurality of filter)

If there are two filters, filter 1 back to the front of the filter 2, they are 执行顺序as follows:

Filter 1
Filter 2
Resources to implement
Filter 2
Filter 1

There are two filters, know how to filter 1 or filter 2 first? Has the problem:和配置有关

1. Configuration Annotations: comparison string comparison rules in accordance with the class name, the first value is smaller. Such as: AFilter and BFilter, AFilter first execution
2.web.xml configuration: <filter-mapping> who defined above whoever

2.Listener (listeners)

2.1. The concept

Listener: Web的三大组件之一, sensing mechanism of events

Event: One thing

Event Source: Local happenings

Listener: An object

Register listeners: event, event source, the listener bound, when an event occurs on the event source, the listener code execution

2.2.ServletContextListener

ServletContextListener: creating and destroying listening ServletContext object, which has two methods:

void contextDestroyed (ServletContextEvent sce): This method is called when the object is destroyed ServletContext
void contextInitialized (ServletContextEvent sce): This method is called after ServletContext object is created