What is servlet filter

The servlet filter is the middle layer component of server and client requests and responses. It is mainly used to filter browser requests and forward the filtered requests to the next resource.

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Basic concept of servlet filter

The literal meaning of Servlet filter is that a level of filtering is required to meet the usage requirements. In fact, Servlet filter is the middle layer component of server and client requests and responses. In actual project development, Servlet filters are mainly used to filter browser requests and then transfer the filtered requests to the next resource.

Filter is a new feature added after Servlet 2.3. When you need to restrict users from accessing certain resources or process certain resources in advance when processing requests, you can use filters.

Filters are bound to WEB applications in the form of a component. Different from other WEB application components, filters are processed in a "chain" manner.

Filter

Servlet filter Fileter is a small web component that intercepts requests and responses in order to View, extract or in some way manipulate the data exchanged between the client and the server to achieve the "filtering" function. Filter usually encapsulates some functional web components. Filter provides an object-oriented modular mechanism to encapsulate tasks into a pluggable component. Filter components are declared through configuration files and dynamically proxied.

Simply put, the Servlet Filter is:

● Declarative: By declaring it in the web.xml configuration file, filters are allowed to be added and deleted without changing any application code or jsp page.

● Flexible: The filter can be used to directly call the client to perform pre-processing and post-processing work. Some flexible functions can be achieved through the filter chain.

● Portable: Since each web container today is designed based on Servlet specifications, Servlet filters are also cross-container.

● Reusable: Based on its portability and declarative configuration method, Filter is reusable.

In general, Servlet filters are modular reusable components that are flexibly declared through a configuration file. Filters dynamically intercept incoming requests and outgoing responses, adding or removing them transparently without modifying program code. It is independent of any platform and web container.

Filter working principle

Based on the description of Filter architecture, we can see the working principle of Filter, which is simply demonstrated through a flow chart:

When the client browser accesses a specific resource of the web server, after the relevant processing of the code1 code block in filter 1, it passes the request request to the filter chain. The next filter 2, (the order of the filter chain is based on the order in the configuration file) after filter 2 is processed, the request completes the corresponding logic for the Servlet passed. The process of returning a response is similar, except that the order of the filter chain is reversed, so I won’t go into details here.

Filter creation process

To write a filter, you must implement the Filter interface. Implement the methods specified by its interface.

★ Implement the javax.servlet.Filter interface

★ Implement the init method and read the initialization parameters of the filter

★ Implement the doFilter method to complete the filtering of requests or responses

★ Call the doFilter method of the FilterChain interface object to pass the request or response to the subsequent filter

A most basic filter implementation code:

java class :

import java.io.IOException;

import javax.servlet.Filter;
import javax.servlet.FilterChain;
import javax.servlet.FilterConfig;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;

public class BasicsFilter implements Filter {

    public BasicsFilter() {
        // 容器（服务器）启动时执行
    }

    /**
     * @see Filter#init(FilterConfig)
     */
    public void init(FilterConfig fConfig) throws ServletException {
        /* 容器（服务器）时执行 */
        System.out.println("======== 初始化方法 ========");
        // 获取的是web.xml中配置Filter时设置的值，参数为设置值得名称，若参数不存在，则返回空
        String initParam = fConfig.getInitParameter("param");
        System.out.println("param ========" + initParam);
    }

    /**
     * @see Filter#doFilter(ServletRequest, ServletResponse, FilterChain)
     */
    public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException {
        /* 在访问的时候执行 */
        System.out.println("======= 开始执行doFilter ========");
        // 转发到下一个组件，进行后续的处理（组件可以是一个过滤器，也可以是一个servlet）
        chain.doFilter(request, response);
        System.out.println("======= 结束执行doFilter ========");
        
    }

    /**
     * @see Filter#destroy()
     */
    public void destroy() {
        // TODO Auto-generated method stub
    }
}

web.xml configuration:

<filter>
    <filter-name>basicsFilter</filter-name>
    <filter-class>com.hudongwu.filter.BasicsFilter</filter-class>
    <init-param><!-- 设置在过滤器中执行初始化方法时，获取的值 -->
        <param-name>param</param-name>
        <param-value>studyFilter</param-value>
    </init-param>
</filter>
<filter-mapping>
    <filter-name>basicsFilter</filter-name>
    <!-- 设置为访问该网站所有地址都需要通过该过滤器 -->
    <url-pattern>/*</url-pattern>
    <!-- 设置为只有访问该网站的/Helloword地址时才通过该过滤器 -->
    <url-pattern>/Helloword</url-pattern>
</filter-mapping>

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