Introduction and detailed explanation of filter Filter

1. Introduction

In Java Web development,FilterIt is a very important component for pre-processing or post-processing of requests and responses before they arrive at the Servlet or before the response returns to the client.

FilterIt can be used to implement various functions, such as logging, permission checking, encoding conversion, request header modification, etc. It’s like the layers of security checks at the airport, where passengers who come check and filter, carry illegal items, and do not buy air tickets, etc., will be blocked from entering.

How it works

Configuration:existConfigure in the file or using annotationsFilter。

• existMedium configuration

<filter>
    <!--set upfilterAlias ​​of-->
    <filter-name>LoggingFilter</filter-name>
    <!--filterbytecode path-->
    <filter-class></filter-class>
</filter>

<filter-mapping>
  	<!--usefilterThe filter path corresponding to the alias，There can be multiple-->
    <filter-name>LoggingFilter</filter-name>
     <!--/*Indicates filtering all paths-->
    <url-pattern>/*</url-pattern>
    <!--What to filterservletAlias ​​of-->
    <servlet-name>servlet1</servlet-name>
</filter-mapping>

Use Notes@WebFilter, It has the following commonly used values:

1. filterName: The alias of filter is equivalent to the tag
2. urlPatterns: The resource url to be filtered is equivalent to the tag
3. ServletNames: The servlet alias to be filtered, equivalent to servletNames

@WebFilter(
        filterName = "loggingFilter",
        urlPatterns = {"/servlet1","*.html"},
        servletNames = {"servlet1","Servlet2"}
)

Intercept：

• When the request arrives,FilterRequests are intercepted and preprocessing logic is executed.
• Some operations performed before the request reaches the target resource, such as checking whether the user has permission to access records, requests, and responses information

Let go：

• FilterYou can choose whether to release the request to the target resource (such as a Servlet). It will be executed at this timeFilterChainofdoFilterMethod represents release.
• FilterChainexpressFilterIf there are other links for this resource, there are other links in the futureFilterTo filter, at this timedoFilterThe method will execute otherFilter;If there is noFilterTo filter, it will be released and the target resource will be processed (such as servlet)

Post-processing：

• After the target resource (such as Servlet) has processed the request
• FilterResponse can be post-processed

Life cycle

Including the firstcreate、Initialize、filteranddestroyFour stages.

When you pay special attention to itFilterCreated and initialized when the web application is started, this process only occurs once.

The order of execution

In a web project, multiple filters can be defined at the same time. When multiple filters filter the same resource, the working positions are successively formed, and a working chain is formed as a whole, which is called the filter chain (FilterChain）

• When configuring with configuration files
• The order of filters in the filter chain is determined by the order of definition of the <filter-mapping> tag
• When configuring using annotations
• Usually all filters are placed in one package. At this time, the execution order is the dictionary sort of class names, which is executed from small to large.

5. An example of a simple Filter

Case requirements:

When a user accesses a resource, check whether it logs in. If it is not logged in, it will jump to the login page. If it is logged in, it will be released.

@WebFilter("/*")
public class CheckLoginFilter implements Filter {
    @Override
    public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain)
            throws IOException, ServletException {
        //Transform downward, realize redirection, obtain session and other functions        HttpServletRequest httpRequest = (HttpServletRequest) request;
        HttpServletResponse httpResponse = (HttpServletResponse) response;

        // Check whether the user is logged in        String user = (String) ().getAttribute("username");
        if (user == null) {
            // The user is not logged in, redirect to the login page            (() + "/login");
        } else {
            // The user has logged in and released the request. Note that the doFilter method in FilterChain is called!  !            (request, response);
        }
    }
}

Summarize

The above is personal experience. I hope you can give you a reference and I hope you can support me more.