Filter，过滤器

有了 servlet 的经验，学习如何编写和使用 filter 就简单多了。

HelloFilter

import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;

public class HelloFilter implements Filter {

    @Override
    public void init(FilterConfig arg) throws ServletException {
        System.out.println("HelloFilter initialized");
    }

    @Override
    public void doFilter(ServletRequest req, ServletResponse res,
                         FilterChain chain) throws IOException, ServletException {
        HttpServletRequest request = (HttpServletRequest)req;
        System.out.println("HelloFilter touched by " + request.getRequestURI());
        chain.doFilter(req, res);
    }

    @Override
    public void destroy() {
        System.out.println("HelloFilter destroyed");
    }
}

编译

Filter 的编译同样依赖 servlet-api：

javac -cp ${CATALINA_BASE}/lib/servlet-api.jar HelloFilter.java

在 Tomcat 中使用 Filter

<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns="http://xmlns.jcp.org/xml/ns/javaee"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee
                             http://xmlns.jcp.org/xml/ns/javaee/web-app_4_0.xsd"
         version="4.0"
         metadata-complete="true">

    <filter>
        <filter-name>hello</filter-name>
        <filter-class>HelloFilter</filter-class>
    </filter>

    <filter-mapping>
        <filter-name>hello</filter-name>
        <url-pattern>/*</url-pattern>
    </filter-mapping>

    <servlet>
        <servlet-name>hello</servlet-name>
        <servlet-class>HelloWorld</servlet-class>
    </servlet>

    <servlet-mapping>
        <servlet-name>hello</servlet-name>
        <url-pattern>/hello</url-pattern>
    </servlet-mapping>

</web-app>