SpringBoot 2.x （6）：使用Filter、Servlet、Listener

Filter的使用：

1.常见的默认Filter有：

characterEncodingFilter

hiddenHttpMethodFilter

httpPutFormContentFilter

requestContextFilter

2.Filter的优先级：

Ordered.HIGHEST_PRECEDENCE：最低优先级，值为int最大值
Ordered.LOWEST_PRECEDENCE：最高优先级，值为int最小值

低位值意味着更高的优先级

因此如果要自定义Filter，应该避免和默认的Filter优先级一样，不然会冲突

3.自定义Filter

可以使用Spring的FilterRegistrationBean，但是这种方式较为麻烦

所以这里采用Servlet3.0的方式：

结合实际应用的一个小Demo

Controller：

package org.dreamtech.springboot.controller;

import java.util.HashMap;
import java.util.Map;

import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RestController;

@RestController
public class DemoController {
    /**
     * 查看当前账户余额
     *
     * @return
     */
    @RequestMapping("/login/account")
    private Object account() {
        Map<String, Object> modelMap = new HashMap<String, Object>();
        modelMap.put("money", 1000);
        return modelMap;
    }
}

Filter：

package org.dreamtech.springboot.filter;

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;
import javax.servlet.annotation.WebFilter;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

@WebFilter(urlPatterns = "/login/*", filterName = "loginFilter")
public class LoginFilter implements Filter {

    /**
     * 容器加载时候调用
     */
    @Override
    public void init(FilterConfig filterConfig) throws ServletException {
        System.out.println("[ LoginFilter Init ]");
    }

    /**
     * 请求被拦截的时候调用
     */
    @Override
    public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain)
            throws IOException, ServletException {
        System.out.println("[ LoginFilter doFilter ]");

        HttpServletRequest req = (HttpServletRequest) request;
        HttpServletResponse resp = (HttpServletResponse) response;

        String username = req.getParameter("username");

        if (username != null) {
            chain.doFilter(req, resp);
        } else {
            System.out.println("[ username is null ]");
            return;
        }
    }

    /**
     * 容易被销毁的时候调用
     */
    @Override
    public void destroy() {
        System.out.println("[ LoginFilter Destroy ]");
    }

}

特别注意：要在SpringBoot启动类中加入@ServletComponentScan注解

package org.dreamtech.springboot;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.boot.web.servlet.ServletComponentScan;

@SpringBootApplication
@ServletComponentScan
public class SpringbootApplication {

    public static void main(String[] args) {
        SpringApplication.run(SpringbootApplication.class, args);
    }

}

启动后的结果：

访问：http://localhost:8080/login/account显示空白页面

访问：http://localhost:8080/login/account?username=xxx显示{"money":1000}

Servlet的使用：

package org.dreamtech.springboot.servlet;

import java.io.IOException;

import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

@WebServlet(urlPatterns = "/servlet/demo", name = "demoServlet")
public class DemoServlet extends HttpServlet {

    private static final long serialVersionUID = -428928425699550712L;

    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        resp.getWriter().print("demo servlet");
        resp.getWriter().flush();
        resp.getWriter().close();
    }

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        this.doGet(req, resp);
    }
}

注意：和Filter使用相同地，要在SpringBoot启动类中加入@ServletComponentScan注解

Listener的使用：

常见的Listener有

ServletContextListener、HttpSessionListener、ServletRequestListener

ServletRequestListener:用于统计（比如文章的浏览次数等）

Controller:

    @RequestMapping("/listener")
    private Object listener() {
        System.out.println("listener controller");
        return "listener";
    }

Listener:

package org.dreamtech.springboot.listener;

import javax.servlet.ServletRequestEvent;
import javax.servlet.ServletRequestListener;
import javax.servlet.annotation.WebListener;

@WebListener("/listener")
public class DemoListener implements ServletRequestListener {
    @Override
    public void requestDestroyed(ServletRequestEvent sre) {
        System.out.println("request destroyed");
    }

    @Override
    public void requestInitialized(ServletRequestEvent sre) {
        System.out.println("request initialized");
    }
}

打印顺序应该是：每一次访问/listener都会打印这三行

request initialized
listener controller
request destroyed

ServletContextListener:用于资源加载

package org.dreamtech.springboot.listener;

import javax.servlet.ServletContextEvent;
import javax.servlet.ServletContextListener;
import javax.servlet.annotation.WebListener;

@WebListener("/listener")
public class DemoListener implements ServletContextListener {
    @Override
    public void contextInitialized(ServletContextEvent sce) {
        System.out.println("context initialized");
    }

    @Override
    public void contextDestroyed(ServletContextEvent sce) {
        System.out.println("context destroyed");
    }
}

打印顺序：

一开始打印context initialized

结束进程的时候打印context destroyed

只打印一遍

至于HttpSessionListener不怎么使用，就不介绍了