十五、在Spring Boot中使用Servlet组件（Servlet、Filter、监听器）

  注意，在Spring Boot中不再直接支持Servlet组件，如若使用，需要一些额外的操作！！
  扩展：监听器有如下接口

javax.servlet.ServletContextListener
javax.servlet.HttpSessionListener
javax.servlet.HttpSessionActivationListener
javax.servlet.ServletRequestListener
javax.servlet.ServletContextAttributeListener
javax.servlet.HttpSessionAttributeListener
javax.servlet.HttpSessionBindingListener
javax.servlet.ServletRequestAttributeListener

一、注解方式注册Servlet组件

（一）注册Servlet

1、自定义Servlet类：MyServlet

/**
 * @WebServlet注解：指定该类为Servlet
 * name：类名
 * urlPatterns：映射路径（访问路径）
 * initParams：Servlet初始化参数，可以在生命周期方法init()中读取该参数
 */
@WebServlet(name = "myServlet", 
        urlPatterns = "/myservlet",
        initParams = {@WebInitParam(name = "initName", value = "initValue")}
)
public class MyServlet extends HttpServlet {

    private String value;
    
    @Override
    public void init(ServletConfig config) throws ServletException {
        //取出@WebServlet注解中的初始化参数
        value = config.getInitParameter("initName");
    }

    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        Writer writer = resp.getWriter();
        writer.write("这里是servlet！");
    }
}

2、在启动类上加​​@ServletComponentScan​​注解

/**
 * @ServletComponentScan：扫描Servlet组件
 * basePackages：指定Servlet类所在包名
 */
@SpringBootApplication
@ServletComponentScan(basePackages = {"org.pc.servlet.servlet"})
public class ServletApplication {

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

（二）注册Filter
  和注册Servlet差不多，不过注册注解换成​​​@WebFilter​​ 1、创建Filter

/**
 * OncePerRequestFilter:该类的作用是保证每次请求，该过滤器只会被调用一次（PS：在不同的请求方式下，
 * 比如forward/redirect等，过滤器的调用可能会有多次）
 *
 * @WebFilter：
 * servletNames：指定过滤哪一个servlet
 * urlPatterns：指定过滤哪一个访问请求
 */
//@WebFilter(servletNames = "myServlet")
@WebFilter(urlPatterns = "/myservlet")
public class MyFilter extends OncePerRequestFilter {
    @Override
    protected void doFilterInternal(HttpServletRequest httpServletRequest, HttpServletResponse httpServletResponse,
                                    FilterChain filterChain) throws ServletException, IOException {
        ServletContext servletContext = httpServletRequest.getServletContext();
        System.out.println("调用Filter1");
        filterChain.doFilter(httpServletRequest, httpServletResponse);

    }
}

2、指定扫描包名

/**
 * @ServletComponentScan：扫描Servlet组件
 * basePackages：指定Servlet组件所在包名
 */
@SpringBootApplication
@ServletComponentScan(basePackages = {"org.pc.servlet.servlet", "org.pc.servlet.filter"})
public class ServletApplication {

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

（三）注册Listener
  和注册Servlet差不多，不过注册注解换成​​​@WebListener​​ 1、自定义Listener

@WebListener
public class MyListener implements ServletRequestListener {

    @Override
    public void requestInitialized(ServletRequestEvent sre) {
        ServletRequest request = sre.getServletRequest();
        ServletContext servletContext = request.getServletContext();
        servletContext.log("Listener initial");
    }

    @Override
    public void requestDestroyed(ServletRequestEvent sre) {
        ServletRequest request = sre.getServletRequest();
        ServletContext servletContext = request.getServletContext();
        servletContext.log("Listener destroy");
    }
}

2、指定扫描包名

/**
 * @ServletComponentScan：扫描Servlet组件
 * basePackages：指定Servlet类所在包名
 */
@SpringBootApplication
@ServletComponentScan(basePackages = {"org.pc.servlet.servlet", "org.pc.servlet.filter", "org.pc.servlet.listener"})
public class ServletApplication {

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

二、Spring Boot API方式注册Servlet组件
  自定义Servlet、Filter、Listener参见前面，这里主要讲如何注册

（一）注册Servlet

/**
   * Spring Boot API方式注册Servlet组件
   */
  @Bean
  public static ServletRegistrationBean  servletRegistrationBean(){
    ServletRegistrationBean servletRegistrationBean = new ServletRegistrationBean();

    servletRegistrationBean.setName("myServlet2");
    servletRegistrationBean.setServlet(new MyServlet2());
    servletRegistrationBean.addUrlMappings("/myservlet2");
    servletRegistrationBean.addInitParameter("initName", "initValue");

    return servletRegistrationBean;
  }

（二）注册Filter

/**
   * Spring Boot API方式注册Filter组件
   */
  @Bean
  public static FilterRegistrationBean filterRegistrationBean(){
    FilterRegistrationBean filterRegistrationBean = new FilterRegistrationBean();

    filterRegistrationBean.setFilter(new MyFilter2());
    filterRegistrationBean.setServletNames(Arrays.asList("myServlet2"));
    /**
     * 设置拦截的请求类型
     */
    filterRegistrationBean.setDispatcherTypes(DispatcherType.REQUEST, DispatcherType.FORWARD, DispatcherType.INCLUDE);

    return filterRegistrationBean;
  }

（二）注册Listener

/**
   * Spring Boot API方式注册Listener组件
   */
  @Bean
  public static ServletListenerRegistrationBean servletListenerRegistrationBean(){
    ServletListenerRegistrationBean servletListenerRegistrationBean = new ServletListenerRegistrationBean();
    servletListenerRegistrationBean.setListener(new MyListener2());
    return servletListenerRegistrationBean;
  }