三个注解都必须在启动类增加@ServletComponentScan才能够被扫描到。

@WebServlet

可用于根据不同条件，重定向请求至不同URL。
示例代码

@WebServlet(name = "/my")
@Slf4j
public class MyWebServlet extends HttpServlet {

    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) {
        log.info("进入get方法");
        log.info(req.getParameter("name"));
        doPost(req,resp);
    }

    @Override
    protected void doHead(HttpServletRequest req, HttpServletResponse resp) {
        log.info("进入get方法");
    }

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) {
        log.info("进入post方法");
        log.info(req.getParameter("name"));
    }

    @Override
    protected void doPut(HttpServletRequest req, HttpServletResponse resp) {
        log.info("进入put方法");
    }

    @Override
    protected void doDelete(HttpServletRequest req, HttpServletResponse resp) {
        log.info("进入delete方法");
    }

    @Override
    protected void doOptions(HttpServletRequest req, HttpServletResponse resp) {
        log.info("进入option方法");
    }

    @Override
    public void destroy() {
        log.info("servlet>>>destroy");
    }

    @Override
    public void init() {
        log.info("servlet>>>init");
    }
}

使用postman分别通过get、post、delete等进行请求http://localhost:8080/my?name=张三，可以查看输出结果

@WebFilter

可用于拦截/过滤请求，提前对请求的request进行判断处理，在doFilter后过滤器放行，调用实际请求URL路径。
示例代码

@WebFilter("/my")
@Slf4j
public class MyWebFilter implements Filter {

    @Override
    public void init(FilterConfig filterConfig) throws ServletException {
        log.info("filter>>>init");
    }

    @Override
    public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain) throws IOException, ServletException {
        log.info("进入doFilter方法");
        filterChain.doFilter(servletRequest,servletResponse);
    }

    @Override
    public void destroy() {
        log.info("filter>>>destroy");
    }
}

再次请求，查看输出结果
[alt type="error"]注意看输出顺序[/alt]

@WebListener

在spring启动之前监听执行初始化操作，可用于提前加载缓存等。

@WebListener
@Slf4j
public class MyWebListener implements ServletRequestListener {

    @Override
    public void requestDestroyed(ServletRequestEvent sre) {
        log.info("requestDestroyed");
    }

    @Override
    public void requestInitialized(ServletRequestEvent sre) {
        log.info("requestInitialized");
    }
}

输出结果