springboot設置過濾器、監聽器、攔截器

其實這篇文章算不上是springboot的東西，咱們在spring普通項目中也是能夠直接使用的

設置過濾器：

之前在普通項目中咱們要在web.xml中進行filter的配置，可是隻從servlet 3.0後，咱們就能夠在直接在項目中進行filter的設置，由於她提供了一個註解@WebFilter（在javax.servlet.annotation包下），使用這個註解咱們就能夠進行filter的設置了，同時也解決了咱們使用springboot項目沒有web.xml的尷尬，使用方法以下所示

@WebFilter(urlPatterns="/*",filterName="corsFilter", asyncSupported = true)
public class CorsFilter implements Filter{

    @Override
    public void init(FilterConfig filterConfig) throws ServletException {

    }

    @Override
    public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse,
            FilterChain chain) throws IOException, ServletException {
        HttpServletResponse response = (HttpServletResponse)servletResponse; 
        HttpServletRequest request = (HttpServletRequest)servletRequest;
        chain.doFilter(servletRequest, servletResponse);
    }

    @Override
    public void destroy() {

    }

}

其實在WebFilter註解中有一些屬性咱們須要進行設置， 好比value、urlPatterns，這兩個屬性其實都是同樣的做用，都是爲了設置攔截路徑，asyncSupported這個屬性是設置配置的filter是否支持異步響應，默認是不支持的，若是咱們的項目須要進行請求的異步響應，請求通過了filter，那麼這個filter的asyncSupported屬性必須設置爲true否則請求的時候會報異常。

設置攔截器：

編寫一個配置類，繼承org.springframework.web.servlet.config.annotation.WebMvcConfigurerAdapter或者org.springframework.web.servlet.config.annotation.WebMvcConfigurationSupport並重寫addInterceptors(InterceptorRegistry registry)方法，其實父類的addInterceptors(InterceptorRegistry registry)方法就是個空方法。使用方法以下：

@Configuration
public class MvcConfig extends WebMvcConfigurationSupport {

    @Override
    public void addInterceptors(InterceptorRegistry registry) {
        InterceptorRegistration registration = registry.addInterceptor(new HandlerInterceptor() {
            @Override
            public boolean preHandle(HttpServletRequest request, HttpServletResponse response, Object handler) throws Exception {
                return true;
            }

            @Override
            public void postHandle(HttpServletRequest request, HttpServletResponse response, Object handler, ModelAndView modelAndView) throws Exception {

            }

            @Override
            public void afterCompletion(HttpServletRequest request, HttpServletResponse response, Object handler, Exception ex) throws Exception {

            }
        });
        // 配置攔截路徑
        registration.addPathPatterns("/**");
        // 配置不進行攔截的路徑
        registration.excludePathPatterns("/static/**");
    }
}

配置監聽器：

通常咱們經常使用的就是request級別的javax.servlet.ServletRequestListener和session級別的javax.servlet.http.HttpSessionListener，下面以ServletRequestListener爲例，編寫一個類實現ServletRequestListener接口並實現requestInitialized(ServletRequestEvent event)方法和requestDestroyed(ServletRequestEvent event)方法，在實現類上加上@WebListener（javax.servlet.annotation包下），以下所示

@WebListener
public class RequestListener implements ServletRequestListener {

    @Override
    public void requestDestroyed(ServletRequestEvent sre) {
        System.out.println("請求結束");
    }

    @Override
    public void requestInitialized(ServletRequestEvent sre) {
        System.out.println("請求開始");
    }
}

這樣每個請求都會被監聽到，在請求處理前equestInitialized(ServletRequestEvent event)方法，在請求結束後調用requestDestroyed(ServletRequestEvent event)方法，其實在spring中有一個很是好的例子，就是org.springframework.web.context.request.RequestContextListener類

public class RequestContextListener implements ServletRequestListener {

    private static final String REQUEST_ATTRIBUTES_ATTRIBUTE =
            RequestContextListener.class.getName() + ".REQUEST_ATTRIBUTES";


    @Override
    public void requestInitialized(ServletRequestEvent requestEvent) {
        if (!(requestEvent.getServletRequest() instanceof HttpServletRequest)) {
            throw new IllegalArgumentException(
                    "Request is not an HttpServletRequest: " + requestEvent.getServletRequest());
        }
        HttpServletRequest request = (HttpServletRequest) requestEvent.getServletRequest();
        ServletRequestAttributes attributes = new ServletRequestAttributes(request);
        request.setAttribute(REQUEST_ATTRIBUTES_ATTRIBUTE, attributes);
        LocaleContextHolder.setLocale(request.getLocale());
        RequestContextHolder.setRequestAttributes(attributes);
    }

    @Override
    public void requestDestroyed(ServletRequestEvent requestEvent) {
        ServletRequestAttributes attributes = null;
        Object reqAttr = requestEvent.getServletRequest().getAttribute(REQUEST_ATTRIBUTES_ATTRIBUTE);
        if (reqAttr instanceof ServletRequestAttributes) {
            attributes = (ServletRequestAttributes) reqAttr;
        }
        RequestAttributes threadAttributes = RequestContextHolder.getRequestAttributes();
        if (threadAttributes != null) {
            // We're assumably within the original request thread...
            LocaleContextHolder.resetLocaleContext();
            RequestContextHolder.resetRequestAttributes();
            if (attributes == null && threadAttributes instanceof ServletRequestAttributes) {
                attributes = (ServletRequestAttributes) threadAttributes;
            }
        }
        if (attributes != null) {
            attributes.requestCompleted();
        }
    }

}

在這個類中，spring將每個請求開始前都將請求進行了一次封裝並設置了一個threadLocal，這樣咱們在請求處理的任何地方均可以經過這個threadLocal獲取到請求對象，好處固然是有的啦，好比咱們在service層須要用到request的時候，能夠不須要調用者傳request對象給咱們，咱們能夠經過一個工具類就能夠獲取，豈不美哉。

擴充：在springboot的啓動類中咱們能夠添加一些ApplicationListener監聽器，例如：

@SpringBootApplication
public class DemoApplication {

    public static void main(String[] args) {
        SpringApplication application = new SpringApplication(DemoApplication.class);
        application.addListeners(new ApplicationListener<ApplicationEvent>() {
            @Override
            public void onApplicationEvent(ApplicationEvent event) {
                System.err.println(event.toString());
            }
        });
        application.run(args);
    }
}

ApplicationEvent是一個抽象類，她的子類有不少好比ServletRequestHandledEvent（發生請求事件的時候觸發）、ApplicationStartedEvent（應用開始前觸發，作一些啓動準備工做）、ContextRefreshedEvent（容器初始化結束後觸發），其餘還有不少，這裏再也不多說，可是這些ApplicationListener只能在springboot項目以main方法啓動的時候纔會生效，也就是說項目要打jar包時才適用，若是打war包，放在Tomcat等web容器中是沒有效果的。