Tomcat 滤波器不支持异步

Question

比如标题

@WebFilter("/*")
@Component
@Slf4j
public class FilterDemo1 implements Filter {

    public void doFilter(ServletRequest req, ServletResponse resp, FilterChain chain) throws ServletException, IOException, IOException {
        new Thread(() -> {
            try {
                chain.doFilter(req, resp);
            } catch (Exception e) {
                log.error("a", e);
            }
        }).start();
     
    }

}

因此，如果Tomcat过滤器中有耗时任务（如RPC或HTTP），则必须等待，不能异步

java.lang.NullPointerException: null
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:231) ~[tomcat-embed-core-9.0.27.jar:9.0.27]
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) ~[tomcat-embed-core-9.0.27.jar:9.0.27]```


[error][1]

  [1]: https://i.stack.imgur.com/tFYTH.png

Answer 1

ServletRequest 、 ServletResponse和FilterChain object 仅在调用doFilter的线程上有效。 如果要异步使用它们，则需要启用异步处理（参见Jakarta EE 教程）。

ServletRequest.startAsync()将ServletRequest和ServletResponse置于异步模式，但FilterChain只能在原线程上使用：

service方法需要与应用于 servlet 的所有过滤器在同一线程中运行。

（参见Servlet 规范）

因此，您需要进行以下操作：

1. 当新请求通过过滤器时，您调用ServletRequest.startAsync()并启动新线程或使用任何其他执行器进行异步过滤（例如AsyncContext.start(Runnable) ），
2. 异步任务完成后，将结果写入请求属性并调用AsyncContext.dispatch() ：这将从头开始重新启动过滤器链，
3. 当再次调用doFilter时，您使用请求的属性来执行您的过滤逻辑并调用FilterChain.doFilter

例如，您可以使用以下内容：

@WebFilter(asyncSupported = true, urlPatterns = {"/*"}, dispatcherTypes = {DispatcherType.ASYNC, DispatcherType.REQUEST})
public class Filter1 implements Filter {

   @Override
   public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException {
      switch (request.getDispatcherType()) {
         case REQUEST :
            // First pass: start asynchronous processing
            final AsyncContext asyncContext = request.startAsync();
            new Thread(() -> {
               try {
                  Thread.currentThread().sleep(5000);
               } catch (Exception e) {
                  request.setAttribute("filter1Error", e);
               }
               asyncContext.dispatch();
            }).start();
            break;
         case ASYNC :
            // Second pass: throw or forward
            Exception e = (Exception) request.getAttribute("filter1Error");
            if (e instanceof IOException) {
               throw (IOException) e;
            } else if (e instanceof ServletException) {
               throw (ServletException) e;
            } else if (e != null) {
               throw new ServletException(e);
            }
            chain.doFilter(request, response);
            break;
         default :
      }
   }