【web.xml】

<filter>
    <filter-name>normalFilter</filter-name>
    <filter-class>net.mypla.controller.filter.AnyRequestFilter</filter-class>
    <async-supported>true</async-supported>  <!--请求启动AsyncContext必要声明-->
</filter>

<filter-mapping>
    <filter-name>normalFilter</filter-name>
    <url-pattern>/*</url-pattern>
    <dispatcher>REQUEST</dispatcher><!--只拦截普通请求-->
</filter-mapping>

<filter>
    <filter-name>forwardFilter</filter-name>
    <filter-class>net.mypla.controller.filter.AnyRequestFilter</filter-class>
    <async-supported>true</async-supported>
</filter>

<filter-mapping>
    <filter-name>forwardFilter</filter-name>
    <url-pattern>/*</url-pattern>
    <dispatcher>FORWARD</dispatcher><!--只拦截转发请求-->
</filter-mapping>

<filter>
    <filter-name>asyncFilter</filter-name>
    <filter-class>net.mypla.controller.filter.AnyRequestFilter</filter-class>
    <async-supported>true</async-supported>
</filter>

<filter-mapping>
    <filter-name>asyncFilter</filter-name>
    <url-pattern>/*</url-pattern>
    <dispatcher>ASYNC</dispatcher><!--拦截由AsyncContext派发的请求-->
</filter-mapping>

【AnyRequestFilter】

@Override
public void doFilter(ServletRequest req, ServletResponse resp, FilterChain filterChain) throws IOException, ServletException {
    System.out.println("Entering " + this.name + ".doFilter().");
    filterChain.doFilter(new HttpServletRequestWrapper((HttpServletRequest) req), new HttpServletResponseWrapper((HttpServletResponse) resp));
    //真正处理async请求
    if (req.isAsyncSupported() && req.isAsyncStarted()) {
        AsyncContext context = req.getAsyncContext();
        System.out.println("Leaving " + this.name + ".doFilter(),async" +
                "context holds wrapped req/resp = " +
                !context.hasOriginalRequestAndResponse());
    } else {
        System.out.println("leaving " + this.name + ".doFilter().");
    }
}
【异步Servlet】异步请求，关键代码

//过滤器有wrap这个动作，我们有收到unwrap的话就是不要wrap
//unwrap 原始参数启动异步  、  没有unwrap就用wrap后的req resp
final AsyncContext context = req.getParameter("unwrap")!=null?
        req.startAsync():req.startAsync(req,resp);
//timeout 是请求参数 ?timeout=10000
context.setTimeout(timeout);

System.out.println("Starting asynchronous thread.Request ID = "+id+".");

AsyncThread thread = new AsyncThread(id,context);
context.start(thread::doWork);

【异步线程】启动一个线程去处理，最终采用异步派发 而不是直接处理

public void doWork(){
    System.out.println("Asynchronous thread started. Request ID = "+
        this.id+".");
    //当前线程休眠5秒 
    try {
        Thread.sleep(5_000L);
    } catch (InterruptedException e) {
        e.printStackTrace();
    }
　　//异步请求
    HttpServletRequest request = (HttpServletRequest) this.context.getRequest();
    System.out.println("Done sleeping. Request ID = "+this.id+
        ",URL="+request.getRequestURL()+".");
    this.context.dispatch("/WEB-INF/jsp/filter/async.jsp");//异步派发，过滤器拦截。（如果AsyncContext直接处理掉请求，过滤器无法拦截。）
    System.out.println("Asynchronous thread completed. Request ID = "+
        this.id+".");
}