关于异步的好处我在这里就不多说了，自从servlet3.1规范发布以来，控制层的异步处理也越来越多的被人提及。而Spring5的webflux诞生也意味着Spring全方位对异步提供了支持。其实早在SpringMVC3.2版本就开始支持异步了，那么这篇文章我们就来探讨一下SpringMVC使用异步的方式。

一、DeferredResult

     DeferredResult这个类代表延迟结果，我们先看一看spring的API文档给我们的解释：

{@code DeferredResult} provides an alternative to using a {@link Callable} for asynchronous request processing. While a {@code Callable} is executed concurrently on behalf of the application, with a {@code DeferredResult} the application can produce the result from a thread of its choice.

     根据文档说明DeferredResult可以替代Callable来进行异步的请求处理。只不过这个类可以从其他线程里拿到对应的结果。当使用DeferredResult，我们可以将DefferedResult的类型并将其保存到可以获取到该对象的地方，比如说队列或者集合当中，这样方便其它线程能够取到并设置DefferedResult的值。

1.1、示例

     我们先定义一个Controller，代码内容如下：

package com.bdqn.lyrk.ssm.study.web.controller;

import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RestController;
import org.springframework.web.context.request.async.DeferredResult;

import java.util.concurrent.ArrayBlockingQueue;
import java.util.concurrent.BlockingQueue;
import java.util.concurrent.Callable;

/**
 * 异步任务的控制器
 *
 * @author chen.nie
 * @date 2018/8/2
 **/
@RestController
public class AsyncController {

    private BlockingQueue<DeferredResult<String>> blockingQueue = new ArrayBlockingQueue(1024);

    /**
     * 返回值是DeferredResult类型，如果没有结果请求阻塞
     *
     * @return
     */
    @GetMapping("/quotes")
    public DeferredResult<String> quotes() {
        //指定超时时间，及出错时返回的值
        DeferredResult<String> result = new DeferredResult(3000L,"error");
        blockingQueue.add(result);
        return result;
    }

    /**
     * 另外一个请求(新的线程)设置值
     *
     * @throws InterruptedException
     */

    @GetMapping("take")
    public void take() throws InterruptedException {
        DeferredResult<String> result = blockingQueue.take();
        result.setResult("route");
    }

    @GetMapping
    public Callable<String> callable() {
        return () -> "callable";
    }


}

     控制器可以从不同的线程异步生成返回值，例如响应外部事件(JMS消息)、计划任务等，那么在这里我先使用另外一个请求来模拟这个过程
     此时我们启动tomcat,先访问地址http://localhost:8080/quotes ,此时我们会看到发送的请求由于等待响应遭到了阻塞：

     当在规定时间内访问http://localhost:8080/take 时，则能成功显示结果：

1.2、DeferredResult处理流程

根据官网描述：

DeferredResult processing:

• Controller returns a DeferredResult and saves it in some in-memory queue or list where it can be accessed.
• Spring MVC calls request.startAsync().
• Meanwhile the DispatcherServlet and all configured Filter’s exit the request processing thread but the response remains open.
• The application sets the DeferredResult from some thread and Spring MVC dispatches the request back to the Servlet container.
• The DispatcherServlet is invoked again and processing resumes with the asynchronously produced return value.

     将Controller返回的DeferredResult值保存到内存队列或集合当中，紧接着SpringMVC调用HttpServletRequest的startAsync()方法,与此同时DispatcherServlet和所有配置的Filter退出当前的请求线程(不过响应时开放的)，当其他线程里设置DeferredResult的值时将重新发送请求，此时DispatcherServlet使用异步生成的返回值继续处理。

     在这里一切的一切还需要通过源代码来解释:

• 当一个请求被DispatcherServlet处理时，会试着获取一个WebAsyncManager对象

protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception {
        HttpServletRequest processedRequest = request;
        HandlerExecutionChain mappedHandler = null;
        boolean multipartRequestParsed = false;

        WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);
        try {
          // ......省略部分代码
          // 执行子控制器的方法
          mv = ha.handle(processedRequest, response, mappedHandler.getHandler());
        //如果当前的请求需要异步处理，则终止当前请求，但是响应是开放的
          if (asyncManager.isConcurrentHandlingStarted()) {
              return;
          }
        //....省略部分代码
       }
        catch (Exception ex) {
            triggerAfterCompletion(processedRequest, response, mappedHandler, ex);
        }
        catch (Throwable err) {
            triggerAfterCompletion(processedRequest, response, mappedHandler,
                new NestedServletException("Handler processing failed", err));
        }
        finally {
            if (asyncManager.isConcurrentHandlingStarted()) {
                // Instead of postHandle and afterCompletion
                if (mappedHandler != null) {
                    mappedHandler.applyAfterConcurrentHandlingStarted(processedRequest, response);
                }
            }
            else {
                // Clean up any resources used by a multipart request.
                if (multipartRequestParsed) {
                    cleanupMultipart(processedRequest);
                }
            }
        }
}

• 对于每一个子控制器的方法返回值，都是HandlerMethodReturnValueHandler接口处理的，其中有一个实现类是DeferredResultMethodReturnValueHandler，关键代码如下：

package org.springframework.web.servlet.mvc.method.annotation;

import java.util.HashMap;
import java.util.Map;
import java.util.concurrent.CompletionStage;
import java.util.function.BiFunction;

import org.springframework.core.MethodParameter;
import org.springframework.lang.UsesJava8;
import org.springframework.util.Assert;
import org.springframework.util.ClassUtils;
import org.springframework.util.concurrent.ListenableFuture;
import org.springframework.util.concurrent.ListenableFutureCallback;
import org.springframework.web.context.request.NativeWebRequest;
import org.springframework.web.context.request.async.DeferredResult;
import org.springframework.web.context.request.async.WebAsyncUtils;
import org.springframework.web.method.support.AsyncHandlerMethodReturnValueHandler;
import org.springframework.web.method.support.ModelAndViewContainer;

/**
 * Handler for return values of type {@link DeferredResult}, {@link ListenableFuture},
 * {@link CompletionStage} and any other async type with a {@link #getAdapterMap()
 * registered adapter}.
 *
 * @author Rossen Stoyanchev
 * @since 3.2
 */
@SuppressWarnings("deprecation")
public class DeferredResultMethodReturnValueHandler implements AsyncHandlerMethodReturnValueHandler {

    //存放DeferredResult的适配集合
    private final Map<Class<?>, DeferredResultAdapter> adapterMap;


    public DeferredResultMethodReturnValueHandler() {
        this.adapterMap = new HashMap<Class<?>, DeferredResultAdapter>(5);
        this.adapterMap.put(DeferredResult.class, new SimpleDeferredResultAdapter());
        this.adapterMap.put(ListenableFuture.class, new ListenableFutureAdapter());
        if (ClassUtils.isPresent("java.util.concurrent.CompletionStage", getClass().getClassLoader())) {
            this.adapterMap.put(CompletionStage.class, new CompletionStageAdapter());
        }
    }


    /**
     * Return the map with {@code DeferredResult} adapters.
     * <p>By default the map contains adapters for {@code DeferredResult}, which
     * simply downcasts, {@link ListenableFuture}, and {@link CompletionStage}.
     * @return the map of adapters
     * @deprecated in 4.3.8, see comments on {@link DeferredResultAdapter}
     */
    @Deprecated
    public Map<Class<?>, DeferredResultAdapter> getAdapterMap() {
        return this.adapterMap;
    }

    private DeferredResultAdapter getAdapterFor(Class<?> type) {
        for (Class<?> adapteeType : getAdapterMap().keySet()) {
            if (adapteeType.isAssignableFrom(type)) {
                return getAdapterMap().get(adapteeType);
            }
        }
        return null;
    }


    @Override
    public boolean supportsReturnType(MethodParameter returnType) {
        return (getAdapterFor(returnType.getParameterType()) != null);
    }

    @Override
    public boolean isAsyncReturnValue(Object returnValue, MethodParameter returnType) {
        return (returnValue != null && (getAdapterFor(returnValue.getClass()) != null));
    }

    @Override
    public void handleReturnValue(Object returnValue, MethodParameter returnType,
            ModelAndViewContainer mavContainer, NativeWebRequest webRequest) throws Exception {

        if (returnValue == null) {
            mavContainer.setRequestHandled(true);
            return;
        }
       //根据返回值的类型获取对应的DeferredResult适配器
        DeferredResultAdapter adapter = getAdapterFor(returnValue.getClass());
        if (adapter == null) {
            throw new IllegalStateException(
                    "Could not find DeferredResultAdapter for return value type: " + returnValue.getClass());
        }
        DeferredResult<?> result = adapter.adaptToDeferredResult(returnValue);
        //开启异步请求
        WebAsyncUtils.getAsyncManager(webRequest).startDeferredResultProcessing(result, mavContainer);
    }

}

     在这里我们关注handleReturnValue的方法，在经过适配包装后获取DeferredResult开启了异步之旅

• 紧接着我们关注一下WebAsyncManager的startDeferredResultProcessing方法

/**
     * Start concurrent request processing and initialize the given
     * {@link DeferredResult} with a {@link DeferredResultHandler} that saves
     * the result and dispatches the request to resume processing of that
     * result. The {@code AsyncWebRequest} is also updated with a completion
     * handler that expires the {@code DeferredResult} and a timeout handler
     * assuming the {@code DeferredResult} has a default timeout result.
     * @param deferredResult the DeferredResult instance to initialize
     * @param processingContext additional context to save that can be accessed
     * via {@link #getConcurrentResultContext()}
     * @throws Exception if concurrent processing failed to start
     * @see #getConcurrentResult()
     * @see #getConcurrentResultContext()
     */
    public void startDeferredResultProcessing(
            final DeferredResult<?> deferredResult, Object... processingContext) throws Exception {

        Assert.notNull(deferredResult, "DeferredResult must not be null");
        Assert.state(this.asyncWebRequest != null, "AsyncWebRequest must not be null");
        //设置超时时间
        Long timeout = deferredResult.getTimeoutValue();
        if (timeout != null) {
            this.asyncWebRequest.setTimeout(timeout);
        }

        //获取所有的延迟结果拦截器
        List<DeferredResultProcessingInterceptor> interceptors = new ArrayList<DeferredResultProcessingInterceptor>();
        interceptors.add(deferredResult.getInterceptor());
        interceptors.addAll(this.deferredResultInterceptors.values());
        interceptors.add(timeoutDeferredResultInterceptor);

        final DeferredResultInterceptorChain interceptorChain = new DeferredResultInterceptorChain(interceptors);
       
        this.asyncWebRequest.addTimeoutHandler(new Runnable() {
            @Override
            public void run() {
                try {
                    interceptorChain.triggerAfterTimeout(asyncWebRequest, deferredResult);
                }
                catch (Throwable ex) {
                    setConcurrentResultAndDispatch(ex);
                }
            }
        });

        this.asyncWebRequest.addCompletionHandler(new Runnable() {
            @Override
            public void run() {
                interceptorChain.triggerAfterCompletion(asyncWebRequest, deferredResult);
            }
        });

        interceptorChain.applyBeforeConcurrentHandling(this.asyncWebRequest, deferredResult);
         //开始异步处理
        startAsyncProcessing(processingContext);

        try {
            interceptorChain.applyPreProcess(this.asyncWebRequest, deferredResult);
            deferredResult.setResultHandler(new DeferredResultHandler() {
                @Override
                public void handleResult(Object result) {
                    result = interceptorChain.applyPostProcess(asyncWebRequest, deferredResult, result);
                    //设置结果并转发
                    setConcurrentResultAndDispatch(result);
                }
            });
        }
        catch (Throwable ex) {
            setConcurrentResultAndDispatch(ex);
        }
    }

    private void startAsyncProcessing(Object[] processingContext) {
        clearConcurrentResult();
        this.concurrentResultContext = processingContext;
        //实际上是执行的是HttpServletRequest对应方法
        this.asyncWebRequest.startAsync();

        if (logger.isDebugEnabled()) {
            HttpServletRequest request = this.asyncWebRequest.getNativeRequest(HttpServletRequest.class);
            String requestUri = urlPathHelper.getRequestUri(request);
            logger.debug("Concurrent handling starting for " + request.getMethod() + " [" + requestUri + "]");
        }
    }

     在这里首先收集所有配置好的DeferredResultProcessingInterceptor ,然后设置asyncRequest的超时处理，完成时的处理等，同时会分阶段执行拦截器中的各个方法。在这里真的佩服Spring框架的扩展机制做的实在是太好了。最后我们关注一下如下代码：

 deferredResult.setResultHandler(new DeferredResultHandler() {
                @Override
                public void handleResult(Object result) {
                    result = interceptorChain.applyPostProcess(asyncWebRequest, deferredResult, result);
                    //设置结果并转发
                    setConcurrentResultAndDispatch(result);
                }
            });

     其最终还是要调用AsyncWebRequest接口中的dispatch方法进行转发，让DispatcherServlet重新处理异步结果:

/**
     * Dispatch the request to the container in order to resume processing after
     * concurrent execution in an application thread.
     */
    void dispatch();

     其实在这里都是封装自HttpServletRequest的异步操作,我们可以看一下StandardServletAsyncWebRequest的类结构图：

     我们可以在其父类ServletRequestAttributes里找到对应的实现：

 
    private final HttpServletRequest request;
/**
     * Exposes the native {@link HttpServletRequest} that we're wrapping.
     */
    public final HttpServletRequest getRequest() {
        return this.request;
    }

     最后我在贴出一段StandardServletAsyncWebRequest 代码，大家就应该知道整个异步是怎么执行的了：

   //java.servlet.AsnycContext
    private AsyncContext asyncContext;
  
    @Override
    public void startAsync() {
        Assert.state(getRequest().isAsyncSupported(),
                "Async support must be enabled on a servlet and for all filters involved " +
                "in async request processing. This is done in Java code using the Servlet API " +
                "or by adding \"<async-supported>true</async-supported>\" to servlet and " +
                "filter declarations in web.xml.");
        Assert.state(!isAsyncComplete(), "Async processing has already completed");

        if (isAsyncStarted()) {
            return;
        }
        this.asyncContext = getRequest().startAsync(getRequest(), getResponse());
        this.asyncContext.addListener(this);
        if (this.timeout != null) {
            this.asyncContext.setTimeout(this.timeout);
        }
    }

    @Override
    public void dispatch() {
        Assert.notNull(this.asyncContext, "Cannot dispatch without an AsyncContext");
        this.asyncContext.dispatch();
    }

二、使用Callable作为返回值

     使用Callable作为返回值来实现异步与DeferredResult类似，我们先看一看官网描述的具体流程：

Callable processing:

• Controller returns a Callable.
• Spring MVC calls request.startAsync() and submits the Callable to a TaskExecutor for processing in a separate thread.
• Meanwhile the DispatcherServlet and all Filter’s exit the Servlet container thread but the response remains open.
• Eventually the Callable produces a result and Spring MVC dispatches the request back to the Servlet container to complete processing.
• The DispatcherServlet is invoked again and processing resumes with the asynchronously produced return value from the Callable.

     流程上大体与DeferredResult类似，只不过Callable是由TaskExecutor来处理的，而TaskExecutor继承自java.util.concurrent.Executor。我们来看一下它的源代码，它也是在WebAysncManager中处理的：

/**
     * Use the given {@link WebAsyncTask} to configure the task executor as well as
     * the timeout value of the {@code AsyncWebRequest} before delegating to
     * {@link #startCallableProcessing(Callable, Object...)}.
     * @param webAsyncTask a WebAsyncTask containing the target {@code Callable}
     * @param processingContext additional context to save that can be accessed
     * via {@link #getConcurrentResultContext()}
     * @throws Exception if concurrent processing failed to start
     */
    public void startCallableProcessing(final WebAsyncTask<?> webAsyncTask, Object... processingContext) throws Exception {
        Assert.notNull(webAsyncTask, "WebAsyncTask must not be null");
        Assert.state(this.asyncWebRequest != null, "AsyncWebRequest must not be null");

        Long timeout = webAsyncTask.getTimeout();
        if (timeout != null) {
            this.asyncWebRequest.setTimeout(timeout);
        }

        AsyncTaskExecutor executor = webAsyncTask.getExecutor();
        if (executor != null) {
            this.taskExecutor = executor;
        }

        List<CallableProcessingInterceptor> interceptors = new ArrayList<CallableProcessingInterceptor>();
        interceptors.add(webAsyncTask.getInterceptor());
        interceptors.addAll(this.callableInterceptors.values());
        interceptors.add(timeoutCallableInterceptor);

        final Callable<?> callable = webAsyncTask.getCallable();
        final CallableInterceptorChain interceptorChain = new CallableInterceptorChain(interceptors);

        this.asyncWebRequest.addTimeoutHandler(new Runnable() {
            @Override
            public void run() {
                logger.debug("Processing timeout");
                Object result = interceptorChain.triggerAfterTimeout(asyncWebRequest, callable);
                if (result != CallableProcessingInterceptor.RESULT_NONE) {
                    setConcurrentResultAndDispatch(result);
                }
            }
        });

        this.asyncWebRequest.addCompletionHandler(new Runnable() {
            @Override
            public void run() {
                interceptorChain.triggerAfterCompletion(asyncWebRequest, callable);
            }
        });

        interceptorChain.applyBeforeConcurrentHandling(this.asyncWebRequest, callable);
        startAsyncProcessing(processingContext);
        //启动线程池的异步处理
        try {
            this.taskExecutor.submit(new Runnable() {
                @Override
                public void run() {
                    Object result = null;
                    try {
                        interceptorChain.applyPreProcess(asyncWebRequest, callable);
                        result = callable.call();
                    }
                    catch (Throwable ex) {
                        result = ex;
                    }
                    finally {
                        result = interceptorChain.applyPostProcess(asyncWebRequest, callable, result);
                    }
                    //设置当前的结果并转发
                    setConcurrentResultAndDispatch(result);
                }
            });
        }
        catch (RejectedExecutionException ex) {
            Object result = interceptorChain.applyPostProcess(this.asyncWebRequest, callable, ex);
            setConcurrentResultAndDispatch(result);
            throw ex;
        }
    }

     对比DeferredResult，在这里刚开始也是添加拦截器，只不过拦截器的名称是CallableProcessingInterceptor ，同时也需要设置WebAsyncRequest的超时处理，完成时处理的响应操作。这其中最大的区别就是使用TaskExecutor来对Callable进行异步处理