springmvc的异步处理

     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