15.SpringMVC之异步请求

SpringMVC中异步请求相关组件

SpringMVC在此基础上对异步请求进行了封装。提供了AsyncWebRequest类型的request,并提供了处理异步请求的管理器WebAsyncManager和工具WebAsyncUtils.

SpringMVC将异步请求返回值细分为了：Callable,WebAsyncTask,,DeferredResult 和 ListenableFuture. 后续会针对这四种不同的类型一一分析。

AsyncWebRequest

AsyncWebRequest,它是专门处理异步请求的request,定义如下：

//org.springframework.web.context.request.async.AsyncWebRequest
public interface AsyncWebRequest extends NativeWebRequest {
    void setTimeout(Long timeout);
        
    //相当于在AsyncListener中的`onTimeout和onComplete`
    void addTimeoutHandler(Runnable runnable);
    void addCompletionHandler(Runnable runnable);
    
    void startAsync();
    
    //判断异步请求是否开启和结束
    boolean isAsyncStarted();
    boolean isAsyncComplete();
    
    void dispatch();
}

AsyncWebRequest 有两个实现类，

• NoSupportAsyncWebRequest: 不支持异步请求
• StandardServletAsyncWebRequest: 支持异步请求。

StandardServletAsyncWebRequest 除了实现了AsyncWebRequest接口外，还实现了AsyncListener,另外它还继承了ServletWebRequest.

public class StandardServletAsyncWebRequest extends ServletWebRequest implements AsyncWebRequest, AsyncListener {
    private Long timeout;
    
    //封装 AsyncContext 属性
    private AsyncContext asyncContext;
    private AtomicBoolean asyncCompleted = new AtomicBoolean(false);
    
    //AsyncListener onTimeout,onCompletion方法 调用如下handlers..
    private final List<Runnable> timeoutHandlers = new ArrayList<Runnable>();
    private final List<Runnable> completionHandlers = new ArrayList<Runnable>();
    
    @Override
    public boolean isAsyncStarted() {
        return ((this.asyncContext != null) && getRequest().isAsyncStarted());
    }
    
    @Override
    public void startAsync() {
        if (isAsyncStarted()) {
            return;
        }
        this.asyncContext = getRequest().startAsync(getRequest(), getResponse());
        this.asyncContext.addListener(this);
        if (this.timeout != null) {
            this.asyncContext.setTimeout(this.timeout);
        }
    }
    
    // ---  实现 AsyncListener 方法----
    @Override
    public void onTimeout(AsyncEvent event) throws IOException {
        for (Runnable handler : this.timeoutHandlers) {
            handler.run();
        }
    }

    @Override
    public void onComplete(AsyncEvent event) throws IOException {
        for (Runnable handler : this.completionHandlers) {
            handler.run();
        }
        //执行完完成时，清空asyncContext
        this.asyncContext = null;
        this.asyncCompleted.set(true);
    }
}

WebAsyncUtils

//org.springframework.web.context.request.async.WebAsyncUtils
public abstract class WebAsyncUtils {
    //第一次获取时，直接创建WebAsyncManager,并设置到setAttribute中。 以后获取，直接从request属性中获取。
    public static WebAsyncManager getAsyncManager(ServletRequest servletRequest) {
        WebAsyncManager asyncManager = (WebAsyncManager) servletRequest.getAttribute(WEB_ASYNC_MANAGER_ATTRIBUTE);
        if (asyncManager == null) {
            asyncManager = new WebAsyncManager();
            servletRequest.setAttribute(WEB_ASYNC_MANAGER_ATTRIBUTE, asyncManager);
        }
        return asyncManager;
    }
    
    public static WebAsyncManager getAsyncManager(WebRequest webRequest) {
        //逻辑类似  getAsyncManager(ServletRequest servletRequest) 略...
    }
    
    //判断ServletRequest是否有方法"startAsync"。 只有servlet环境3.0以上版本才有此方法
    public static AsyncWebRequest createAsyncWebRequest(HttpServletRequest request, HttpServletResponse response) {
        return ClassUtils.hasMethod(ServletRequest.class, "startAsync") ?
                createStandardServletAsyncWebRequest(request, response) : new NoSupportAsyncWebRequest(request, response);
    }
    
    private static AsyncWebRequest createStandardServletAsyncWebRequest(HttpServletRequest request, HttpServletResponse response) {
        if (standardAsyncRequestConstructor == null) {
            String className = "org.springframework.web.context.request.async.StandardServletAsyncWebRequest";
            Class<?> clazz = ClassUtils.forName(className, WebAsyncUtils.class.getClassLoader());
            standardAsyncRequestConstructor = clazz.getConstructor(HttpServletRequest.class, HttpServletResponse.class);
        }
        return (AsyncWebRequest) BeanUtils.instantiateClass(standardAsyncRequestConstructor, request, response);
    }
}

WebAsyncManager

WebAsyncManager是SpringMVC处理异步请求过程中最核心的类，它管理着整个异步处理的过程。

//org.springframework.web.context.request.async
public final class WebAsyncManager {
    //两种类型的 超时 Interceptors
    private static final CallableProcessingInterceptor timeoutCallableInterceptor = new TimeoutCallableProcessingInterceptor();
    private static final DeferredResultProcessingInterceptor timeoutDeferredResultInterceptor = new TimeoutDeferredResultProcessingInterceptor();
    
    //持有 asyncWebRequest 对象
    private AsyncWebRequest asyncWebRequest;
    private AsyncTaskExecutor taskExecutor = new SimpleAsyncTaskExecutor(this.getClass().getSimpleName());
    
    //两种类型的 处理请求Interceptors
    private final Map<Object, CallableProcessingInterceptor> callableInterceptors = new LinkedHashMap<Object, CallableProcessingInterceptor>();
    private final Map<Object, DeferredResultProcessingInterceptor> deferredResultInterceptors = new LinkedHashMap<Object, DeferredResultProcessingInterceptor>();    
    
    //用来处理Callable 和 WebAsyncTask 类型的异步请求
    public void startCallableProcessing(final WebAsyncTask<?> webAsyncTask, Object... processingContext) throws Exception {   }
    
    //用来处理 DeferredResult 和 ListenableFuture 类型的请求 
    public void startDeferredResultProcessing(final DeferredResult<?> deferredResult, Object... processingContext) throws Exception 
}

它最重要的两个方法是：startCallableProcessing和startDeferredResultProcessing,这两个方法是启动异步处理的入口方法，它们一共做三件事：

1. 给Request设置属性(timeout,timeoutHandler,completionHandler…)
2. 在相应位置，执行interceptors逻辑
3. 启动异步处理

这里重点分析下startCallableProcessing：

public void startCallableProcessing(final WebAsyncTask<?> webAsyncTask, Object... processingContext) throws Exception {
    //设置asyncWebRequest 属性...
    Long timeout = webAsyncTask.getTimeout();
    if (timeout != null) {
        this.asyncWebRequest.setTimeout(timeout);
    }

    AsyncTaskExecutor executor = webAsyncTask.getExecutor();
    if (executor != null) {
        this.taskExecutor = executor;
    }
    
    //初始化 interceptors
   
   //在asyncWebRequest 执行前后，执行完成，超时 等关键时间节点 执行 interceptors 逻辑...
   
   //启动异步处理
    startAsyncProcessing(processingContext);

    // 线程池 执行callable方法....
    this.taskExecutor.submit(new Runnable() {
        @Override
        public void run() {
            // interceptors 略....
            Object result = callable.call();
            
            //设置处理结果，并发送请求
            setConcurrentResultAndDispatch(result);
        }
    });
}

    
//调用asyncWebRequest.startAsync()启动异步处理 
private void startAsyncProcessing(Object[] processingContext) {
    clearConcurrentResult();
    this.concurrentResultContext = processingContext;
    this.asyncWebRequest.startAsync();
}    

//判断是否已经有异步处理结果
public boolean hasConcurrentResult() {
    //concurrentResult 初始化时 = RESULT_NONE
    return (this.concurrentResult != RESULT_NONE);
}

 //设置处理结果，并发送请求
private void setConcurrentResultAndDispatch(Object result) {
    synchronized (WebAsyncManager.this) {
        //判断是否已经有异步处理结果
        if (hasConcurrentResult()) {
            return;
        }
        
        //将result设置为当前处理结果
        this.concurrentResult = result;
    }
    
    //如果异步请求在这里已经被设置为异步处理完成状态，则记录错误日志。(网络异常会造成此种问题)
    if (this.asyncWebRequest.isAsyncComplete()) {
        logger.error("Could not complete async processing due to timeout or network error");
        return;
    }
    //再次发送请求：SpringMVC请求处理完成之后再次发送一个相同的请求。在HandlerAdapter做特殊处理
    this.asyncWebRequest.dispatch();
}

SpringMVC 对异步的支持

SpringMVC想要支持异步处理，首先DispatchServlet要配置:<async-supported>true</async-supported>,其次请求方法的返回值为：Callable,WebAsyncTask,,DeferredResult 和 ListenableFuture

@Controller
@RequestMapping("/async")
public class AsyncController {

    @RequestMapping(value = "/callable",produces = "text/plain;charset=UTF-8")
    @ResponseBody
    public Callable<String> callable(){
        System.out.println("Callable进入主线程...");
        Callable<String> result =  new Callable<String>() {
            @Override
            public String call() throws Exception {
                Thread.sleep(5 * 1000);
                System.out.println("Callable子线程执行ing...");
                return "Callable:"+"久等了";
            }
        };
        System.out.println("Callable主线程退出...");
        return result;
    }

    @RequestMapping(value = "/web",produces = "text/plain;charset=UTF-8")
    @ResponseBody
    public WebAsyncTask<String> web(){
        System.out.println("WebAsyncTask 进入主线程...");
        WebAsyncTask task = new WebAsyncTask(new Callable() {
            @Override
            public Object call() throws Exception {
                Thread.sleep(5 * 1000);
                System.out.println("WebAsyncTask 子线程执行ing...");
                return "WebAsyncTask:"+"久等了";
            }
        });
        System.out.println("WebAsyncTask 主线程退出...");
        return task;
    }

    @RequestMapping(value = "/deferred",produces = "text/plain;charset=UTF-8")
    @ResponseBody
    public DeferredResult<String> deferred(){
        //这里的 7 * 1000 L ，是指主线程结束之后的超时时间。
        DeferredResult<String> result = new DeferredResult<String>(7 * 1000L , "超时了");
        approve(result);
        try {
            Thread.sleep(10 * 1000); //在主线程执行这段代码，并不会抛出"超时了"
        } catch (InterruptedException e) {
        }
        return result;
    }

    private void approve(final DeferredResult<String> result) {
        new Thread(() -> {
            try {
                Thread.sleep(5 * 1000);
                result.setResult("同意:" + LocalDateTime.now());
            } catch (InterruptedException e) {
            }
        }).start();
    }

    @RequestMapping(value = "/future",produces = "text/plain;charset=UTF-8")
    public ListenableFuture<ResponseEntity<String>> future(){
        ListenableFuture<ResponseEntity<String>> future = new AsyncRestTemplate().getForEntity("http://www.baidu.com", String.class);
        return future;
    }
}

源码跟踪

springMVC异步处理请求的过程是总体上可以拆分为2次：

• 第一次，启动异步请求，并设置timeout,completion等事件的监听，直接返回 null;
• 第二次，当监听到completion 时，直接在发送一次相同的请求，并将执行结果返回。

SpringMVC执行请求方法的过程都是在HandlerAdater中进行的。

在之前解析RequestMappingHandlerAdapter#invokeHandleMethod()处理请求时，将异步请求部分给剔除了，现在回看此方法：

//org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter
private ModelAndView invokeHandleMethod(HttpServletRequest request,
            HttpServletResponse response, HandlerMethod handlerMethod) throws Exception {

        ServletWebRequest webRequest = new ServletWebRequest(request, response);

        WebDataBinderFactory binderFactory = getDataBinderFactory(handlerMethod);
        ModelFactory modelFactory = getModelFactory(handlerMethod, binderFactory);
        ServletInvocableHandlerMethod requestMappingMethod = createRequestMappingMethod(handlerMethod, binderFactory);

        //mavContainer相关略......

        AsyncWebRequest asyncWebRequest = WebAsyncUtils.createAsyncWebRequest(request, response);
        asyncWebRequest.setTimeout(this.asyncRequestTimeout);

        final WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);
        asyncManager.setTaskExecutor(this.taskExecutor);
        asyncManager.setAsyncWebRequest(asyncWebRequest);
        asyncManager.registerCallableInterceptors(this.callableInterceptors);
        asyncManager.registerDeferredResultInterceptors(this.deferredResultInterceptors);
        
        //异步请求是否已经完成
        if (asyncManager.hasConcurrentResult()) {
            //如果异步请求已经处理完成，则获取执行结果  --- 1
            Object result = asyncManager.getConcurrentResult();
            mavContainer = (ModelAndViewContainer) asyncManager.getConcurrentResultContext()[0];
            
            //清空执行结果
            asyncManager.clearConcurrentResult();
            
            //覆盖原有的requestMappingMethod方法;  --- 2
            requestMappingMethod = requestMappingMethod.wrapConcurrentResult(result);
        }
        
        //执行方法 -- 3
        requestMappingMethod.invokeAndHandle(webRequest, mavContainer);
        
        //asyncManager是否已经启动
        if (asyncManager.isConcurrentHandlingStarted()) {
            //-- 4
            return null;
        }
        // --- 5
        return getModelAndView(mavContainer, modelFactory, webRequest);
    }

• 第一次执行时： 会执行上述代码中的 3,4
• 第二次执行时： 执行上述代码中的1,2,3,5 。 注意步骤2,会将原有的requestMappingMethod重写.接下来会分析。

ServletInvocableHandlerMethod.invokeAndHandle(webRequest, mavContainer)

springMVC在使用RequestMappingHandlerAdapter#invokeHandleMethod()处理请求时，会调用ServletInvocableHandlerMethod#invokeAndHandle()方法，该方法在处理完毕之后，会调用

this.returnValueHandlers.handleReturnValue(returnValue, getReturnValueType(returnValue), mavContainer, webRequest);`

处理返回值，针对上述四种类型的结果，匹配不同的XXReturnValueHandler.

Callable : CallableMethodReturnValueHandler

//org.springframework.web.servlet.mvc.method.annotation.CallableMethodReturnValueHandler
@Override
public void handleReturnValue(Object returnValue, MethodParameter returnType,
        ModelAndViewContainer mavContainer, NativeWebRequest webRequest) throws Exception {
    //if 略...
    Callable<?> callable = (Callable<?>) returnValue;
    WebAsyncUtils.getAsyncManager(webRequest).startCallableProcessing(callable, mavContainer);
}

WebAsyncTask : AsyncTaskMethodReturnValueHandler

//org.springframework.web.servlet.mvc.method.annotation.AsyncTaskMethodReturnValueHandler
@Override
public void handleReturnValue(Object returnValue, MethodParameter returnType,
        ModelAndViewContainer mavContainer, NativeWebRequest webRequest) throws Exception {
    //if 略...
    WebAsyncTask<?> webAsyncTask = (WebAsyncTask<?>) returnValue;
    webAsyncTask.setBeanFactory(this.beanFactory);
    WebAsyncUtils.getAsyncManager(webRequest).startCallableProcessing(webAsyncTask, mavContainer);
}

这里可以看出 Callable和webAsyncTask都是用了startCallableProcessing方法。

DeferredResult: DeferredResultMethodReturnValueHandler

//org.springframework.web.servlet.mvc.method.annotation.DeferredResultMethodReturnValueHandler
@Override
public void handleReturnValue(Object returnValue, MethodParameter returnType,
        ModelAndViewContainer mavContainer, NativeWebRequest webRequest) throws Exception {
    //if 略...
    DeferredResult<?> deferredResult = (DeferredResult<?>) returnValue;
    WebAsyncUtils.getAsyncManager(webRequest).startDeferredResultProcessing(deferredResult, mavContainer);
}

DeferredResult: DeferredResultMethodReturnValueHandler

//org.springframework.web.servlet.mvc.method.annotation.ListenableFutureReturnValueHandler
@Override
public void handleReturnValue(Object returnValue, MethodParameter returnType,
        ModelAndViewContainer mavContainer, NativeWebRequest webRequest) throws Exception {
    //if 略...
    final DeferredResult<Object> deferredResult = new DeferredResult<Object>();
    WebAsyncUtils.getAsyncManager(webRequest).startDeferredResultProcessing(deferredResult, mavContainer);

    ListenableFuture<?> future = (ListenableFuture<?>) returnValue;
    future.addCallback(new ListenableFutureCallback<Object>() {
        @Override
        public void onSuccess(Object result) {
            deferredResult.setResult(result);
        }
        @Override
        public void onFailure(Throwable ex) {
            deferredResult.setErrorResult(ex);
        }
    });
}

自此可以说明看 DeferredResult 和 ListenableFuture都是用了startDeferredResultProcessing方法。

ServletInvocableHandlerMethod.wrapConcurrentResult(result)

第二次请求时，要重点关注此行：requestMappingMethod.wrapConcurrentResult(result)，此时的result已经是异步执行后的最终结果，不是DeferredResult.

//org.springframework.web.servlet.mvc.method.annotation.ServletInvocableHandlerMethod
private static final Method CALLABLE_METHOD = ClassUtils.getMethod(Callable.class, "call");

public ServletInvocableHandlerMethod(Object handler, Method method) {
    super(handler, method);
    initResponseStatus();
}
ServletInvocableHandlerMethod wrapConcurrentResult(Object result) {
    return new ConcurrentResultHandlerMethod(result, new ConcurrentResultMethodParameter(result));
}

//org.springframework.web.servlet.mvc.method.annotation.ServletInvocableHandlerMethod $ ConcurrentResultMethodParameter
private class ConcurrentResultMethodParameter extends HandlerMethodParameter {
    private final Object returnValue;
    private final ResolvableType returnType;
    
    //直接传入返回值returnValue， 返回值的类型为 returnValue的类型
    public ConcurrentResultMethodParameter(Object returnValue) {
        super(-1);
        this.returnValue = returnValue;
        this.returnType = ResolvableType.forType(super.getGenericParameterType()).getGeneric(0);
    }
}

//org.springframework.web.servlet.mvc.method.annotation.ServletInvocableHandlerMethod $ ConcurrentResultHandlerMethod
private class ConcurrentResultHandlerMethod extends ServletInvocableHandlerMethod {
    public ConcurrentResultHandlerMethod(final Object result, ConcurrentResultMethodParameter returnType) {
        //调用父类的构造方法(handler,method)，最终调用 method.invoke();
        super(new Callable<Object>() {
            @Override
            public Object call() throws Exception {
                if (result instanceof Exception) {
                    throw (Exception) result;
                }
                else if (result instanceof Throwable) {
                    throw new NestedServletException("Async processing failed", (Throwable) result);
                }
                //此时的result即为最终异步处理的结果.
                return result;
            }
        }, CALLABLE_METHOD);
        setHandlerMethodReturnValueHandlers(ServletInvocableHandlerMethod.this.returnValueHandlers);
        this.returnType = returnType;
    }
}

第二次执行 requestMappingMethod.invokeAndHandle(webRequest, mavContainer);，此时的requestMappingMethod已经是伪造后的结果，该方法的返回值也被伪造为ConcurrentResultMethodParameter,最终调用的为ConcurrentResultHandlerMethod在构造函数中定义的Callable.call();

SpringMVC想要支持异步处理，首先DispatchServlet要配置:<async-supported>true</async-supported>,其次请求方法的返回值为：Callable,WebAsyncTask,,DeferredResult 和 ListenableFuture

<task:executor />配置参数：

• id：当配置多个executor时，被@Async(“id”)指定使用；也被作为线程名的前缀。
• pool-size：
  
  • core size：最小的线程数，缺省：1
  • max size：最大的线程数，缺省：Integer.MAX_VALUE
• queue-capacity：当最小的线程数已经被占用满后，新的任务会被放进queue里面，当这个queue的capacity也被占满之后，pool里面会创建新线程处理这个任务，直到总线程数达到了max size，这时系统会拒绝这个任务并抛出TaskRejectedException异常（缺省配置的情况下，可以通过rejection-policy来决定如何处理这种情况）。缺省值为：Integer.MAX_VALUE
• keep-alive：超过core size的那些线程，任务完成后，再经过这个时长（秒）会被结束掉
• rejection-policy：当pool已经达到max size的时候，如何处理新任务
  
  • ABORT（缺省）：抛出TaskRejectedException异常，然后不执行
  • DISCARD：不执行，也不抛出异常
  • DISCARD_OLDEST：丢弃queue中最旧的那个任务
  • CALLER_RUNS：不在新线程中执行任务，而是有调用者所在的线程来执行

Java编程方式的配置方法：

@Configuration  
@EnableAsync  
public class SpringConfig {  

    /** Set the ThreadPoolExecutor's core pool size. */  
    private int corePoolSize = 10;  
    /** Set the ThreadPoolExecutor's maximum pool size. */  
    private int maxPoolSize = 200;  
    /** Set the capacity for the ThreadPoolExecutor's BlockingQueue. */  
    private int queueCapacity = 10;  

    private String ThreadNamePrefix = "MyLogExecutor-";  

    @Bean  
    public Executor logExecutor() {  
        ThreadPoolTaskExecutor executor = new ThreadPoolTaskExecutor();  
        executor.setCorePoolSize(corePoolSize);  
        executor.setMaxPoolSize(maxPoolSize);  
        executor.setQueueCapacity(queueCapacity);  
        executor.setThreadNamePrefix(ThreadNamePrefix);  

        // rejection-policy：当pool已经达到max size的时候，如何处理新任务  
        // CALLER_RUNS：不在新线程中执行任务，而是有调用者所在的线程来执行  
        executor.setRejectedExecutionHandler(new ThreadPoolExecutor.CallerRunsPolicy());  
        executor.initialize();  
        return executor;  
    }  

}