15.SpringMVC之异步请求

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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：

给Request设置属性(timeout,timeoutHandler,completionHandler…)
在相应位置，执行interceptors逻辑
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;
} }