你的響應阻塞了沒有？--Spring-WebFlux源碼分析

1. Spring WebFlux是什么？

Spring WebFlux是Spring Framework 5.0中引入的新的反應式Web框架。 與Spring MVC不同，它不需要Servlet API，完全異步和非阻塞， 並通過Reactor項目實現Reactive Streams規范。 並且可以在諸如Netty，Undertow和Servlet 3.1+容器的服務器上運行。
Reactor 也是 Spring 5 中反應式編程的基礎，它一個新的反應式編程庫。

2. Reactor是什么？

Reactor offers non-blocking and backpressure-ready network runtimes including local TCP/HTTP/UDP client & servers based on the robust Netty framework.

Reactor提供了一個非阻塞的，高並發的基於健壯的Netty框架的網絡運行API，包括本地tcp/http/udp 客戶端和服務端。

重要的兩個概念

Flux 和 Mono 是 Reactor 中的兩個基本概念。Flux 表示的是包含 0 到 N 個元素的異步序列。在該序列中可以包含三種不同類型的消息通知：正常的包含元素的消息、序列結束的消息和序列出錯的消息。當消息通知產生時，訂閱者中對應的方法 onNext(), onComplete()和 onError()會被調用。Mono 表示的是包含 0 或者 1 個元素的異步序列。該序列中同樣可以包含與 Flux 相同的三種類型的消息通知。Flux 和 Mono 之間可以進行轉換。對一個 Flux 序列進行計數操作，得到的結果是一個 Mono<Long>對象。把兩個 Mono 序列合並在一起，得到的是一個 Flux 對象。

簡單說Mono返回單個元素，Flux返回多個元素

3. spring webflux處理請求流程

核心控制器DispatcherHandler，等同於阻塞方式的DispatcherServlet

/**
 * Central dispatcher for HTTP request handlers/controllers. Dispatches to
 * registered handlers for processing a request, providing convenient mapping
 * facilities.
 *
 * <p>{@code DispatcherHandler} discovers the delegate components it needs from
 * Spring configuration. It detects the following in the application context:
 * <ul>
 * <li>{@link HandlerMapping} -- map requests to handler objects
 * <li>{@link HandlerAdapter} -- for using any handler interface
 * <li>{@link HandlerResultHandler} -- process handler return values
 * </ul>
 *
 * <p>{@code DispatcherHandler} is also designed to be a Spring bean itself and
 * implements {@link ApplicationContextAware} for access to the context it runs
 * in. If {@code DispatcherHandler} is declared with the bean name "webHandler"
 * it is discovered by {@link WebHttpHandlerBuilder#applicationContext} which
 * creates a processing chain together with {@code WebFilter},
 * {@code WebExceptionHandler} and others.
 *
 * <p>A {@code DispatcherHandler} bean declaration is included in
 * {@link org.springframework.web.reactive.config.EnableWebFlux @EnableWebFlux}
 * configuration.
 *
 * @author Rossen Stoyanchev
 * @author Sebastien Deleuze
 * @author Juergen Hoeller
 * @since 5.0
 * @see WebHttpHandlerBuilder#applicationContext(ApplicationContext)
 */

3.1 初始化

獲取HandlerMapping，HandlerAdapter，HandlerResultHandler的所有實例

 

    protected void initStrategies(ApplicationContext context) {
 Map<String, HandlerMapping> mappingBeans = BeanFactoryUtils.beansOfTypeIncludingAncestors(
                context, HandlerMapping.class, true, false); //1 

        ArrayList<HandlerMapping> mappings = new ArrayList<>(mappingBeans.values());
        AnnotationAwareOrderComparator.sort(mappings);
        this.handlerMappings = Collections.unmodifiableList(mappings);

 Map<String, HandlerAdapter> adapterBeans = BeanFactoryUtils.beansOfTypeIncludingAncestors(
                context, HandlerAdapter.class, true, false);  //2 

        this.handlerAdapters = new ArrayList<>(adapterBeans.values());
        AnnotationAwareOrderComparator.sort(this.handlerAdapters);

 Map<String, HandlerResultHandler> beans = BeanFactoryUtils.beansOfTypeIncludingAncestors(
                context, HandlerResultHandler.class, true, false); //3

        this.resultHandlers = new ArrayList<>(beans.values());
        AnnotationAwareOrderComparator.sort(this.resultHandlers);
    }

其中，1.獲取所有HandlerMapping實例

　　　2.獲取所有HandlerAdapter實例

　　　3.獲取所有HandlerResultHandler實例

3.2 流式處理請求

    public Mono<Void> handle(ServerWebExchange exchange) {
        if (this.handlerMappings == null) {
            return createNotFoundError();
        }
        return Flux.fromIterable(this.handlerMappings)
 .concatMap(mapping -> mapping.getHandler(exchange))//1
 .next() .switchIfEmpty(createNotFoundError())//2
 .flatMap(handler -> invokeHandler(exchange, handler))//3
 .flatMap(result -> handleResult(exchange, result));//4
    }

其中，第一步，從handlerMapping這個map中獲取HandlerMapping

　　　第二步，觸發HandlerApter的handle方法

    private Mono<HandlerResult> invokeHandler(ServerWebExchange exchange, Object handler) {
        if (this.handlerAdapters != null) {
            for (HandlerAdapter handlerAdapter : this.handlerAdapters) {
                if (handlerAdapter.supports(handler)) {
                    return handlerAdapter.handle(exchange, handler);
                }
            }
        }
        return Mono.error(new IllegalStateException("No HandlerAdapter: " + handler));
    }

　　　第三步，觸發HandlerResultHandler 的handleResult方法

    private Mono<Void> handleResult(ServerWebExchange exchange, HandlerResult result) {
        return getResultHandler(result).handleResult(exchange, result)
                .onErrorResume(ex -> result.applyExceptionHandler(ex).flatMap(exceptionResult ->
                        getResultHandler(exceptionResult).handleResult(exchange, exceptionResult)));
    }

    private HandlerResultHandler getResultHandler(HandlerResult handlerResult) {
        if (this.resultHandlers != null) {
            for (HandlerResultHandler resultHandler : this.resultHandlers) {
                if (resultHandler.supports(handlerResult)) {
                    return resultHandler;
                }
            }
        }
        throw new IllegalStateException("No HandlerResultHandler for " + handlerResult.getReturnValue());
    }

4.HandlerMapping實現

5.HanlderAdapter的實現

6.HandlerResultHandler的實現

 7.不同容器的實現

 7.1 Reactor實現ReactorHttpHandlerAdapter

執行apply方法

    @Override
    public Mono<Void> apply(HttpServerRequest reactorRequest, HttpServerResponse reactorResponse) {
        NettyDataBufferFactory bufferFactory = new NettyDataBufferFactory(reactorResponse.alloc());
        try {
            ReactorServerHttpRequest request = new ReactorServerHttpRequest(reactorRequest, bufferFactory);
            ServerHttpResponse response = new ReactorServerHttpResponse(reactorResponse, bufferFactory);

            if (request.getMethod() == HttpMethod.HEAD) {
                response = new HttpHeadResponseDecorator(response);
            }

            return this.httpHandler.handle(request, response)
                    .doOnError(ex -> logger.trace(request.getLogPrefix() + "Failed to complete: " + ex.getMessage()))
                    .doOnSuccess(aVoid -> logger.trace(request.getLogPrefix() + "Handling completed"));
        }
        catch (URISyntaxException ex) {
            if (logger.isDebugEnabled()) {
                logger.debug("Failed to get request URI: " + ex.getMessage());
            }
            reactorResponse.status(HttpResponseStatus.BAD_REQUEST);
            return Mono.empty();
        }
    }

其中，HttpHandler的定義

**
 * Lowest level contract for reactive HTTP request handling that serves as a
 * common denominator across different runtimes.
 *
 * <p>Higher-level, but still generic, building blocks for applications such as
 * {@code WebFilter}, {@code WebSession}, {@code ServerWebExchange}, and others
 * are available in the {@code org.springframework.web.server} package.
 *
 * <p>Application level programming models such as annotated controllers and
 * functional handlers are available in the {@code spring-webflux} module.
 *
 * <p>Typically an {@link HttpHandler} represents an entire application with
 * higher-level programming models bridged via
 * {@link org.springframework.web.server.adapter.WebHttpHandlerBuilder}.
 * Multiple applications at unique context paths can be plugged in with the
 * help of the {@link ContextPathCompositeHandler}.
 *
 * @author Arjen Poutsma
 * @author Rossen Stoyanchev
 * @since 5.0
 * @see ContextPathCompositeHandler
 */

具體的實現類為：ContextPathCompositeHandler

/**
 * {@code HttpHandler} delegating requests to one of several {@code HttpHandler}'s
 * based on simple, prefix-based mappings.
 *
 * <p>This is intended as a coarse-grained mechanism for delegating requests to
 * one of several applications -- each represented by an {@code HttpHandler}, with
 * the application "context path" (the prefix-based mapping) exposed via
 * {@link ServerHttpRequest#getPath()}.
 *
 * @author Rossen Stoyanchev
 * @since 5.0
 */
    @Override
    public Mono<Void> handle(ServerHttpRequest request, ServerHttpResponse response) {
        // Remove underlying context path first (e.g. Servlet container)
        String path = request.getPath().pathWithinApplication().value();
        return this.handlerMap.entrySet().stream()
                .filter(entry -> path.startsWith(entry.getKey()))
                .findFirst()
                .map(entry -> {
                    String contextPath = request.getPath().contextPath().value() + entry.getKey();
                    ServerHttpRequest newRequest = request.mutate().contextPath(contextPath).build();
                    return entry.getValue().handle(newRequest, response);
                })
                .orElseGet(() -> {
                    response.setStatusCode(HttpStatus.NOT_FOUND);
                    return response.setComplete();
                });
    }

基於前綴的映射Handler

 7.2 Jetty實現JettyHttpHandlerAdapter

繼承自ServletHttpHandlerAdapter 實現了Servlet，執行service方法

@Override
    public void service(ServletRequest request, ServletResponse response) throws ServletException, IOException {
        // Check for existing error attribute first
        if (DispatcherType.ASYNC.equals(request.getDispatcherType())) {
            Throwable ex = (Throwable) request.getAttribute(WRITE_ERROR_ATTRIBUTE_NAME);
            throw new ServletException("Failed to create response content", ex);
        }

        // Start async before Read/WriteListener registration
        AsyncContext asyncContext = request.startAsync();
        asyncContext.setTimeout(-1);

        ServletServerHttpRequest httpRequest;
        try {
            httpRequest = createRequest(((HttpServletRequest) request), asyncContext);//1
        }
        catch (URISyntaxException ex) {
            if (logger.isDebugEnabled()) {
                logger.debug("Failed to get request  URL: " + ex.getMessage());
            }
            ((HttpServletResponse) response).setStatus(400);
            asyncContext.complete();
            return;
        }

        ServerHttpResponse httpResponse = createResponse(((HttpServletResponse) response), asyncContext, httpRequest);//2
        if (httpRequest.getMethod() == HttpMethod.HEAD) {
            httpResponse = new HttpHeadResponseDecorator(httpResponse);
        }

        AtomicBoolean isCompleted = new AtomicBoolean();
        HandlerResultAsyncListener listener = new HandlerResultAsyncListener(isCompleted, httpRequest);
        asyncContext.addListener(listener);

        HandlerResultSubscriber subscriber = new HandlerResultSubscriber(asyncContext, isCompleted, httpRequest);
        this.httpHandler.handle(httpRequest, httpResponse).subscribe(subscriber);//3
    }

其中，1.創建request

　　　 2.創建response

            3.handler執行的結果進行subscribe

　　JettyHttpHandlerAdapter是ServletHttpHandlerAdapter 的擴展，重寫了創建request  創建response方法

 7.3 Tomcat實現TomcatHttpHandlerAdapter

　　TomcatHttpHandlerAdapter是ServletHttpHandlerAdapter 的擴展，重寫了創建request  創建response方法

 7.4 AbstractReactiveWebInitializer抽象類

繼承自AbstractReactiveWebInitializer的類可以在servlet容器中安裝一個Spring Reactive Web Application。

@Override
    public void onStartup(ServletContext servletContext) throws ServletException {
        String servletName = getServletName();
        Assert.hasLength(servletName, "getServletName() must not return null or empty");

        ApplicationContext applicationContext = createApplicationContext();
        Assert.notNull(applicationContext, "createApplicationContext() must not return null");

        refreshApplicationContext(applicationContext);
        registerCloseListener(servletContext, applicationContext);

 HttpHandler httpHandler = WebHttpHandlerBuilder.applicationContext(applicationContext).build();
        ServletHttpHandlerAdapter servlet = new ServletHttpHandlerAdapter(httpHandler);

        ServletRegistration.Dynamic registration = servletContext.addServlet(servletName, servlet);
        if (registration == null) {
            throw new IllegalStateException("Failed to register servlet with name '" + servletName + "'. " +
                    "Check if there is another servlet registered under the same name.");
        }

        registration.setLoadOnStartup(1);
        registration.addMapping(getServletMapping());
        registration.setAsyncSupported(true);
    }

它通過將ServletHttpHandlerAdapter實例作為一個servlet安裝到servler容器中。

8.總結

 　　DispatcherHandler的流程是

　　　　1.通過 HandlerMapping（和DispathcherServlet中的HandlerMapping不同）獲取到HandlerAdapter放到ServerWebExchange的屬性中

　　　　2.獲取到HandlerAdapter后觸發handle方法，得到HandlerResult

　　　　3.通過HandlerResult，觸發handleResult，針對不同的返回類找到不同的HandlerResultHandler如

　視圖渲染ViewResolutionResultHandler，

ServerResponseResultHandler,

ResponseBodyResultHandler,

ResponseEntityResultHandler

  不同容器有不同的實現，如Reactor，Jetty，Tomcat等。

參考文獻：

【1】https://blog.csdn.net/qq_15144655/article/details/80708915

【2】https://www.ibm.com/developerworks/cn/java/j-cn-with-reactor-response-encode/index.html