你的响应阻塞了没有？--Spring-WebFlux源码分析

1. Spring WebFlux是什么？

Spring WebFlux是Spring Framework 5.0中引入的新的反应式Web框架。 与Spring MVC不同，它不需要Servlet API，完全异步和非阻塞， 并通过Reactor项目实现Reactive Streams规范。 并且可以在诸如Netty，Undertow和Servlet 3.1+容器的服务器上运行。
Reactor 也是 Spring 5 中反应式编程的基础，它一个新的反应式编程库。

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提供了一个非阻塞的，高并发的基于健壮的Netty框架的网络运行API，包括本地tcp/http/udp 客户端和服务端。

重要的两个概念

Flux 和 Mono 是 Reactor 中的两个基本概念。Flux 表示的是包含 0 到 N 个元素的异步序列。在该序列中可以包含三种不同类型的消息通知：正常的包含元素的消息、序列结束的消息和序列出错的消息。当消息通知产生时，订阅者中对应的方法 onNext(), onComplete()和 onError()会被调用。Mono 表示的是包含 0 或者 1 个元素的异步序列。该序列中同样可以包含与 Flux 相同的三种类型的消息通知。Flux 和 Mono 之间可以进行转换。对一个 Flux 序列进行计数操作，得到的结果是一个 Mono<Long>对象。把两个 Mono 序列合并在一起，得到的是一个 Flux 对象。

简单说Mono返回单个元素，Flux返回多个元素

3. spring webflux处理请求流程

核心控制器DispatcherHandler，等同于阻塞方式的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

获取HandlerMapping，HandlerAdapter，HandlerResultHandler的所有实例

    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.获取所有HandlerMapping实例

　　　2.获取所有HandlerAdapter实例

　　　3.获取所有HandlerResultHandler实例

3.2 流式处理请求

    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，从handlerMapping这个map中获取HandlerMapping

　　　第二步，触发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，触发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实现

5.HanlderAdapter的实现

6.HandlerResultHandler的实现

7.不同容器的实现

7.1 Reactor实现ReactorHttpHandlerAdapter

执行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的定义

**
 * 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
 */

具体的实现类为：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();
                });
    }

基于前缀的映射Handler

7.2 Jetty实现JettyHttpHandlerAdapter

继承自ServletHttpHandlerAdapter 实现了Servlet，执行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.创建request

　　　 2.创建response

3.handler执行的结果进行subscribe

　　JettyHttpHandlerAdapter是ServletHttpHandlerAdapter 的扩展，重写了创建request  创建response方法

7.3 Tomcat实现TomcatHttpHandlerAdapter

　　TomcatHttpHandlerAdapter是ServletHttpHandlerAdapter 的扩展，重写了创建request  创建response方法

7.4 AbstractReactiveWebInitializer抽象类

继承自AbstractReactiveWebInitializer的类可以在servlet容器中安装一个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);
    }

它通过将ServletHttpHandlerAdapter实例作为一个servlet安装到servler容器中。

8.总结

　　DispatcherHandler的流程是

　　　　1.通过 HandlerMapping（和DispathcherServlet中的HandlerMapping不同）获取到HandlerAdapter放到ServerWebExchange的属性中

　　　　2.获取到HandlerAdapter后触发handle方法，得到HandlerResult

　　　　3.通过HandlerResult，触发handleResult，针对不同的返回类找到不同的HandlerResultHandler如

　视图渲染ViewResolutionResultHandler，

ServerResponseResultHandler,

ResponseBodyResultHandler,

ResponseEntityResultHandler

不同容器有不同的实现，如Reactor，Jetty，Tomcat等。

参考文献：

【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