Feign使用及源码深度解析（下）

版权声明：本文为博主原创文章，未经博主允许不得转载。 https://blog.csdn.net/qq_26323323/article/details/82379252

前言：

    在上一篇文章中，我们对@EnableFeignClients注解的源码进行了分析，了解到其主要作用就是把带有@FeignClient注解的类或接口用FeignClientFactoryBean类注册到Spring中。

    本文中，便开始介绍FeignClientFactoryBean是如何实现Ribbon+Hystrix的功能的

1.FeignClientFactoryBean源码结构分析

    可知，其实现了FactoryBean接口，那么当从ApplicationContext中获取该bean的时候，实际调用的是其getObject()方法

class FeignClientFactoryBean implements FactoryBean<Object>, InitializingBean,
		ApplicationContextAware{
            
	public Object getObject() throws Exception {
        // 1.获取容器中的FeignContext实现
        // 默认实现在FeignAutoConfiguration类中
		FeignContext context = applicationContext.getBean(FeignContext.class);
        // 2.主要使用构造者模式来构建一个Feign
        // 在1）中详细分析
		Feign.Builder builder = feign(context);

        // 3.本例中没有指定URL，故执行if
		if (!StringUtils.hasText(this.url)) {
			String url;
			if (!this.name.startsWith("http")) {
				url = "http://" + this.name;
			}
			else {
				url = this.name;
			}
			url += cleanPath();
            // 4.关键方法
            // 在2）中详细分析
			return loadBalance(builder, context, new HardCodedTarget<>(this.type,
					this.name, url));
		}
		if (StringUtils.hasText(this.url) && !this.url.startsWith("http")) {
			this.url = "http://" + this.url;
		}
		String url = this.url + cleanPath();
		Client client = getOptional(context, Client.class);
		if (client != null) {
			if (client instanceof LoadBalancerFeignClient) {
				// not lod balancing because we have a url,
				// but ribbon is on the classpath, so unwrap
				client = ((LoadBalancerFeignClient)client).getDelegate();
			}
			builder.client(client);
		}
		Targeter targeter = get(context, Targeter.class);
		return targeter.target(this, builder, context, new HardCodedTarget<>(
				this.type, this.name, url));
	}
}

    1）feign(context)构造者模式构建Feign.Builder

protected Feign.Builder feign(FeignContext context) {
    FeignLoggerFactory loggerFactory = get(context, FeignLoggerFactory.class);
    Logger logger = loggerFactory.create(this.type);

    // @formatter:off
    Feign.Builder builder = get(context, Feign.Builder.class)
        // required values
        .logger(logger)
        .encoder(get(context, Encoder.class))
        .decoder(get(context, Decoder.class))
        .contract(get(context, Contract.class));
    // @formatter:on

    // 下面的几项getOptional()主要功能就是从ApplicationContext中获取对应类的实现
    // 这些类用户可自定义，默认的话是从FeignAutoConfiguration中获取对应bean
    Logger.Level level = getOptional(context, Logger.Level.class);
    if (level != null) {
        builder.logLevel(level);
    }
    Retryer retryer = getOptional(context, Retryer.class);
    if (retryer != null) {
        builder.retryer(retryer);
    }
    ErrorDecoder errorDecoder = getOptional(context, ErrorDecoder.class);
    if (errorDecoder != null) {
        builder.errorDecoder(errorDecoder);
    }
    Request.Options options = getOptional(context, Request.Options.class);
    if (options != null) {
        builder.options(options);
    }
    Map<String, RequestInterceptor> requestInterceptors = context.getInstances(
        this.name, RequestInterceptor.class);
    if (requestInterceptors != null) {
        builder.requestInterceptors(requestInterceptors.values());
    }

    if (decode404) {
        builder.decode404();
    }

    return builder;
}

    2）loadBalance(builder, context, new HardCodedTarget<>(this.type,this.name, url))获取负载均衡后的对象

protected <T> T loadBalance(Feign.Builder builder, FeignContext context,
                            HardCodedTarget<T> target) {
    // 1.获取Client的实现类，默认为LoadBalancerFeignClient类
    // 实现在FeignRibbonClientAutoConfiguration中
    Client client = getOptional(context, Client.class);
    if (client != null) {
        // 2.将LoadBalancerFeignClient包装到Feign.Builder
        builder.client(client);
        
        // 3.获取ApplicationContext中的Targeter实现
        // 默认实现为HystrixTargeter，实现在FeignAutoConfiguration类中
        Targeter targeter = get(context, Targeter.class);
        
        // 4.重点在这里
        // 我们来看下这个方法
        return targeter.target(this, builder, context, target);
    }
    ...
}

// HystrixTargeter.target()
public <T> T target(FeignClientFactoryBean factory, Feign.Builder feign, FeignContext context,
                    Target.HardCodedTarget<T> target) {
    // Feign.Builder为feign包路径下，不是 feign.hystrix.HystrixFeign.Builder，故直接走return方法
    if (!(feign instanceof feign.hystrix.HystrixFeign.Builder)) {
        // 我们来看下这个方法，在feign.Feign.Builder类中
        return feign.target(target);
    }
    ...
}

// feign.Feign.Builder.target(target)
public <T> T target(Target<T> target) {
    return build().newInstance(target);
}

// feign.Feign.Builder.build()
public Feign build() {
    // 注意这个工厂类，下面还是会用到的
    SynchronousMethodHandler.Factory synchronousMethodHandlerFactory =
        new SynchronousMethodHandler.Factory(client, retryer, requestInterceptors, logger,
                                             logLevel, decode404);
    ParseHandlersByName handlersByName =
        new ParseHandlersByName(contract, options, encoder, decoder,
                                errorDecoder, synchronousMethodHandlerFactory);
    // 在这里可以看到真正返回的是ReflectiveFeign
    return new ReflectiveFeign(handlersByName, invocationHandlerFactory);
}

// ReflectiveFeign.newInstance(target)
public <T> T newInstance(Target<T> target) {
    // 1.分析出具体方法和对应的Handler处理，
    Map<String, MethodHandler> nameToHandler = targetToHandlersByName.apply(target);
    Map<Method, MethodHandler> methodToHandler = new LinkedHashMap<Method, MethodHandler>();
    List<DefaultMethodHandler> defaultMethodHandlers = new LinkedList<DefaultMethodHandler>();

    for (Method method : target.type().getMethods()) {
        if (method.getDeclaringClass() == Object.class) {
            continue;
        } else if(Util.isDefault(method)) {
            DefaultMethodHandler handler = new DefaultMethodHandler(method);
            defaultMethodHandlers.add(handler);
            methodToHandler.put(method, handler);
        } else {
            methodToHandler.put(method, nameToHandler.get(Feign.configKey(target.type(), method)));
        }
    }
    // 2.由factory创建一个InvocationHandler，实现为FeignInvocationHandler
    // 可知其代理的target为HardCodedTarget
    InvocationHandler handler = factory.create(target, methodToHandler);
    // 3.可以看到最终返回的是一个代理
    T proxy = (T) Proxy.newProxyInstance(target.type().getClassLoader(), new Class<?>[]{target.type()}, handler);

    for(DefaultMethodHandler defaultMethodHandler : defaultMethodHandlers) {
        defaultMethodHandler.bindTo(proxy);
    }
    return proxy;
}

总结：由以上分析可知，FeignClientFactoryBean.getObject()具体返回的是一个代理类，具体为FeignInvocationHandler

3.FeignInvocationHandler接收请求方法过程分析

public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
    if ("equals".equals(method.getName())) {
        try {
            Object
                otherHandler =
                args.length > 0 && args[0] != null ? Proxy.getInvocationHandler(args[0]) : null;
            return equals(otherHandler);
        } catch (IllegalArgumentException e) {
            return false;
        }
    } else if ("hashCode".equals(method.getName())) {
        return hashCode();
    } else if ("toString".equals(method.getName())) {
        return toString();
    }
    // 非Object方法，则默认执行该句
    // dispatch为map，方法的实现类为SynchronousMethodHandler
    // 我们来分析SynchronousMethodHandler.invoke()方法
    return dispatch.get(method).invoke(args);
}

    1）SynchronousMethodHandler.invoke()

public Object invoke(Object[] argv) throws Throwable {
    // 1.根据请求参数创建一个feign.RequestTemplate
    RequestTemplate template = buildTemplateFromArgs.create(argv);
    // 2.用户定义的重试策略
    Retryer retryer = this.retryer.clone();
    while (true) {
        try {
            // 重要方法在这里
            return executeAndDecode(template);
        } catch (RetryableException e) {
            retryer.continueOrPropagate(e);
            if (logLevel != Logger.Level.NONE) {
                logger.logRetry(metadata.configKey(), logLevel);
            }
            continue;
        }
    }
}

    2）executeAndDecode(template)执行请求

Object executeAndDecode(RequestTemplate template) throws Throwable {
    // 1.封装请求信息，feign.Request，会将请求封装为以下信息
    // GET http://part-1-sms-interface/sms/test HTTP/1.1
    Request request = targetRequest(template);

    if (logLevel != Logger.Level.NONE) {
        logger.logRequest(metadata.configKey(), logLevel, request);
    }

    Response response;
    long start = System.nanoTime();
    try {
        // 2.真正的执行在这里
        // client为LoadBalancerFeignClient
        // 继续在3）中详细分析
        response = client.execute(request, options);
        // ensure the request is set. TODO: remove in Feign 10
        response.toBuilder().request(request).build();
    } catch (IOException e) {
        if (logLevel != Logger.Level.NONE) {
            logger.logIOException(metadata.configKey(), logLevel, e, elapsedTime(start));
        }
        throw errorExecuting(request, e);
    }
    long elapsedTime = TimeUnit.NANOSECONDS.toMillis(System.nanoTime() - start);

    // 响应处理
    ...
}

    3）LoadBalancerFeignClient.execute(request, options)请求负载均衡

public Response execute(Request request, Request.Options options) throws IOException {
    try {
        // 1.获取URI
        URI asUri = URI.create(request.url());
        String clientName = asUri.getHost();
        URI uriWithoutHost = cleanUrl(request.url(), clientName);
        
        // 2.封装成RibbonRequest请求
        FeignLoadBalancer.RibbonRequest ribbonRequest = new FeignLoadBalancer.RibbonRequest(
            this.delegate, request, uriWithoutHost);

        // 3.封装请求参数信息
        IClientConfig requestConfig = getClientConfig(options, clientName);
        
        // 4.执行请求，并进行负载均衡
        // 本方法可分为三步：
        // 1)lbClient(clientName)获取执行类，本例中为FeignLoadBalancer
        // 2)FeignLoadBalancer.executeWithLoadBalancer()执行请求
        // 3)toResponse()获取响应
        return lbClient(clientName).executeWithLoadBalancer(ribbonRequest,
                                                            requestConfig).toResponse();
    }
    catch (ClientException e) {
        IOException io = findIOException(e);
        if (io != null) {
            throw io;
        }
        throw new RuntimeException(e);
    }
}

    4）FeignLoadBalancer.executeWithLoadBalancer()执行请求

public T executeWithLoadBalancer(final S request, final IClientConfig requestConfig) throws ClientException {
    RequestSpecificRetryHandler handler = getRequestSpecificRetryHandler(request, requestConfig);
    LoadBalancerCommand<T> command = LoadBalancerCommand.<T>builder()
        .withLoadBalancerContext(this)
        .withRetryHandler(handler)
        .withLoadBalancerURI(request.getUri())
        .build();

    try {
        // 在这里可以看到Hystrix的相关代码，
        return command.submit(
            new ServerOperation<T>() {
                @Override
                public Observable<T> call(Server server) {
                    URI finalUri = reconstructURIWithServer(server, request.getUri());
                    S requestForServer = (S) request.replaceUri(finalUri);
                    try {
                        // 执行ribbon负载均衡请求
                        return Observable.just(AbstractLoadBalancerAwareClient.this.execute(requestForServer, requestConfig));
                    } 
                    catch (Exception e) {
                        return Observable.error(e);
                    }
                }
            })
            .toBlocking()
            .single();
    } catch (Exception e) {
        Throwable t = e.getCause();
        if (t instanceof ClientException) {
            throw (ClientException) t;
        } else {
            throw new ClientException(e);
        }
    }

}

总结：

    1.@EnableFeignClients注解将所有带有@FeignClient的类或接口注册到Spring中，注册类为FeignClientFactoryBean

    2.FeignClientFactoryBean.getObject()方法返回的是一个代理类，InvocationHandler中包含类中每个方法对应的MethodHandler，也就是SynchronousMethodHandler，方法真正执行就是SynchronousMethodHandler.invoke()方法

    3.LoadBalancerFeignClient.execute()方法进行业务的处理，在这一步操作中就用到了ribbon和Hystrix功能

以下摘自博文 https://blog.csdn.net/w8452960/article/details/77150111

Feign的关键标签类：

1. Client(接口) - Feign(抽象类) - ReflectiveFeign(实现类)。 

2. RequestTemplate 

3. InvocationHandlerFactory(接口) - SynchronousMethodHandler(实现类) 

4. Decoder与Encoder

调用栈大约如下 

1. ReflectiveFeign 被反射实例化 

2. 调用ReflectiveFeign.invoke 

3. 调用SynchronousMethodHandler.invoke。此处实例化RequestTemplate 

4. 调用SynchronousMethodHandler.executeAndDecode 

5. 将RequestTemplate build为request,调用http客户端执行 

6. 将Response Decode为Object并返回