解决SpringCloud使用Feign跨服调用时header请求头中的信息丢失

在使用SpringCloud进行Feign跨服调用时header请求头中的信息会丢失，是因为Feign是不会带上当前请求的Cookie信息和头信息的，这个时候就需要重写请求拦截。

1、需要重写RequestInterceptor接口中的apply方法（前提是Feign的隔离策略为SEMAPHORE）

@Component
public class FeignInterceptor implements RequestInterceptor{

    @Override
    public void apply(RequestTemplate template) {
        ServletRequestAttributes attributes = (ServletRequestAttributes) RequestContextHolder.getRequestAttributes();
        if (attributes != null) {
            HttpServletRequest request = attributes.getRequest();
           //将token信息放入header中
            template.header("access_token",request.getHeader("access_token"));
        }
    }
}    

注意当Feign的隔离策略为THREAD时，由于当使用该隔离策略时，是没办法拿到 ThreadLocal 中的值的，但是RequestContextHolder 源码中，使用了两个ThreadLocal，因此当使用该隔离策略时是没有办法通过RequestContextHolder获取到request对象的，这时如果你还坚持使用THREAD这个隔离策略就需要自定义策略（重写THREAD隔离策略），代码如下：

/**
 * 自定义并发策略
 * 将现有的并发策略作为新并发策略的成员变量
 * 在新并发策略中，返回现有并发策略的线程池、Queue
 *
 * hystrix.command.default.execution.isolation.strategy=THREAD   Hystrix的默认隔离策略（官方推荐，当使用该隔离策略时，是没办法拿到 ThreadLocal 中的值的，但是
 *                                                               RequestContextHolder 源码中，使用了两个ThreadLocal）
 * hystrix.command.default.execution.isolation.strategy=SEMAPHORE （将隔离策略改为SEMAPHORE 也可以解决这个问题，但是官方并不推荐这个策略，因为这个策略对网络资源消耗比较大）
 *
 * 主要是解决当 Hystrix的默认隔离策略是THREAD时，不能通过RequestContextHolder获取到request对象的问题
 *
 * Create By yxl on 2018/5/22
 */
@Component
public class FeignConfig extends HystrixConcurrencyStrategy {
    private static final Logger log = LoggerFactory.getLogger(FeignConfig.class);
    private HystrixConcurrencyStrategy delegate;

    public FeignConfig() {
        try {
            this.delegate = HystrixPlugins.getInstance().getConcurrencyStrategy();
            if (this.delegate instanceof FeignConfig) {
                // Welcome to singleton hell...
                return;
            }
            HystrixCommandExecutionHook commandExecutionHook =
                    HystrixPlugins.getInstance().getCommandExecutionHook();
            HystrixEventNotifier eventNotifier = HystrixPlugins.getInstance().getEventNotifier();
            HystrixMetricsPublisher metricsPublisher = HystrixPlugins.getInstance().getMetricsPublisher();
            HystrixPropertiesStrategy propertiesStrategy =
                    HystrixPlugins.getInstance().getPropertiesStrategy();
            this.logCurrentStateOfHystrixPlugins(eventNotifier, metricsPublisher, propertiesStrategy);
            HystrixPlugins.reset();
            HystrixPlugins.getInstance().registerConcurrencyStrategy(this);
            HystrixPlugins.getInstance().registerCommandExecutionHook(commandExecutionHook);
            HystrixPlugins.getInstance().registerEventNotifier(eventNotifier);
            HystrixPlugins.getInstance().registerMetricsPublisher(metricsPublisher);
            HystrixPlugins.getInstance().registerPropertiesStrategy(propertiesStrategy);
        } catch (Exception e) {
            log.error("Failed to register Sleuth Hystrix Concurrency Strategy", e);
        }
    }

    private void logCurrentStateOfHystrixPlugins(HystrixEventNotifier eventNotifier,
                                                 HystrixMetricsPublisher metricsPublisher, HystrixPropertiesStrategy propertiesStrategy) {
        if (log.isDebugEnabled()) {
            log.debug("Current Hystrix plugins configuration is [" + "concurrencyStrategy ["
                    + this.delegate + "]," + "eventNotifier [" + eventNotifier + "]," + "metricPublisher ["
                    + metricsPublisher + "]," + "propertiesStrategy [" + propertiesStrategy + "]," + "]");
            log.debug("Registering Sleuth Hystrix Concurrency Strategy.");
        }
    }

    @Override
    public <T> Callable<T> wrapCallable(Callable<T> callable) {
        RequestAttributes requestAttributes = RequestContextHolder.getRequestAttributes();
        return new WrappedCallable<>(callable, requestAttributes);
    }

    @Override
    public ThreadPoolExecutor getThreadPool(HystrixThreadPoolKey threadPoolKey,
                                            HystrixProperty<Integer> corePoolSize, HystrixProperty<Integer> maximumPoolSize,
                                            HystrixProperty<Integer> keepAliveTime, TimeUnit unit, BlockingQueue<Runnable> workQueue) {
        return this.delegate.getThreadPool(threadPoolKey, corePoolSize, maximumPoolSize, keepAliveTime,
                unit, workQueue);
    }

    @Override
    public BlockingQueue<Runnable> getBlockingQueue(int maxQueueSize) {
        return this.delegate.getBlockingQueue(maxQueueSize);
    }

    @Override
    public <T> HystrixRequestVariable<T> getRequestVariable(HystrixRequestVariableLifecycle<T> rv) {
        return this.delegate.getRequestVariable(rv);
    }

    static class WrappedCallable<T> implements Callable<T> {
        private final Callable<T> target;
        private final RequestAttributes requestAttributes;

        public WrappedCallable(Callable<T> target, RequestAttributes requestAttributes) {
            this.target = target;
            this.requestAttributes = requestAttributes;
        }

        @Override
        public T call() throws Exception {
            try {
                RequestContextHolder.setRequestAttributes(requestAttributes);
                return target.call();
            } finally {
                RequestContextHolder.resetRequestAttributes();
            }
        }
    }
}