解决feign调用session传不出去的问题

微服务使用feign相互之间调用时，存在session丢失的问题。例如，使用Feign调用某个远程API，这个远程API需要传递一个鉴权信息，我们可以把cookie里面的session信息放到Header里面，这个Header是动态的，跟你的HttpRequest相关，我们选择编写一个拦截器来实现Header的传递，也就是需要实现RequestInterceptor接口，具体代码如下：

@Configuration  
@EnableFeignClients(basePackages = "com.xxx.xxx.client")  
public class FeignClientsConfigurationCustom implements RequestInterceptor {  

  @Override  
  public void apply(RequestTemplate template) {  

    RequestAttributes requestAttributes = RequestContextHolder.getRequestAttributes();  
    if (requestAttributes == null) {  
      return;  
    }  

    HttpServletRequest request = ((ServletRequestAttributes) requestAttributes).getRequest();  
    Enumeration<String> headerNames = request.getHeaderNames();  
    if (headerNames != null) {  
      while (headerNames.hasMoreElements()) {  
        String name = headerNames.nextElement();  
        Enumeration<String> values = request.getHeaders(name);  
        while (values.hasMoreElements()) {  
          String value = values.nextElement();  
          template.header(name, value);  
        }  
      }  
    }  

  }  

}  

经过测试，上面的解决方案可以正常的使用；
但是，当引入Hystrix熔断策略时，出现了一个新的问题；

RequestAttributes requestAttributes = RequestContextHolder.getRequestAttributes();

获取不到request信息，从而无法传递session信息，最终发现RequestContextHolder.getRequestAttributes()该方法是从ThreadLocal变量里面取得对应信息的，这就找到问题原因了，由于Hystrix熔断机制导致的。
Hystrix有隔离策略：THREAD以及SEMAPHORE，当隔离策略为 THREAD 时，是没办法拿到 ThreadLocal 中的值的。

因此有两种解决方案：

方案一：调整格隔离策略

hystrix.command.default.execution.isolation.strategy: SEMAPHORE

这样配置后，Feign可以正常工作。

但该方案不是特别好。原因是Hystrix官方强烈建议使用THREAD作为隔离策略！ 可以参考官方文档说明

方案二：自定义策略

查看源码找到了
org.springframework.cloud.sleuth.instrument.hystrix.SleuthHystrixConcurrencyStrategy
这个类，查看SleuthHystrixConcurrencyStrategy的源码，继承了HystrixConcurrencyStrategy，用来实现了自己的并发策略。

/**

• Abstract class for defining different behavior or implementations for concurrency related aspects of the system with default implementations.
• For example, every {@link Callable} executed by {@link HystrixCommand} will call {@link #wrapCallable(Callable)} to give a chance for custom implementations to decorate the {@link Callable} with
• additional behavior.
• When you implement a concrete {@link HystrixConcurrencyStrategy}, you should make the strategy idempotent w.r.t ThreadLocals.
• Since the usage of threads by Hystrix is internal, Hystrix does not attempt to apply the strategy in an idempotent way.
• Instead, you should write your strategy to work idempotently. See https://github.com/Netflix/Hystrix/issues/351 for a more detailed discussion.
• See {@link HystrixPlugins} or the Hystrix GitHub Wiki for information on configuring plugins:
• href=“https://github.com/Netflix/Hystrix/wiki/Plugins”>https://github.com/Netflix/Hystrix/wiki/Plugins.
  */
  public abstract class HystrixConcurrencyStrategy

搜索发现有好几个地方继承HystrixConcurrencyStrategy类

其中就有我们熟悉的Spring Security和刚才提到的Sleuth都是使用了自定义的策略，同时由于Hystrix只允许有一个并发策略，因此为了不影响Spring Security和Sleuth，我们可以参考他们的策略实现自己的侧路，大致思路：
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；
代码如下：

public class FeignHystrixConcurrencyStrategy extends HystrixConcurrencyStrategy {  

  private static final Logger log = LoggerFactory.getLogger(FeignHystrixConcurrencyStrategy.class);  
  private HystrixConcurrencyStrategy delegate;  

  public FeignHystrixConcurrencyStrategy() {  
    try {  
      this.delegate = HystrixPlugins.getInstance().getConcurrencyStrategy();  
      if (this.delegate instanceof FeignHystrixConcurrencyStrategy) {  
        // 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 ThreadPoolExecutor getThreadPool(HystrixThreadPoolKey threadPoolKey,  
      HystrixThreadPoolProperties threadPoolProperties) {  
    return this.delegate.getThreadPool(threadPoolKey, threadPoolProperties);  
  }  

  @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();  
      }  
    }  
  }  
}  

至此，结合FeignClientsConfigurationCustom配置feign调用session丢失的问题完美解决