最近在做專案的時候發現，微服務使用feign相互之間呼叫時，存在session丟失的問題。例如，使用Feign呼叫某個遠端API，這個遠端API需要傳遞一個鑑權資訊，我們可以把cookie裡面的session資訊放到Header裡面，這個Header是動態的，跟你的HttpRequest相關，我們選擇編寫一個攔截器來實現Header的傳遞，也就是需要實現RequestInterceptor介面，具體程式碼如下：

@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);  
        }  
      }  
    }  

  }  

}  
 

經過測試，上面的解決方案可以正常的使用；
但是，當引入Hystrix熔斷策略時，出現了一個新的問題；

RequestAttributes requestAttributes = RequestContextHolder.getRequestAttributes();

獲取不到request資訊，從而無法傳遞session資訊，最終發現RequestContextHolder.getRequestAttributes()該方法是從ThreadLocal變數裡面取得對應資訊的，這就找到問題原因了，由於Hystrix熔斷機制導致的。
Hystrix有隔離策略：THREAD以及SEMAPHORE，當隔離策略為 THREAD 時，是沒辦法拿到 ThreadLocal 中的值的。

因此有兩種解決方案：

方案一：調整格隔離策略

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

這樣配置後，Feign可以正常工作。

但該方案不是特別好。原因是Hystrix官方強烈建議使用THREAD作為隔離策略！ 可以參考官方文件說明

方案二：自定義策略

記得之前在研究zipkin日誌追蹤的時候，看到過Sleuth有自己的熔斷機制，用來在thread之間傳遞Trace資訊，Sleuth是可以拿到自己上下文資訊的，檢視原始碼找到了
org.springframework.cloud.sleuth.instrument.hystrix.SleuthHystrixConcurrencyStrategy
這個類，檢視SleuthHystrixConcurrencyStrategy的原始碼，繼承了HystrixConcurrencyStrategy，用來實現了自己的併發策略。

/**
 * Abstract class for defining different behavior or implementations for concurrency related aspects of the system with default implementations.
 * <p>
 * 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.
 * <p>
 * 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.
 * <p>
 * See {@link HystrixPlugins} or the Hystrix GitHub Wiki for information on configuring plugins: <a
 * href="https://github.com/Netflix/Hystrix/wiki/Plugins">https://github.com/Netflix/Hystrix/wiki/Plugins</a>.
 */
public abstract class HystrixConcurrencyStrategy 

搜尋發現有好幾個地方繼承HystrixConcurrencyStrategy類

其中就有我們熟悉的Spring Security和剛才提到的Sleuth都是使用了自定義的策略，同時由於Hystrix只允許有一個併發策略，因此為了不影響Spring Security和Sleuth，我們可以參考他們的策略實現自己的側路，大致思路：
將現有的併發策略作為新併發策略的成員變數；
在新併發策略中，返回現有併發策略的執行緒池、Queue；
程式碼如下：

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，結合FeignClientsConfigurationCustom配置feign呼叫session丟失的問題完美解決