feign调用session丢失解决方案

       最近在做项目的时候发现，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 headerNames = request.getHeaderNames();  
    if (headerNames != null) {  
      while (headerNames.hasMoreElements()) {  
        String name = headerNames.nextElement();  
        Enumeration 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();

此时requestAttributes会返回null，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，是由于Hystrix熔断机制导致的。 
Hystrix有2个隔离策略：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

记得之前在研究zipkin日志追踪的时候，看到过Sleuth有自己的熔断机制，用来在thread之间传递Trace信息，Sleuth是可以拿到自己上下文信息的，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: https://github.com/Netflix/Hystrix/wiki/Plugins.
 */
public abstract class HystrixConcurrencyStrategy 

搜索发现有好几个地方继承了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策略，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  Callable wrapCallable(Callable callable) {  
    RequestAttributes requestAttributes = RequestContextHolder.getRequestAttributes();  
    return new WrappedCallable<>(callable, requestAttributes);  
  }  

  @Override  
  public ThreadPoolExecutor getThreadPool(HystrixThreadPoolKey threadPoolKey,  
      HystrixProperty corePoolSize, HystrixProperty maximumPoolSize,  
      HystrixProperty keepAliveTime, TimeUnit unit, BlockingQueue 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 getBlockingQueue(int maxQueueSize) {  
    return this.delegate.getBlockingQueue(maxQueueSize);  
  }  

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

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

    public WrappedCallable(Callable 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();  
      }  
    }  
  }  
}  

最后，将这个策略类作为bean配置到feign的配置类FeignClientsConfigurationCustom中

  @Bean
  public FeignHystrixConcurrencyStrategy feignHystrixConcurrencyStrategy() {
    return new FeignHystrixConcurrencyStrategy();
  }

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