详解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 * 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();

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.
* <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" rel="external nofollow" >https://github.com/Netflix/Hystrix/wiki/Plugins</a>.
*/
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 <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();
  }
 }
}
}

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。

来源：https://blog.csdn.net/zl1zl2zl3/article/details/79084368