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();

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。

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