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。

以上就是本文的全部内容，希望对大家的学习有所帮助，也希望大家多多支持。