? ? ? ?最近在做項目的時候發現，微服務使用feign相互之間調用時，存在session丟失的問題。例如，使用Feign調用某個遠程API，這個遠程API需要傳遞一個鑒權信息，我們可以把cookie里面的session信息放到Header里面，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接口，具體代碼如下：

@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();

此時requestAttributes會返回null，從而無法傳遞session信息，最終發現RequestContextHolder.getRequestAttributes()，該方法是從ThreadLocal變量里面取得對應信息的，這就找到問題原因了，是由于Hystrix熔斷機制導致的。?
Hystrix有2個隔離策略：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

這樣配置后，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！ 可以參考官方文檔說明。

方案二：自定義策略

記得之前在研究zipkin日志追蹤的時候，看到過Sleuth有自己的熔斷機制，用來在thread之間傳遞Trace信息，Sleuth是可以拿到自己上下文信息的，查看源碼找到了?
org.springframework.cloud.sleuth.instrument.hystrix.SleuthHystrixConcurrencyStrategy?
這個類，查看SleuthHystrixConcurrencyStrategy的源碼，繼承了HystrixConcurrencyStrategy，用來實現了自己的并發策略。

/*** 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

搜索發現有好幾個地方繼承了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，同時由于Hystrix只允許有一個并發策略，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策略，大致思路：?
將現有的并發策略作為新并發策略的成員變量；?
在新并發策略中，返回現有并發策略的線程池、Queue；?
代碼如下：

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(); } } } }

最后，將這個策略類作為bean配置到feign的配置類FeignClientsConfigurationCustom中

@Beanpublic FeignHystrixConcurrencyStrategy feignHystrixConcurrencyStrategy() {return new FeignHystrixConcurrencyStrategy();}

至此，結合FeignClientsConfigurationCustom配置feign調用session丟失的問題完美解決。

?

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