? ? ? ?最近在做項(xiàng)目的時(shí)候發(fā)現(xiàn)，微服務(wù)使用feign相互之間調(diào)用時(shí)，存在session丟失的問(wèn)題。例如，使用Feign調(diào)用某個(gè)遠(yuǎn)程API，這個(gè)遠(yuǎn)程API需要傳遞一個(gè)鑒權(quán)信息，我們可以把cookie里面的session信息放到Header里面，這個(gè)Header是動(dòng)態(tài)的，跟你的HttpRequest相關(guān)，我們選擇編寫(xiě)一個(gè)攔截器來(lái)實(shí)現(xiàn)Header的傳遞，也就是需要實(shí)現(xiàn)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(jīng)過(guò)測(cè)試，上面的解決方案可以正常的使用；?

但是，當(dāng)引入Hystrix熔斷策略時(shí)，出現(xiàn)了一個(gè)新的問(wèn)題：

RequestAttributes requestAttributes = RequestContextHolder.getRequestAttributes();

此時(shí)requestAttributes會(huì)返回null，從而無(wú)法傳遞session信息，最終發(fā)現(xiàn)RequestContextHolder.getRequestAttributes()，該方法是從ThreadLocal變量里面取得對(duì)應(yīng)信息的，這就找到問(wèn)題原因了，是由于Hystrix熔斷機(jī)制導(dǎo)致的。?
Hystrix有2個(gè)隔離策略：THREAD以及SEMAPHORE，當(dāng)隔離策略為 THREAD 時(shí)，是沒(méi)辦法拿到 ThreadLocal 中的值的。

因此有兩種解決方案：

方案一：調(diào)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(qiáng)烈建議使用THREAD作為隔離策略！ 可以參考官方文檔說(shuō)明。

方案二：自定義策略

記得之前在研究zipkin日志追蹤的時(shí)候，看到過(guò)Sleuth有自己的熔斷機(jī)制，用來(lái)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(gè)類(lèi)，查看SleuthHystrixConcurrencyStrategy的源碼，繼承了HystrixConcurrencyStrategy，用來(lái)實(shí)現(xiàn)了自己的并發(fā)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(fā)現(xiàn)有好幾個(gè)地方繼承了HystrixConcurrencyStrategy類(lèi)?

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(shí)由于Hystrix只允許有一個(gè)并發(fā)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(shí)現(xiàn)自己的策略，大致思路：?
將現(xiàn)有的并發(fā)策略作為新并發(fā)策略的成員變量；?
在新并發(fā)策略中，返回現(xiàn)有并發(fā)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(gè)策略類(lèi)作為bean配置到feign的配置類(lèi)FeignClientsConfigurationCustom中

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

至此，結(jié)合FeignClientsConfigurationCustom配置feign調(diào)用session丟失的問(wèn)題完美解決。

?

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