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Hystrix线程隔离

在微服务框架中，可能一个服务需要调用多个微服务，在tomcat中运行时，tomcat只是分配了100个线程，由于多个服务之间调用的时间消耗过长，可能会导致线程耗尽，而在Hystrix中存在线程隔离，对于每个微服务分配一个线程池，访问某个微服务时就从对应的线程池中取线程，如果对应线程池中的线程都用光了，那么就认为该服务不可用了，如果在需要请求该微服务，则直接返回

那么这个线程池是存在于哪里呢？

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既然Hystrix的Command都是在线程池中执行的，就会遇到当前的RequestContextHolder获取不到RequestAttributes，没办法，跨线程了呀(RequestContextHolder中使用了ThreadLocal)，这怎么解决呢？Hystrix中提供了一个HystrixConcurrencyStrategy类，HystrixConcurrencyStrategy提供了一套默认的并发策略实现。我们可以根据我们自己不同需求通过装饰去扩展它。如每次执行HystrixCommand的时候都会去调用wrapCallable(Callable) 方法，这里我们就可以通过装饰Callable使它提供一些额外的功能（如ThreadLocal上下文传递）

public class FeignHystrixConcurrencyStrategy extends HystrixConcurrencyStrategy
{private static final Logger log;private HystrixConcurrencyStrategy delegate;public FeignHystrixConcurrencyStrategy() {try {this.delegate = HystrixPlugins.getInstance().getConcurrencyStrategy();if (this.delegate instanceof FeignHystrixConcurrencyStrategy) {return;}HystrixCommandExecutionHook commandExecutionHook = HystrixPlugins.getInstance().getCommandExecutionHook();HystrixEventNotifier eventNotifier = HystrixPlugins.getInstance().getEventNotifier();HystrixMetricsPublisher metricsPublisher = HystrixPlugins.getInstance().getMetricsPublisher();HystrixPropertiesStrategy propertiesStrategy = HystrixPlugins.getInstance().getPropertiesStrategy();HystrixPlugins.reset();// 注册并发策略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);}}// 这个时候还在主线程了，所以通过RequestContextHolder.getRequestAttributes()是能拿到上下文的拿到后hold住，等到run执行的时候再绑定即可public <T> Callable<T> wrapCallable( Callable<T> callable) {// 获取当前请求的requestAttributesRequestAttributes requestAttributes = RequestContextHolder.getRequestAttributes();return new WrappedCallable<T>(callable, requestAttributes);}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);}public ThreadPoolExecutor getThreadPool( HystrixThreadPoolKey threadPoolKey,  HystrixThreadPoolProperties threadPoolProperties) {return this.delegate.getThreadPool(threadPoolKey, threadPoolProperties);}public BlockingQueue<Runnable> getBlockingQueue( int maxQueueSize) {return this.delegate.getBlockingQueue(maxQueueSize);}public <T> HystrixRequestVariable<T> getRequestVariable( HystrixRequestVariableLifecycle<T> rv) {return this.delegate.getRequestVariable(rv);}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;}@Overridepublic T call() throws Exception {try {// 执行之前绑定上下文，执行完成后释放RequestContextHolder.setRequestAttributes(this.requestAttributes);return this.target.call();}finally {RequestContextHolder.resetRequestAttributes();}}}
}

这时候大家就奇怪了，为什么在wrapCallable方法中可以获取到当前请求呢，来看源码是怎么调用HystrixConcurrencyStrategy的

public class HystrixContextRunnable implements Runnable {private final Callable<Void> actual;// 父线程的上下文private final HystrixRequestContext parentThreadState;public HystrixContextRunnable(Runnable actual) {this(HystrixPlugins.getInstance().getConcurrencyStrategy(), actual);}public HystrixContextRunnable(HystrixConcurrencyStrategy concurrencyStrategy, final Runnable actual) {// 实例化HystrixContextRunnable的时候去调用的concurrencyStrategy.wrapCallable，此时还没有切换线程呢this.actual = concurrencyStrategy.wrapCallable(new Callable<Void>() {@Overridepublic Void call() throws Exception {actual.run();return null;}});this.parentThreadState = HystrixRequestContext.getContextForCurrentThread();}@Overridepublic void run() {HystrixRequestContext existingState = HystrixRequestContext.getContextForCurrentThread();try {// set the state of this thread to that of its parentHystrixRequestContext.setContextOnCurrentThread(parentThreadState);// execute actual Callable with the state of the parenttry {actual.call();} catch (Exception e) {throw new RuntimeException(e);}} finally {// restore this thread back to its original stateHystrixRequestContext.setContextOnCurrentThread(existingState);}}}

参考文献

• Hystrix线程隔离导致的问题