Spring Cloud全解析：熔断之Hystrix线程隔离导致

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？

• [ ]

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requestAttributes
         RequestAttributes 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;
        }
        
        @Override
        public 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>() {

            @Override
            public Void call() throws Exception {
                actual.run();
                return null;
            }

        });
        this.parentThreadState = HystrixRequestContext.getContextForCurrentThread();
    }

    @Override
    public void run() {
        HystrixRequestContext existingState = HystrixRequestContext.getContextForCurrentThread();
        try {
            // set the state of this thread to that of its parent
            HystrixRequestContext.setContextOnCurrentThread(parentThreadState);
            // execute actual Callable with the state of the parent
            try {
                actual.call();
            } catch (Exception e) {
                throw new RuntimeException(e);
            }
        } finally {
            // restore this thread back to its original state
            HystrixRequestContext.setContextOnCurrentThread(existingState);
        }
    }

}

https://zhhll.icu/2021/框架/微服务/springcloud/熔断/Hystrix断路器/6.Hystrix线程隔离导致的问题/