前段时间项目中加入了
Hystrix做了服务接口的降级和熔断，随之引发了一些问题。
发现问题
当Hystrix隔离策略为THREAD模式时，是无法获取到ThreadLocal中的值的；
当时我们使用Feign调用接口时RequestInterceptor作为拦截器来实现远程调用间Header信息的传递，但是当我打开熔断策略开关后，RequestContextHolder.getRequestAttributes()无法获取到当前请求的头信息，导致后置服务无法通过鉴权。如下代码：
@Configuration
@Slf4j
public class FeignConfig implements RequestInterceptor {
@Override
public void apply(RequestTemplate template) {
//开启熔断后，这里是无法获取到值的
ServletRequestAttributes attributes = (ServletRequestAttributes) RequestContextHolder.getRequestAttributes();
...
...
}
}
原因在于，我们使用的隔离策略是THREAD 。而 RequestContextHolder 源码中，使用了两个血淋淋的ThreadLocal 。
解决问题
最简单无脑的解决方式就是将隔离策略修改为SEMAPHORE即可，信号量的方式不会导致获取不到当前线程的请求头信息。
但是这种解决方式略微牵强，因为Hystrix官方强烈建议使用THREAD作为隔离策略。
那么有什么办法可以在不改变隔离策略的前提下解决这个问题吗？答案是自定义并发策略：
- 编写自定义并发策略，编写一个类，让其继承
HystrixConcurrencyStrategy，并重写wrapCallable方法即可。wrapCallable方法拿到RequestContextHolder.getRequestAttributes()，也就是我们想传播的对象；- 在
WrappedCallable类中，我们将要传播的对象作为成员变量，并在其中的call方法中，为静态方法设值。 - 这样，在
Hystrix包裹的方法中，就可以使用RequestContextHolder.getRequestAttributes()获取到相关属性——也就是说，可以拿到RequestContextHolder中的ThreadLocal属性。
@Component
public class RequestAttributeHystrixConcurrencyStrategy extends HystrixConcurrencyStrategy {
private static final Log log = LogFactory.getLog(RequestHystrixConcurrencyStrategy.class);
public RequestHystrixConcurrencyStrategy() {
HystrixPlugins.reset();
HystrixPlugins.getInstance().registerConcurrencyStrategy(this);
}
@Override
public <T> Callable<T> wrapCallable(Callable<T> callable) {
RequestAttributes requestAttributes = RequestContextHolder.getRequestAttributes();
return new WrappedCallable<>(callable, requestAttributes);
}
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();
}
}
}
}
- 至此，已经实现了
ThreadLocal属性的传递，然而Hystrix只允许有一个并发策略！这意味着——如果不做任何处理，Sleuth、Spring Security将无法正常拿到上下文！（目前Sleuth、Spring Security都是通过自定义并发策略的方式来传递ThreadLocal对象的。）
如何解决这个问题呢？参考下Sleuth以及Spring Security的实现：- org.springframework.cloud.sleuth.instrument.hystrix.SleuthHystrixConcurrencyStrategy
- org.springframework.cloud.netflix.hystrix.security.SecurityContextConcurrencyStrategy
- 模仿它们的写法，改编上面的并发策略：
- 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。
public class RequestAttributeHystrixConcurrencyStrategy extends HystrixConcurrencyStrategy {
private static final Log log = LogFactory.getLog(RequestAttributeHystrixConcurrencyStrategy.class);
private HystrixConcurrencyStrategy delegate;
public RequestAttributeHystrixConcurrencyStrategy() {
try {
this.delegate = HystrixPlugins.getInstance().getConcurrencyStrategy();
if (this.delegate instanceof RequestAttributeHystrixConcurrencyStrategy) {
// 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();
}
}
}
}
- 至此，解决
Hystrix传播ThreadLocal对象的问题。
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