二十一.SpringCloud源码剖析-Hystrix的初始化

前言

Hystrix不是停更了吗？你在这写什么？是，Hystrix是停止更新版本了，说不定后面又继续更新了呢？比如阿里的dubbo不也是停更一段时间后又继续更新了么。Hystrix只是停止开发新的版本，并不是完全停止维护，有Bug依然会修复，Hystrix已经是比较稳定的，很多项目依旧在使用它。

再者说Hystrix是SpringCloud 第一代技术标准中的非常重要的一个组件，可以看做是我们学习SpringCloud全家桶的一个必不可少的过程。在实际项目中你当然可以使用Spring Cloud Alibaba 等更先进的技术来作为微服务架构，使用sentinel代替Hystrix，但是如果你只会使用SpringCloudAlibaba,却不懂netflix技术体系，那我觉得你的技术栈是不完整的。即使你不使用Hystrix，那么它的一些思想和原理也是你需要掌握的，你觉得呢？？？

所以本着技术研究的态度，我还是写了这篇文章，Hystrix 源码剖析，对于Hystrix的使用请移步《熔断器Hystrix》

1.回顾Hystrix使用

为了防止服务之间的调用异常造成的连锁反应，在SpringCloud中提供了Hystrix组件来实现服务调用异常的处理，或对高并发情况下的服务降级处理 。这里简单回顾一下Hystrix的使用：

1.要使用 Hystrix熔断机制处理引入它本身的依赖之外，我们需要在主程序配置类上贴 @EnableCircuitBreaker标签 开启Hystrix功能，如下

@EnableCircuitBreaker
@EnableEurekaClient
@SpringBootApplication
...
public class ConsumerApplication {

2.开启Hystrix熔断机制后，对方法进行熔断处理

@Service
public class HelloService {@Autowiredprivate RestTemplate restTemplate;//该注解对该方法创建了熔断器的功能，并指定了fallbackMethod熔断方法@HystrixCommand(fallbackMethod = "hiError")public String hiService(String name){//调用接口进行消费String result = restTemplate.getForObject("http://PRODUCER/hello?name="+name,String.class);return result;}public String hiError(String name) {return "hi,"+name+"error!";}
}

当hiService方法第调用异常，会触发 fallbackMethod做降级处理，那么我们就沿着我们的使用方式来跟踪一下 @HystrixCommand注解的背后工作原理。

2.Hystrix的自动配置

首先我们看一下标签：@EnableCircuitBreaker ，他的作用从名字就能看出就是开启Hystrix ，我们看一下它的源码

/*** Annotation to enable a CircuitBreaker implementation.* http://martinfowler.com/bliki/CircuitBreaker.html* @author Spencer Gibb*/
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@Import(EnableCircuitBreakerImportSelector.class)
public @interface EnableCircuitBreaker {}

@EnableCircuitBreaker标签引入了一个@Import(EnableCircuitBreakerImportSelector.class) 类，翻译类的名字就是 ， 开启熔断器的导入选择器 ，导入什么东西呢？看源码

/*** Import a single circuit breaker implementation Configuration* @author Spencer Gibb*/
@Order(Ordered.LOWEST_PRECEDENCE - 100)
public class EnableCircuitBreakerImportSelector extendsSpringFactoryImportSelector<EnableCircuitBreaker> {@Overrideprotected boolean isEnabled() {return getEnvironment().getProperty("spring.cloud.circuit.breaker.enabled", Boolean.class, Boolean.TRUE);}}

选择器中的isEnabled方法用来读取Hytrix配置，判断是否开启熔断支持，默认是true，
翻译类上的注释 “Import a single circuit breaker implementation Configuration”，其实EnableCircuitBreakerImportSelector的作用就是去导入熔断器的配置

这个就是走的SpringBoot自动配置了，其实Spring中也有类似于JAVA SPI 的加载机制, 即会自动加载 jar包 spring-cloud-netflix-core 中的META-INF/spring.factories 中的Hystrix相关的自动配置类
注：SPI : 通过将服务的接口与实现分离以实现解耦，提高程序拓展性的机制，达到插拔式的效果 。

HystrixCircuitBreakerConfiguration 就是针对于 Hystrix熔断器的配置

/*** @author Spencer Gibb* @author Christian Dupuis* @author Venil Noronha*/
@Configuration
public class HystrixCircuitBreakerConfiguration {@Beanpublic HystrixCommandAspect hystrixCommandAspect() {return new HystrixCommandAspect();}@Beanpublic HystrixShutdownHook hystrixShutdownHook() {return new HystrixShutdownHook();}@Beanpublic HasFeatures hystrixFeature() {return HasFeatures.namedFeatures(new NamedFeature("Hystrix", HystrixCommandAspect.class));}
......

3.HystrixCommandAspect Aop切面类

在该配置类中创建了 HystrixCommandAspect ,它其实就是针对于@HystrixCommand的切面类，看一下源码：

/*** AspectJ aspect to process methods which annotated with {@link HystrixCommand} annotation.*/
@Aspect
public class HystrixCommandAspect {private static final Map<HystrixPointcutType, MetaHolderFactory> META_HOLDER_FACTORY_MAP;static {META_HOLDER_FACTORY_MAP = ImmutableMap.<HystrixPointcutType, MetaHolderFactory>builder().put(HystrixPointcutType.COMMAND, new CommandMetaHolderFactory()).put(HystrixPointcutType.COLLAPSER, new CollapserMetaHolderFactory()).build();}//定义切点，切到 @HystrixCommand标签所在的方法  @Pointcut("@annotation(com.netflix.hystrix.contrib.javanica.annotation.HystrixCommand)")public void hystrixCommandAnnotationPointcut() {}@Pointcut("@annotation(com.netflix.hystrix.contrib.javanica.annotation.HystrixCollapser)")public void hystrixCollapserAnnotationPointcut() {}//针对切点：@hystrixCommand切点的处理@Around("hystrixCommandAnnotationPointcut() || hystrixCollapserAnnotationPointcut()")public Object methodsAnnotatedWithHystrixCommand(final ProceedingJoinPoint joinPoint) throws Throwable {//获取到目标方法Method method = getMethodFromTarget(joinPoint);Validate.notNull(method, "failed to get method from joinPoint: %s", joinPoint);//判断方法上不能同时存在@HystrixCommand标签和HystrixCollapser标签if (method.isAnnotationPresent(HystrixCommand.class) && method.isAnnotationPresent(HystrixCollapser.class)) {throw new IllegalStateException("method cannot be annotated with HystrixCommand and HystrixCollapser " +"annotations at the same time");}MetaHolderFactory metaHolderFactory = META_HOLDER_FACTORY_MAP.get(HystrixPointcutType.of(method));//获取到@HystrixCommand注解的元数据MetaHolder metaHolder = metaHolderFactory.create(joinPoint);//把方法封装成 HystrixInvokable ,HystrixInvokable就是调用代理HystrixInvokable invokable = HystrixCommandFactory.getInstance().create(metaHolder);//这里在判断方法上是否使用 @HystrixCollapser ,我们使用的是@HystrixCommand//所以这里的执行类型ExecutionType是 SYNCHRONOUS 同步的方式去调用ExecutionType executionType = metaHolder.isCollapserAnnotationPresent() ?metaHolder.getCollapserExecutionType() : metaHolder.getExecutionType();Object result;try {// 通过CommandExecutor来执行方法if (!metaHolder.isObservable()) {//没有isObservable，走这里result = CommandExecutor.execute(invokable, executionType, metaHolder);} else {result = executeObservable(invokable, executionType, metaHolder);}} catch (HystrixBadRequestException e) {throw e.getCause() != null ? e.getCause() : e;} catch (HystrixRuntimeException e) {throw hystrixRuntimeExceptionToThrowable(metaHolder, e);}return result;

HystrixCommandAspect 其实就是对 贴了@HystrixCommand标签的方法使用 Aop机制实现处理 。代码中通过把目标方法封装成 HystrixInvokable对象，通过CommandExecutor工具来执行目标方法。

3.1.HystrixCommand创建

HystrixInvokable 是HystrixCommand的父接口，HystrixCommand是hystrix的核心组件，Hystrix的初始化、执行、限流、熔断等都是在这个组件中完成的，具体在run()方法中执行具体业务逻辑，那么他是如何创建的我们看代码HystrixInvokable invokable = HystrixCommandFactory.getInstance().create(metaHolder);的create方法

   public HystrixInvokable create(MetaHolder metaHolder) {HystrixInvokable executable;...省略代码...executable = new GenericCommand(HystrixCommandBuilderFactory.getInstance().create(metaHolder));}return executable;}

其实是new了一个 GenericCommand 对象，很明显他们是实现关系，我们看一下关系图

GenericCommand主要是通过AbstractCommand进行初始化,见：com.netflix.hystrix.AbstractCommand#AbstractCommand

protected AbstractCommand(HystrixCommandGroupKey group, HystrixCommandKey key, HystrixThreadPoolKey threadPoolKey, HystrixCircuitBreaker circuitBreaker, HystrixThreadPool threadPool,HystrixCommandProperties.Setter commandPropertiesDefaults, HystrixThreadPoolProperties.Setter threadPoolPropertiesDefaults,HystrixCommandMetrics metrics, TryableSemaphore fallbackSemaphore, TryableSemaphore executionSemaphore,HystrixPropertiesStrategy propertiesStrategy, HystrixCommandExecutionHook executionHook) {//各种初始化this.commandGroup = initGroupKey(group);this.commandKey = initCommandKey(key, getClass());this.properties = initCommandProperties(this.commandKey, propertiesStrategy, commandPropertiesDefaults);//初始化线程池this.threadPoolKey = initThreadPoolKey(threadPoolKey, this.commandGroup, this.properties.executionIsolationThreadPoolKeyOverride().get());this.metrics = initMetrics(metrics, this.commandGroup, this.threadPoolKey, this.commandKey, this.properties);//初始化熔断器this.circuitBreaker = initCircuitBreaker(this.properties.circuitBreakerEnabled().get(), circuitBreaker, this.commandGroup, this.commandKey, this.properties, this.metrics);//初始化线程池this.threadPool = initThreadPool(threadPool, this.threadPoolKey, threadPoolPropertiesDefaults);//Strategies from pluginsthis.eventNotifier = HystrixPlugins.getInstance().getEventNotifier();this.concurrencyStrategy = HystrixPlugins.getInstance().getConcurrencyStrategy();HystrixMetricsPublisherFactory.createOrRetrievePublisherForCommand(this.commandKey, this.commandGroup, this.metrics, this.circuitBreaker, this.properties);this.executionHook = initExecutionHook(executionHook);//初始化请求缓存this.requestCache = HystrixRequestCache.getInstance(this.commandKey, this.concurrencyStrategy);this.currentRequestLog = initRequestLog(this.properties.requestLogEnabled().get(), this.concurrencyStrategy);/* fallback semaphore override if applicable */this.fallbackSemaphoreOverride = fallbackSemaphore;/* execution semaphore override if applicable */this.executionSemaphoreOverride = executionSemaphore;}

3.2.熔断器的创建

HystrixCircuitBreaker是处理熔断业务的熔断器，跟一下 com.netflix.hystrix.AbstractCommand#initCircuitBreaker 方法，看一下熔断器是如何创建的

private static HystrixCircuitBreaker initCircuitBreaker(boolean enabled, HystrixCircuitBreaker fromConstructor,HystrixCommandGroupKey groupKey, HystrixCommandKey commandKey,HystrixCommandProperties properties, HystrixCommandMetrics metrics) {if (enabled) {if (fromConstructor == null) {// get the default implementation of HystrixCircuitBreakerreturn HystrixCircuitBreaker.Factory.getInstance(commandKey, groupKey, properties, metrics);} else {return fromConstructor;}} else {return new NoOpCircuitBreaker();}}

• HystrixCommandKey：一个HystrixCommandKey一个HystrixCommand，服务熔断，监控都会用到这个key
• HystrixCommandGroupKey：一组HystrixCommand可以用HystrixCommandGroupKey来配置
• HystrixCommandProperties ：读取和设置HystrixCommand配置的配置类
• HystrixCommandMetrics ：用于统计HystrixCommand的调用，包括成功、超时、异常、线程池队列/semaphore满、短路器打开等

这里使用 HystrixCircuitBreaker.Factory 来创建HystrixCircuitBreaker 熔断器，跟一下源码：

  class Factory {// String is HystrixCommandKey.name() (we can't use HystrixCommandKey directly as we can't guarantee it implements hashcode/equals correctly)//存放熔断器的容器ConcurrentHashMapprivate static ConcurrentHashMap<String, HystrixCircuitBreaker> circuitBreakersByCommand = new ConcurrentHashMap<String, HystrixCircuitBreaker>();public static HystrixCircuitBreaker getInstance(HystrixCommandKey key, HystrixCommandGroupKey group, HystrixCommandProperties properties, HystrixCommandMetrics metrics) {// this should find it for all but the first timeHystrixCircuitBreaker previouslyCached = circuitBreakersByCommand.get(key.name());if (previouslyCached != null) {return previouslyCached;}// if we get here this is the first time so we need to initialize// Create and add to the map ... use putIfAbsent to atomically handle the possible race-condition of// 2 threads hitting this point at the same time and let ConcurrentHashMap provide us our thread-safety// If 2 threads hit here only one will get added and the other will get a non-null response instead.//熔断器使用的是HystrixCircuitBreakerImplHystrixCircuitBreaker cbForCommand = circuitBreakersByCommand.putIfAbsent(key.name(), new HystrixCircuitBreakerImpl(key, group, properties, metrics));if (cbForCommand == null) {// this means the putIfAbsent step just created a new one so let's retrieve and return itreturn circuitBreakersByCommand.get(key.name());} else {// this means a race occurred and while attempting to 'put' another one got there before// and we instead retrieved it and will now return itreturn cbForCommand;}}

HystrixCircuitBreaker熔断器使用了ConcurrentHashMap来存储，具体的实现类是 HystrixCircuitBreakerImpl

3.3.线程池的创建

回到com.netflix.hystrix.AbstractCommand#initThreadPool方法看一下线程池是如何创建的

private static HystrixThreadPool initThreadPool(HystrixThreadPool fromConstructor, HystrixThreadPoolKey threadPoolKey, HystrixThreadPoolProperties.Setter threadPoolPropertiesDefaults) {if (fromConstructor == null) {// get the default implementation of HystrixThreadPoolreturn HystrixThreadPool.Factory.getInstance(threadPoolKey, threadPoolPropertiesDefaults);} else {return fromConstructor;}}

• HystrixThreadPool ： 对线程池进行统一管理，任务提交、队列限制等
• HystrixThreadPoolKey：一个key对应着hystrix的一个HystrixThreadPool
• HystrixThreadPoolProperties：读取线程池配置

这里使用HystrixThreadPool.Factory来创建HystrixThreadPool ，它其实和熔断器创建很相似：

static class Factory {/** Use the String from HystrixThreadPoolKey.name() instead of the HystrixThreadPoolKey instance as it's just an interface and we can't ensure the object* we receive implements hashcode/equals correctly and do not want the default hashcode/equals which would create a new threadpool for every object we get even if the name is the same*//* package */final static ConcurrentHashMap<String, HystrixThreadPool> threadPools = new ConcurrentHashMap<String, HystrixThreadPool>();/*** Get the {@link HystrixThreadPool} instance for a given {@link HystrixThreadPoolKey}.* 
* This is thread-safe and ensures only 1 {@link HystrixThreadPool} per {@link HystrixThreadPoolKey}.** @return {@link HystrixThreadPool} instance*//* package */static HystrixThreadPool getInstance(HystrixThreadPoolKey threadPoolKey, HystrixThreadPoolProperties.Setter propertiesBuilder) {// get the key to use instead of using the object itself so that if people forget to implement equals/hashcode things will still workString key = threadPoolKey.name();// this should find it for all but the first timeHystrixThreadPool previouslyCached = threadPools.get(key);if (previouslyCached != null) {return previouslyCached;}// if we get here this is the first time so we need to initializesynchronized (HystrixThreadPool.class) {if (!threadPools.containsKey(key)) {threadPools.put(key, new HystrixThreadPoolDefault(threadPoolKey, propertiesBuilder));}}return threadPools.get(key);}

线程池使用的是HystrixThreadPoolDefault创建实例，然后使用一个ConcurrentHashMap进行存储的。

4. HystrixCommand的执行流程

回到 HystrixCommandAspect的methodsAnnotatedWithHystrixCommand方法中，我们看下 CommandExecutor.execute是如何执行的

public class CommandExecutor {public CommandExecutor() {}public static Object execute(HystrixInvokable invokable, ExecutionType executionType, MetaHolder metaHolder) throws RuntimeException {Validate.notNull(invokable);Validate.notNull(metaHolder);switch(executionType) {//异步case SYNCHRONOUS:return castToExecutable(invokable, executionType).execute();//同步case ASYNCHRONOUS:HystrixExecutable executable = castToExecutable(invokable, executionType);if (metaHolder.hasFallbackMethodCommand() && ExecutionType.ASYNCHRONOUS == metaHolder.getFallbackExecutionType()) {return new FutureDecorator(executable.queue());}return executable.queue();case OBSERVABLE:HystrixObservable observable = castToObservable(invokable);return ObservableExecutionMode.EAGER == metaHolder.getObservableExecutionMode() ? observable.observe() : observable.toObservable();default:throw new RuntimeException("unsupported execution type: " + executionType);}}private static HystrixExecutable castToExecutable(HystrixInvokable invokable, ExecutionType executionType) {if (invokable instanceof HystrixExecutable) {return (HystrixExecutable)invokable;} else {throw new RuntimeException("Command should implement " + HystrixExecutable.class.getCanonicalName() + " interface to execute in: " + executionType + " mode");}}

这里有两种执行方式 SYNCHRONOUS 异步 ，ASYNCHRONOUS同步 ，我们先看异步： castToExecutable(invokable, executionType).execute(); 这里代码把HystrixInvokable对象转成 HystrixExecutable并调用execute方法执行 ，跟踪execute方法进入HystrixCommand.execute方法中

 public R execute() {try {return queue().get();} catch (Exception e) {throw Exceptions.sneakyThrow(decomposeException(e));}}
--------------public Future<R> queue() {/** The Future returned by Observable.toBlocking().toFuture() does not implement the* interruption of the execution thread when the "mayInterrupt" flag of Future.cancel(boolean) is set to true;* thus, to comply with the contract of Future, we must wrap around it.*/final Future<R> delegate = toObservable().toBlocking().toFuture();final Future<R> f = new Future<R>() {@Overridepublic boolean cancel(boolean mayInterruptIfRunning) {if (delegate.isCancelled()) {return false;}if (HystrixCommand.this.getProperties().executionIsolationThreadInterruptOnFutureCancel().get()) {/** The only valid transition here is false -> true. If there are two futures, say f1 and f2, created by this command* (which is super-weird, but has never been prohibited), and calls to f1.cancel(true) and to f2.cancel(false) are* issued by different threads, it's unclear about what value would be used by the time mayInterruptOnCancel is checked.* The most consistent way to deal with this scenario is to say that if *any* cancellation is invoked with interruption,* than that interruption request cannot be taken back.*/interruptOnFutureCancel.compareAndSet(false, mayInterruptIfRunning);}final boolean res = delegate.cancel(interruptOnFutureCancel.get());if (!isExecutionComplete() && interruptOnFutureCancel.get()) {final Thread t = executionThread.get();if (t != null && !t.equals(Thread.currentThread())) {t.interrupt();}}return res;}
....省略...

在 HystrixCommand.execute方法中 其实是Future 来异步执行，调用过程中会触发 GenericCommand来完成调用，执行完成后调用 Future.get()方法拿到执行结果 。

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