但执行HystrixCommand时是有默认threadPoolKey的，那么这个默认值从何而来，command又是怎么初始化线程池的呢？？
通过metaHolder构造HystrixCommandBuilder //----------HystrixCommandFactory#create() public HystrixInvokable create(MetaHolder metaHolder) { HystrixInvokable executable; if (metaHolder.isCollapserAnnotationPresent()) { executable = new CommandCollapser(metaHolder); } else if (metaHolder.isObservable()) { executable = new GenericObservableCommand(HystrixCommandBuilderFactory.getInstance().create(metaHolder)); } else { //通过metaHolder构造HystrixCommandBuilder，再创建GenericCommand executable = new GenericCommand(HystrixCommandBuilderFactory.getInstance().create(metaHolder)); } return executable; } //----------HystrixCommandBuilderFactory#create() 创建HystrixCommandBuilder //在创建HystrixCommandBuilder时，createGenericSetterBuilder(metaHolder)构造了Setter，是用于设置groupKey、commandKey、threadPoolKey的 public <ResponseType> HystrixCommandBuilder create(MetaHolder metaHolder, Collection<HystrixCollapser.CollapsedRequest<ResponseType, Object>> collapsedRequests) { validateMetaHolder(metaHolder); return HystrixCommandBuilder.builder() .setterBuilder(createGenericSetterBuilder(metaHolder)) //重点：设置setterBuilder .commandActions(createCommandActions(metaHolder)) .collapsedRequests(collapsedRequests) .cacheResultInvocationContext(createCacheResultInvocationContext(metaHolder)) .cacheRemoveInvocationContext(createCacheRemoveInvocationContext(metaHolder)) .ignoreExceptions(metaHolder.getCommandIgnoreExceptions()) .executionType(metaHolder.getExecutionType()) .build(); } //----------createGenericSetterBuilder() 创建SetterBuilder private GenericSetterBuilder createGenericSetterBuilder(MetaHolder metaHolder) { GenericSetterBuilder.Builder setterBuilder = GenericSetterBuilder.builder() .groupKey(metaHolder.getCommandGroupKey()) .threadPoolKey(metaHolder.getThreadPoolKey()) //查看从metaHolder如何获取threadPoolKey .commandKey(metaHolder.getCommandKey()) .collapserKey(metaHolder.getCollapserKey()) .commandProperties(metaHolder.getCommandProperties()) .threadPoolProperties(metaHolder.getThreadPoolProperties()) .collapserProperties(metaHolder.getCollapserProperties()); if (metaHolder.isCollapserAnnotationPresent()) { setterBuilder.scope(metaHolder.getHystrixCollapser().scope()); } return setterBuilder.build(); } //如果使用了Command注解，从注解指定的threadPoolKey 和 defaultThreadPoolKey二选一，以前者为主 //本例中，既没有通过注解指定threadPoolKey，也没有defaultThreadPoolKey public String getThreadPoolKey() { return isCommandAnnotationPresent() ? get(hystrixCommand.threadPoolKey(), defaultThreadPoolKey) : ""; }从上面看，HystrixCommandBuilder都构造完成了，还没有设置threadPoolKey
通过HystrixCommandBuilder创建HystrixCommand下面是通过HystrixCommandBuilder作为参数创建GenericCommand()，即通过HystrixCommandBuilder创建HystrixCommand
GenericCommand的类图为：
可见GenericCommand集成关系，从AbstractHystrixCommand --> HystrixCommand --> AbstractCommand，最终他们都是HystrixInvokeable接口的实现了，即可被Hystrix调用的
向上进入父类构造，HystrixCommand(Setter setter)
protected HystrixCommand(Setter setter) { // use 'null' to specify use the default this(setter.groupKey, setter.commandKey, setter.threadPoolKey, null, null, setter.commandPropertiesDefaults, setter.threadPoolPropertiesDefaults, null, null, null, null, null); }从setter中获取了groupKey、commandKey、threadPoolKey、commandPropertiesDefaults、threadPoolPropertiesDefaults，其它参数为null
AbstractCommand构造进入到 AbstractCommand构造方法，封装了构造一个HystrixCommand的基本上所有元素的逻辑
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) { this.commandGroup = initGroupKey(group); //初始化commandGroupKey this.commandKey = initCommandKey(key, getClass()); //初始化commandKey this.properties = initCommandProperties(this.commandKey, propertiesStrategy, commandPropertiesDefaults); //初始化commandProperties this.threadPoolKey = initThreadPoolKey(threadPoolKey, this.commandGroup, this.properties.executionIsolationThreadPoolKeyOverride().get()); //初始化threadPoolKey this.metrics = initMetrics(metrics, this.commandGroup, this.threadPoolKey, this.commandKey, this.properties); //初始化metrics this.circuitBreaker = initCircuitBreaker(this.properties.circuitBreakerEnabled().get(), circuitBreaker, this.commandGroup, this.commandKey, this.properties, this.metrics); //初始化断路器 this.threadPool = initThreadPool(threadPool, this.threadPoolKey, threadPoolPropertiesDefaults); //初始化线程池 //Strategies from plugins this.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); 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; }接下来主要看是如何初始化threadPoolKey，以及threadPool的
initThreadPoolKey()initThreadPoolKey(threadPoolKey, this.commandGroup, this.properties.executionIsolationThreadPoolKeyOverride().get())
参数：
threadPoolKey -- 指定的 或 默认的threadPoolKey
this.commandGroup -- 当前的groupKey