this.properties.executionIsolationThreadPoolKeyOverride().get()) -- 字符串类型，允许动态覆盖修改HystrixThreadPoolKey的值，并将动态更新HystrixCommand执行的HystrixThreadPool，这个override值的典型值是null，并且在构造HystrixCommandProperties时override全局的配置为null
// threadpool doesn't have a global override, only instance level makes sense this.executionIsolationThreadPoolKeyOverride = forString().add(propertyPrefix + ".command." + key.name() + ".threadPoolKeyOverride", null).build();接着看 initThreadPoolKey() 方法内部
/* * ThreadPoolKey * * This defines which thread-pool this command should run on. * * It uses the HystrixThreadPoolKey if provided, then defaults to use HystrixCommandGroup. * 如果提供了threadPoolKey，就使用，否则默认使用groupKey * * It can then be overridden by a property if defined so it can be changed at runtime. * 可以被threadPoolKeyOverride在运行时动态覆盖 */ private static HystrixThreadPoolKey initThreadPoolKey(HystrixThreadPoolKey threadPoolKey, HystrixCommandGroupKey groupKey, String threadPoolKeyOverride) { if (threadPoolKeyOverride == null) { // we don't have a property overriding the value so use either HystrixThreadPoolKey or HystrixCommandGroup if (threadPoolKey == null) { /* * use HystrixCommandGroup if HystrixThreadPoolKey is null * 如果HystrixThreadPoolKey为空，使用groupKey作为threadPoolKey */ return HystrixThreadPoolKey.Factory.asKey(groupKey.name()); } else { return threadPoolKey; } } else { // we have a property defining the thread-pool so use it instead return HystrixThreadPoolKey.Factory.asKey(threadPoolKeyOverride); } }可见，在最开始构造HystrixCommand时，threadPoolKeyOverride为null，且没有自己指定的threadPoolKey，也没有默认的threadPoolKey，那么将使用groupKey作为threadPoolKey
所以，默认使用groupKey作为threadPoolKey，而group默认值是标注了@HystrixCommand的类名
最后，看一下如何根据threadPoolKey，初始化threadPool
initThreadPool() //----------AbstractCommand#initThreadPool() private static HystrixThreadPool initThreadPool(HystrixThreadPool fromConstructor, HystrixThreadPoolKey threadPoolKey, HystrixThreadPoolProperties.Setter threadPoolPropertiesDefaults) { // fromConstructor为null，使用HystrixThreadPool.Factory创建线程池 if (fromConstructor == null) { // get the default implementation of HystrixThreadPool return HystrixThreadPool.Factory.getInstance(threadPoolKey, threadPoolPropertiesDefaults); } else { return fromConstructor; } } //----------HystrixThreadPool.Factory#getInstance() 获取HystrixThreadPool实例 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 work String key = threadPoolKey.name(); // this should find it for all but the first time // 从缓存threadPools中获取HystrixThreadPool，有则直接返回 HystrixThreadPool previouslyCached = threadPools.get(key); if (previouslyCached != null) { return previouslyCached; } // if we get here this is the first time so we need to initialize // 第一次初始化HystrixThreadPool synchronized (HystrixThreadPool.class) { if (!threadPools.containsKey(key)) { threadPools.put(key, new HystrixThreadPoolDefault(threadPoolKey, propertiesBuilder)); } } return threadPools.get(key); }