关于threadPoolKey默认值的疑问

使用SpingCloud必然会用到Hystrix做熔断降级，也必然会用到@HystrixCommand注解，@HystrixCommand注解可以配置的除了常用的groupKey、commandKey、fallbackMethod等，还有一个很关键的就是threadPoolKey，就是使用Hystrix线程隔离策略时的线程池Key

/**
 * This annotation used to specify some methods which should be processes as hystrix commands.
 */
@Target({ElementType.METHOD})
@Retention(RetentionPolicy.RUNTIME)
@Inherited
@Documented
public @interface HystrixCommand {

    /**
     * The command group key is used for grouping together commands such as for reporting,
     * alerting, dashboards or team/library ownership.
     * <p/>
     * default => the runtime class name of annotated method
     *
     * @return group key
     */
    String groupKey() default "";

    /**
     * Hystrix command key.
     * <p/>
     * default => the name of annotated method. for example:
     * <code>
     *     ...
     *     @HystrixCommand
     *     public User getUserById(...)
     *     ...
     *     the command name will be: 'getUserById'
     * </code>
     *
     * @return command key
     */
    String commandKey() default "";

    /**
     * The thread-pool key is used to represent a
     * HystrixThreadPool for monitoring, metrics publishing, caching and other such uses.
     *
     * @return thread pool key
     */
    String threadPoolKey() default "";
    
    ......省略
}

而使用中我们常常只指定fallbackMethod回退方法，而不会指定所有属性，从@HystrixCommand的源码注释来看

• groupKey的默认值是使用@HystrixCommand标注的方法所在的类名
• commandKey的默认值是@HystrixCommand标注的方法名，即每个方法会被当做一个HystrixCommand
• threadPoolKey没有默认值

但threadPoolKey却没有说明默认值，而threadPoolKey是和执行HystrixCommand的线程池直接相关的

所以我的疑问就是，threadPoolKey有默认值吗？ 默认值是什么？ 执行HystrixCommand的线程池又是怎么初始化的？ 可以动态调整吗？

测试论证

测试代码

spring-cloud-example-consumer-ribbon-hystrix-threadpool

测试端点及方式

首先需要启动 spring-cloud-example-eureka-server-standalone注册中心 和 spring-cloud-example-simple-provider服务提供者

再启动 spring-cloud-example-consumer-ribbon-hystrix-threadpool

测试端点

http://127.0.0.1:20006/testDefaultThreadPoolKey

http://127.0.0.1:20006/testDefaultThreadPoolKey2

testDefaultThreadPoolKey 和 testDefaultThreadPoolKey2 是同一个service的两个方法，分别使用@HystrixCommand指定fallback方法

线程池大小设置为2，且不使用队列暂存，服务提供方sleep 30秒，通过chrome多窗口调用 /testDefaultThreadPoolKey 端点

或同时调用 /testDefaultThreadPoolKey、/testDefaultThreadPoolKey2 端点
通过大于线程池最大值的请求被线程池拒绝进入fallback，判断线程池是方法级，还是类级的，以及threadPoolKey默认值

注意：

使用firefox浏览器测试有问题，多标签页必须等待一个GET请求完成，才能继续下一个，测不出并发的效果。

一开始以为是程序上的限制，后来才发现是浏览器，使用chrome问题解决

线程池配置

hystrix.threadpool.default.coreSize = 2
hystrix.threadpool.default.maximumSize = 2
hystrix.threadpool.default.maxQueueSize = -1

线程池的coreSize和maximumSize都设置为2（1.5.9版本后才添加maximumSize），且线程池队列大小为-1，即使用SynchronousQueue

Hystrix线程池的其它属性： Hystrix Thread Pool Properties

测试结果

1542010160835.png

chrome浏览器连续GET请求调用6次，分别调用3次 /testDefaultThreadPoolKey，3次 /testDefaultThreadPoolKey2，以绿色线问分隔，可见前两次调用成功，后4次均直接拒绝，进入fallback

具体异常信息为：

java.util.concurrent.RejectedExecutionException: Task java.util.concurrent.FutureTask@6d72dcf rejected from java.util.concurrent.ThreadPoolExecutor@431bfebc[Running, pool size = 2, active threads = 2, queued tasks = 0, completed tasks = 1]

线程池大小为2起到了作用，将大于并发数的请求拒绝了，并且无论是只调用 /testDefaultThreadPoolKey，还是轮询调用 /testDefaultThreadPoolKey 和 /testDefaultThreadPoolKey2 ，测试结果都是这样。再根据hystrix线程的名字 hystrix-ConsumerRibbonHystrixThreadPoolService-n，可以猜想：Hystrix的 threadPoolKey是和hystrixCommand执行的类相关的，可能一个类使用一个线程池，所以两个service方法才会共用线程池

原理分析

HystrixCommandAspect

首先，被@HystrixCommand注解标注的方法会被AOP拦截，具体逻辑在 HystrixCommandAspect

private static final Map<HystrixPointcutType, MetaHolderFactory> META_HOLDER_FACTORY_MAP;

// 初始化用于处理 HystrixCommand 和 HystrixCollapser 的 MetaHolderFactory
// HystrixCommand -- CommandMetaHolderFactory
// HystrixCollapser -- CollapserMetaHolderFactory
static {
    META_HOLDER_FACTORY_MAP = ImmutableMap.<HystrixPointcutType, MetaHolderFactory>builder()
            .put(HystrixPointcutType.COMMAND, new CommandMetaHolderFactory())
            .put(HystrixPointcutType.COLLAPSER, new CollapserMetaHolderFactory())
            .build();
}


// HystrixCommand Pointcut切入点
@Pointcut("@annotation(com.netflix.hystrix.contrib.javanica.annotation.HystrixCommand)")
public void hystrixCommandAnnotationPointcut() {
}

// HystrixCollapser Pointcut切入点
@Pointcut("@annotation(com.netflix.hystrix.contrib.javanica.annotation.HystrixCollapser)")
public void hystrixCollapserAnnotationPointcut() {
}

// HystrixCommand 和 HystrixCollapser 的环绕通知
@Around("hystrixCommandAnnotationPointcut() || hystrixCollapserAnnotationPointcut()")
public Object methodsAnnotatedWithHystrixCommand(final ProceedingJoinPoint joinPoint) throws Throwable {
    Method method = getMethodFromTarget(joinPoint);
    Validate.notNull(method, "failed to get method from joinPoint: %s", joinPoint);
    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");
    }
    
    // 创建metaHolder，用于保存元数据
    MetaHolderFactory metaHolderFactory = META_HOLDER_FACTORY_MAP.get(HystrixPointcutType.of(method));
    MetaHolder metaHolder = metaHolderFactory.create(joinPoint);
    
    // 创建HystrixCommand，HystrixInvokable是父接口
    HystrixInvokable invokable = HystrixCommandFactory.getInstance().create(metaHolder);
    ExecutionType executionType = metaHolder.isCollapserAnnotationPresent() ?
            metaHolder.getCollapserExecutionType() : metaHolder.getExecutionType();

    // 执行HystrixCommand
    Object result;
    try {
        if (!metaHolder.isObservable()) {
            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;
}

重点是methodsAnnotatedWithHystrixCommand()环绕通知的实现方法

其中MetaHolder metaHolder = metaHolderFactory.create(joinPoint)根据joinPoint的信息创建元数据时肯定会有初始化默认groupKey、默认commandKey以及默认threadPoolKey的逻辑

创建MetaHolder元数据

private static class CommandMetaHolderFactory extends MetaHolderFactory {
    @Override
    public MetaHolder create(Object proxy, Method method, Object obj, Object[] args, final ProceedingJoinPoint joinPoint) {
        HystrixCommand hystrixCommand = method.getAnnotation(HystrixCommand.class);
        ExecutionType executionType = ExecutionType.getExecutionType(method.getReturnType());
        
        // 创建MetaHolderBuilder
        MetaHolder.Builder builder = metaHolderBuilder(proxy, method, obj, args, joinPoint);
        if (isCompileWeaving()) {
            builder.ajcMethod(getAjcMethodFromTarget(joinPoint));
        }
        
        return builder.defaultCommandKey(method.getName())  //默认commandKey是方法名
                        .hystrixCommand(hystrixCommand)
                        .observableExecutionMode(hystrixCommand.observableExecutionMode())
                        .executionType(executionType)
                        .observable(ExecutionType.OBSERVABLE == executionType)
                        .build();
    }
}


//----------再来看看创建MetaHolderBuilder - metaHolderBuilder()
//== MetaHolderFactory#metaHolderBuilder()
MetaHolder.Builder metaHolderBuilder(Object proxy, Method method, Object obj, Object[] args, final ProceedingJoinPoint joinPoint) {
    MetaHolder.Builder builder = MetaHolder.builder()
            .args(args).method(method).obj(obj).proxyObj(proxy)
            .joinPoint(joinPoint);

    // 设置fallback方法
    setFallbackMethod(builder, obj.getClass(), method);
    // 设置默认配置
    builder = setDefaultProperties(builder, obj.getClass(), joinPoint);
    
    return builder;
}

//== 设置默认配置 setDefaultProperties()
private static MetaHolder.Builder setDefaultProperties(MetaHolder.Builder builder, Class<?> declaringClass, final ProceedingJoinPoint joinPoint) {
    //根据@DefaultProperties注解获取配置
    Optional<DefaultProperties> defaultPropertiesOpt = AopUtils.getAnnotation(joinPoint, DefaultProperties.class);
    //设置 默认groupKey为类名simpleName
    builder.defaultGroupKey(declaringClass.getSimpleName());
    
    //如果存在@DefaultProperties，使用其指定的groupKey、threadPoolKey
    if (defaultPropertiesOpt.isPresent()) {
        DefaultProperties defaultProperties = defaultPropertiesOpt.get();
        builder.defaultProperties(defaultProperties);
        if (StringUtils.isNotBlank(defaultProperties.groupKey())) {
            builder.defaultGroupKey(defaultProperties.groupKey());
        }
        if (StringUtils.isNotBlank(defaultProperties.threadPoolKey())) {
            builder.defaultThreadPoolKey(defaultProperties.threadPoolKey());
        }
    }
    return builder;
}

由此可见，在构造metaHolder元数据时，通过@HystrixCommand标准方法所在的类名作为goutpKey，通过方法名作为commandKey，但没有指定threadPoolKey

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：

1542074599975.png

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

• 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);
}

先根据threadPoolKey尝试从threadPools这个ConcurrentHashMap<String, HystrixThreadPool>中获取，即从线程池缓存中获取，有就直接返回previouslyCached之前的缓存，如果没有，synchromized对HystrixThreadPool类上锁后，再次判断还是没有threadPoolKey的缓存，就 new HystrixThreadPoolDefault(threadPoolKey, propertiesBuilder)

//----------new HystrixThreadPoolDefault(threadPoolKey, propertiesBuilder)
public HystrixThreadPoolDefault(HystrixThreadPoolKey threadPoolKey, HystrixThreadPoolProperties.Setter propertiesDefaults) {
    this.properties = HystrixPropertiesFactory.getThreadPoolProperties(threadPoolKey, propertiesDefaults); //threadPoolProperties
    HystrixConcurrencyStrategy concurrencyStrategy = HystrixPlugins.getInstance().getConcurrencyStrategy(); //并发策略
    this.queueSize = properties.maxQueueSize().get(); //线程池队列大小

    //创建HystrixThreadPoolMetrics，其中concurrencyStrategy.getThreadPool()会创建线程池
    this.metrics = HystrixThreadPoolMetrics.getInstance(threadPoolKey,
            concurrencyStrategy.getThreadPool(threadPoolKey, properties),
            properties);
    this.threadPool = this.metrics.getThreadPool();
    this.queue = this.threadPool.getQueue();

    /* strategy: HystrixMetricsPublisherThreadPool */
    HystrixMetricsPublisherFactory.createOrRetrievePublisherForThreadPool(threadPoolKey, this.metrics, this.properties);
}


//----------HystrixConcurrencyStrategy#getThreadPool(HystrixThreadPoolKey, HystrixThreadPoolProperties)
// concurrencyStrategy.getThreadPool()时会创建ThreadPoolExecutor
public ThreadPoolExecutor getThreadPool(final HystrixThreadPoolKey threadPoolKey, HystrixThreadPoolProperties threadPoolProperties) {
    final ThreadFactory threadFactory = getThreadFactory(threadPoolKey);

    final boolean allowMaximumSizeToDivergeFromCoreSize = threadPoolProperties.getAllowMaximumSizeToDivergeFromCoreSize().get(); //是否允许maximumSize生效
    final int dynamicCoreSize = threadPoolProperties.coreSize().get(); //动态coreSize
    final int keepAliveTime = threadPoolProperties.keepAliveTimeMinutes().get(); //大于coreSize的线程，未使用的保活时间
    final int maxQueueSize = threadPoolProperties.maxQueueSize().get(); //线程队列最大值
    final BlockingQueue<Runnable> workQueue = getBlockingQueue(maxQueueSize);

    //允许使用maximumSize
    if (allowMaximumSizeToDivergeFromCoreSize) {
        final int dynamicMaximumSize = threadPoolProperties.maximumSize().get();
        
        //dynamicCoreSize > dynamicMaximumSize，打印error
        if (dynamicCoreSize > dynamicMaximumSize) {
            logger.error("Hystrix ThreadPool configuration at startup for : " + threadPoolKey.name() + " is trying to set coreSize = " +
                    dynamicCoreSize + " and maximumSize = " + dynamicMaximumSize + ".  Maximum size will be set to " +
                    dynamicCoreSize + ", the coreSize value, since it must be equal to or greater than the coreSize value");
            return new ThreadPoolExecutor(dynamicCoreSize, dynamicCoreSize, keepAliveTime, TimeUnit.MINUTES, workQueue, threadFactory);
        } 
        //dynamicCoreSize <= dynamicMaximumSize，正常
        else { 
            return new ThreadPoolExecutor(dynamicCoreSize, dynamicMaximumSize, keepAliveTime, TimeUnit.MINUTES, workQueue, threadFactory);
        }
    } 
    else { //不允许使用maximumSize
        return new ThreadPoolExecutor(dynamicCoreSize, dynamicCoreSize, keepAliveTime, TimeUnit.MINUTES, workQueue, threadFactory);
    }
}

至此，线程池创建完毕

结论

• threadPoolKey的默认值是groupKey，而groupKey默认值是@HystrixCommand标注的方法所在类名

• 可以通过在类上加@DefaultProperties( threadPoolKey="xxx" )设置默认的threadPoolKey

• 可以通过@HystrixCommand( threadPoolKey="xxx" ) 指定当前HystrixCommand实例的threadPoolKey

• threadPoolKey用于从线程池缓存中获取线程池 和 初始化创建线程池，由于默认以groupKey即类名为threadPoolKey，那么默认所有在一个类中的HystrixCommand共用一个线程池

• 动态配置线程池 -- 可以通过hystrix.command.HystrixCommandKey.threadPoolKeyOverride=线程池key动态设置threadPoolKey，对应的HystrixCommand所使用的线程池也会重新创建，还可以继续通过hystrix.threadpool.HystrixThreadPoolKey.coreSize=n和hystrix.threadpool.HystrixThreadPoolKey.maximumSize=n动态设置线程池大小

  注意： 通过threadPoolKeyOverride动态修改threadPoolKey之后，hystrixCommand会使用新的threadPool，但是老的线程池还会一直存在，并没有触发shutdown的机制