Hystrix Plugins插件机制解析

Hystrix Plugins插件机制

Hystrix通过SPI提供了插件扩展机制，有如下几种插件：

HystrixConcurrencyStrategy：并发
HystrixEventNotifier：通知
HystrixMetricsPublisher：度量
HystrixPropertiesStrategy：Properties配置
HystrixCommandExecutionHook：回调函数
HystrixDynamicProperties：配置

以上插件都由HystrixPlugins统一管理，负责加载和实例化。

SPI插件机制

Hystrix插件机制实现

举例HystrixConcurrencyStrategy来进行讲解

HystrixConcurrencyStrategy插件讲解

插件实现的相关类如下

实现默认的抽象类HystrixConcurrencyStrategy
官方仅提供了HystrixConcurrencyStrategyDefault的默认实现，继承了1中的抽象类

HystrixConcurrencyStrategy类的作用解析

Hystrix只有一个作用，实例化线程执行器。其核心方法为getThreadPool()，其本质就是把参数传进来，然后实例化一个ThreadPoolExecutor
实际上HystrixConcurrencyStrategyDefault是直接继承HystrixConcurrencyStrategy，并且没有重写getThreadPool()方法，所以直接调用到下面的getThreadPool()逻辑的

/**
 * 目前只有这个使用，只分析这个就好
 * @param threadPoolKey key
 * @param threadPoolProperties 参数
 * @return 线程池执行器
 */
public ThreadPoolExecutor getThreadPool(final HystrixThreadPoolKey threadPoolKey, HystrixThreadPoolProperties threadPoolProperties) {
    // 获取ThreadFactory
    final ThreadFactory threadFactory = getThreadFactory(threadPoolKey);

    // 获取请求参数
    final boolean allowMaximumSizeToDivergeFromCoreSize = threadPoolProperties.getAllowMaximumSizeToDivergeFromCoreSize().get();
    final int dynamicCoreSize = threadPoolProperties.coreSize().get();
    final int keepAliveTime = threadPoolProperties.keepAliveTimeMinutes().get();
    final int maxQueueSize = threadPoolProperties.maxQueueSize().get();
    final BlockingQueue<Runnable> workQueue = getBlockingQueue(maxQueueSize);

    // allowMaximumSizeToDivergeFromCoreSize = true，就允许核心线程起作用
    if (allowMaximumSizeToDivergeFromCoreSize) {
        final int dynamicMaximumSize = threadPoolProperties.maximumSize().get();
        // allowMaximumSizeToDivergeFromCoreSize=true情况下，核心线程数必须小于等于最大线程数
        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);
        } else {
            return new ThreadPoolExecutor(dynamicCoreSize, dynamicMaximumSize, keepAliveTime, TimeUnit.MINUTES, workQueue, threadFactory);
        }
    } else {
        return new ThreadPoolExecutor(dynamicCoreSize, dynamicCoreSize, keepAliveTime, TimeUnit.MINUTES, workQueue, threadFactory);
    }
}

怎么加载HystrixConcurrencyStrategy插件

初始化HystrixThreadPool时，默认会调用到HystrixThreadPoolDefault，并进行实例化

class HystrixThreadPoolDefault implements HystrixThreadPool {
    // ....其他逻辑
    public HystrixThreadPoolDefault(HystrixThreadPoolKey threadPoolKey, HystrixThreadPoolProperties.Setter propertiesDefaults) {
        this.properties = HystrixPropertiesFactory.getThreadPoolProperties(threadPoolKey, propertiesDefaults);
        // ！！！实例化HystrixConcurrencyStrategy！！
        HystrixConcurrencyStrategy concurrencyStrategy = HystrixPlugins.getInstance().getConcurrencyStrategy();
        this.queueSize = properties.maxQueueSize().get();
        // 实例化了一个线程池：concurrencyStrategy.getThreadPool(threadPoolKey, properties)
        this.metrics = HystrixThreadPoolMetrics.getInstance(threadPoolKey,
                concurrencyStrategy.getThreadPool(threadPoolKey, properties),
                properties);
        // 最后线程池在这里赋值给this.threadPool
        this.threadPool = this.metrics.getThreadPool();
        this.queue = this.threadPool.getQueue();

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

可见最终调用了HystrixPlugins.getInstance().getConcurrencyStrategy()来及性能获取具体的策略实例

首先从属性"hystrix.plugin.HystrixConcurrencyStrategy.implementation=xxx“，看看有没自定义的策略类，如果有就用Class.forName加载
1.1 举个参数例子：hystrix.plugin.HystrixConcurrencyStrategy.implementation=com.lds.hystrix.MyHystrixConcurrencyStrategy
如果没有返回null，就会用CAS来吧策略实例替换为默认的HystrixConcurrencyStrategyDefault，否则用自定义的

public class HystrixPlugins {
    // ....其他逻辑
    
    public HystrixConcurrencyStrategy getConcurrencyStrategy() {
        // 获取策略实例
        if (concurrencyStrategy.get() == null) {
            // 走到这部说明不存在
            // check for an implementation from Archaius first
            // 不存在则通过getPluginImplementation获取，这个会从"hystrix.plugin.HystrixConcurrencyStrategy.implementation“属性值里面获取
            //   如果没有设置对应的实例返回，就会返回null
            Object impl = getPluginImplementation(HystrixConcurrencyStrategy.class);
            if (impl == null) {
                // 返回null，就会默认采用HystrixConcurrencyStrategyDefault来进行实例化对应的线程池
                // nothing set via Archaius so initialize with default
                concurrencyStrategy.compareAndSet(null, HystrixConcurrencyStrategyDefault.getInstance());
                // we don't return from here but call get() again in case of thread-race so the winner will always get returned
            } else {
                // we received an implementation from Archaius so use it
                // 否则用自定义的类来实例化线程池
                concurrencyStrategy.compareAndSet(null, (HystrixConcurrencyStrategy) impl);
            }
        }
        return concurrencyStrategy.get();
    }
    // ....其他逻辑
}

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