聊聊HystrixPlugins

序

本文主要研究下HystrixPlugins

HystrixPlugins

hystrix-core-1.5.12-sources.jar!/com/netflix/hystrix/strategy/HystrixPlugins.java

/**
 * Registry for plugin implementations that allows global override and handles the retrieval of correct implementation based on order of precedence:
 * <ol>
 * <li>plugin registered globally via <code>register</code> methods in this class</li>
 * <li>plugin registered and retrieved using the resolved {@link HystrixDynamicProperties} (usually Archaius, see get methods for property names)</li>
 * <li>plugin registered and retrieved using the JDK {@link ServiceLoader}</li>
 * <li>default implementation</li>
 * </ol>
 * 
 * The exception to the above order is the {@link HystrixDynamicProperties} implementation 
 * which is only loaded through <code>System.properties</code> or the ServiceLoader (see the {@link HystrixPlugins#getDynamicProperties() getter} for more details).
 * <p>
 * See the Hystrix GitHub Wiki for more information: <a href="https://github.com/Netflix/Hystrix/wiki/Plugins">https://github.com/Netflix/Hystrix/wiki/Plugins</a>.
 */
public class HystrixPlugins {
    
    //We should not load unless we are requested to. This avoids accidental initialization. @agentgt
    //See https://en.wikipedia.org/wiki/Initialization-on-demand_holder_idiom
    private static class LazyHolder { private static final HystrixPlugins INSTANCE = HystrixPlugins.create(); }
    private final ClassLoader classLoader;
    /* package */ final AtomicReference<HystrixEventNotifier> notifier = new AtomicReference<HystrixEventNotifier>();
    /* package */ final AtomicReference<HystrixConcurrencyStrategy> concurrencyStrategy = new AtomicReference<HystrixConcurrencyStrategy>();
    /* package */ final AtomicReference<HystrixMetricsPublisher> metricsPublisher = new AtomicReference<HystrixMetricsPublisher>();
    /* package */ final AtomicReference<HystrixPropertiesStrategy> propertiesFactory = new AtomicReference<HystrixPropertiesStrategy>();
    /* package */ final AtomicReference<HystrixCommandExecutionHook> commandExecutionHook = new AtomicReference<HystrixCommandExecutionHook>();
    private final HystrixDynamicProperties dynamicProperties;

    
    private HystrixPlugins(ClassLoader classLoader, LoggerSupplier logSupplier) {
        //This will load Archaius if its in the classpath.
        this.classLoader = classLoader;
        //N.B. Do not use a logger before this is loaded as it will most likely load the configuration system.
        //The configuration system may need to do something prior to loading logging. @agentgt
        dynamicProperties = resolveDynamicProperties(classLoader, logSupplier);
    }

    /**
     * For unit test purposes.
     * @ExcludeFromJavadoc
     */
    /* private */ static HystrixPlugins create(ClassLoader classLoader, LoggerSupplier logSupplier) {
        return new HystrixPlugins(classLoader, logSupplier);
    }
    
    /**
     * For unit test purposes.
     * @ExcludeFromJavadoc
     */
    /* private */ static HystrixPlugins create(ClassLoader classLoader) {
        return new HystrixPlugins(classLoader, new LoggerSupplier() {
            @Override
            public Logger getLogger() {
                return LoggerFactory.getLogger(HystrixPlugins.class);
            }
        });
    }
    /**
     * @ExcludeFromJavadoc
     */
    /* private */ static HystrixPlugins create() {
        return create(HystrixPlugins.class.getClassLoader());
    }

    public static HystrixPlugins getInstance() {
        return LazyHolder.INSTANCE;
    }

    /**
     * Reset all of the HystrixPlugins to null.  You may invoke this directly, or it also gets invoked via <code>Hystrix.reset()</code>
     */
    public static void reset() {
        getInstance().notifier.set(null);
        getInstance().concurrencyStrategy.set(null);
        getInstance().metricsPublisher.set(null);
        getInstance().propertiesFactory.set(null);
        getInstance().commandExecutionHook.set(null);
        HystrixMetricsPublisherFactory.reset();
    }

    /**
     * Register a {@link HystrixEventNotifier} implementation as a global override of any injected or default implementations.
     * 
     * @param impl
     *            {@link HystrixEventNotifier} implementation
     * @throws IllegalStateException
     *             if called more than once or after the default was initialized (if usage occurs before trying to register)
     */
    public void registerEventNotifier(HystrixEventNotifier impl) {
        if (!notifier.compareAndSet(null, impl)) {
            throw new IllegalStateException("Another strategy was already registered.");
        }
    }

    /**
     * Register a {@link HystrixConcurrencyStrategy} implementation as a global override of any injected or default implementations.
     * 
     * @param impl
     *            {@link HystrixConcurrencyStrategy} implementation
     * @throws IllegalStateException
     *             if called more than once or after the default was initialized (if usage occurs before trying to register)
     */
    public void registerConcurrencyStrategy(HystrixConcurrencyStrategy impl) {
        if (!concurrencyStrategy.compareAndSet(null, impl)) {
            throw new IllegalStateException("Another strategy was already registered.");
        }
    }

    /**
     * Register a {@link HystrixMetricsPublisher} implementation as a global override of any injected or default implementations.
     * 
     * @param impl
     *            {@link HystrixMetricsPublisher} implementation
     * @throws IllegalStateException
     *             if called more than once or after the default was initialized (if usage occurs before trying to register)
     */
    public void registerMetricsPublisher(HystrixMetricsPublisher impl) {
        if (!metricsPublisher.compareAndSet(null, impl)) {
            throw new IllegalStateException("Another strategy was already registered.");
        }
    }

    /**
     * Register a {@link HystrixPropertiesStrategy} implementation as a global override of any injected or default implementations.
     * 
     * @param impl
     *            {@link HystrixPropertiesStrategy} implementation
     * @throws IllegalStateException
     *             if called more than once or after the default was initialized (if usage occurs before trying to register)
     */
    public void registerPropertiesStrategy(HystrixPropertiesStrategy impl) {
        if (!propertiesFactory.compareAndSet(null, impl)) {
            throw new IllegalStateException("Another strategy was already registered.");
        }
    }

    /**
     * Register a {@link HystrixCommandExecutionHook} implementation as a global override of any injected or default implementations.
     * 
     * @param impl
     *            {@link HystrixCommandExecutionHook} implementation
     * @throws IllegalStateException
     *             if called more than once or after the default was initialized (if usage occurs before trying to register)
     * 
     * @since 1.2
     */
    public void registerCommandExecutionHook(HystrixCommandExecutionHook impl) {
        if (!commandExecutionHook.compareAndSet(null, impl)) {
            throw new IllegalStateException("Another strategy was already registered.");
        }
    }
    //......
}

• hystrix提供了一個plugin機制用於修改hystrix的行爲，能夠做用到全部的HystrixCommand、HystrixObservableCommand、HystrixCollapser的實現
• 提供了HystrixEventNotifier、HystrixConcurrencyStrategy、HystrixMetricsPublisher、HystrixPropertiesStrategy、HystrixCommandExecutionHook這幾個plugin能夠本身去實現
• 這裏採用了static holder的模式來實現HystrixPlugins的單例

getPluginImplementation

private <T> T getPluginImplementation(Class<T> pluginClass) {
        T p = getPluginImplementationViaProperties(pluginClass, dynamicProperties);
        if (p != null) return p;        
        return findService(pluginClass, classLoader);
    }
    
    @SuppressWarnings("unchecked")
    private static <T> T getPluginImplementationViaProperties(Class<T> pluginClass, HystrixDynamicProperties dynamicProperties) {
        String classSimpleName = pluginClass.getSimpleName();
        // Check Archaius for plugin class.
        String propertyName = "hystrix.plugin." + classSimpleName + ".implementation";
        String implementingClass = dynamicProperties.getString(propertyName, null).get();
        if (implementingClass != null) {
            try {
                Class<?> cls = Class.forName(implementingClass);
                // narrow the scope (cast) to the type we're expecting
                cls = cls.asSubclass(pluginClass);
                return (T) cls.newInstance();
            } catch (ClassCastException e) {
                throw new RuntimeException(classSimpleName + " implementation is not an instance of " + classSimpleName + ": " + implementingClass);
            } catch (ClassNotFoundException e) {
                throw new RuntimeException(classSimpleName + " implementation class not found: " + implementingClass, e);
            } catch (InstantiationException e) {
                throw new RuntimeException(classSimpleName + " implementation not able to be instantiated: " + implementingClass, e);
            } catch (IllegalAccessException e) {
                throw new RuntimeException(classSimpleName + " implementation not able to be accessed: " + implementingClass, e);
            }
        } else {
            return null;
        }
    }

    private static <T> T findService(
            Class<T> spi, 
            ClassLoader classLoader) throws ServiceConfigurationError {
        
        ServiceLoader<T> sl = ServiceLoader.load(spi,
                classLoader);
        for (T s : sl) {
            if (s != null)
                return s;
        }
        return null;
    }

• 先從動態配置找實現類，若是找不到則使用ServiceLoader進行加載
• 配置實現類的格式爲："hystrix.plugin." + classSimpleName + ".implementation"，好比

hystrix.plugin.HystrixEventNotifier.implementation=com.example.hystrix.hook.CustomHystrixEventNotifierHook

配置能夠放到配置文件hystrix-plugins.properties中

• 從配置文件找到實現類的，採用的是Class.forName的形式來獲取Class，而後使用newInstance來實例化

小結

hystrix內置了一個plugin機制，內置了HystrixEventNotifier、HystrixConcurrencyStrategy、HystrixMetricsPublisher、HystrixPropertiesStrategy、HystrixCommandExecutionHook這幾個plugin，提供了默認的實現，並容許開發者在指定配置文件自定義該插件的實現類，經過類尋找及實例化的方式來加載，而後做用在HystrixCommand、HystrixObservableCommand及HystrixCollapser上。

doc

• Hystrix/wiki/Plugins