其实只是Hystrix初始化部分,我们从源码的角度分析一下@EnableCircuitBreaker以及@HystrixCommand注解的初始化过程。

从@EnableCircuitBreaker入手

我们是通过在启动类添加@EnableCircuitBreaker注解启用Hystrix的,所以,源码解析也要从这个注解入手。

该注解已经被标了@Deprecated了,不过,挡不住......

Spring关于@Enablexxxx注解的套路:通过@Import注解引入了EnableCircuitBreakerImportSelector.class:

@Deprecated
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@Import(EnableCircuitBreakerImportSelector.class)
public @interface EnableCircuitBreaker {

}

跟踪EnableCircuitBreakerImportSelector,继承自SpringFactoryImportSelector<EnableCircuitBreaker>,注意SpringFactoryImportSelector类的泛型类型为EnableCircuitBreaker:

@Order(Ordered.LOWEST_PRECEDENCE - 100)
public class EnableCircuitBreakerImportSelector extends SpringFactoryImportSelector<EnableCircuitBreaker> {

    @Override
    protected boolean isEnabled() {
        return getEnvironment().getProperty("spring.cloud.circuit.breaker.enabled", Boolean.class, Boolean.TRUE);
    }

}

看一下类图:
在这里插入图片描述

属性annotationClass通过GenericTypeResolver.resolveTypeArgument方法获取当前对象的泛型参数的具体类型,现在我们知道是EnableCircuitBreaker(在org.springframework.cloud.client.circuitbreaker包下)。

public abstract class SpringFactoryImportSelector<T>
        implements DeferredImportSelector, BeanClassLoaderAware, EnvironmentAware {

    private final Log log = LogFactory.getLog(SpringFactoryImportSelector.class);

    private ClassLoader beanClassLoader;

    private Class<T> annotationClass;

    private Environment environment;

    @SuppressWarnings("unchecked")
    protected SpringFactoryImportSelector() {
        this.annotationClass = (Class<T>) GenericTypeResolver.resolveTypeArgument(this.getClass(),
                SpringFactoryImportSelector.class);
    }

@Import注解我们前面做过详细分析了,实现了DeferredImportSelector接口表示延迟加载全限定名称为selectImports方法返回的类。看selectImports方法:

@Override
    public String[] selectImports(AnnotationMetadata metadata) {
        //Enable参数没有打开的话就不加载
        if (!isEnabled()) {
            return new String[0];
        }
        AnnotationAttributes attributes = AnnotationAttributes
                .fromMap(metadata.getAnnotationAttributes(this.annotationClass.getName(), true));

        Assert.notNull(attributes, "No " + getSimpleName() + " attributes found. Is " + metadata.getClassName()
                + " annotated with @" + getSimpleName() + "?");

        // Find all possible auto configuration classes, filtering duplicates
        //SPI机制调用spring.factories文件下的EnableCircuitBreaker
        List<String> factories = new ArrayList<>(new LinkedHashSet<>(
                SpringFactoriesLoader.loadFactoryNames(this.annotationClass, this.beanClassLoader)));

        if (factories.isEmpty() && !hasDefaultFactory()) {
            throw new IllegalStateException("Annotation @" + getSimpleName()
                    + " found, but there are no implementations. Did you forget to include a starter?");
        }

        if (factories.size() > 1) {
            // there should only ever be one DiscoveryClient, but there might be more than
            // one factory
            this.log.warn("More than one implementation " + "of @" + getSimpleName()
                    + " (now relying on @Conditionals to pick one): " + factories);
        }

        return factories.toArray(new String[factories.size()]);
    }

selectImports方法的主要功能就是调用SpringFactoriesLoader.loadFactoryNames方法,该方法的目的是通过SPI机制读取spring.factories文件中的org.springframework.cloud.client.circuitbreaker相关配置,返回。

返回的信息是类全限定名,从selectImports方法作用可知,这些类会加载到Spring Ioc容器中。

org.springframework.cloud.client.circuitbreaker在spring-cloud-netflix-hystrix-2.2.10.RELEASE包下:
org.springframework.cloud.client.circuitbreaker
所以该配置下的org.springframework.cloud.netflix.hystrix.HystrixCircuitBreakerConfiguration会被调用并加载到Spring IoC容器中。

HystrixCircuitBreakerConfiguration

跟踪配置类HystrixCircuitBreakerConfiguration:


/**
 * @author Spencer Gibb
 * @author Christian Dupuis
 * @author Venil Noronha
 */
@Configuration(proxyBeanMethods = false)
public class HystrixCircuitBreakerConfiguration {

    @Bean
    public HystrixCommandAspect hystrixCommandAspect() {
        return new HystrixCommandAspect();
    }

    @Bean
    public HystrixShutdownHook hystrixShutdownHook() {
        return new HystrixShutdownHook();
    }

注入了一个叫HystrixCommandAspect 的bean,从名字上看,应该是关于HystrixCommand的切面,用到了AOP。其实也容易理解,加了@HystrixCommand注解的方法的执行逻辑发生了变化:方法增强了执行时长是否超时、执行是否成功(是否返回异常)、超时或异常的情况下调用fallback......方法功能的增强正是AOP的强项。

继续跟踪HystrixCommandAspect 类。

HystrixCommandAspect

打开代码,首先是非常熟悉的@Aspect注解:

@Aspect
public class HystrixCommandAspect {

表明当前类是AOP的切面。

继续看代码:


    @Pointcut("@annotation(com.netflix.hystrix.contrib.javanica.annotation.HystrixCommand)")

    public void hystrixCommandAnnotationPointcut() {
    }

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

    @Around("hystrixCommandAnnotationPointcut() || hystrixCollapserAnnotationPointcut()")
    public Object methodsAnnotatedWithHystrixCommand(final ProceedingJoinPoint joinPoint) throws Throwable {
    

添加了针对注解HystrixCommand的切点,以及针对该切点的环绕增强方法methodsAnnotatedWithHystrixCommand。

最终的熔断、限流、服务降级功能,都是在这个methodsAnnotatedWithHystrixCommand方法里实现的,继续向下研究这个方法的代码逻辑,需要RxJava背景知识做支撑。

有空再聊:) ~!


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