Spring Cloud整合Hystrix的源码分析_`spring cloud hystix代码

我们在Spring Cloud项目中使用Hystrix时，引入相应的依赖

<dependency>
    <groupId>org.springframework.cloud</groupId>
    <artifactId>spring-cloud-starter-netflix-hystrix</artifactId>
</dependency>
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然后在启动类上添加@EnableCircuitBreaker注解，就可以使用Hystrix实现熔断降级了。这一切是如何实现的呢？我们来看这个注解

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

}
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继续看EnableCircuitBreakerImportSelector

@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);
   }
}
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EnableCircuitBreakerImportSelector继承了SpringFactoryImportSelector，传入的泛型EnableCircuitBreaker的全限定名会作为Key在spring.factories文件中查找对应的配置类，然后注入到IOC容器中。
下面是spring-cloud-netflix-hystrix-2.1.3.RELEASE.jar包中的spring.factories文件

org.springframework.boot.autoconfigure.EnableAutoConfiguration=\
org.springframework.cloud.netflix.hystrix.HystrixAutoConfiguration,\
org.springframework.cloud.netflix.hystrix.security.HystrixSecurityAutoConfiguration

org.springframework.cloud.client.circuitbreaker.EnableCircuitBreaker=\
org.springframework.cloud.netflix.hystrix.HystrixCircuitBreakerConfiguration
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这里会注入HystrixCircuitBreakerConfiguration配置类

@Configuration
public class HystrixCircuitBreakerConfiguration {

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

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

   @Bean
   public HasFeatures hystrixFeature() {
      return HasFeatures
            .namedFeatures(new NamedFeature("Hystrix", HystrixCommandAspect.class));
   }

   /**
    * {@link DisposableBean} that makes sure that Hystrix internal state is cleared when
    * {@link ApplicationContext} shuts down.
    */
   private class HystrixShutdownHook implements DisposableBean {
      @Override
      public void destroy() throws Exception {
         // Just call Hystrix to reset thread pool etc.
         Hystrix.reset();
      }
   }
}
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在HystrixCircuitBreakerConfiguration配置类中注入了HystrixCommandAspect切面，

@Aspect
public class HystrixCommandAspect {

    private static final Map<HystrixPointcutType, MetaHolderFactory> META_HOLDER_FACTORY_MAP;

    static {
        META_HOLDER_FACTORY_MAP = ImmutableMap.<HystrixPointcutType, MetaHolderFactory>builder()
                .put(HystrixPointcutType.COMMAND, new CommandMetaHolderFactory())
                .put(HystrixPointcutType.COLLAPSER, new CollapserMetaHolderFactory())
                .build();
    }

    @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 {
        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");
        }
        MetaHolderFactory metaHolderFactory = META_HOLDER_FACTORY_MAP.get(HystrixPointcutType.of(method));
        MetaHolder metaHolder = metaHolderFactory.create(joinPoint);
        HystrixInvokable invokable = HystrixCommandFactory.getInstance().create(metaHolder);
        ExecutionType executionType = metaHolder.isCollapserAnnotationPresent() ?
                metaHolder.getCollapserExecutionType() : metaHolder.getExecutionType();

        Object result;
        try {
            if (!metaHolder.isObservable()) {
                result = CommandExecutor.execute(invokable, executionType, metaHolder);
            } else {
                result = executeObservable(invokable, executionType, metaHolder);
            }
        } catch (HystrixBadRequestException e) {
            throw e.getCause();
        } catch (HystrixRuntimeException e) {
            throw hystrixRuntimeExceptionToThrowable(metaHolder, e);
        }
        return result;
    }
}
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在项目中添加了HystrixCommand注解和HystrixCollapser注解的方法都会成为HystrixCommandAspect切面的切入点，methodsAnnotatedWithHystrixCommand方法实现了横切逻辑，这就是为什么给方法添加HystrixCommand注解可以实现熔断降级策略。

@HystrixCommand(
    threadPoolKey = "testHystrix",
    threadPoolProperties = {
        @HystrixProperty(name="coreSize",value ="1"), // 线程数
        @HystrixProperty(name="maxQueueSize",value="20") // 等待队列⻓度
    },
    commandProperties = {
        @HystrixProperty(name="execution.isolation.thread.timeoutInMilliseconds",value="2000")
    }
)
@GetMapping("/testHystrix/{userId}")
public Integer testHystrix(@PathVariable Long userId) {
    ......
}
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版本信息
JDK：1.8
Spring Boot：2.1.9.RELEASE
Spring Cloud：Greenwich.SR3