Spring Cloud整合Hystrix的源码分析

本是古典何须时尚 2022-08-31 12:29 182阅读 0赞

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>

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 { 
    
    }

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);
       }
    }

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

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();
          }
       }
    }

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;
        }
    }

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) { 
        ......
    }

**版本信息**  
JDK：1.8  
Spring Boot：2.1.9.RELEASE  
Spring Cloud：Greenwich.SR3