背景
Hystrix 旨在通过控制那些访问远程系统、服务和第三方库的节点，从而对延迟和故障提供更强大的容错能力。Hystrix具备拥有回退机制和断路器功能的线程和信号隔离，请求缓存和请求打包，以及监控和配置等功能。
Dubbo是Alibaba开源的，目前国内最流行的java rpc框架。
本文介绍在spring应用里，怎么把Dubbo和Hystrix结合起来使用。
spring boot官方提供了对hystrix的集成，直接在pom.xml里加入依赖：
<dependency>
<groupId>org.springframework.cloud</groupId>
<artifactId>spring-cloud-starter-netflix-hystrix</artifactId>
<version>1.4.4.RELEASE</version>
</dependency>
然后在Application类上增加@EnableHystrix来启用hystrix starter：
@SpringBootApplication
@EnableHystrix
publicclassProviderApplication {
在Dubbo的Provider上增加@HystrixCommand配置，这样子调用就会经过Hystrix代理。
@Service(version = "1.0.0")
publicclassHelloServiceImpl implementsHelloService {
@HystrixCommand(commandProperties = {
@HystrixProperty(name = "circuitBreaker.requestVolumeThreshold", value = "10"),
@HystrixProperty(name = "execution.isolation.thread.timeoutInMilliseconds", value = "2000") })
@Override
publicString sayHello(String name) {
// System.out.println("async provider received: " + name);
// return "annotation: hello, " + name;
thrownewRuntimeException("Exception to show hystrix enabled.");
}
}
对于Consumer端，则可以增加一层method调用，并在method上配置@HystrixCommand。当调用出错时，会走到fallbackMethod = "reliable"的调用里。
@Reference(version = "1.0.0")
privateHelloService demoService;
@HystrixCommand(fallbackMethod = "reliable")
publicString doSayHello(String name) {
returndemoService.sayHello(name);
}
publicString reliable(String name) {
return"hystrix fallback value";
}
通过上面的配置，很简单地就完成了Spring Boot里Dubbo + Hystrix的集成。
传统spring annotation应用的配置其实也很简单，和spring boot应用不同的是：
显式配置Spring AOP支持：@EnableAspectJAutoProxy
显式通过@Configuration配置HystrixCommandAspectBean。
@Configuration
@EnableDubbo(scanBasePackages = "com.alibaba.dubbo.samples.annotation.action")
@PropertySource("classpath:/spring/dubbo-consumer.properties")
@ComponentScan(value = {"com.alibaba.dubbo.samples.annotation.action"})
@EnableAspectJAutoProxy
staticpublicclassConsumerConfiguration {
@Bean
publicHystrixCommandAspect hystrixCommandAspect() {
returnnewHystrixCommandAspect();
}
}
在上面的例子里可以看到，Hystrix对Spring的集成是通过Spring AOP来实现的。下面简单分析下实现。
@Aspect
publicclassHystrixCommandAspect {
@Pointcut("@annotation(com.netflix.hystrix.contrib.javanica.annotation.HystrixCommand)")
publicvoidhystrixCommandAnnotationPointcut() {
}
@Pointcut("@annotation(com.netflix.hystrix.contrib.javanica.annotation.HystrixCollapser)")
publicvoidhystrixCollapserAnnotationPointcut() {
}
@Around("hystrixCommandAnnotationPointcut() || hystrixCollapserAnnotationPointcut()")
publicObject methodsAnnotatedWithHystrixCommand(finalProceedingJoinPoint joinPoint) throwsThrowable {
Method method = getMethodFromTarget(joinPoint);
Validate.notNull(method, "failed to get method from joinPoint: %s", joinPoint);
if(method.isAnnotationPresent(HystrixCommand.class) && method.isAnnotationPresent(HystrixCollapser.class)) {
thrownewIllegalStateException("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) {
throwe.getCause() != null? e.getCause() : e;
} catch(HystrixRuntimeException e) {
throwhystrixRuntimeExceptionToThrowable(metaHolder, e);
}
returnresult;
}
HystrixCommandAspect里定义了两个注解的AspectJ Pointcut：@HystrixCommand,@HystrixCollapser。所有带这两个注解的spring bean都会经过AOP处理
在@AroundAOP处理函数里，可以看到Hystrix会创建出HystrixInvokable，再通过CommandExecutor来执行
spring-cloud-starter-netflix-hystrix的代码分析
@EnableHystrix引入了@EnableCircuitBreaker，@EnableCircuitBreaker引入了EnableCircuitBreakerImportSelector
@EnableCircuitBreaker
public@interfaceEnableHystrix {
}
@Import(EnableCircuitBreakerImportSelector.class)
public@interfaceEnableCircuitBreaker {
}
EnableCircuitBreakerImportSelector继承了SpringFactoryImportSelector<EnableCircuitBreaker>，使spring加载META-INF/spring.factories里的EnableCircuitBreaker声明的配置在META-INF/spring.factories里可以找到下面的配置，也就是引入了HystrixCircuitBreakerConfiguration。
org.springframework.cloud.client.circuitbreaker.EnableCircuitBreaker=\
org.springframework.cloud.netflix.hystrix.HystrixCircuitBreakerConfiguration
在HystrixCircuitBreakerConfiguration里可以发现创建了HystrixCommandAspect
@Configuration
publicclassHystrixCircuitBreakerConfiguration {
@Bean
publicHystrixCommandAspect hystrixCommandAspect() {
returnnewHystrixCommandAspect();
}
可见spring-cloud-starter-netflix-hystrix实际上也是创建了HystrixCommandAspect来集成Hystrix。
另外spring-cloud-starter-netflix-hystrix里还有metrics, health, dashboard等集成。
对于dubbo provider的@Service是一个spring bean，直接在上面配置@HystrixCommand即可
对于dubbo consumer的@Reference，可以通过加一层简单的spring method包装，配置@HystrixCommand即可
Hystrix本身提供HystrixCommandAspect来集成Spring AOP，配置了@HystrixCommand和@HystrixCollapser的spring method都会被Hystrix处理
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