SpringCloud-源码分析 Hystrix 熔断器

回顾
为了防止服务之间的调用异常造成的连锁反应，在SpringCloud中提供了Hystrix组件来实现服务调用异常的处理，或对高并发情况下的服务降级处理 。简单回顾一下Hystrix的使用：
1.要使用 Hystrix熔断机制处理引入它本身的依赖之外，我们需要在主程序配置类上贴 @EnableHystrix 标签 开启Hystrix功能，如下

@EnableHystrix @EnableEurekaClient @SpringBootApplication … public
class ConsumerApplication {

2.开启Hystrix熔断机制后，对方法进行熔断处理

@Service public class HelloService {

@Autowired
private RestTemplate restTemplate;

//该注解对该方法创建了熔断器的功能，并指定了fallbackMethod熔断方法
@HystrixCommand(fallbackMethod = "hiError")
public String hiService(String name){
    //调用接口进行消费
    String result = restTemplate.getForObject("http://PRODUCER/hello?name="+name,String.class);
    return result;
}
public String hiError(String name) {
    return "hi,"+name+"error!";
} }

当hiService方法第调用异常，会触发 fallbackMethod执行的hiError方法做成一些补救处理。
那么我们就沿着我们的使用方式来跟踪一下 Hystrix的 工作原理。
首先我们看一下标签：@EnableHystrix ，他的作用从名字就能看出就是开启Hystrix ，我们看一下它的源码

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

}

它上面有一个注解：@ EnableCircuitBreaker ，翻译单词意思就是启用熔断器(断路器),那么@ EnableHystrix标签的本质其实是@ EnableCircuitBreaker ,我们看一下他的源码

/** * Annotation to enable a CircuitBreaker implementation. *
http://martinfowler.com/bliki/CircuitBreaker.html * @author Spencer
Gibb */ @Target(ElementType.TYPE) @Retention(RetentionPolicy.RUNTIME)
@Documented @Inherited
@Import(EnableCircuitBreakerImportSelector.class) public @interface
EnableCircuitBreaker {

}

@EnableCircuitBreaker标签引入了一个@Import(EnableCircuitBreakerImportSelector.class) 类，翻译类的名字就是 ， 开启熔断器的导入选择器 ，导入什么东西呢？看源码

/** * Import a single circuit breaker implementation Configuration *
@author Spencer Gibb */ @Order(Ordered.LOWEST_PRECEDENCE - 100)
public class EnableCircuitBreakerImportSelector extends
SpringFactoryImportSelector {

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

}

翻译类上的注释 “Import a single circuit breaker implementation Configuration”，其实EnableCircuitBreakerImportSelector的作用就是去导入熔断器的配置 。其实Spring中也有类似于JAVA SPI 的加载机制, 即会自动加载 jar包 spring-cloud-netflix-core 中的META-INF/spring.factories 中的Hystrix相关的自动配置类
注：SPI : 通过将服务的接口与实现分离以实现解耦，提高程序拓展性的机制，达到插拔式的效果 。

HystrixCircuitBreakerConfiguration 就是针对于 Hystrix熔断器的配置

/**

• @author Spencer Gibb * @author Christian Dupuis * @author Venil Noronha */ @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));
  } …

在该配置类中创建了 HystrixCommandAspect

/** * AspectJ aspect to process methods which annotated with {@link
HystrixCommand} annotation. */ @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();
}

//定义切点，切到 @HystrixCommand标签所在的方法
@Pointcut("@annotation(com.netflix.hystrix.contrib.javanica.annotation.HystrixCommand)")

public void hystrixCommandAnnotationPointcut() {
}

@Pointcut("@annotation(com.netflix.hystrix.contrib.javanica.annotation.HystrixCollapser)")
public void hystrixCollapserAnnotationPointcut() {
} //针对切点：@hystrixCommand切点的处理
@Around("hystrixCommandAnnotationPointcut() || hystrixCollapserAnnotationPointcut()")
public Object methodsAnnotatedWithHystrixCommand(final ProceedingJoinPoint joinPoint) throws Throwable { //获取到目标方法   Method

method = getMethodFromTarget(joinPoint);
Validate.notNull(method, “failed to get method from joinPoint: %s”, joinPoint); //判断方法上不能同时存在@HystrixCommand标签和HystrixCollapser标签
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); //把方法封装成 HystrixInvokable
HystrixInvokable invokable = HystrixCommandFactory.getInstance().create(metaHolder);
ExecutionType executionType = metaHolder.isCollapserAnnotationPresent() ?
metaHolder.getCollapserExecutionType() : metaHolder.getExecutionType();

    Object result;
    try { // 通过CommandExecutor来执行方法
        if (!metaHolder.isObservable()) {
            result = CommandExecutor.execute(invokable, executionType, metaHolder);
        } else {
            result = executeObservable(invokable, executionType, metaHolder);
        }
    } catch (HystrixBadRequestException e) {
        throw e.getCause() != null ? e.getCause() : e;
    } catch (HystrixRuntimeException e) {
        throw hystrixRuntimeExceptionToThrowable(metaHolder, e);
    }
    return result;

HystrixCommandAspect 其实就是对 贴了@HystrixCommand标签的方法使用 Aop机制实现处理 。代码中通过把目标方法封装成 HystrixInvokable对象，通过CommandExecutor工具来执行目标方法。
HystrixInvokable是用来干嘛的？看源码知道，其实他是一个空行法的接口，他的目的只是用来标记可被执行，那么他是如何创建的我们看代码HystrixInvokable invokable = HystrixCommandFactory.getInstance().create(metaHolder);的create方法

public HystrixInvokable create(MetaHolder metaHolder) {
HystrixInvokable executable;
…省略代码…
executable = new GenericCommand(HystrixCommandBuilderFactory.getInstance().create(metaHolder));
}
return executable;
}

其实是new了一个 GenericCommand 对象，很明显他们是实现关系，我们看一下关系图

跟踪 GenericCommand 的源码

@ThreadSafe public class GenericCommand extends
AbstractHystrixCommand {
private static final Logger LOGGER = LoggerFactory.getLogger(GenericCommand.class);

public GenericCommand(HystrixCommandBuilder builder) {
    super(builder);
}

protected Object run() throws Exception {
    LOGGER.debug("execute command: {}", this.getCommandKey().name());
    return this.process(new AbstractHystrixCommand<Object>.Action() {
        Object execute() {
            return GenericCommand.this.getCommandAction().execute(GenericCommand.this.getExecutionType());
        }
    });
}

protected Object getFallback() {
    final CommandAction commandAction = this.getFallbackAction();
    if (commandAction != null) {
        try {
            return this.process(new AbstractHystrixCommand<Object>.Action() {
                Object execute() {
                    MetaHolder metaHolder = commandAction.getMetaHolder();
                    Object[] args = CommonUtils.createArgsForFallback(metaHolder,

GenericCommand.this.getExecutionException());
return commandAction.executeWithArgs(metaHolder.getFallbackExecutionType(),
args);
}
});
} catch (Throwable var3) {
LOGGER.error(FallbackErrorMessageBuilder.create().append(commandAction,
var3).build());
throw new FallbackInvocationException(ExceptionUtils.unwrapCause(var3));
}
} else {
return super.getFallback();
}
} }

它本身对目标方法的正常执行和对 fallback方法的 执行做了实现 。
GenericCommand.this.getCommandAction().execute(…)获取到目标方法并执行，底层会交给 MethodExecutionAction 使用反射去执行方法，
回到 HystrixCommandAspect的methodsAnnotatedWithHystrixCommand方法中，我们看下 CommandExecutor.execute是如何执行的

public class CommandExecutor {
public CommandExecutor() {
}

public static Object execute(HystrixInvokable invokable, ExecutionType executionType, MetaHolder metaHolder) throws

RuntimeException {
Validate.notNull(invokable);
Validate.notNull(metaHolder);
switch(executionType) { //异步
case SYNCHRONOUS:
return castToExecutable(invokable, executionType).execute(); //同步
case ASYNCHRONOUS:
HystrixExecutable executable = castToExecutable(invokable, executionType);
if (metaHolder.hasFallbackMethodCommand() && ExecutionType.ASYNCHRONOUS == metaHolder.getFallbackExecutionType()) {
return new FutureDecorator(executable.queue());
}

        return executable.queue();
    case OBSERVABLE:
        HystrixObservable observable = castToObservable(invokable);
        return ObservableExecutionMode.EAGER == metaHolder.getObservableExecutionMode() ? observable.observe() :

observable.toObservable();
default:
throw new RuntimeException("unsupported execution type: " + executionType);
}
}

private static HystrixExecutable castToExecutable(HystrixInvokable invokable, ExecutionType executionType) {
    if (invokable instanceof HystrixExecutable) {
        return (HystrixExecutable)invokable;
    } else {
        throw new RuntimeException("Command should implement " + HystrixExecutable.class.getCanonicalName() + " interface to execute

in: " + executionType + " mode");
}
}

这里有两种执行方式 SYNCHRONOUS 异步 ，ASYNCHRONOUS同步 ，我们先看异步： castToExecutable(invokable, executionType).execute(); 这里代码把HystrixInvokable对象转成 HystrixExecutable并调用execute方法执行 ，跟踪execute方法进入HystrixCommand.execute方法中

public R execute() {
try {
return queue().get();
} catch (Exception e) {
throw Exceptions.sneakyThrow(decomposeException(e));
}
}
-------------- public Future queue() {
/*
* The Future returned by Observable.toBlocking().toFuture() does not implement the
* interruption of the execution thread when the “mayInterrupt” flag of Future.cancel(boolean) is set to true;
* thus, to comply with the contract of Future, we must wrap around it.
*/
final Future delegate = toObservable().toBlocking().toFuture();

    final Future<R> f = new Future<R>() {

        @Override
        public boolean cancel(boolean mayInterruptIfRunning) {
            if (delegate.isCancelled()) {
                return false;
            }

            if (HystrixCommand.this.getProperties().executionIsolationThreadInterruptOnFutureCancel().get())

{
/*
* The only valid transition here is false -> true. If there are two futures, say f1 and f2, created by this command
* (which is super-weird, but has never been prohibited), and calls to f1.cancel(true) and to f2.cancel(false) are
* issued by different threads, it’s unclear about what value would be used by the time mayInterruptOnCancel is checked.
* The most consistent way to deal with this scenario is to say that if any cancellation is invoked with
interruption,
* than that interruption request cannot be taken back.
*/
interruptOnFutureCancel.compareAndSet(false, mayInterruptIfRunning);
}

            final boolean res = delegate.cancel(interruptOnFutureCancel.get());

            if (!isExecutionComplete() && interruptOnFutureCancel.get()) {
                final Thread t = executionThread.get();
                if (t != null && !t.equals(Thread.currentThread())) {
                    t.interrupt();
                }
            }

            return res;
        } ....省略...

在 HystrixCommand.execute方法中 其实是Future 来异步执行，调用过程中会触发 GenericCommand来完成调用，执行完成后调用 Future.get()方法拿到执行结果 。