一、Hystrix源码总结

Hystrix在底层使用了Spring提供的切面技术:

• 通过HystrixCommandAspect.java定义了一个切面（该类有@Aspect注解），专门用来处理那些标注了@HystrixCommand的方法

  /**
   * AspectJ aspect to process methods which annotated with {@link HystrixCommand} annotation.
   */
  @Aspect
  public class HystrixCommandAspect {...}

• 更详细的讲，该类中定义了一个切点。切点是@HystrixCommand。

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

• 而针对该切点，还设置一个环绕函数
  

  @Around("hystrixCommandAnnotationPointcut()") 
  public Object methodsAnnotatedWithHystrixCommand()

• 环绕函数中，最关键的一句是execute()。这里会根据executionType有SYNCHRONOUS，ASYNCHRONOUS，OBSERVABLE调用不同逻辑，使用rxjava的观察者模式并最终在这里计算出结果返回。

  result = CommandExecutor.execute(invokable, executionType, metaHolder);

• 当调用execute()出现失败时，会调用getFallBack()方法。而如果没有设置降级方法则调用父类的getFallBack()方法，父类的getFallBack方法会抛出一个找不到降级方法的异常

  protected T getFallback() {
      throw new RuntimeException("No fallback available.", getExecutionException());
  }

   

二、详细解析

Hystrix在底层使用了Spring提供的切面技术。在HystrixCommandAspect.java文件中可以看到定义的切面：

/**
 * 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();
    }

    @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 {
        //通过切点获取被拦截的方法
        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中保存了很多和切点相关的信息，详见后文的贴图
        MetaHolder metaHolder = metaHolderFactory.create(joinPoint);
        HystrixInvokable invokable = HystrixCommandFactory.getInstance().create(metaHolder);
        ExecutionType executionType = metaHolder.isCollapserAnnotationPresent() ?
                metaHolder.getCollapserExecutionType() : metaHolder.getExecutionType();
        
        //result中保留操作的结果，可能是成功操作的返回值也可能是fallback方法的返回值
        Object result;
        try {
            //如果返回的结果使用了observable模式则执行以下代码
            if (!metaHolder.isObservable()) {
                result = CommandExecutor.execute(invokable, executionType, metaHolder);
            } else {
            //否则执行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;
    }
    ..........
}

可以看到在这个类中首先是定义了两个切面，由于我们这里讨论的是HystrixCommand，所以只关注第一个切面，在这个切面里可以看到切点是被HystrixCommand注解的方法，然后对方法的具体增强是在接下来的一个名为methodsAnnotatedWithHystrixCommand()的方法里，具体的流程用注释写在了代码中。

metaHolder的内容：

可以看到在以上代码中，最重要的便是获取到result具体结果的那一步，由于我们的例子不是使用，所以重点放在：

result = CommandExecutor.execute(invokable, executionType, metaHolder);

接下来跳转到CommandExecutor.java中的execute()方法：

/**
     * Calls a method of {@link HystrixExecutable} in accordance with specified execution type.
     *
     * @param invokable  {@link HystrixInvokable}
     * @param metaHolder {@link MetaHolder}
     * @return the result of invocation of specific method.
     * @throws RuntimeException
     */
    public static Object execute(HystrixInvokable invokable, ExecutionType executionType, MetaHolder metaHolder) throws RuntimeException {
        Validate.notNull(invokable);
        Validate.notNull(metaHolder);

        switch (executionType) {
            case SYNCHRONOUS: {
                return castToExecutable(invokable, executionType).execute();
            }
            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);
        }
    }

在execute()方法中使用了rxjava的观察者模式并最终在这里计算出结果返回。当出现失败是，会调用GenericCommand.java中的getFallBack()方法，代码如下：

/**
     * The fallback is performed whenever a command execution fails.
     * Also a fallback method will be invoked within separate command in the case if fallback method was annotated with
     * HystrixCommand annotation, otherwise current implementation throws RuntimeException and leaves the caller to deal with it
     * (see {@link super#getFallback()}).
     * The getFallback() is always processed synchronously.
     * Since getFallback() can throw only runtime exceptions thus any exceptions are thrown within getFallback() method
     * are wrapped in {@link FallbackInvocationException}.
     * A caller gets {@link com.netflix.hystrix.exception.HystrixRuntimeException}
     * and should call getCause to get original exception that was thrown in getFallback().
     *
     * @return result of invocation of fallback method or RuntimeException
     */
    @Override
    protected Object getFallback() {
        final CommandAction commandAction = getFallbackAction();
        if (commandAction != null) {
            try {
                return process(new Action() {
                    @Override
                    Object execute() {
                        MetaHolder metaHolder = commandAction.getMetaHolder();
                        Object[] args = createArgsForFallback(metaHolder, getExecutionException());
                        return commandAction.executeWithArgs(metaHolder.getFallbackExecutionType(), args);
                    }
                });
            } catch (Throwable e) {
                LOGGER.error(FallbackErrorMessageBuilder.create()
                        .append(commandAction, e).build());
                throw new FallbackInvocationException(unwrapCause(e));
            }
        } else {
            return super.getFallback();
        }
    }

在这个方法中首先会对是否设置了commandAction进行判断，commandAction就是我们之前所设置的fallback字段指向的方法，如果发现有降级方法，则调用降级方法获取结果，进入process()方法，代码如下：

/**
     * Executes an action. If an action has failed and an exception is ignorable then propagate it as HystrixBadRequestException
     * otherwise propagate original exception to trigger fallback method.
     * Note: If an exception occurred in a command directly extends {@link java.lang.Throwable} then this exception cannot be re-thrown
     * as original exception because HystrixCommand.run() allows throw subclasses of {@link java.lang.Exception}.
     * Thus we need to wrap cause in RuntimeException, anyway in this case the fallback logic will be triggered.
     *
     * @param action the action
     * @return result of command action execution
     */
    Object process(Action action) throws Exception {
        Object result;
        try {
            result = action.execute();
            flushCache();
        } catch (CommandActionExecutionException throwable) {
            Throwable cause = throwable.getCause();
            if (isIgnorable(cause)) {
                throw new HystrixBadRequestException(cause.getMessage(), cause);
            }
            if (cause instanceof RuntimeException) {
                throw (RuntimeException) cause;
            } else if (cause instanceof Exception) {
                throw (Exception) cause;
            } else {
                // instance of Throwable
                throw new CommandActionExecutionException(cause);
            }
        }
        return result;
    }

最终在process()方法中获取降级方法执行的结果，而如果没有设置降级方法则调用父类的getFallBack()方法，父类的getFallBack方法会抛出一个找不到降级方法的异常，代码如下：

protected T getFallback() {
    throw new RuntimeException("No fallback available.", getExecutionException());
}

至于何时会跳转到降级方法，则是在AbstractCommand.java中，在这里定义了很多种执行失败的情况，通过rxjava框架的观察者模式对错误进行监听，根据不同的情况会进入不同的处理方法，最终这些处理方法都会调用HystrixCommand.java中的getFallbackObservable()方法，并最终进入上文所述的真正执行fallback方法的代码。

以上就是对Hystrix降级机制的说明，想要深入理解Hystrix，必须要对rxjava有比较深刻的认识，在Hystrix中大量运用了rxjava。

参考文献

版权声明：本文为CSDN博主「栗子栗」的原创文章，遵循CC 4.0 BY-SA版权协议，转载请附上原文出处链接及本声明。
原文链接：https://blog.csdn.net/turbo_zone/article/details/83831992