Hystrix的工作原理

原文：https://www.cnblogs.com/sglx/p/15771838.html

一、简介

　　hystrix经常被我们用于服务的熔断，降级等领域，基于RxJava（一种基于观察者模式的响应式编程框架）实现，具备服务降级、服务熔断、线程与信号隔离、请求缓存、请求合并以及服务监控等强大功能。

二、基本原理

　　当我们需要调用某个方法时（一般是远程调用），通过 Hystrix 将方法调用包裹起来，交由 Hystrix 来完成，从而享受 Hystrix 带来保护。

Hystrix 提供了两个请求命令：HystrixCommand、HystrixObservableCommand，可以使用这两个对象来包裹待执行的任务。

HystrixCommand用在依赖服务返回单个操作结果的时候：

　　execute()：同步执行，从依赖的服务返回一个单一的结果对象，或是在发生错误的时候抛出异常。

　　queue()：异步执行，直接返回一个Future对象，其中包含了服务执行结束时要返回的单一结果对象。

HystrixObservableCommand用在依赖服务返回多个操作结果的时候：

　　observe()：返回Obervable对象，他代表了操作的多个结果，它是一个Hot Observable。

　　toObservable()：同样返回Observable对象，也代表了操作多个结果，但它返回的是一个Cold Observable。

三、基本用法

　　hystrix可以使用手动自定义Command、注解、结合feign的方式来实现，手动创建和结合feign的这里就不介绍了，主要看一下注解的实现方式。

1、开启hystrix，启动类上加上@EnableHystrix注解

2、在需要降级的方法上使用@HystrixCommand

public class UserServiceImpl{
　　　@Autowired
　　　private UserDao userDao;

    @HystrixCommand(fallbackMethod = "getUserNameFallBack")
    @Override
    public String getUserName(String userId){
        int i = 1/0
        return userDao.getNameById(userId);
    }
     
    public String getUserNameFallBack(String userId){
        return "服务暂时不可用，请稍后再试";
    }

} 

四、初始化

从@EnableHystrix注解看起

//开启EnableCircuitBreaker
@EnableCircuitBreaker
public @interface EnableHystrix {
}

//导入EnableCircuitBreakerImportSelector
@Import(EnableCircuitBreakerImportSelector.class)
public @interface EnableCircuitBreaker {
}

在注解上又开启了一个注解@EnableCircuitBreaker，并导入了一个Selector

@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);
    }

}

这是hystrix生效的一个关键点，继承了SpringFactoryImportSelector，此类在初始化后，会执行selectImports(AnnotationMetadata metadata)的方法。此方法会根据注解启动的注解（这里指@EnableCircuitBreaker）从spring.factories文件中获取其配置需要初始化@Configuration类，看下关键代码

List<String> factories = new ArrayList<>(new LinkedHashSet<>(SpringFactoriesLoader
.loadFactoryNames(this.annotationClass, this.beanClassLoader)));

看一下spring.factories文件

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

org.springframework.cloud.client.circuitbreaker.EnableCircuitBreaker=\
org.springframework.cloud.netflix.hystrix.HystrixCircuitBreakerConfiguration

配置类HystrixCircuitBreakerConfiguration

@Configuration(proxyBeanMethods = false)
public class HystrixCircuitBreakerConfiguration {

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

}

向spring注入了HystrixCommandAspect

@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();
    }
    
    //切点是所有使用了HystrixCommand注解的地方
    @Pointcut("@annotation(com.netflix.hystrix.contrib.javanica.annotation.HystrixCommand)")
    public void hystrixCommandAnnotationPointcut() {
    }
    //切点是所有使用了HystrixCollapser注解的地方
    @Pointcut("@annotation(com.netflix.hystrix.contrib.javanica.annotation.HystrixCollapser)")
    public void hystrixCollapserAnnotationPointcut() {
    }
    
    //环绕通知
    @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);
        //两个注解不能同时作用
        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");
        }
        //META_HOLDER_FACTORY_MAP预先初始化了两个工厂类
        //@HystrixCommand：CommandMetaHolderFactory
        //@HystrixCollapser：CollapserMetaHolderFactory
        MetaHolderFactory metaHolderFactory = META_HOLDER_FACTORY_MAP.get(HystrixPointcutType.of(method));
        //创建，把切点封装进了MetaHolder
        MetaHolder metaHolder = metaHolderFactory.create(joinPoint);
        // 创建HystrixInvokable,只是一个空接口，没有任何方法，只是用来标记具备可执行的能力
        // 具体的执行由实现类来做
        HystrixInvokable invokable = HystrixCommandFactory.getInstance().create(metaHolder);
        //执行类型
        ExecutionType executionType = metaHolder.isCollapserAnnotationPresent() ?
                metaHolder.getCollapserExecutionType() : metaHolder.getExecutionType();

        Object result;
        try {
            if (!metaHolder.isObservable()) {
                // 利用工具CommandExecutor来执行
                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;
    }

   ···

}

通过AOP编程，创建方法的代理，决定执行何种逻辑，创建参数包装类

public MetaHolder create(final ProceedingJoinPoint joinPoint) {
  //获取方法对象
  Method method = getMethodFromTarget(joinPoint);
  //目标对象
  Object obj = joinPoint.getTarget();
  //方法的参数列表
  Object[] args = joinPoint.getArgs();
  //代理对象
  Object proxy = joinPoint.getThis();
  //调用子类create方法
  return create(proxy, method, obj, args, joinPoint);
}

public MetaHolder create(Object proxy, Method method, Object obj, Object[] args, final ProceedingJoinPoint joinPoint) {
  //获取HystrixCommand注解的信息
  HystrixCommand hystrixCommand = method.getAnnotation(HystrixCommand.class);
  //判断方法的返回值类型Future：异步，Observable：rxjava中的被观察者，其他：同步
  ExecutionType executionType = ExecutionType.getExecutionType(method.getReturnType());
  //获取MetaHolder的builder对象
  MetaHolder.Builder builder = metaHolderBuilder(proxy, method, obj, args, joinPoint);
  if (isCompileWeaving()) {
    builder.ajcMethod(getAjcMethodFromTarget(joinPoint));
  }
  //建造者模式，创建MetaHolder包装类
  return builder.defaultCommandKey(method.getName())
    .hystrixCommand(hystrixCommand)
    .observableExecutionMode(hystrixCommand.observableExecutionMode())
    .executionType(executionType)
    .observable(ExecutionType.OBSERVABLE == executionType)
    .build();
} 

创建命令执行器

    public HystrixInvokable create(MetaHolder metaHolder) {
        HystrixInvokable executable;
        //@HystrixCollapser
        if (metaHolder.isCollapserAnnotationPresent()) {
            executable = new CommandCollapser(metaHolder);
        //@HystrixCommand 并且 返回值类型是Observable
        } else if (metaHolder.isObservable()) {
            executable = new GenericObservableCommand(HystrixCommandBuilderFactory.getInstance().create(metaHolder));
        } else {
            //其他情况 把HystrixCommandBuilder封装进GenericCommand
            executable = new GenericCommand(HystrixCommandBuilderFactory.getInstance().create(metaHolder));
        }
        return executable;
    }
    public GenericCommand(HystrixCommandBuilder builder) {
        super(builder);
    }
    
    protected AbstractHystrixCommand(HystrixCommandBuilder builder) {
        super(builder.getSetterBuilder().build());
        //命令形式 包含需要执行的方法 fallback方法
        this.commandActions = builder.getCommandActions();
        this.collapsedRequests = builder.getCollapsedRequests();
        this.cacheResultInvocationContext = builder.getCacheResultInvocationContext();
        this.cacheRemoveInvocationContext = builder.getCacheRemoveInvocationContext();
        this.ignoreExceptions = builder.getIgnoreExceptions();
        //执行类型 ASYNCHRONOUS SYNCHRONOUS OBSERVABLE
        this.executionType = builder.getExecutionType();
    }
    
    public HystrixCommand.Setter build() throws HystrixPropertyException {
        //分组key：类名，命令key：方法名，线程池key
        HystrixCommand.Setter setter = HystrixCommand.Setter
                .withGroupKey(HystrixCommandGroupKey.Factory.asKey(groupKey))
                .andCommandKey(HystrixCommandKey.Factory.asKey(commandKey));
        if (StringUtils.isNotBlank(threadPoolKey)) {
            setter.andThreadPoolKey(HystrixThreadPoolKey.Factory.asKey(threadPoolKey));
        }
        try {
            //初始化线程池的配置
            setter.andThreadPoolPropertiesDefaults(HystrixPropertiesManager.initializeThreadPoolProperties(threadPoolProperties));
        } catch (IllegalArgumentException e) {
            throw new HystrixPropertyException("Failed to set Thread Pool properties. " + getInfo(), e);
        }
        try {
            //初始化命令执行配置
            setter.andCommandPropertiesDefaults(HystrixPropertiesManager.initializeCommandProperties(commandProperties));
        } catch (IllegalArgumentException e) {
            throw new HystrixPropertyException("Failed to set Command properties. " + getInfo(), e);
        }
        return setter;
    }

执行命令

    public static Object execute(HystrixInvokable invokable, ExecutionType executionType, MetaHolder metaHolder)
　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　 throws RuntimeException {
        Validate.notNull(invokable);
        Validate.notNull(metaHolder);

        switch (executionType) {
            case SYNCHRONOUS: {
                //转换成子接口HystrixExecutable
                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);
        }
    }
    public R execute() {
        try {
            return queue().get();
        } catch (Exception e) {
            throw Exceptions.sneakyThrow(decomposeException(e));
        }
    }

HystrixCommand的四种执行方式

测试类

public class HystrixCommandTest {

    private <T> com.netflix.hystrix.HystrixCommand<T> createCommand(T message) {
        com.netflix.hystrix.HystrixCommand.Setter setter = com.netflix.hystrix.HystrixCommand.Setter
                .withGroupKey(HystrixCommandGroupKey.Factory.asKey("serviceA"))
                .andCommandKey(HystrixCommandKey.Factory.asKey("methodA"));
        return new com.netflix.hystrix.HystrixCommand<T>(setter) {
            @Override
            protected T run() throws Exception {
                System.out.println("HystrixCommand执行了！！！" + System.currentTimeMillis());
                return message;
            }
        };
    }

    @Test
    public void test01() {
        com.netflix.hystrix.HystrixCommand<String> command = createCommand("this is test01");
        System.out.println(command.execute());
    }


    @Test
    public void test02() throws ExecutionException, InterruptedException {
        com.netflix.hystrix.HystrixCommand<String> command = createCommand("this is test02");
        Future<String> f = command.queue();
        System.out.println("queue之后，command执行:" + System.currentTimeMillis());
        Thread.sleep(1000);
        System.out.println(f.get());
    }

    @Test
    public void test03() throws InterruptedException {
        HystrixCommand<String> command = createCommand("this is test03");
        // observe直接执行run方法，称为Hot Observable
        Observable<String> observe = command.observe();
        System.out.println("observe之后，command执行：" + System.currentTimeMillis());
        Thread.sleep(1000);
        observe.subscribe(new Action1<String>() {
            @Override
            public void call(String s) {
                System.out.println("通过订阅，获取执行结果：" + s);
            }
        });
    }

    @Test
    public void test04() throws InterruptedException {
        HystrixCommand<String> command = createCommand("this is test04");
        // toObservable不直接执行run方法
        Observable<String> observe = command.toObservable();
        System.out.println("未订阅，command不执行：" + System.currentTimeMillis());
        Thread.sleep(1000);
        observe.subscribe();
        System.out.println("订阅后，command执行了" + System.currentTimeMillis());
        Thread.sleep(1000);
    }

    @Test
    public void test05() throws InterruptedException, ExecutionException {
        HystrixCommand<String> command = createCommand("this is test05");
        // toObservable不直接执行run方法
        Future<String> f = command.toObservable().toBlocking().toFuture();
        System.out.println("转成future执行：" + System.currentTimeMillis());
        Thread.sleep(1000);
        System.out.println(f.get());
    }
}

回到hystrix源码中queue()方法

    public Future<R> queue() {
        // toObservable转换为Observable
        // toBlocking转换为BlockingObservable
        // toFuture转换为Future
        // 完成了Observable的创建和订阅
        final Future<R> delegate = toObservable().toBlocking().toFuture();
        // 代理future，对于cancel操作做特殊处理
        // 因为toObservable().toBlocking().toFuture()返回的future无法通过cancel方法中断执行线程。
        final Future<R> f = new Future<R>() {

            @Override
            public boolean cancel(boolean mayInterruptIfRunning) {
                if (delegate.isCancelled()) {
                    return false;
                }
                // 如果 execution.isolation.thread.interruptOnFutureCancel = true（默认false）
                if (HystrixCommand.this.getProperties().executionIsolationThreadInterruptOnFutureCancel().get()) {
                    // 设置标志位
                    interruptOnFutureCancel.compareAndSet(false, mayInterruptIfRunning);
                }
                // 执行目标future的cancel
                final boolean res = delegate.cancel(interruptOnFutureCancel.get());
                // 如果command还没执行完成 且 需要中断执行的线程
                if (!isExecutionComplete() && interruptOnFutureCancel.get()) {
                    // 获取执行线程
                    final Thread t = executionThread.get();
                    // 执行线程非当前线程则中断线程
                    if (t != null && !t.equals(Thread.currentThread())) {
                        t.interrupt();
                    }
                }

                return res;
            }

            ```
            
        };

        //判断是否执行完成
        if (f.isDone()) {
            try {
                //获取结果
                f.get();
                return f;
            } catch (Exception e) {
                ···
            }
        }

        return f;
    }

这里用到了RxJava框架的响应式编程，会执行到具体Command（之前封装的GenericCommand）的run方法

public class GenericCommand extends AbstractHystrixCommand<Object> {

    private static final Logger LOGGER = LoggerFactory.getLogger(GenericCommand.class);

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

    //执行目标的方法
    @Override
    protected Object run() throws Exception {
        LOGGER.debug("execute command: {}", getCommandKey().name());
        return process(new Action() {
            @Override
            Object execute() {
                return getCommandAction().execute(getExecutionType());
            }
        });
    }
    
    //执行fallback方法
    @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();
        }
    }

}

五、Hystrix上下文

　　当feign结合Hystrix使用线程隔离时，如果我们想要使用ThreadLocal传递参数是不行的，存在跨线程传递的问题，Hystrix提供了一个上下文类HystrixRequestContext，以传递traceId为例

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