@RabbitListener起作用的原理是什么

这篇文章主要讲解了“@RabbitListener起作用的原理是什么”，文中的讲解内容简单清晰，易于学习与理解，下面请大家跟着小编的思路慢慢深入，一起来研究和学习“@RabbitListener起作用的原理是什么”吧！

一、前言

在spring中，定义rabbitMq的消费者可以相当方便，只需要在消息处理类或者类方法加上@RabbitListener注解，指定队列名称即可。

如下代码：

@Component
public class RabbitMqListener1 {
    @RabbitListener(queues = "queue1")
    public void consumer1(Message message) {
    }
    @RabbitListener(queues = "queue2")
    public void consumer2(String messsageBody) {
    }
}
@Component
@RabbitListener(queues = "queue3")
public class RabbitMqListener2 {
    @RabbitHandler(isDefault=true)
    public void consumer3() {
    }
}

注意！！！如果@RabbitListener加在类上面，需要有一个默认的处理方法@RabbitHandler(isDefault=true)，默认是false。

不设置一个true，消费mq消息的时候会出现“Listener method ‘no match’ threw exception”异常。

原因在RabbitListenerAnnotationBeanPostProcessor.processMultiMethodListeners方法，有兴趣的可以看下。

可以看到代码相当的简单。但是！！！为什么加上这个注解，就能作为一个consumer接受mq的消息呢？为啥处理mq消息的方法，入参可以那么随意？

有经验的程序员，可能会有这样的设想：

1、单纯看这些listener的代码，只是定义了由spring管理的bean，要能监听rabbitMq的消息，肯定需要有另外一个类，这个类会扫描所有加了@RabbitListener的bean，进行加工。

2、看这些listener的代码，可以发现处理mq消息的，都是具体的某个方法。那加工的过程，应该就是利用反射拿到对象、方法和@RabbitListener中的queue属性，然后建立一个绑定关系（对象+方法）——>（queue的consumer）。queue的consumer在接收到mq消息后，找到绑定的“对象+方法”，再通过反射的方式，调用真正的处理方法。

3、mq消息的处理方法，可以那么随意，应该是queue的consumer在调用真正处理方法之前，需要根据处理方法的参数类型，做一次数据转换。

接下来，就去看看源码，看一下设想是不是正确的~~

二、源码分析

1、谁来扫描@RabbitListener注解的bean

在springBoot使用rabbit，一般是在@Configuration类上加上@EnableRabbit注解来开启rabbit功能。那我们就去看看@EnableRabbit注解的源码，看这个注解的作用

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Import(RabbitBootstrapConfiguration.class)
public @interface EnableRabbit {
}

可以看到，这个注解的作用，是导入RabbitBootstrapConfiguration配置类

@Configuration
public class RabbitBootstrapConfiguration {
    @Bean(name = RabbitListenerConfigUtils.RABBIT_LISTENER_ANNOTATION_PROCESSOR_BEAN_NAME)
    @Role(BeanDefinition.ROLE_INFRASTRUCTURE)
    public RabbitListenerAnnotationBeanPostProcessor rabbitListenerAnnotationProcessor() {
        return new RabbitListenerAnnotationBeanPostProcessor();
    }
    @Bean(name = RabbitListenerConfigUtils.RABBIT_LISTENER_ENDPOINT_REGISTRY_BEAN_NAME)
    public RabbitListenerEndpointRegistry defaultRabbitListenerEndpointRegistry() {
        return new RabbitListenerEndpointRegistry();
    }
}

RabbitBootstrapConfiguration 配置类的作用，就是定义了RabbitListenerAnnotationBeanPostProcessor 和RabbitListenerEndpointRegistry 两个bean。

看到RabbitListenerAnnotationBeanPostProcessor 这个类名，就可以猜到，该类的实例bean就是用来扫描加了@RabbitListener 的类，并做一些加工。

（“RabbitListenerAnnotationBean”——针对添加了@RabbitListener注解的bean； “PostProcessor”——后置加工）

2、怎么建立（对象+方法）——>（queue的consumer）的映射关系

分析一下RabbitListenerAnnotationBeanPostProcessor类的源码

// 实现了BeanPostProcessor、Ordered、BeanFactoryAware、BeanClassLoaderAware、EnvironmentAware和SmartInitializingSingleton 6个接口
public class RabbitListenerAnnotationBeanPostProcessor
        implements BeanPostProcessor, Ordered, BeanFactoryAware, BeanClassLoaderAware, EnvironmentAware,
        SmartInitializingSingleton {
        
    .......
    
    // 完成初始化bean之后，调用该方法
    @Override
    public Object postProcessAfterInitialization(final Object bean, final String beanName) throws BeansException {
        Class<?> targetClass = AopUtils.getTargetClass(bean);
        TypeMetadata metadata = this.typeCache.get(targetClass);
        if (metadata == null) {
            metadata = buildMetadata(targetClass);
            this.typeCache.putIfAbsent(targetClass, metadata);
        }
        for (ListenerMethod lm : metadata.listenerMethods) {
            for (RabbitListener rabbitListener : lm.annotations) {
                processAmqpListener(rabbitListener, lm.method, bean, beanName);
            }
        }
        if (metadata.handlerMethods.length > 0) {
            processMultiMethodListeners(metadata.classAnnotations, metadata.handlerMethods, bean, beanName);
        }
        return bean;
    }
    // 根据Class，获取元数据
    private TypeMetadata buildMetadata(Class<?> targetClass) {
        Collection<RabbitListener> classLevelListeners = findListenerAnnotations(targetClass);
        final boolean hasClassLevelListeners = classLevelListeners.size() > 0;
        final List<ListenerMethod> methods = new ArrayList<ListenerMethod>();
        final List<Method> multiMethods = new ArrayList<Method>();
        ReflectionUtils.doWithMethods(targetClass, new ReflectionUtils.MethodCallback() {
            @Override
            public void doWith(Method method) throws IllegalArgumentException, IllegalAccessException {
                Collection<RabbitListener> listenerAnnotations = findListenerAnnotations(method);
                if (listenerAnnotations.size() > 0) {
                    methods.add(new ListenerMethod(method,
                            listenerAnnotations.toArray(new RabbitListener[listenerAnnotations.size()])));
                }
                if (hasClassLevelListeners) {
                    RabbitHandler rabbitHandler = AnnotationUtils.findAnnotation(method, RabbitHandler.class);
                    if (rabbitHandler != null) {
                        multiMethods.add(method);
                    }
                }
            }
        }, ReflectionUtils.USER_DECLARED_METHODS);
        if (methods.isEmpty() && multiMethods.isEmpty()) {
            return TypeMetadata.EMPTY;
        }
        return new TypeMetadata(
                methods.toArray(new ListenerMethod[methods.size()]),
                multiMethods.toArray(new Method[multiMethods.size()]),
                classLevelListeners.toArray(new RabbitListener[classLevelListeners.size()]));
    }
    // 检查一下是否使用jdk代理，使用jdk代理方式必须实现了接口
    // new一个MethodRabbitListenerEndpoint对象，交由processListener方法进行处理
    protected void processAmqpListener(RabbitListener rabbitListener, Method method, Object bean, String beanName) {
        Method methodToUse = checkProxy(method, bean);
        MethodRabbitListenerEndpoint endpoint = new MethodRabbitListenerEndpoint();
        endpoint.setMethod(methodToUse);
        endpoint.setBeanFactory(this.beanFactory);
        processListener(endpoint, rabbitListener, bean, methodToUse, beanName);
    }
// 前面大半代码都是对MethodRabbitListenerEndpoint对象的属性设置：处理消息的bean、消息处理方法的工厂类、监听的队列名。。。。
// 通过beanFactory获取RabbitListenerContainerFactory类的bean
// 调用RabbitListenerEndpointRegistar的registerEndpoint方法注册mq消息消费端点
protected void processListener(MethodRabbitListenerEndpoint endpoint, RabbitListener rabbitListener, Object bean,
            Object adminTarget, String beanName) {
        endpoint.setBean(bean);
        endpoint.setMessageHandlerMethodFactory(this.messageHandlerMethodFactory);
        endpoint.setId(getEndpointId(rabbitListener));
        endpoint.setQueueNames(resolveQueues(rabbitListener));
        String group = rabbitListener.group();
        if (StringUtils.hasText(group)) {
            Object resolvedGroup = resolveExpression(group);
            if (resolvedGroup instanceof String) {
                endpoint.setGroup((String) resolvedGroup);
            }
        }
        endpoint.setExclusive(rabbitListener.exclusive());
        String priority = resolve(rabbitListener.priority());
        if (StringUtils.hasText(priority)) {
            try {
                endpoint.setPriority(Integer.valueOf(priority));
            }
            catch (NumberFormatException ex) {
                throw new BeanInitializationException("Invalid priority value for " +
                        rabbitListener + " (must be an integer)", ex);
            }
        }
        String rabbitAdmin = resolve(rabbitListener.admin());
        if (StringUtils.hasText(rabbitAdmin)) {
            Assert.state(this.beanFactory != null, "BeanFactory must be set to resolve RabbitAdmin by bean name");
            try {
                endpoint.setAdmin(this.beanFactory.getBean(rabbitAdmin, RabbitAdmin.class));
            }
            catch (NoSuchBeanDefinitionException ex) {
                throw new BeanInitializationException("Could not register rabbit listener endpoint on [" +
                        adminTarget + "], no " + RabbitAdmin.class.getSimpleName() + " with id '" +
                        rabbitAdmin + "' was found in the application context", ex);
            }
        }
        RabbitListenerContainerFactory<?> factory = null;
        String containerFactoryBeanName = resolve(rabbitListener.containerFactory());
        if (StringUtils.hasText(containerFactoryBeanName)) {
            Assert.state(this.beanFactory != null, "BeanFactory must be set to obtain container factory by bean name");
            try {
                factory = this.beanFactory.getBean(containerFactoryBeanName, RabbitListenerContainerFactory.class);
            }
            catch (NoSuchBeanDefinitionException ex) {
                throw new BeanInitializationException("Could not register rabbit listener endpoint on [" +
                        adminTarget + "] for bean " + beanName + ", no " +
                        RabbitListenerContainerFactory.class.getSimpleName() + " with id '" +
                        containerFactoryBeanName + "' was found in the application context", ex);
            }
        }
        this.registrar.registerEndpoint(endpoint, factory);
    }
    ........
}

这个类的代码比较长，只贴部分比较主要的部分，其他的，可以自己查看源码进行了解。

RabbitListenerAnnotationBeanPostProcessor实现了BeanPostProcessor（bean初始化后的后置处理）、Ordered（后置处理的排序）、BeanFactoryAware（注入BeanFactory）、BeanClassLoaderAware(注入BeanClassLoader)、EnvironmentAware（注入spring环境）和SmartInitializingSingleton（单例bean初始化后的回调） 6个接口。

我们需要关注的是BeanPostProcessor接口定义的方法，看postProcessAfterInitialization方法的代码，大致流程为：

1、通过AopUtils得到bean代理的对象的class

2、判断缓存中是否有该class的类型元数据，如果没有则调用buildMetadata方法生成类型元数据并放入缓存

3、遍历加了@RabbitListener注解的方法，调用processAmqpListener方法进行处理

4、调用processMultiMethodListeners方法对加了@RabbitHandler的方法进行处理

关于buildMetadata方法：

代码不复杂，就是利用反射，拿到class中，添加了@RabbitListener和@RabbitHandler注解的方法。另外，从代码中也可以看出，@RabbitHandler注解要生效，必须在class上增加@RabbitListener注解

关于processAmqpListener方法：

没有什么实际内容，就干两个事情：

1、检查一下是否使用jdk代理，使用jdk代理方式必须实现了接口

2、new一个MethodRabbitListenerEndpoint对象，交由processListener方法进行处理

关于processListener方法：

1、前面大半代码都是对MethodRabbitListenerEndpoint对象的属性设置：处理消息的bean、消息处理方法的工厂类、监听的队列名。。。。

其中要关注一下setMessageHandlerMethodFactory方法，查看MessageHandlerMethodFactory接口的源码

public interface MessageHandlerMethodFactory {
    InvocableHandlerMethod createInvocableHandlerMethod(Object bean, Method method);

从入参和返回值可以看出来，这个工厂的作用就是将spring的bean对象和方法包装成一个InvocableHandlerMethod对象，也就是我们上面提到的（对象+方法）。

2、通过beanFactory获取RabbitListenerContainerFactory类的bean。

3、调用RabbitListenerEndpointRegistar的registerEndpoint方法注册mq消息消费端点。

继续往下追，看一下RabbitListenerEndpointRegistar的代码：

public class RabbitListenerEndpointRegistrar implements BeanFactoryAware, InitializingBean {
    // 将整个endpointDescriptors数组进行注册
    protected void registerAllEndpoints() {
        synchronized (this.endpointDescriptors) {
            for (AmqpListenerEndpointDescriptor descriptor : this.endpointDescriptors) {
                this.endpointRegistry.registerListenerContainer(
                        descriptor.endpoint, resolveContainerFactory(descriptor));
            }
            this.startImmediately = true;  // trigger immediate startup
        }
    }
    
    // 解析得到RabbitListenerContainerFactory
    // 如果AmqpListenerEndpointDescriptor 的containerFactory属性不为空，直接返回containerFactory
    // 如果为空，尝试从beanFactory获取
    private RabbitListenerContainerFactory<?> resolveContainerFactory(AmqpListenerEndpointDescriptor descriptor) {
        if (descriptor.containerFactory != null) {
            return descriptor.containerFactory;
        }
        else if (this.containerFactory != null) {
            return this.containerFactory;
        }
        else if (this.containerFactoryBeanName != null) {
            Assert.state(this.beanFactory != null, "BeanFactory must be set to obtain container factory by bean name");
            this.containerFactory = this.beanFactory.getBean(
                    this.containerFactoryBeanName, RabbitListenerContainerFactory.class);
            return this.containerFactory;  // Consider changing this if live change of the factory is required
        }
        else {
            throw new IllegalStateException("Could not resolve the " +
                    RabbitListenerContainerFactory.class.getSimpleName() + " to use for [" +
                    descriptor.endpoint + "] no factory was given and no default is set.");
        }
    }
    
    // new一个AmqpListenerEndpointDescriptor对象
    // 如果立即启动，则调用RabbitListenerEndpointRegistry注册器来注册消息监听
    // 如果不是立即启动，则添加到endpointDescriptors列表中，后面通过registerAllEndpoints方法统一启动
    public void registerEndpoint(RabbitListenerEndpoint endpoint, RabbitListenerContainerFactory<?> factory) {
        Assert.notNull(endpoint, "Endpoint must be set");
        Assert.hasText(endpoint.getId(), "Endpoint id must be set");
        // Factory may be null, we defer the resolution right before actually creating the container
        AmqpListenerEndpointDescriptor descriptor = new AmqpListenerEndpointDescriptor(endpoint, factory);
        synchronized (this.endpointDescriptors) {
            if (this.startImmediately) { // Register and start immediately
                this.endpointRegistry.registerListenerContainer(descriptor.endpoint,
                        resolveContainerFactory(descriptor), true);
            }
            else {
                this.endpointDescriptors.add(descriptor);
            }
        }
    }
}

从上面的代码可以看出，我们关心的内容，应该是在RabbitListenerEndpointRegistry类的registerListenerContainer方法！！

public class RabbitListenerEndpointRegistry implements DisposableBean, SmartLifecycle, ApplicationContextAware,
        ApplicationListener<ContextRefreshedEvent> {
        // 检查是否被注册过，注册过就不能注册第二次
        // 调用createListenerContainer创建消息监听
        // 关于分组消费的，我们不关心
        // 是否立即启动，是的话，同步调用startIfNecessary方法
        public void registerListenerContainer(RabbitListenerEndpoint endpoint, RabbitListenerContainerFactory<?> factory,
                                          boolean startImmediately) {
        Assert.notNull(endpoint, "Endpoint must not be null");
        Assert.notNull(factory, "Factory must not be null");
        String id = endpoint.getId();
        Assert.hasText(id, "Endpoint id must not be empty");
        synchronized (this.listenerContainers) {
            Assert.state(!this.listenerContainers.containsKey(id),
                    "Another endpoint is already registered with id '" + id + "'");
            MessageListenerContainer container = createListenerContainer(endpoint, factory);
            this.listenerContainers.put(id, container);
            if (StringUtils.hasText(endpoint.getGroup()) && this.applicationContext != null) {
                List<MessageListenerContainer> containerGroup;
                if (this.applicationContext.containsBean(endpoint.getGroup())) {
                    containerGroup = this.applicationContext.getBean(endpoint.getGroup(), List.class);
                }
                else {
                    containerGroup = new ArrayList<MessageListenerContainer>();
                    this.applicationContext.getBeanFactory().registerSingleton(endpoint.getGroup(), containerGroup);
                }
                containerGroup.add(container);
            }
            if (startImmediately) {
                startIfNecessary(container);
            }
        }
    // 其实就是调用了RabbitListenerContainerFactory的createListenerContainer生成了一个MessageListenerContainer对象
    protected MessageListenerContainer createListenerContainer(RabbitListenerEndpoint endpoint,
            RabbitListenerContainerFactory<?> factory) {
        MessageListenerContainer listenerContainer = factory.createListenerContainer(endpoint);
        if (listenerContainer instanceof InitializingBean) {
            try {
                ((InitializingBean) listenerContainer).afterPropertiesSet();
            }
            catch (Exception ex) {
                throw new BeanInitializationException("Failed to initialize message listener container", ex);
            }
        }
        int containerPhase = listenerContainer.getPhase();
        if (containerPhase < Integer.MAX_VALUE) {  // a custom phase value
            if (this.phase < Integer.MAX_VALUE && this.phase != containerPhase) {
                throw new IllegalStateException("Encountered phase mismatch between container factory definitions: " +
                        this.phase + " vs " + containerPhase);
            }
            this.phase = listenerContainer.getPhase();
        }
        return listenerContainer;
    }
}

createListenerContainer方法调用了RabbitListenerContainerFactory接口的createListenerContainer方法创建一个MessageListenerContainer对象。

在这里，如果是通过RabbitAutoConfiguration自动配置的，那么RabbitListenerContainerFactory接口的具体实现类是SimpleRabbitListenerContainerFactory，MessageListenerContainer接口的具体实现类是SimpleMessageListenerContainer。有兴趣的话，可以去看下rabbitMq自动配置的几个类。

RabbitListenerContainerFactory接口的createListenerContainer方法是由AbstractRabbitListenerContainerFactory抽象类实现，代码如下：

    @Override
    public C createListenerContainer(RabbitListenerEndpoint endpoint) {
        C instance = createContainerInstance();
        if (this.connectionFactory != null) {
            instance.setConnectionFactory(this.connectionFactory);
        }
        if (this.errorHandler != null) {
            instance.setErrorHandler(this.errorHandler);
        }
        if (this.messageConverter != null) {
            instance.setMessageConverter(this.messageConverter);
        }
        if (this.acknowledgeMode != null) {
            instance.setAcknowledgeMode(this.acknowledgeMode);
        }
        if (this.channelTransacted != null) {
            instance.setChannelTransacted(this.channelTransacted);
        }
        if (this.autoStartup != null) {
            instance.setAutoStartup(this.autoStartup);
        }
        if (this.phase != null) {
            instance.setPhase(this.phase);
        }
        instance.setListenerId(endpoint.getId());
        // 最重要的一行！！！
        endpoint.setupListenerContainer(instance);
        initializeContainer(instance);
        return instance;
    }

乍一看，都是对MessageListenerContainer实例的初始化，实际上有一行，相当重要“ endpoint.setupListenerContainer(instance); ”，这一行最终是走到

AbstractRabbitListenerEndpoint.setupListenerContainer

public abstract class AbstractRabbitListenerEndpoint implements RabbitListenerEndpoint, BeanFactoryAware {
    ......
    
    // 设置MessageListenerContainer，最重要的就是设置监听的队列名称！！！
    @Override
    public void setupListenerContainer(MessageListenerContainer listenerContainer) {
        SimpleMessageListenerContainer container = (SimpleMessageListenerContainer) listenerContainer;
        boolean queuesEmpty = getQueues().isEmpty();
        boolean queueNamesEmpty = getQueueNames().isEmpty();
        if (!queuesEmpty && !queueNamesEmpty) {
            throw new IllegalStateException("Queues or queue names must be provided but not both for " + this);
        }
        if (queuesEmpty) {
            Collection<String> names = getQueueNames();
            container.setQueueNames(names.toArray(new String[names.size()]));
        }
        else {
            Collection<Queue> instances = getQueues();
            container.setQueues(instances.toArray(new Queue[instances.size()]));
        }
        container.setExclusive(isExclusive());
        if (getPriority() != null) {
            Map<String, Object> args = new HashMap<String, Object>();
            args.put("x-priority", getPriority());
            container.setConsumerArguments(args);
        }
        if (getAdmin() != null) {
            container.setRabbitAdmin(getAdmin());
        }
        setupMessageListener(listenerContainer);
    }
    
    // 创建MessageListener
    protected abstract MessageListener createMessageListener(MessageListenerContainer container);
    // 创建MessageListener，设置到MessageListenerContainer 里
    private void setupMessageListener(MessageListenerContainer container) {
        MessageListener messageListener = createMessageListener(container);
        Assert.state(messageListener != null, "Endpoint [" + this + "] must provide a non null message listener");
        container.setupMessageListener(messageListener);
    }
    ......
}

用@RabbitLinstener注解的方法，使用的endpoint是MethodRabbitListenerEndpoint继承自AbstractRabbitListenerEndpoint，所以看看AbstractRabbitListenerEndpoint的createMessageListener方法

public class MethodRabbitListenerEndpoint extends AbstractRabbitListenerEndpoint {
    ......
    @Override
    protected MessagingMessageListenerAdapter createMessageListener(MessageListenerContainer container) {
        Assert.state(this.messageHandlerMethodFactory != null,
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