Spring MVC 之 @Autowired

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版权声明：本文为博主原创文章，转载请注明出处，谢谢!

版权声明：本文为博主原创文章，转载请注明出处：http://blog.jerkybible.com/2015/05/21/Spring MVC 之 @Autowired/

访问原文「Spring MVC 之 @Autowired」

一、准备工作

POJO类Parent

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package com.test.mvc;
public class Parent {
}

Controller类HomeController

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@Controller
public class HomeController {
	@Autowired
	Parent parent;
	private static final Logger logger = LoggerFactory.getLogger(HomeController.class);
	/**
	 * Simply selects the home view to render by returning its name.
	 */
	@RequestMapping(value = "/", method = RequestMethod.GET)
	public String home(Locale locale, Model model) {
		logger.info("Welcome home! The client locale is {}. ", locale);
		logger.info(parent.toString());		
		Date date = new Date();
		DateFormat dateFormat = DateFormat.getDateTimeInstance(DateFormat.LONG, DateFormat.LONG, locale);		
		String formattedDate = dateFormat.format(date);		
		model.addAttribute("serverTime", formattedDate );
		return "home"; 
	}
}

root-context.xml

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<bean id="parent" class="com.test.mvc.Parent" ></bean>

web.xml略。

二、回忆

在Spring 使用简单Demo进行源码调试（六）这篇文章中介绍了bean参数的赋值，AutoWired也是在这个地方实现自动加载的。首先展开文章中的doCreateBean。这里省略了很多代码。首先看pplyMergedBeanDefinitionPostProcessors这个方法。

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protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final Object[] args) {
	// Instantiate the bean.
	BeanWrapper instanceWrapper = null;
	if (mbd.isSingleton()) {
		instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
	}
	//.......
        //.......
	synchronized (mbd.postProcessingLock) {
		if (!mbd.postProcessed) {
			applyMergedBeanDefinitionPostProcessors(mbd, beanType, beanName);
 			mbd.postProcessed = true;
		}
	}
	//......
        //......
	// Initialize the bean instance.
	Object exposedObject = bean;
	try {
		populateBean(beanName, mbd, instanceWrapper);
		if (exposedObject != null) {
			exposedObject = initializeBean(beanName, exposedObject, mbd);
		}
	}
	catch (Throwable ex) {
		if (ex instanceof BeanCreationException && beanName.equals(((BeanCreationException) ex).getBeanName())) {
			throw (BeanCreationException) ex;
		}
		else {
			throw new BeanCreationException(mbd.getResourceDescription(), beanName, "Initialization of bean failed", ex);
		}
	}
	//......
        //......
	// Register bean as disposable.
	try {
		registerDisposableBeanIfNecessary(beanName, bean, mbd);
	}
	catch (BeanDefinitionValidationException ex) {
		throw new BeanCreationException(mbd.getResourceDescription(), beanName, "Invalid destruction signature", ex);
	}
	return exposedObject;
}

断点运行到后去homeController的，后处理为AutowiredAnnotationBeanPostProcessor，如下图。
断点在homeController
展开AutowiredAnnotationBeanPostProcessor中的findAutowiringMetadata方法。这一步中的重点是在buildAutowiringMetadata，构建元数据。buildAutowiringMetadata首先去缓存里面找元数据，找不到就遍历bean的和父类的字段域和方法，如果别标记为@Autowired并且不是静态的就添加到InjectionMetadata中，并添加到缓存中。

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private InjectionMetadata findAutowiringMetadata(String beanName, Class<?> clazz, PropertyValues pvs) {
	// 回到使用类名作为缓存键,保持自定义命令向后兼容性
	String cacheKey = (StringUtils.hasLength(beanName) ? beanName : clazz.getName());
	InjectionMetadata metadata = this.injectionMetadataCache.get(cacheKey);
	if (InjectionMetadata.needsRefresh(metadata, clazz)) {
		synchronized (this.injectionMetadataCache) {
			metadata = this.injectionMetadataCache.get(cacheKey);
			if (InjectionMetadata.needsRefresh(metadata, clazz)) {
				if (metadata != null) {
					metadata.clear(pvs);
				}
				try {
                                        //构建元数据
					metadata = buildAutowiringMetadata(clazz);
					this.injectionMetadataCache.put(cacheKey, metadata);
				}
				catch (NoClassDefFoundError err) {
					throw new IllegalStateException("Failed to introspect bean class [" + clazz.getName() +
							"] for autowiring metadata: could not find class that it depends on", err);
				}
			}
		}
	}
	return metadata;
}

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private InjectionMetadata buildAutowiringMetadata(Class<?> clazz) {
	LinkedList<InjectionMetadata.InjectedElement> elements = new LinkedList<InjectionMetadata.InjectedElement>();
	Class<?> targetClass = clazz;
	do {//这里一个循环，因为要考虑父类的字段和方法
		LinkedList<InjectionMetadata.InjectedElement> currElements = new LinkedList<InjectionMetadata.InjectedElement>();
                //这里循环字段，找到被标记为@Autowired的字段
		for (Field field : targetClass.getDeclaredFields()) {
			AnnotationAttributes ann = findAutowiredAnnotation(field);
			if (ann != null) {
				if (Modifier.isStatic(field.getModifiers())) {
					if (logger.isWarnEnabled()) {
						logger.warn("Autowired annotation is not supported on static fields: " + field);
					}
					continue;
				}
				boolean required = determineRequiredStatus(ann);
                                //创建AutowiredFieldElement。
				currElements.add(new AutowiredFieldElement(field, required));
			}
		}
                 //这里循环方法
		for (Method method : targetClass.getDeclaredMethods()) {
			Method bridgedMethod = BridgeMethodResolver.findBridgedMethod(method);
			if (!BridgeMethodResolver.isVisibilityBridgeMethodPair(method, bridgedMethod)) {
				continue;
			}
			AnnotationAttributes ann = findAutowiredAnnotation(bridgedMethod);
			if (ann != null && method.equals(ClassUtils.getMostSpecificMethod(method, clazz))) {
				if (Modifier.isStatic(method.getModifiers())) {
					if (logger.isWarnEnabled()) {
						logger.warn("Autowired annotation is not supported on static methods: " + method);
					}
					continue;
				}
				if (method.getParameterTypes().length == 0) {
					if (logger.isWarnEnabled()) {
						logger.warn("Autowired annotation should be used on methods with actual parameters: " + method);
					}
				}
				boolean required = determineRequiredStatus(ann);
				PropertyDescriptor pd = BeanUtils.findPropertyForMethod(bridgedMethod, clazz);
                                //创建AutowiredMethodElement。
				currElements.add(new AutowiredMethodElement(method, required, pd));
			}
		}
		elements.addAll(0, currElements);
		targetClass = targetClass.getSuperclass();
	}
	while (targetClass != null && targetClass != Object.class);
        //将InjectionElement集合添加到新建的InjectionMetadata中。
	return new InjectionMetadata(clazz, elements);
}

三、自动装载

代码运行到populateBean方法。这个方法太长，这里就不贴出来了，直接走到postProcessPropertyValues方法,获得InjectionMetadata对象后便遍历其中的所有InjectionElement对象，调用其中的inject方法。接着对每一个injectedElement进行inject。最后的inject进行真正的属性赋值。

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public PropertyValues postProcessPropertyValues(
		PropertyValues pvs, PropertyDescriptor[] pds, Object bean, String beanName) throws BeansException {
	InjectionMetadata metadata = findAutowiringMetadata(beanName, bean.getClass(), pvs);
	try {
		metadata.inject(bean, beanName, pvs);
	}
	catch (Throwable ex) {
		throw new BeanCreationException(beanName, "Injection of autowired dependencies failed", ex);
	}
	return pvs;
}

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public void inject(Object target, String beanName, PropertyValues pvs) throws Throwable {
	Collection<InjectedElement> elementsToIterate =
			(this.checkedElements != null ? this.checkedElements : this.injectedElements);
	if (!elementsToIterate.isEmpty()) {
		boolean debug = logger.isDebugEnabled();
		for (InjectedElement element : elementsToIterate) {
			if (debug) {
				logger.debug("Processing injected element of bean '" + beanName + "': " + element);
			}
			element.inject(target, beanName, pvs);
		}
	}
}
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protected void inject(Object bean, String beanName, PropertyValues pvs) throws Throwable {
		Field field = (Field) this.member;
		try {
			Object value;
			if (this.cached) {
				value = resolvedCachedArgument(beanName, this.cachedFieldValue);
			}
			else {
				DependencyDescriptor desc = new DependencyDescriptor(field, this.required);
				desc.setContainingClass(bean.getClass());
				Set<String> autowiredBeanNames = new LinkedHashSet<String>(1);
				TypeConverter typeConverter = beanFactory.getTypeConverter();
                                //获取真正的属性值。
				value = beanFactory.resolveDependency(desc, beanName, autowiredBeanNames, typeConverter);
				synchronized (this) {
					if (!this.cached) {
						if (value != null || this.required) {
							this.cachedFieldValue = desc;
							registerDependentBeans(beanName, autowiredBeanNames);
							if (autowiredBeanNames.size() == 1) {
								String autowiredBeanName = autowiredBeanNames.iterator().next();
								if (beanFactory.containsBean(autowiredBeanName)) {
									if (beanFactory.isTypeMatch(autowiredBeanName, field.getType())) {
										this.cachedFieldValue = new RuntimeBeanReference(autowiredBeanName);
									}
								}
							}
						}
						else {
							this.cachedFieldValue = null;
						}
						this.cached = true;
					}
				}
			}
			if (value != null) {
				ReflectionUtils.makeAccessible(field);
                                //最终赋值
				field.set(bean, value);
			}
		}
		catch (Throwable ex) {
			throw new BeanCreationException("Could not autowire field: " + field, ex);
		}
	}
}

看一下resolveDependency。 这个方法其实就是根据类型到bean工厂中查找类型匹配的bean实例，然后就看到了这几个条件分支语句，如果是数组，集合，映射表，自定义类型都执行了差不多的操作，findAutowireCandidate方法。这个方法会去工厂中执行类型匹配的查找，将匹配的结果集返回，不同的是，集合数组类型会通过TypeConverter进行结果的转换。

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public Object doResolveDependency(DependencyDescriptor descriptor, String beanName,
	Set<String> autowiredBeanNames, TypeConverter typeConverter) throws BeansException {
	Class<?> type = descriptor.getDependencyType();
	Object value = getAutowireCandidateResolver().getSuggestedValue(descriptor);
	if (value != null) {
		if (value instanceof String) {
			String strVal = resolveEmbeddedValue((String) value);
			BeanDefinition bd = (beanName != null && containsBean(beanName) ? getMergedBeanDefinition(beanName) : null);
			value = evaluateBeanDefinitionString(strVal, bd);
		}
		TypeConverter converter = (typeConverter != null ? typeConverter : getTypeConverter());
		return (descriptor.getField() != null ?
				converter.convertIfNecessary(value, type, descriptor.getField()) :
				converter.convertIfNecessary(value, type, descriptor.getMethodParameter()));
	}
	if (type.isArray()) {
                //如果属性类型是数组
		//......
		return result;
	}
	else if (Collection.class.isAssignableFrom(type) && type.isInterface()) {
                //如果属性是集合，并且是接口
		//......
		return result;
	}
	else if (Map.class.isAssignableFrom(type) && type.isInterface()) {
                //如果属性是Map并且是接口
		//......
		return matchingBeans;
	}
	else {//自定义类型
		Map<String, Object> matchingBeans = findAutowireCandidates(beanName, type, descriptor);
		if (matchingBeans.isEmpty()) {
			if (descriptor.isRequired()) {
				raiseNoSuchBeanDefinitionException(type, "", descriptor);
			}
			return null;
		}
		if (matchingBeans.size() > 1) {
			String primaryBeanName = determineAutowireCandidate(matchingBeans, descriptor);
			if (primaryBeanName == null) {
				throw new NoUniqueBeanDefinitionException(type, matchingBeans.keySet());
			}
			if (autowiredBeanNames != null) {
				autowiredBeanNames.add(primaryBeanName);
			}
			return matchingBeans.get(primaryBeanName);
		}
		// We have exactly one match.
		Map.Entry<String, Object> entry = matchingBeans.entrySet().iterator().next();
		if (autowiredBeanNames != null) {
			autowiredBeanNames.add(entry.getKey());
		}
		return entry.getValue();
	}
}

到此为止，找到了属性的匹配值，然后反射赋值就完成了整个的自动装配过程。可以看出，@Autowired是通过类型来进行自动装配的。

上面是属性的赋值过程也就是InjectionFieldElement的inject方法，InjectionMethodElement的inject方法大致相同只是对每一个方法参数执行一次resolveDependency方法来获取参数值，然后反射执行方法。

到此为止，整个实例化和装配过程也就讲完了，我们总结一下：

1）一切都是从bean工厂的getBean方法开始的，一旦该方法调用总会返回一个bean实例，无论当前是否存在，不存在就实例化一个并装配，否则直接返回。

2）实例化和装配过程中会多次递归调用getBean方法来解决类之间的依赖。

3）Spring几乎考虑了所有可能性，所以方法特别复杂但完整有条理。

4）@Autowired最终是根据类型来查找和装配元素的，但是我们设置了后会影响最终的类型匹配查找。因为在前面有根据BeanDefinition的autowire类型设置PropertyValue值得一步，其中会有新实例的创建和注册。就是那个autowireByName方法。

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