Spring Framework（十四）Spring Framework 获取 Bean 之一探究竟

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GetBeanByClass

    public <T> T getBean(Class<T> requiredType, Object... args) throws BeansException {// 1. 解析为namedBeanNamedBeanHolder<T> namedBean = resolveNamedBean(requiredType, args);if (namedBean != null) {return namedBean.getBeanInstance();}// 2. 获取父BeanFactory,然后从父BeanFactory获取BeanBeanFactory parent = getParentBeanFactory();if (parent != null) {return parent.getBean(requiredType, args);}// 3. 还是找不到的话，抛出异常。throw new NoSuchBeanDefinitionException(requiredType);}

在这里面可以看到实际的他和类加载机制不一样，而是优先从本地获取Bean。所以这里是可以简单的理解为，存在这么以下的场景的时候比较合适的，父工厂里面存放一些commonbean，然后子工厂可以根据自己的实际需求去覆盖。

下面详细看一下。

resolveNamedBean

private <T> NamedBeanHolder<T> resolveNamedBean(Class<T> requiredType, Object... args) throws BeansException {Assert.notNull(requiredType, "Required type must not be null");// 1. 获取候选者的bean名称String[] candidateNames = getBeanNamesForType(requiredType);// 2. if (candidateNames.length > 1) {List<String> autowireCandidates = new ArrayList<String>(candidateNames.length);for (String beanName : candidateNames) {if (!containsBeanDefinition(beanName) || getBeanDefinition(beanName).isAutowireCandidate()) {autowireCandidates.add(beanName);}}if (!autowireCandidates.isEmpty()) {candidateNames = autowireCandidates.toArray(new String[autowireCandidates.size()]);}}if (candidateNames.length == 1) {String beanName = candidateNames[0];return new NamedBeanHolder<T>(beanName, getBean(beanName, requiredType, args));}else if (candidateNames.length > 1) {Map<String, Object> candidates = new LinkedHashMap<String, Object>(candidateNames.length);for (String beanName : candidateNames) {if (containsSingleton(beanName)) {candidates.put(beanName, getBean(beanName, requiredType, args));}else {candidates.put(beanName, getType(beanName));}}String candidateName = determinePrimaryCandidate(candidates, requiredType);if (candidateName == null) {candidateName = determineHighestPriorityCandidate(candidates, requiredType);}if (candidateName != null) {Object beanInstance = candidates.get(candidateName);if (beanInstance instanceof Class) {beanInstance = getBean(candidateName, requiredType, args);}return new NamedBeanHolder<T>(candidateName, (T) beanInstance);}throw new NoUniqueBeanDefinitionException(requiredType, candidates.keySet());}return null;}

获取这个Class类型的所有的Bean名称数组。
如果候选者只有一个的话，直接返回。
如果存在多个Bean，是否指定了PrimaryBean（@Primary），就返回Primary，
否则继续按照优先级进行排序，还是搞不出来的话抛出异常NoUniqueBeanDefinitionException。
正常会返回一个NamedHBeanHodler,然后执行整个getBean流程。这个方式根据名称获取Bean的方法。所以这也是为什么叫做NamedBean了。

public class NamedBeanHolder<T> implements NamedBean {private final String beanName;private final T beanInstance;}

下面看看这个方法。

GetBeanByName

	@Overridepublic Object getBean(String name) throws BeansException {return doGetBean(name, null, null, false);}@Overridepublic <T> T getBean(String name, Class<T> requiredType) throws BeansException {return doGetBean(name, requiredType, null, false);}@Overridepublic Object getBean(String name, Object... args) throws BeansException {return doGetBean(name, null, args, false);}public <T> T getBean(String name, Class<T> requiredType, Object... args) throws BeansException {return doGetBean(name, requiredType, args, false);}

可以看到这里是直接调用了doGetBean方法。

doGetBean 真正执行Bean的方法

这里有个有意思的东西，在Spring里面在真正的执行逻辑的方法基本上都叫做doXxx。

protected <T> T doGetBean(final String name, final Class<T> requiredType, final Object[] args, boolean typeCheckOnly)throws BeansException {final String beanName = transformedBeanName(name);Object bean;// Eagerly check singleton cache for manually registered singletons.Object sharedInstance = getSingleton(beanName);if (sharedInstance != null && args == null) {if (logger.isDebugEnabled()) {if (isSingletonCurrentlyInCreation(beanName)) {logger.debug("Returning eagerly cached instance of singleton bean '" + beanName +"' that is not fully initialized yet - a consequence of a circular reference");}