spring在创建对象(org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory#createBeanInstance)的时候, 使用了这个 构造函数后置处理器, 用来选择使用哪个构造函数的.
所以这个后置处理器的执行时机是: 对象实例化之前执行
org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory#determineConstructorsFromBeanPostProcessors
@Nullable protected Constructor<?>[] determineConstructorsFromBeanPostProcessors(@Nullable Class<?> beanClass, String beanName) throws BeansException { if (beanClass != null && hasInstantiationAwareBeanPostProcessors()) { for (BeanPostProcessor bp : getBeanPostProcessors()) { if (bp instanceof SmartInstantiationAwareBeanPostProcessor) { SmartInstantiationAwareBeanPostProcessor ibp = (SmartInstantiationAwareBeanPostProcessor) bp; Constructor<?>[] ctors = ibp.determineCandidateConstructors(beanClass, beanName); if (ctors != null) { return ctors; } } } } return null; }
这里满足条件的后置处理器, 就两个:
1.
2.
ImportAwareBeanPostProcessor
@Override @Nullable public Constructor<?>[] determineCandidateConstructors(Class<?> beanClass, String beanName) throws BeansException { return null; }
这个类并没有实现这个接口, 而是父类去实现的:
org.springframework.beans.factory.config.InstantiationAwareBeanPostProcessorAdapter#determineCandidateConstructors
然而, 他也是 啥也没干.
怎么感觉这个后置处理器, 哪哪都有他, 却哪哪都不干活.
AutowiredAnnotationBeanPostProcessor
@Override @Nullable public Constructor<?>[] determineCandidateConstructors(Class<?> beanClass, final String beanName) throws BeanCreationException { // Let's check for lookup methods here... if (!this.lookupMethodsChecked.contains(beanName)) { try { ReflectionUtils.doWithMethods(beanClass, method -> { Lookup lookup = method.getAnnotation(Lookup.class); if (lookup != null) { Assert.state(this.beanFactory != null, "No BeanFactory available"); LookupOverride override = new LookupOverride(method, lookup.value()); try { RootBeanDefinition mbd = (RootBeanDefinition) this.beanFactory.getMergedBeanDefinition(beanName); mbd.getMethodOverrides().addOverride(override); } catch (NoSuchBeanDefinitionException ex) { throw new BeanCreationException(beanName, "Cannot apply @Lookup to beans without corresponding bean definition"); } } }); } catch (IllegalStateException ex) { throw new BeanCreationException(beanName, "Lookup method resolution failed", ex); } this.lookupMethodsChecked.add(beanName); } // Quick check on the concurrent map first, with minimal locking. Constructor<?>[] candidateConstructors = this.candidateConstructorsCache.get(beanClass); if (candidateConstructors == null) { // Fully synchronized resolution now... synchronized (this.candidateConstructorsCache) { candidateConstructors = this.candidateConstructorsCache.get(beanClass); if (candidateConstructors == null) { Constructor<?>[] rawCandidates; try { //反射获取所有构造函数 rawCandidates = beanClass.getDeclaredConstructors(); } catch (Throwable ex) { throw new BeanCreationException(beanName, "Resolution of declared constructors on bean Class [" + beanClass.getName() + "] from ClassLoader [" + beanClass.getClassLoader() + "] failed", ex); } //候选构造方法 List<Constructor<?>> candidates = new ArrayList<>(rawCandidates.length); Constructor<?> requiredConstructor = null; Constructor<?> defaultConstructor = null; //这个貌似是 Kotlin 上用的, 不用管它 Constructor<?> primaryConstructor = BeanUtils.findPrimaryConstructor(beanClass); int nonSyntheticConstructors = 0; //遍历这些构造函数 for (Constructor<?> candidate : rawCandidates) { //判断构造方法是否是合成的 if (!candidate.isSynthetic()) { nonSyntheticConstructors++; } else if (primaryConstructor != null) { continue; } //查看是否有 @Autowired 注解 //如果有多个构造方法, 可以通过标注 @Autowired 的方式来指定使用哪个构造方法 AnnotationAttributes ann = findAutowiredAnnotation(candidate); if (ann == null) { Class<?> userClass = ClassUtils.getUserClass(beanClass); if (userClass != beanClass) { try { Constructor<?> superCtor = userClass.getDeclaredConstructor(candidate.getParameterTypes()); ann = findAutowiredAnnotation(superCtor); } catch (NoSuchMethodException ex) { // Simply proceed, no equivalent superclass constructor found... } } } //有 @Autowired 的情况 if (ann != null) { if (requiredConstructor != null) { throw new BeanCreationException(beanName, "Invalid autowire-marked constructor: " + candidate + ". Found constructor with 'required' Autowired annotation already: " + requiredConstructor); } boolean required = determineRequiredStatus(ann); if (required) { if (!candidates.isEmpty()) { throw new BeanCreationException(beanName, "Invalid autowire-marked constructors: " + candidates + ". Found constructor with 'required' Autowired annotation: " + candidate); } requiredConstructor = candidate; } candidates.add(candidate); } //无参构造函数的情况 else if (candidate.getParameterCount() == 0) { //构造函数没有参数, 则设置为默认的构造函数 defaultConstructor = candidate; } } //到这里, 已经循环完了所有的构造方法 //候选者不为空时 if (!candidates.isEmpty()) { // Add default constructor to list of optional constructors, as fallback. if (requiredConstructor == null) { if (defaultConstructor != null) { candidates.add(defaultConstructor); } else if (candidates.size() == 1 && logger.isInfoEnabled()) { logger.info("Inconsistent constructor declaration on bean with name '" + beanName + "': single autowire-marked constructor flagged as optional - " + "this constructor is effectively required since there is no " + "default constructor to fall back to: " + candidates.get(0)); } } candidateConstructors = candidates.toArray(new Constructor<?>[0]); } //类的构造方法只有1个, 且该构造方法有多个参数 else if (rawCandidates.length == 1 && rawCandidates[0].getParameterCount() > 0) { candidateConstructors = new Constructor<?>[] {rawCandidates[0]}; } //这里不会进, 因为 primaryConstructor = null else if (nonSyntheticConstructors == 2 && primaryConstructor != null && defaultConstructor != null && !primaryConstructor.equals(defaultConstructor)) { candidateConstructors = new Constructor<?>[] {primaryConstructor, defaultConstructor}; } //这里也不会进, 因为 primaryConstructor = null else if (nonSyntheticConstructors == 1 && primaryConstructor != null) { candidateConstructors = new Constructor<?>[] {primaryConstructor}; } else { //如果方法进了这里, 就是没找到合适的构造方法 //1. 类定义了多个构造方法, 且没有 @Autowired , 则有可能会进这里 candidateConstructors = new Constructor<?>[0]; } this.candidateConstructorsCache.put(beanClass, candidateConstructors); } } }
//这里如果没找到, 则会返回 null, 而不会返回空数组 return (candidateConstructors.length > 0 ? candidateConstructors : null); }
这个方法比较长, 但是仔细看, 是能看懂的. 大致可以划分为几个步骤:
1. 获取类的所有构造方法
2. 遍历构造方法
|-> 只有一个无参构造方法, 则返回null
|-> 只有一个有参构造方法, 则返回这个构造方法
|-> 有多个构造方法且没有@Autowired, 此时spring则会蒙圈了, 不知道使用哪一个了. 这里的后置处理器, 翻译过来, 叫智能选择构造方法后置处理器.
当选择不了的时候, 干脆返回 null
|-> 有多个构造方法, 且在其中一个方法上标注了 @Autowired , 则会返回这个标注的构造方法
|-> 有多个构造方法, 且在多个方法上标注了@Autowired, 则spring会抛出异常, spring会认为, 你指定了几个给我, 是不是你弄错了
注:
这地方有个问题需要注意一下, 如果你写了多个构造方法, 且没有写 无参构造方法, 那么此处返回null,
在回到 createBeanInstance 方法中, 如果不能走 autowireConstructor(), 而走到 instantiateBean() 中去的话, 会报错的.
因为类已经没有无参构造函数了