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.ConfigurationClassPostProcessor$ImportAwareBeanPostProcessor
2. AutowiredAnnotationBeanPostProcessor
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
来源：oschina
链接：https://my.oschina.net/u/4346770/blog/4443793