本篇内容介绍了“spring自动注入AutowiredAnnotationBeanPostProcessor源码分析”的有关知识，在实际案例的操作过程中，不少人都会遇到这样的困境，接下来就让小编带领大家学习一下如何处理这些情况吧！希望大家仔细阅读，能够学有所成！
一、MergedBeanDefinitionPostProcessor
1.1、postProcessMergedBeanDefinition
在Bean属性赋值前，缓存属性字段上的@Autowired和@Value注解信息。
public void postProcessMergedBeanDefinition(RootBeanDefinition beanDefinition, Class<?> beanType, String beanName) {
//1.1.1 查询属性或方法上有@Value和@Autowired注解的元素
InjectionMetadata metadata = findAutowiringMetadata(beanName, beanType, null);
//1.1.2 检查元数据信息
metadata.checkConfigMembers(beanDefinition);
}1.1.1 findAutowiringMetadata 查询属性或方法上有@Value和@Autowired注解的元素
private InjectionMetadata findAutowiringMetadata(String beanName, Class<?> clazz, @Nullable PropertyValues pvs) {
// Fall back to class name as cache key, for backwards compatibility with custom callers.
//获取Bean名称作为缓存key
String cacheKey = (StringUtils.hasLength(beanName) ? beanName : clazz.getName());
// Quick check on the concurrent map first, with minimal locking.
//使用双重检查机制获取缓存
InjectionMetadata metadata = this.injectionMetadataCache.get(cacheKey);
//判断是否有元数据
if (InjectionMetadata.needsRefresh(metadata, clazz)) {
synchronized (this.injectionMetadataCache) {
metadata = this.injectionMetadataCache.get(cacheKey);
if (InjectionMetadata.needsRefresh(metadata, clazz)) {
if (metadata != null) {
metadata.clear(pvs);
}
//构建元数据
metadata = buildAutowiringMetadata(clazz);
this.injectionMetadataCache.put(cacheKey, metadata);
}
}
}
return metadata;
}这个 do-while 循环是用来一步一步往父类上爬的（可以看到这个循环体的最后一行是获取父类，判断条件是判断是否爬到了
Object）
private InjectionMetadata buildAutowiringMetadata(final Class<?> clazz) {
// 判断是不是候选者类，比如说类名是以 java.开头的则不是候选者类 Order的实现类也不是候选者类
// 但是如果this.autowiredAnnotationTypes 中有以java.开头的注解就返回true了
if (!AnnotationUtils.isCandidateClass(clazz, this.autowiredAnnotationTypes)) {
return InjectionMetadata.EMPTY;
}
List<InjectionMetadata.InjectedElement> elements = new ArrayList<>();
Class<?> targetClass = clazz;
do {
final List<InjectionMetadata.InjectedElement> currElements = new ArrayList<>();
// 循环获取类上的属性，如果类属性上有@Value和@Autowired包装成AutowiredFieldElement放入结果集
ReflectionUtils.doWithLocalFields(targetClass, field -> {
MergedAnnotation<?> ann = findAutowiredAnnotation(field);
if (ann != null) {
if (Modifier.isStatic(field.getModifiers())) {
if (logger.isInfoEnabled()) {
logger.info("Autowired annotation is not supported on static fields: " + field);
}
return;
}
boolean required = determineRequiredStatus(ann);
currElements.add(new AutowiredFieldElement(field, required));
}
});
// 循环获取类上的方法，如果类方法上有@Value和@Autowired包装成AutowiredMethodElement放入结果集
ReflectionUtils.doWithLocalMethods(targetClass, method -> {
Method bridgedMethod = BridgeMethodResolver.findBridgedMethod(method);
if (!BridgeMethodResolver.isVisibilityBridgeMethodPair(method, bridgedMethod)) {
return;
}
MergedAnnotation<?> ann = findAutowiredAnnotation(bridgedMethod);
if (ann != null && method.equals(ClassUtils.getMostSpecificMethod(method, clazz))) {
if (Modifier.isStatic(method.getModifiers())) {
if (logger.isInfoEnabled()) {
logger.info("Autowired annotation is not supported on static methods: " + method);
}
return;
}
if (method.getParameterCount() == 0) {
if (logger.isInfoEnabled()) {
logger.info("Autowired annotation should only be used on methods with parameters: " +
method);
}
}
boolean required = determineRequiredStatus(ann);
PropertyDescriptor pd = BeanUtils.findPropertyForMethod(bridgedMethod, clazz);
currElements.add(new AutowiredMethodElement(method, required, pd));
}
});
elements.addAll(0, currElements);
targetClass = targetClass.getSuperclass();
}
while (targetClass != null && targetClass != Object.class);
return InjectionMetadata.forElements(elements, clazz);
}1.1.2 检查元数据信息
检查是否有重复的元数据，去重处理，如一个属性上既有@Autowired注解，又有@Resource注解 。只使用一种方式进行注入，由于@Resource先进行解析，所以会选择@Resource的方式
public void checkConfigMembers(RootBeanDefinition beanDefinition) {
Set<InjectedElement> checkedElements = new LinkedHashSet<>(this.injectedElements.size());
for (InjectedElement element : this.injectedElements) {
Member member = element.getMember();
if (!beanDefinition.isExternallyManagedConfigMember(member)) {
beanDefinition.registerExternallyManagedConfigMember(member);
checkedElements.add(element);
if (logger.isTraceEnabled()) {
logger.trace("Registered injected element on class [" + this.targetClass.getName() + "]: " + element);
}
}
}
this.checkedElements = checkedElements;
}二、SmartInstantiationAwareBeanPostProcessor
2.1、determineCandidateConstructors
在bean实例化前选择@Autowired注解的构造函数，同时注入属性，从而完成自定义构造函数的选择。
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);
}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
注意：
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。