探索Spring - @Autowired注入原理分析
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疑问：
- @Autowired什么时候注入？
- @Autowired怎么注入？
- 运行流程是什么？
开始探索:
@Autowired实现类:
先看关于@Autowired的三张图片：
- 根据@Autowired的@see提示找到AutowiredAnnotationBeanPostProcessor类，从注释可以看出是BeanPostProcessor是实现了自动注入的关键。是通过@Autowired跟@Value来检测哪些需要要注入。
- 该类继承了【InstantiationAwareBeanPostProcessorAdapter】
实现了【MergedBeanDefinitionPostProcessor】
这俩类先不进行分析，多读几遍，在refresh()中的registerBeanPostProcessors()丶finishBeanFactoryInitialization()中会用到这两个类
refresh()方法：
IOC的核心方法不用说就是refresh()了，我们就从这个方法开始入手(如果不知道refresh()方法，建议先去了解一下)。
标红的方法为实现@Autowired自动注入的关键方法：registerBeanPostProcessors与finishBeanFactoryInitialization下面对这俩方法进行具体分析。
registerBeanPostProcessors:
看方法名是不是很熟悉？之前@Autowired的实现类(AutowiredAnnotationBeanPostProcessor)就是实现了BeanPostProcessor，我们@Autowired的实现类将在这里进行注册，是在初始化所有bean之前执行。
【PostProcessorRegistrationDelegate类】
public static void registerBeanPostProcessors(
ConfigurableListableBeanFactory beanFactory, AbstractApplicationContext applicationContext) {
// 1.找到BeanPostProcessor的子类
String[] postProcessorNames = beanFactory.getBeanNamesForType(BeanPostProcessor.class, true, false);
...(其它代码省略)
// 2.从子类postProcessorNames里找到实现MergedBeanDefinitionPostProcessor的子类，其中就包含AutowiredAnnotationBeanPostProcessor类。
List internalPostProcessors = new ArrayList<>();
for (String ppName : postProcessorNames) {
if (beanFactory.isTypeMatch(ppName, PriorityOrdered.class)) {
BeanPostProcessor pp = beanFactory.getBean(ppName, BeanPostProcessor.class);
priorityOrderedPostProcessors.add(pp);
if (pp instanceof MergedBeanDefinitionPostProcessor) {
internalPostProcessors.add(pp);
}
}
else if (beanFactory.isTypeMatch(ppName, Ordered.class)) {
orderedPostProcessorNames.add(ppName);
}
else {
nonOrderedPostProcessorNames.add(ppName);
}
}
...
// 3.在这被注册
sortPostProcessors(internalPostProcessors, beanFactory);
registerBeanPostProcessors(beanFactory, internalPostProcessors);
}
// 4. registerBeanPostProcessors方法调到addBeanPostProcessor方法
【AbstractBeanFactory类】
public void addBeanPostProcessor(BeanPostProcessor beanPostProcessor) {
...
// 5.将BeanPostProcessor放到beanPostProcessors中，供后续程序调用。
this.beanPostProcessors.add(beanPostProcessor);
}
以上就是注册AutowiredAnnotationBeanPostProcessor的流程：
1.找到BeanPostProcessor的子类
2.找到MergedBeanDefinitionPostProcessor子类
3.放到beanPostProcessors中供后续使用
finishBeanFactoryInitialization：
【AbstractApplicationContext类】
protected void finishBeanFactoryInitialization(ConfigurableListableBeanFactory beanFactory) {
...
// 1. 实例化所有非懒加载的单例
beanFactory.preInstantiateSingletons();
}
【DefaultListableBeanFactory类】
public void preInstantiateSingletons() throws BeansException {
// 2.获取beanName的集合，beanDefinitionNames是在refresh()中的invokeBeanFactoryPostProcessors()中定义好的。
List beanNames = new ArrayList<>(this.beanDefinitionNames);
for (String beanName : beanNames) {
RootBeanDefinition bd = getMergedLocalBeanDefinition(beanName);
if (!bd.isAbstract() && bd.isSingleton() && !bd.isLazyInit()) {
...
}
else {
// 3. 根据beanName获取bean
getBean(beanName);
}
}
}
...
}
// 4. 通过getBean() -> doGetBean()
【AbstractBeanFactory类】
protected T doGetBean(final String name, @Nullable final Class requiredType, @Nullable final Object[] args, boolean typeCheckOnly) throws BeansException {
...
try {
...
if (mbd.isSingleton()) {
// 5. 通过beanName去缓存里找,有就返回，没有就执行第6步
sharedInstance = getSingleton(beanName, () -> {
try {
// 6. 执行createBean，创建bean
return createBean(beanName, mbd, args);
}
catch (BeansException ex) {
...
}
...
}
}
catch (BeansException ex) {
...
}
return (T) bean;
}
【AbstractAutowireCapableBeanFactory类】
protected Object createBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args)throws BeanCreationException {
try {
...
// 7. 真正创建bean的方法
Object beanInstance = doCreateBean(beanName, mbdToUse, args);
...
return beanInstance;
}
}
protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final @Nullable Object[] args) throws BeanCreationException {
...
synchronized (mbd.postProcessingLock) {
if (!mbd.postProcessed) {
try {
// 8. 这个方法会调用到AutowiredAnnotationBeanPostProcessor，这里并不会进行注入，只是定义哪些类的哪些属性需要注入。
applyMergedBeanDefinitionPostProcessors(mbd, beanType, beanName);
}
catch (Throwable ex) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName,
"Post-processing of merged bean definition failed", ex);
}
mbd.postProcessed = true;
}
}
...
try {
// 9. 填充Bean
populateBean(beanName, mbd, instanceWrapper);
// 10. 实例化Bean
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);
}
}
关键的方法是第8步跟第9步
分析applyMergedBeanDefinitionPostProcessors：
【AutowiredAnnotationBeanPostProcessor】类
public void postProcessMergedBeanDefinition(RootBeanDefinition beanDefinition, Class beanType, String beanName) {
// 找到需要需要的属性，并封装成InjectMetadata
InjectionMetadata metadata = findAutowiringMetadata(beanName, beanType, null);
// 这里会对InjectionMetadata的checkedElements属性赋值，之后在注入的时候会用到
metadata.checkConfigMembers(beanDefinition);
}
private InjectionMetadata findAutowiringMetadata(String beanName, Class clazz, @Nullable PropertyValues pvs) {
// Fall back to class name as cache key, for backwards compatibility with custom callers.
String cacheKey = (StringUtils.hasLength(beanName) ? beanName : clazz.getName());
// Quick check on the concurrent map first, with minimal locking.
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);
}
// 创建InjectionMetadata实例
metadata = buildAutowiringMetadata(clazz);
this.injectionMetadataCache.put(cacheKey, metadata);
}
}
}
return metadata;
}
private InjectionMetadata buildAutowiringMetadata(final Class clazz) {
List elements = new ArrayList<>();
Class targetClass = clazz;
// do while获取需要注入的属性，封装成InjectionMetadata
do {
final List currElements = new ArrayList<>();
ReflectionUtils.doWithLocalFields(targetClass, field -> {
// 这里会找到被@Autowired 跟@Value标注的字段
AnnotationAttributes 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);
// 将对应的field封装到InjectedElement的Member上，
之后会用到filed进行属性注入
currElements.add(new AutowiredFieldElement(field, required));
}
});
ReflectionUtils.doWithLocalMethods(targetClass, method -> {
Method bridgedMethod = BridgeMethodResolver.findBridgedMethod(method);
if (!BridgeMethodResolver.isVisibilityBridgeMethodPair(method, bridgedMethod)) {
return;
}
AnnotationAttributes 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 new InjectionMetadata(clazz, elements);
}
applyMergedBeanDefinitionPostProcessors方法：
1.循环beanPostProcessors(registerBeanPostProcessors中注册的)，找到实现了MergedBeanDefinitionPostProcessor的类，然后调用postProcessMergedBeanDefinition方法
2.postProcessMergedBeanDefinition方法中的findAutowiringMetadata方法，会找到被@Autowired修饰的字段，封装成InjectionMetadata，放到缓存中。
分析populateBean：
【AbstractAutowireCapableBeanFactory类】
protected void populateBean(String beanName, RootBeanDefinition mbd, @Nullable BeanWrapper bw) {
...
if (hasInstAwareBpps) {
if (pvs == null) {
pvs = mbd.getPropertyValues();
}
for (BeanPostProcessor bp : getBeanPostProcessors()) {
if (bp instanceof InstantiationAwareBeanPostProcessor) {
InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) bp;
PropertyValues pvsToUse = ibp.postProcessProperties(pvs, bw.getWrappedInstance(), beanName);
if (pvsToUse == null) {
if (filteredPds == null) {
filteredPds = filterPropertyDescriptorsForDependencyCheck(bw, mbd.allowCaching);
}
// 这里进行属性注入
pvsToUse = ibp.postProcessPropertyValues(pvs, filteredPds, bw.getWrappedInstance(), beanName);
if (pvsToUse == null) {
return;
}
}
pvs = pvsToUse;
}
}
}
...
}
【AutowiredAnnotationBeanPostProcessor类】
public PropertyValues postProcessProperties(PropertyValues pvs, Object bean, String beanName) {
// 在applyMergedBeanDefinitionPostProcessors中已经将封装好的InjectionMetadata
放到了缓存里，这里就直接从缓存里取值
InjectionMetadata metadata = findAutowiringMetadata(beanName, bean.getClass(), pvs);
try {
// 开始注入
metadata.inject(bean, beanName, pvs);
}
catch (BeanCreationException ex) {
throw ex;
}
catch (Throwable ex) {
throw new BeanCreationException(beanName, "Injection of autowired dependencies failed", ex);
}
return pvs;
}
【InjectionMetadata】类：
public void inject(Object target, @Nullable String beanName, @Nullable PropertyValues pvs) throws Throwable {
// 这个checkedElements就是在postProcessMergedBeanDefinition中进行赋值的
Collection checkedElements = this.checkedElements;
Collection elementsToIterate =
(checkedElements != null ? checkedElements : this.injectedElements);
if (!elementsToIterate.isEmpty()) {
for (InjectedElement element : elementsToIterate) {
if (logger.isTraceEnabled()) {
logger.trace("Processing injected element of bean '" + beanName + "': " + element);
}
// 对每个需要注入的字段进行赋值
element.inject(target, beanName, pvs);
}
}
}
【AutowiredAnnotationBeanPostProcessor】类
protected void inject(Object bean, @Nullable String beanName, @Nullable PropertyValues pvs) throws Throwable {
Field field = (Field) this.member;
Object value;
if (this.cached) {
value = resolvedCachedArgument(beanName, this.cachedFieldValue);
}
else {
// 根据field封装成DependencyDescriptor
DependencyDescriptor desc = new DependencyDescriptor(field, this.required);
desc.setContainingClass(bean.getClass());
Set autowiredBeanNames = new LinkedHashSet<>(1);
Assert.state(beanFactory != null, "No BeanFactory available");
TypeConverter typeConverter = beanFactory.getTypeConverter();
try {
// 根据DependencyDescriptor中getDependencyType的class类型去beanFactory获取实例
value = beanFactory.resolveDependency(desc, beanName, autowiredBeanNames, typeConverter);
}
catch (BeansException ex) {
throw new UnsatisfiedDependencyException(null, beanName, new InjectionPoint(field), ex);
}
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) &&
beanFactory.isTypeMatch(autowiredBeanName, field.getType())) {
this.cachedFieldValue = new ShortcutDependencyDescriptor(
desc, autowiredBeanName, field.getType());
}
}
}
else {
this.cachedFieldValue = null;
}
this.cached = true;
}
}
}
if (value != null) {
// 最后一步，利用反射，完成属性值的注入！
ReflectionUtils.makeAccessible(field);
field.set(bean, value);
}
}
}
populateBean方法：
1.回调到AutowiredAnnotationBeanPostProcessor中的postProcessProperties方法
2.获取之前定义的InjectionMetadata对象，获取到里面封装的InjectedElement
3.从工厂获取到InjectedElement里需要注入的实例对象
3.利用反射对字段进行赋值
总结：
1. @Autowired什么时候注入？
项目启动后调用容器的refresh()方法，当运行完finishBeanFactoryInitialization()后完成的注入。
2. @Autowired怎么注入？
从容器中获取到需要注入的实例，使用反射完成的注入
3.运行流程？
循环注册每个bean的时候，会看这个bean是否有需要依赖注入的属性，如果有则从容器中获取到需要注入的属性，然后通过反射进行赋值。