基于springboot bean的实例化过程和属性注入过程
作者：leileibest_437147623
这篇文章主要介绍了基于springboot bean的实例化过程和属性注入过程，具有很好的参考价值，希望对大家有所帮助。如有错误或未考虑完全的地方，望不吝赐教
bean的实例化过程和属性注入过程
了解过springboot的加载流程的都知道springboot初始化bean都在refresh方法中。
这个方法代码如下：
// Prepare this context for refreshing.
prepareRefresh();
// Tell the subclass to refresh the internal bean factory.新建beanFactory
ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();
// Prepare the bean factory for use in this context.
prepareBeanFactory(beanFactory);
try {
// Allows post-processing of the bean factory in context subclasses.
postProcessBeanFactory(beanFactory);
// Invoke factory processors registered as beans in the context.
// 加载实现beanFactoryPostProcessor的bean，bean定义的时候执行
invokeBeanFactoryPostProcessors(beanFactory);
// Register bean processors that intercept bean creation.
// 加载实现了beanPostProcessor，在bean实例化前、后执行
registerBeanPostProcessors(beanFactory);
// Initialize message source for this context.
initMessageSource();
// Initialize event multicaster for this context.
initApplicationEventMulticaster();
// Initialize other special beans in specific context subclasses.
onRefresh();
// Check for listener beans and register them.
registerListeners();
// Instantiate all remaining (non-lazy-init) singletons.
//实例化非懒加载的bean、bean封装、属性注入、注解注入(主要使用BeanPostProcessor或子类实现)等
finishBeanFactoryInitialization(beanFactory);
// Last step: publish corresponding event.
finishRefresh();
这里我们主要看下finishBeanfactoryInitialization方法。此方法实现了bean的实例和属性注入等。进入此方法的最后一行。
// Instantiate all remaining (non-lazy-init) singletons. beanFactory.preInstantiateSingletons();
注释写的很清楚，实例化剩余非懒加载的bean。此方法的实现是核心类DefaultLisListableBeanFactory实现。这个方法中有个判断：bean是否是beanFactory的实现类。
如果是则获取bean的时候会从beanFactory实现类的getObject中获取，我们重点看看getBean这个方法。
getBean是spring中最重要、最牛逼的方法之一，具体的逻辑是通过doGetBean方法处理的。
我们看下doGetBean方法，方法很长。我们分成几个部分去看。
1、先判断是否已经存在缓存中，代码如下：
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");
}
else {
logger.debug("Returning cached instance of singleton bean '" + beanName + "'");
}
}
bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
2、从父beanfactory中获取
BeanFactory parentBeanFactory = getParentBeanFactory();
if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {
// Not found -> check parent.
String nameToLookup = originalBeanName(name);
if (parentBeanFactory instanceof AbstractBeanFactory) {
return ((AbstractBeanFactory) parentBeanFactory).doGetBean(
nameToLookup, requiredType, args, typeCheckOnly);
}
else if (args != null) {
// Delegation to parent with explicit args.
return (T) parentBeanFactory.getBean(nameToLookup, args);
}
else {
// No args -> delegate to standard getBean method.
return parentBeanFactory.getBean(nameToLookup, requiredType);
}
}
3、直接创建RootBeanDefinition
//mark 给此bean 马克一下。防止重复创建
if (!typeCheckOnly) {
markBeanAsCreated(beanName);
}
try {
final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
checkMergedBeanDefinition(mbd, beanName, args);
// Guarantee initialization of beans that the current bean depends on.
String[] dependsOn = mbd.getDependsOn();
if (dependsOn != null) {
for (String dep : dependsOn) {
if (isDependent(beanName, dep)) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName,
"Circular depends-on relationship between '" + beanName + "' and '" + dep + "'");
}
registerDependentBean(dep, beanName);
getBean(dep);
}
}
4、是singleton还是prototype类型的，根据不同类型去实例化bean,代码只贴了单例的类型：
// Create bean instance.
if (mbd.isSingleton()) {
sharedInstance = getSingleton(beanName, () -> {
try {
return createBean(beanName, mbd, args);
}
catch (BeansException ex) {
// Explicitly remove instance from singleton cache: It might have been put there
// eagerly by the creation process, to allow for circular reference resolution.
// Also remove any beans that received a temporary reference to the bean.
destroySingleton(beanName);
throw ex;
}
});
bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
5、检查bean的类型是否匹配
// Check if required type matches the type of the actual bean instance.
if (requiredType != null && !requiredType.isInstance(bean)) {
try {
T convertedBean = getTypeConverter().convertIfNecessary(bean, requiredType);
if (convertedBean == null) {
throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
}
return convertedBean;
}
catch (TypeMismatchException ex) {
if (logger.isDebugEnabled()) {
logger.debug("Failed to convert bean '" + name + "' to required type '" +
ClassUtils.getQualifiedName(requiredType) + "'", ex);
}
throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
}
}
整个doGetBean方法改成这5个部分，重点看下第4个部分中的createBean和getObjectForBeanInstance方法。
1、createBean方法，里面主要是2个部分,bean直接是通过BeanPostProcessor的postProcessBeforeIntantiation方法获取的。
注释也是描述的很清楚：Give BeanPostProcessors a chance to return a proxy instead of the bean instance(英语渣渣不敢瞎翻译误导大家，理解就好)
代码如下：
try {
// Give BeanPostProcessors a chance to return a proxy instead of the target bean instance.
Object bean = resolveBeforeInstantiation(beanName, mbdToUse);
if (bean != null) {
return bean;
}
}
catch (Throwable ex) {
throw new BeanCreationException(mbdToUse.getResourceDescription(), beanName,
"BeanPostProcessor before instantiation of bean failed", ex);
}
第一个部分自己实现，那么createBean方法第2个部分毋庸置疑肯定是spring去实例化bean，代码如下：
try {
Object beanInstance = doCreateBean(beanName, mbdToUse, args);
if (logger.isDebugEnabled()) {
logger.debug("Finished creating instance of bean '" + beanName + "'");
}
return beanInstance;
}
又是do开头的方法，说明这个也是一个创建方法，spring中一般do开头的都是用于创建某个对象。跟着代码走，看下doCreateBean方法，在查看此方法之前，可能需要了解下BeanWarpper这个封装类。bean的封装、属性注入等都是用BeanWarpper去完成的。
看下代码：
// Instantiate the bean.
BeanWrapper instanceWrapper = null;
if (mbd.isSingleton()) {
instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
}
if (instanceWrapper == null) {
instanceWrapper = createBeanInstance(beanName, mbd, args);
}
进入createBeanInstance方法中，就是调用构造器去实例化bean，返回beanWrapper。通过构造器获取实例代码如下：
// Need to determine the constructor...
Constructor<?>[] ctors = determineConstructorsFromBeanPostProcessors(beanClass, beanName);
if (ctors != null ||
mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_CONSTRUCTOR ||
mbd.hasConstructorArgumentValues() || !ObjectUtils.isEmpty(args)) {
return autowireConstructor(beanName, mbd, ctors, args);
}
// No special handling: simply use no-arg constructor.
return instantiateBean(beanName, mbd);
这里实例化我一开始以为就是简单的反射，后面我点进去时候发现里面一直提策略实例化，我目前了解的是可以解决Override的问题等。如果有兴趣的可以自行查看。到这里为止。我们终于实例化bean了。下面看下第二步bean的属性注入等。
代码如下：
Object exposedObject = bean;
try {
populateBean(beanName, mbd, instanceWrapper);
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);
}
}
populateBean方法名称就暴露他是干啥的：填充bean。我放一段比较重要的部分：
if (hasInstAwareBpps || needsDepCheck) {
if (pvs == null) {
pvs = mbd.getPropertyValues();
}
PropertyDescriptor[] filteredPds = filterPropertyDescriptorsForDependencyCheck(bw, mbd.allowCaching);
if (hasInstAwareBpps) {
for (BeanPostProcessor bp : getBeanPostProcessors()) {
if (bp instanceof InstantiationAwareBeanPostProcessor) {
InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) bp;
pvs = ibp.postProcessPropertyValues(pvs, filteredPds, bw.getWrappedInstance(), beanName);
if (pvs == null) {
return;
}
}
}
}
if (needsDepCheck) {
checkDependencies(beanName, mbd, filteredPds, pvs);
}
}
这段代码貌似没有什么注入、填充什么的，我们看下InstantiationAwareBeanPostProcessor这个接口，发现这个接口有个很熟悉的实现类是：AutowiredAnnotationBeanPostProcessor。这个类的方法中我们看到终于看到了jnject方法。但是在inject之前需要调用下findAutowiringMatedata方法获取一下元数据：
// 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);
}
metadata = buildAutowiringMetadata(clazz);
this.injectionMetadataCache.put(cacheKey, metadata);
}
}
}
return metadata;
那么问题又来了，这个injectionMetadataCache集合值是从那里来的呢？AutowiredAnnotationBeanPostProcessor实现了MergeBeandefinitionPostProcessor,那么就好了，肯定是在bean实例的时候调用了postProcessMergedBeanDefintion这个方法。
果然在doCreateBean方法中有这么一段：
// Allow post-processors to modify the merged bean definition.
synchronized (mbd.postProcessingLock) {
if (!mbd.postProcessed) {
try {
//通过 BeanPostProcessor将需要注解的元数据 放到Map中
applyMergedBeanDefinitionPostProcessors(mbd, beanType, beanName);
}
catch (Throwable ex) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName,
"Post-processing of merged bean definition failed", ex);
}
mbd.postProcessed = true;
}
}
哎，spring缜密啊。元数据有了，下面我们看下element.inject()方法是如何操作的：
if (this.isField) {
Field field = (Field) this.member;
ReflectionUtils.makeAccessible(field);
field.set(target, getResourceToInject(target, requestingBeanName));
}
else {
if (checkPropertySkipping(pvs)) {
return;
}
try {
Method method = (Method) this.member;
ReflectionUtils.makeAccessible(method);
method.invoke(target, getResourceToInject(target, requestingBeanName));
}
catch (InvocationTargetException ex) {
throw ex.getTargetException();
}
}
方法简单明了，直接使用反射将值set到属性中，至此 bean的实例、属性注入基本完成，下面我们回头来看doGetBean的另一个方法getObjectForBeanInstance。
2、getObjectForBeanInstance方法。对于是FactoryBean类型的 bean通过getObject获取到bean的代理实例，跟着方法一直走下去会到getObejct()方法中。
if (System.getSecurityManager() != null) {
AccessControlContext acc = getAccessControlContext();
try {
object = AccessController.doPrivileged((PrivilegedExceptionAction<Object>) factory::getObject, acc);
}
catch (PrivilegedActionException pae) {
throw pae.getException();
}
}
else {
//从FactoryBean实现bean中getObejct获取到bean
object = factory.getObject();
}
到此 finishBeanfactoryInitialization方法执行结束！
bean实例化流程说明
bean实例化流程流程是在onRefresh方法的finishBeanFactoryInitialization中。进入该方法
protected void finishBeanFactoryInitialization(ConfigurableListableBeanFactory beanFactory) {
// Initialize conversion service for this context.
if (beanFactory.containsBean(CONVERSION_SERVICE_BEAN_NAME) &&
beanFactory.isTypeMatch(CONVERSION_SERVICE_BEAN_NAME, ConversionService.class)) {
beanFactory.setConversionService(
beanFactory.getBean(CONVERSION_SERVICE_BEAN_NAME, ConversionService.class));
}
// Register a default embedded value resolver if no bean post-processor
// (such as a PropertyPlaceholderConfigurer bean) registered any before:
// at this point, primarily for resolution in annotation attribute values.
if (!beanFactory.hasEmbeddedValueResolver()) {
beanFactory.addEmbeddedValueResolver(strVal -> getEnvironment().resolvePlaceholders(strVal));
}
// Initialize LoadTimeWeaverAware beans early to allow for registering their transformers early.
String[] weaverAwareNames = beanFactory.getBeanNamesForType(LoadTimeWeaverAware.class, false, false);
for (String weaverAwareName : weaverAwareNames) {
getBean(weaverAwareName);
}
// Stop using the temporary ClassLoader for type matching.
beanFactory.setTempClassLoader(null);
// Allow for caching all bean definition metadata, not expecting further changes.
beanFactory.freezeConfiguration();
// Instantiate all remaining (non-lazy-init) singletons.
beanFactory.preInstantiateSingletons();
}
1、beanFactory.freezeConfiguration(); 标记正在实例化
当前有183个bean正在实例化中。
2、调用beanFactory.preInstantiateSingletons(); 实例化bean
以上为个人经验，希望能给大家一个参考，也希望大家多多支持脚本之家。