目录
一、前言
二、源码分析
2.1、getBean()的过程
2.2、createBean()的过程
2.2.1、createBeanInstance()：Bean实例的创建
2.2.2、populateBean();属性设置（依赖注入）
2.2.3、initializeBean()：调用Bean的初始化方法
三、总结
一、前言
前面我们对IoC容器的初始化过程进行了详细的分析，这个初始化过程完成的主要工作是在IoC容器中建立BeanDefinition数据映射。在此过程中并没有看到IoC容器对Bean依赖关系进行注入，接下来分析一下IoC容器是怎样对Bean的依赖关系进行注入的。
前面在refresh()-->invokeBeanFactoryPostProcessors(beanFactory);方法中已经完成了IoC容器的初始化并已经载入了我们定义的Bean的信息（BeanDefinition），现在我们开始分析依赖注入的原理。首先需要说明的是依赖注入在用户第一次向IoC容器索要Bean时触发，当然也有例外，我们可以在BeanDefinition中中通过控制lazy-init属性来让容器完成对Bean的预实例化。这个预实例化实际上也是一个依赖注入的过程，但它是在初始化过程中完成的。
二、源码分析
2.1、getBean()的过程
接着前面看refresh()方法，这已经是refresh()方法的第三篇博文了，别迷糊我们还没走出refresh()方法。
// AbstractApplicationContext类
@Override
public void refresh() throws BeansException, IllegalStateException {
synchronized (this.startupShutdownMonitor) {
...
try {
...
// Instantiate all remaining (non-lazy-init) singletons.
finishBeanFactoryInitialization(beanFactory);
...
}
...
}
}
// AbstractApplicationContext类
protected void finishBeanFactoryInitialization(ConfigurableListableBeanFactory beanFactory) {
...
// Instantiate all remaining (non-lazy-init) singletons.
// 实例化所有剩余的(non-lazy-init)单例。
beanFactory.preInstantiateSingletons();
}
// DefaultListableBeanFactory类
@Override
public void preInstantiateSingletons() throws BeansException {
...
// Iterate over a copy to allow for init methods which in turn register new bean definitions.
// While this may not be part of the regular factory bootstrap, it does otherwise work fine.
List<String> beanNames = new ArrayList<>(this.beanDefinitionNames);
// Trigger initialization of all non-lazy singleton beans...
for (String beanName : beanNames) {
RootBeanDefinition bd = getMergedLocalBeanDefinition(beanName);
if (!bd.isAbstract() && bd.isSingleton() && !bd.isLazyInit()) {
if (isFactoryBean(beanName)) {
Object bean = getBean(FACTORY_BEAN_PREFIX + beanName);
if (bean instanceof FactoryBean) {
final FactoryBean<?> factory = (FactoryBean<?>) bean;
boolean isEagerInit;
if (System.getSecurityManager() != null && factory instanceof SmartFactoryBean) {
isEagerInit = AccessController.doPrivileged((PrivilegedAction<Boolean>)
((SmartFactoryBean<?>) factory)::isEagerInit,
getAccessControlContext());
} else {
isEagerInit = (factory instanceof SmartFactoryBean &&
((SmartFactoryBean<?>) factory).isEagerInit());
}
if (isEagerInit) {
getBean(beanName);
}
}
} else {
// 这里就是触发依赖注入的地方
getBean(beanName);
}
}
}
...
}
跟踪其调用栈，看到上面第52行的getBean(beanName);方法，我们再梳理一下getBean()方法，前面总结过该方法在IoC容器的顶层接口BeanFactory中定义，然后在IoC容器的具体产品DefaultListableBeanFactory类的基类AbstractBeanFactory实现了getBean()方法。接着看代码。
// AbstractBeanFactory类
@Override
public Object getBean(String name) throws BeansException {
return doGetBean(name, null, null, false);
}
@Override
public <T> T getBean(String name, @Nullable Class<T> requiredType) throws BeansException {
return doGetBean(name, requiredType, null, false);
}
@Override
public Object getBean(String name, Object... args) throws BeansException {
return doGetBean(name, null, args, false);
}
public <T> T getBean(String name, @Nullable Class<T> requiredType, @Nullable Object... args)
throws BeansException {
return doGetBean(name, requiredType, args, false);
}
从上面代码可知大致可分为两种获取Bean的参数，一种是按名获取，一种是按类获取。但是最终都进入到了doGetBean()方法。
// AbstractBeanFactory类
protected <T> T doGetBean(final String name, @Nullable final Class<T> requiredType,
@Nullable final Object[] args, boolean typeCheckOnly) throws BeansException {
// bean获取过程：先获取bean名字
// 会把带有&前缀的去掉，或者去aliasMap中找这个是不是别名，最终确定bean的id是什么
final String beanName = transformedBeanName(name);
Object bean;
// 1.检查缓存中或者实例工厂中是否有对应的实例
// 因为在创建单例bean的时候会存在依赖注入的情况，而在创建依赖的时候为了避免循环依赖
// Spring在创建bean的时候不会等bean创建完成就会将bean的ObjectFactory提早曝光
// 也就是将ObjectFactory加入到缓存中，一旦下一个要创建的bean需要依赖上个bean则直接使用ObjectFactory
// 2.spring 默认是单例的，如果能获取到直接返回，提高效率。
// Eagerly check singleton cache for manually registered singletons.
Object sharedInstance = getSingleton(beanName);
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 + "'");
}
}
// 用于检测bean的正确性，同时如果获取的是FactoryBean的话还需要调用getObject()方法获取最终的那个bean实例
bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
}
else {
// Fail if we're already creating this bean instance:
// We're assumably within a circular reference.
if (isPrototypeCurrentlyInCreation(beanName)) {
throw new BeanCurrentlyInCreationException(beanName);
}
// Check if bean definition exists in this factory.
//这里对IoC容器中的BeanDefinition是否存在进行检查，检查是否能在当前的BeanFactory中取得需要的Bean。
// 如果当前的工厂中取不到，则到双亲BeanFactory中去取。如果当前的双亲工厂取不到，那就顺着双亲BeanFactory
// 链一直向上查找。
BeanFactory parentBeanFactory = getParentBeanFactory();
if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {
// Not found -> check parent.
String nameToLookup = originalBeanName(name);
if (parentBeanFactory instanceof AbstractBeanFactory) {
// 递归调用父bean的doGetBean查找
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);
}
}
if (!typeCheckOnly) {
markBeanAsCreated(beanName);
}
try {
//这里根据Bean的名字取得BeanDefinition
final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
checkMergedBeanDefinition(mbd, beanName, args);
// Guarantee initialization of beans that the current bean depends on.
//获取当前Bean的所有依赖Bean，这里会触发getBean的递归调用。知道取到一个没有任何依赖的Bean为止。
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);
try {
getBean(dep);
}
catch (NoSuchBeanDefinitionException ex) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName,
"'" + beanName + "' depends on missing bean '" + dep + "'", ex);
}
}
}
// 这里通过createBean方法创建singleton Bean的实例 这里还有一个回调函数
// Create bean instance.
if (mbd.isSingleton()) {
sharedInstance = getSingleton(beanName, () -> {
try {
// 最后在getSingleton中又会调用这个方法
// TODO createBean的入口
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);
}
// 这里是创建prototype bean的地方
else if (mbd.isPrototype()) {
// It's a prototype -> create a new instance.
Object prototypeInstance = null;
try {
beforePrototypeCreation(beanName);
prototypeInstance = createBean(beanName, mbd, args);
}
finally {
afterPrototypeCreation(beanName);
}
bean = getObjectForBeanInstance(prototypeInstance, name, beanName, mbd);
}
else {
String scopeName = mbd.getScope();
final Scope scope = this.scopes.get(scopeName);
if (scope == null) {
throw new IllegalStateException("No Scope registered for scope name '" + scopeName + "'");
}
try {
Object scopedInstance = scope.get(beanName, () -> {
beforePrototypeCreation(beanName);
try {
return createBean(beanName, mbd, args);
}
finally {
afterPrototypeCreation(beanName);
}
});
bean = getObjectForBeanInstance(scopedInstance, name, beanName, mbd);
}
catch (IllegalStateException ex) {
throw new BeanCreationException(beanName,
"Scope '" + scopeName + "' is not active for the current thread; consider " +
"defining a scoped proxy for this bean if you intend to refer to it from a singleton",
ex);
}
}
}
catch (BeansException ex) {
cleanupAfterBeanCreationFailure(beanName);
throw ex;
}
}
// Check if required type matches the type of the actual bean instance.
//这里对创建的Bean进行类型检查，如果没有问题，就返回这个新创建的Bean，这个Bean已经是包含了依赖关系的Bean
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());
}
}
return (T) bean;
}
这个就是依赖注入的入口了，依赖注入是在容器的BeanDefinition数据已经建立好的前提下进行的。“程序=数据+算法”，很经典的一句话，前面我们详细介绍了BeanDefinition的注册过程，BeanDefinition就是数据。如上面代码所示，doGetBean()方法不涉及复杂的算法，但是这个过程也不是很简单，因为我们都知道，对于IoC容器的使用，Spring提供了很多的配置参数，每一个配置参数实际上就代表了一个IoC容器的实现特征，这些特征很多都需要在依赖注入的过程或者对Bean进行生命周期管理的过程中完成。虽然我们可以简单的将IoC容器描述成一个ConcurrentHashMap，ConcurrentHashMap只是它的数据结构而不是IoC容器的全部。
TIPS:
- 我在工程中简单写了一个controller和一个service，我们在后面debug看看依赖注入的过程是怎么样的，
也不知道我能不能说清楚。希望大家看到这一块多debug一下，debug技巧如下图所示，写了debug的条件，
因为这边到处都是递归和回调函数，再加上有很多Spring的Bean，但是我们只关心自己的Bean，
所以就写了这样的过滤条件：
beanName.equals("webController")||beanName.equals("webService")
@RestController
public class WebController {
@Autowired
private WebService webService;
@RequestMapping("/web")
public String web(){
return webService.hello();
}
}
image.png
下面我们通过代码看看获取bean的过程。
OK，看代码，Object sharedInstance = getSingleton(beanName);如注释所说，首先回去找在容器中是不是已经存在该单例。具体在哪找我们在前面的文章中已经说得很清楚了。看一下getSingleton()方法
// DefaultSingletonBeanRegistry类
protected Object getSingleton(String beanName, boolean allowEarlyReference) {
// 由于scope是singleton，所以先从缓存中取单例对象的实例，如果取到直接返回，没有取到加载bean
Object singletonObject = this.singletonObjects.get(beanName);
// 当想要获取的bean没有被加载，并且也没有正在被创建的时候，主动去加载bean
if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {
// 锁住单例缓存区加载bean
synchronized (this.singletonObjects) {
// singletonObjects ，earlySingletonObjects ，singletonFactories是一个单例实例的三种存在状态
// 再去earlySingletonObjects中去找
singletonObject = this.earlySingletonObjects.get(beanName);
if (singletonObject == null && allowEarlyReference) {
// 去singletonFactories中去找对象的实例
ObjectFactory<?> singletonFactory = this.singletonFactories.get(beanName);
if (singletonFactory != null) {
singletonObject = singletonFactory.getObject();
this.earlySingletonObjects.put(beanName, singletonObject);
this.singletonFactories.remove(beanName);
}
}
}
}
return singletonObject;
}
在DefaultSingletonBeanRegistry类中的singletonObjects属性就是存singleton bean的地方。
如果getSingleton()为 null继续往下看，会在当前的BeanFactory中获取BeanDefinition，也就是这行方法代码：final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);在这行代码拿到BeanDefinition后，首先判断是不是singleton Bean，如果是的话，开始执行创建Bean，正是return createBean(beanName, mbd, args);这行代码。如果是原型(Prototype)Bean我们就不分析了。原型bean每次执行getBean()都会创建一个实例。接下来我们看createBean()方法。
2.2、createBean()的过程
首先看一下create bean的过程
1，Bean实例的创建
2，为Bean实例设置属性(属性注入，其实就是依赖注入真正发生的地方)
3，调用Bean的初始化方法
前面说了getBean()是依赖注入的起点，之后会调用createBean()，下面通过createBean()代码来了解这个过程。在这个过程中，Bean对象会根据BeanDefinition定义的要求生成。
// AbstractAutowireCapableBeanFactory类
@Override
protected Object createBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args)
throws BeanCreationException {
...
try {
// 验证以及准备override的方法
mbdToUse.prepareMethodOverrides();
}
catch (BeanDefinitionValidationException ex) {
throw new BeanDefinitionStoreException(mbdToUse.getResourceDescription(),
beanName, "Validation of method overrides failed", ex);
}
try {
// Give BeanPostProcessors a chance to return a proxy instead of the target bean instance.
// createBean之前调用BeanPostProcessor的postProcessBeforeInitialization和postProcessAfterInitialization方法
// 默认不做任何处理所以会返回null
// 但是如果我们重写了这两个方法，那么bean的创建过程就结束了，这里就为以后的annotation自动注入提供了钩子
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);
}
try {
// 实际执行createBean的是doCreateBean()方法
Object beanInstance = doCreateBean(beanName, mbdToUse, args);
if (logger.isDebugEnabled()) {
logger.debug("Finished creating instance of bean '" + beanName + "'");
}
return beanInstance;
}
catch (BeanCreationException | ImplicitlyAppearedSingletonException ex) {
// A previously detected exception with proper bean creation context already,
// or illegal singleton state to be communicated up to DefaultSingletonBeanRegistry.
throw ex;
}
catch (Throwable ex) {
throw new BeanCreationException(
mbdToUse.getResourceDescription(), beanName, "Unexpected exception during bean creation", ex);
}
}
接着往下看doCreateBean()方法。
// AbstractAutowireCapableBeanFactory类
protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final @Nullable Object[] args)
throws BeanCreationException {
// BeanWrapper是用来持有创建出来的Bean对象
// Instantiate the bean.
BeanWrapper instanceWrapper = null;
// 如果是单例，先把缓存中的同名Bean清除
if (mbd.isSingleton()) {
instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
}
// 这里是创建Bean的地方，由createBeanInstance完成。
// TODO 完成Bean初始化过程的第一步：创建实例
if (instanceWrapper == null) {
instanceWrapper = createBeanInstance(beanName, mbd, args);
}
final Object bean = instanceWrapper.getWrappedInstance();
Class<?> beanType = instanceWrapper.getWrappedClass();
if (beanType != NullBean.class) {
mbd.resolvedTargetType = beanType;
}
// Allow post-processors to modify the merged bean definition.
synchronized (mbd.postProcessingLock) {
if (!mbd.postProcessed) {
try {
applyMergedBeanDefinitionPostProcessors(mbd, beanType, beanName);
}
catch (Throwable ex) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName,
"Post-processing of merged bean definition failed", ex);
}
mbd.postProcessed = true;
}
}
// Eagerly cache singletons to be able to resolve circular references
// even when triggered by lifecycle interfaces like BeanFactoryAware.
// 是否自动解决循环引用
// 当bean条件为： 单例&&允许循环引用&&正在创建中这样的话提早暴露一个ObjectFactory
boolean earlySingletonExposure = (mbd.isSingleton() && this.allowCircularReferences &&
isSingletonCurrentlyInCreation(beanName));
if (earlySingletonExposure) {
if (logger.isDebugEnabled()) {
logger.debug("Eagerly caching bean '" + beanName +
"' to allow for resolving potential circular references");
}
// 把ObjectFactory放进singletonFactories中
// 这里在其他bean在创建的时候会先去singletonFactories中查找有没有beanName到ObjectFactory的映射
// 如果有ObjectFactory就调用它的getObject方法获取实例
// 但是在这里就可以对一个bean进行保证，代理等等AOP就可以在getEarlyBeanReference这里实现
addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));
}
// Initialize the bean instance.
Object exposedObject = bean;
try {
// TODO 完成Bean初始化过程的第二步：为Bean的实例设置属性
// Bean依赖注入发生的地方
// 对bean进行属性填充，如果存在依赖于其他的bean的属性，则会递归的调用初始化依赖的bean
populateBean(beanName, mbd, instanceWrapper);
// TODO 完成Bean初始化过程的第三步：调用Bean的初始化方法（init-method）
// 调用初始化方法，比如init-method方法指定的方法
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);
}
}
if (earlySingletonExposure) {
Object earlySingletonReference = getSingleton(beanName, false);
if (earlySingletonReference != null) {
if (exposedObject == bean) {
exposedObject = earlySingletonReference;
}
else if (!this.allowRawInjectionDespiteWrapping && hasDependentBean(beanName)) {
String[] dependentBeans = getDependentBeans(beanName);
Set<String> actualDependentBeans = new LinkedHashSet<>(dependentBeans.length);
for (String dependentBean : dependentBeans) {
if (!removeSingletonIfCreatedForTypeCheckOnly(dependentBean)) {
actualDependentBeans.add(dependentBean);
}
}
if (!actualDependentBeans.isEmpty()) {
throw new BeanCurrentlyInCreationException(beanName,
"Bean with name '" + beanName + "' has been injected into other beans [" +
StringUtils.collectionToCommaDelimitedString(actualDependentBeans) +
"] in its raw version as part of a circular reference, but has eventually been " +
"wrapped. This means that said other beans do not use the final version of the " +
"bean. This is often the result of over-eager type matching - consider using " +
"'getBeanNamesOfType' with the 'allowEagerInit' flag turned off, for example.");
}
}
}
}
// Register bean as disposable.
try {
// 注册销毁方法,比如：可以在配置bean的时候指定destory-method方法
registerDisposableBeanIfNecessary(beanName, bean, mbd);
}
catch (BeanDefinitionValidationException ex) {
throw new BeanCreationException(
mbd.getResourceDescription(), beanName, "Invalid destruction signature", ex);
}
return exposedObject;
}
结合上面的代码，我们再来看一下创建Bean的三个步骤，是不是有点豁然开朗的感觉。别着急继续往下看。
1，Bean实例的创建，instanceWrapper = createBeanInstance(beanName, mbd, args);
2，为Bean实例设置属性，populateBean(beanName, mbd, instanceWrapper);
3，调用Bean的初始化方法，exposedObject = initializeBean(beanName, exposedObject, mbd);
2.2.1、createBeanInstance()：Bean实例的创建
看代码
// AbstractAutowireCapableBeanFactory类
protected BeanWrapper createBeanInstance(String beanName, RootBeanDefinition mbd, @Nullable Object[] args) {
// Make sure bean class is actually resolved at this point.
// 确认需要创建的Bean的实例的类可以实例化
Class<?> beanClass = resolveBeanClass(mbd, beanName);
if (beanClass != null && !Modifier.isPublic(beanClass.getModifiers()) && !mbd.isNonPublicAccessAllowed()) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName,
"Bean class isn't public, and non-public access not allowed: " + beanClass.getName());
}
Supplier<?> instanceSupplier = mbd.getInstanceSupplier();
if (instanceSupplier != null) {
return obtainFromSupplier(instanceSupplier, beanName);
}
// 当有工厂方法的时候使用工厂方法初始化Bean，就是配置的时候指定FactoryMethod属性，类似注解中的@Bean把方法的返回值作为Bean
if (mbd.getFactoryMethodName() != null) {
return instantiateUsingFactoryMethod(beanName, mbd, args);
}
// Shortcut when re-creating the same bean...
boolean resolved = false;
boolean autowireNecessary = false;
if (args == null) {
synchronized (mbd.constructorArgumentLock) {
if (mbd.resolvedConstructorOrFactoryMethod != null) {
resolved = true;
autowireNecessary = mbd.constructorArgumentsResolved;
}
}
}
if (resolved) {
if (autowireNecessary) {
// 如果有有参数的构造函数，构造函数自动注入
// 这里spring会花费大量的精力去进行参数的匹配
return autowireConstructor(beanName, mbd, null, null);
}
else {
// 如果没有有参构造函数，使用默认构造函数构造
return instantiateBean(beanName, mbd);
}
}
// Need to determine the constructor...
// 使用构造函数进行实例化
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.
// 使用默认的构造函数对Bean进行实例化
return instantiateBean(beanName, mbd);
}
我们可以看到在instantiateBean()方法中生成了Bean所包含的Java对象，这个对象的生成有很多种不同的方式，可以通过工厂方法生成，也可以通过容器的autowire特性生成，这些生成方式都是由BeanDefinition决定的。对于上面我们的WebController和WebService两个类是通过最后一行，使用默认的构造函数进行Bean的实例化。
接着看instantiateBean()方法。
// AbstractAutowireCapableBeanFactory类
protected BeanWrapper instantiateBean(final String beanName, final RootBeanDefinition mbd) {
// 使用默认的实例化策略对Bean进行实例化，默认的实例化策略是CglibSubclassingInstantiationStrategy，
// 也就是常说的CGLIB来对Bean进行实例化。PS：面试官常问的字节码增强
try {
Object beanInstance;
final BeanFactory parent = this;
if (System.getSecurityManager() != null) {
beanInstance = AccessController.doPrivileged((PrivilegedAction<Object>) () ->
getInstantiationStrategy().instantiate(mbd, beanName, parent),
getAccessControlContext());
}
else {
// getInstantiationStrategy()会返回CglibSubclassingInstantiationStrategy类的实例
beanInstance = getInstantiationStrategy().instantiate(mbd, beanName, parent);
}
BeanWrapper bw = new BeanWrapperImpl(beanInstance);
initBeanWrapper(bw);
return bw;
}
catch (Throwable ex) {
throw new BeanCreationException(
mbd.getResourceDescription(), beanName, "Instantiation of bean failed", ex);
}
}
这里使用CGLIB进行Bean的实例化。CGLIB是一个常用的字节码生成器的类库，其提供了一系列的API来提供生成和转换Java字节码的功能。在Spring AOP中同样也是使用的CGLIB对Java的字节码进行增强。在IoC容器中，使用SimpleInstantiationStrategy类。这个类是Spring用来生成Bean对象的默认类，它提供了两种实例化Java对象的方法，一种是通过BeanUtils，它使用的是JVM的反射功能，一种是通过CGLIB来生成。
getInstantiationStrategy()方法获取到CglibSubclassingInstantiationStrategy实例，instantiate()是CglibSubclassingInstantiationStrategy类的父类SimpleInstantiationStrategy实现的。
继续看代码
// SimpleInstantiationStrategy类
@Override
public Object instantiate(RootBeanDefinition bd, @Nullable String beanName, BeanFactory owner) {
// Don't override the class with CGLIB if no overrides.
// 如果BeanFactory重写了Bean内的方法，则使用CGLIB，否则使用BeanUtils
if (!bd.hasMethodOverrides()) {
// 如果bean没有需要动态替换的方法就直接反射进行创建实例
Constructor<?> constructorToUse;
synchronized (bd.constructorArgumentLock) {
constructorToUse = (Constructor<?>) bd.resolvedConstructorOrFactoryMethod;
if (constructorToUse == null) {
final Class<?> clazz = bd.getBeanClass();
if (clazz.isInterface()) {
throw new BeanInstantiationException(clazz, "Specified class is an interface");
}
try {
if (System.getSecurityManager() != null) {
constructorToUse = AccessController.doPrivileged(
(PrivilegedExceptionAction<Constructor<?>>) clazz::getDeclaredConstructor);
} else {
constructorToUse = clazz.getDeclaredConstructor();
}
bd.resolvedConstructorOrFactoryMethod = constructorToUse;
} catch (Throwable ex) {
throw new BeanInstantiationException(clazz, "No default constructor found", ex);
}
}
}
// 通过BeanUtils进行实例化，这个BeanUtils的实例化通过Constructor类实例化Bean
// 在BeanUtils中可以看到具体的调用ctor.newInstances(args)
return BeanUtils.instantiateClass(constructorToUse);
} else {
// Must generate CGLIB subclass.
// TODO 使用CGLIB实例化对象
return instantiateWithMethodInjection(bd, beanName, owner);
}
}
在SpringBoot中我们一般采用@Autowire的方式进行依赖注入，很少采用像SpringMVC那种在xml中使用<lookup-method>或者<replaced-method>等标签的方式对注入的属性进行override，所以在上面的代码中if(!bd.hasMethodOverrides())中的判断为true，会采用BeanUtils的实例化方式。
2.2.2、populateBean();属性设置（依赖注入）
看代码
protected void populateBean(String beanName, RootBeanDefinition mbd, @Nullable BeanWrapper bw) {
if (bw == null) {
if (mbd.hasPropertyValues()) {
throw new BeanCreationException(
mbd.getResourceDescription(), beanName, "Cannot apply property values to null instance");
}
else {
// Skip property population phase for null instance.
return;
}
}
// Give any InstantiationAwareBeanPostProcessors the opportunity to modify the
// state of the bean before properties are set. This can be used, for example,
// to support styles of field injection.
boolean continueWithPropertyPopulation = true;
// 调用InstantiationAwareBeanPostProcessor Bean的后置处理器，在Bean注入属性前改变BeanDefinition的信息
if (!mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors()) {
for (BeanPostProcessor bp : getBeanPostProcessors()) {
if (bp instanceof InstantiationAwareBeanPostProcessor) {
InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) bp;
if (!ibp.postProcessAfterInstantiation(bw.getWrappedInstance(), beanName)) {
continueWithPropertyPopulation = false;
break;
}
}
}
}
if (!continueWithPropertyPopulation) {
return;
}
// 这里取得在BeanDefinition中设置的property值，这些property来自对BeanDefinition的解析
// 用于在配置文件中通过<property>配置的属性并且显示在配置文件中配置了autowireMode属性
PropertyValues pvs = (mbd.hasPropertyValues() ? mbd.getPropertyValues() : null);
if (mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_BY_NAME ||
mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_BY_TYPE) {
MutablePropertyValues newPvs = new MutablePropertyValues(pvs);
// Add property values based on autowire by name if applicable.
// 这里对autowire注入的处理，autowire by name
if (mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_BY_NAME) {
autowireByName(beanName, mbd, bw, newPvs);
}
// Add property values based on autowire by type if applicable.
// 这里对autowire注入的处理， autowire by type
// private List<Test> tests;
if (mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_BY_TYPE) {
autowireByType(beanName, mbd, bw, newPvs);
}
pvs = newPvs;
}
boolean hasInstAwareBpps = hasInstantiationAwareBeanPostProcessors();
boolean needsDepCheck = (mbd.getDependencyCheck() != RootBeanDefinition.DEPENDENCY_CHECK_NONE);
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;
// TODO @Autowire @Resource @Value @Inject 等注解的依赖注入过程
pvs = ibp.postProcessPropertyValues(pvs, filteredPds, bw.getWrappedInstance(), beanName);
if (pvs == null) {
return;
}
}
}
}
if (needsDepCheck) {
checkDependencies(beanName, mbd, filteredPds, pvs);
}
}
if (pvs != null) {
// 注入配置文件中<property>配置的属性
applyPropertyValues(beanName, mbd, bw, pvs);
}
}
上面方法中的31-52行以及78行的applyPropertyValues()方法基本都是用于SpringMVC中采用xml配置Bean的方法。所以我们不做介绍了。看注释知道干嘛的就行了。我们主要看的是pvs = ibp.postProcessPropertyValues(pvs, filteredPds, bw.getWrappedInstance(), beanName);这行代码，这行代码是真正执行采用@Autowire @Resource @Value @Inject 等注解的依赖注入过程。
接着往下看
// AutowiredAnnotationBeanPostProcessor类
@Override
public PropertyValues postProcessPropertyValues(
PropertyValues pvs, PropertyDescriptor[] pds, Object bean, String beanName) throws BeanCreationException {
// 遍历，获取@Autowire，@Resource，@Value，@Inject等具备注入功能的注解
InjectionMetadata metadata = findAutowiringMetadata(beanName, bean.getClass(), pvs);
try {
// 属性注入
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;
}
AutowiredAnnotationBeanPostProcessor类实现了postProcessPropertyValues()方法。findAutowiringMetadata(beanName, bean.getClass(), pvs);方法会寻找在当前类中的被@Autowire，@Resource，@Value，@Inject等具备注入功能的注解的属性。
debug看一下结果，如下图所示，成功得到了@Autowire注解的属性webService。
image
metadata.inject(bean, beanName, pvs);方法开始执行注入的逻辑。
// AutowiredAnnotationBeanPostProcessor类
@Override
protected void inject(Object bean, @Nullable String beanName, @Nullable PropertyValues pvs) throws Throwable {
// 需要注入的字段
Field field = (Field) this.member;
// 需要注入的属性值
Object value;
if (this.cached) {
value = resolvedCachedArgument(beanName, this.cachedFieldValue);
} else {
// @Autowired(required = false)，当在该注解中设置为false的时候，如果有直接注入，没有跳过，不会报错。
DependencyDescriptor desc = new DependencyDescriptor(field, this.required);
desc.setContainingClass(bean.getClass());
Set<String> autowiredBeanNames = new LinkedHashSet<>(1);
Assert.state(beanFactory != null, "No BeanFactory available");
TypeConverter typeConverter = beanFactory.getTypeConverter();
try {
// 通过BeanFactory 解决依赖关系
// 比如在webController中注入了webService，这个会去BeanFactory中去获取webService，也就是getBean()的逻辑。
// 如果存在直接返回，不存在再执行createBean()逻辑。
// 如果在webService中依然依赖，依然会去递归。
// 这里是一个复杂的递归逻辑。
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) {
ReflectionUtils.makeAccessible(field);
field.set(bean, value);
}
}
debug看一下field。如下图所示正是我们的webService
image
看这行代码：value = beanFactory.resolveDependency(desc, beanName, autowiredBeanNames, typeConverter);注意beanFactory依旧是我们熟悉的IoC容器的具体产品，也就是实现类DefaultListableBeanFactory。见到就说一遍，方便大家记住它，很重要。
在resolveDependency()方法中经过一顿操作，最终又会来到上面的getBean()方法。以上就是依赖注入的整个过程。注意看代码中的注释哦。
2.2.3、initializeBean()：调用Bean的初始化方法
设置Bean的初始化方法有两种方法，一种是在xml或者@Bean指定init-method方法。另一种是让bean实现InitializingBean接口重写afterPropertiesSet()方法。
// AbstractAutowireCapableBeanFactory类
protected Object initializeBean(final String beanName, final Object bean, @Nullable RootBeanDefinition mbd) {
if (System.getSecurityManager() != null) {
AccessController.doPrivileged((PrivilegedAction<Object>) () -> {
invokeAwareMethods(beanName, bean);
return null;
}, getAccessControlContext());
}
else {
//在调用Bean的初始化方法之前，调用一系列的aware接口实现，把相关的BeanName，BeanClassLoader，以及BeanFactory注入到Bean中去。
invokeAwareMethods(beanName, bean);
}
Object wrappedBean = bean;
if (mbd == null || !mbd.isSynthetic()) {
// 这些都是钩子方法，在反复的调用，给Spring带来了极大的可拓展性
// 初始化之前调用BeanPostProcessor
wrappedBean = applyBeanPostProcessorsBeforeInitialization(wrappedBean, beanName);
}
try {
// 调用指定的init-method方法
invokeInitMethods(beanName, wrappedBean, mbd);
}
catch (Throwable ex) {
throw new BeanCreationException(
(mbd != null ? mbd.getResourceDescription() : null),
beanName, "Invocation of init method failed", ex);
}
if (mbd == null || !mbd.isSynthetic()) {
// 这些都是钩子方法，在反复的调用，给Spring带来了极大的可拓展性
// 初始化之后调用BeanPostProcessor
wrappedBean = applyBeanPostProcessorsAfterInitialization(wrappedBean, beanName);
}
return wrappedBean;
}
在调用Bean的初始化方法之前，调用一系列的aware接口实现，把相关的BeanName，BeanClassLoader，以及BeanFactory注入到Bean中去。接着会执行invokeInitMethods()方法。
// AbstractAutowireCapableBeanFactory类
protected void invokeInitMethods(String beanName, final Object bean, @Nullable RootBeanDefinition mbd)
throws Throwable {
// 除了使用init-method指定的初始化方法，还可以让bean实现InitializingBean接口重写afterPropertiesSet()方法
boolean isInitializingBean = (bean instanceof InitializingBean);
if (isInitializingBean && (mbd == null || !mbd.isExternallyManagedInitMethod("afterPropertiesSet"))) {
if (logger.isDebugEnabled()) {
logger.debug("Invoking afterPropertiesSet() on bean with name '" + beanName + "'");
}
if (System.getSecurityManager() != null) {
try {
AccessController.doPrivileged((PrivilegedExceptionAction<Object>) () -> {
((InitializingBean) bean).afterPropertiesSet();
return null;
}, getAccessControlContext());
}
catch (PrivilegedActionException pae) {
throw pae.getException();
}
}
else {
// 执行afterPropertiesSet()方法进行初始化
((InitializingBean) bean).afterPropertiesSet();
}
}
// 先执行afterPropertiesSet()方法，再进行init-method
if (mbd != null && bean.getClass() != NullBean.class) {
String initMethodName = mbd.getInitMethodName();
if (StringUtils.hasLength(initMethodName) &&
!(isInitializingBean && "afterPropertiesSet".equals(initMethodName)) &&
!mbd.isExternallyManagedInitMethod(initMethodName)) {
invokeCustomInitMethod(beanName, bean, mbd);
}
}
}
可见该方法中首先判断Bean是否配置了init-method方法，如果有，那么通过invokeCustomInitMethod()方法来直接调用。其中在invokeCustomInitMethod()方法中是通过JDK的反射机制得到method对象，然后调用的init-method。最终完成Bean的初始化。
三、总结
SpringBoot启动流程相关的博文到这里就结束了，在前面的文章中，我们详细介绍了IoC容器的设计与实现，并结合SpringBoot的启动流程介绍了IoC容器的初始化过程，及IoC容器的依赖注入，及大家都很关心的SpringBoot是如何实现自动装配的。
