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前言
上一篇文章中，我们介绍了Spring IoC 的容器初始化过程 - IoC 容器初始化
本篇文章中，我们继续介绍Spring IoC 依赖注入的过程和源码解读。
还是如之前一样，为大家梳理一下步骤流程，以便于大家能在心里有个大概的脉络，更容易读懂源码，更容易抓住重点。
主要内容：
- beanName 解析转换
- 手动注册Bean检测
- 双亲容器检测
- 依赖初始化（递归）
- ★ 创建singleton 实例
- 对象实例化
- 属性装配
- 处理Bean创建之后的各种回调事件
- ...
源码解析
上一章最后一节，容器初始化的倒数第二步，finishBeanFactoryInitialization(beanFactory)实例化所有单例，调用了getBean()方法来做singleton bean 的实例化操作。这就是Spring IoC 依赖注入的入口。
在开始之前，有一点需要提一下。前面我们是从容器初始化之后进来的，但实际操作中，我们有可能是在程序普通运行情况下，用ApplicationContext.getBean()去获取容器中bean。不要局限于刚刚的视角中。
现在让我们开始吧。
首先看看getBean()
源码位置：AbstractBeanFactory#getBean(String name)
@Override
public Object getBean(String name) throws BeansException {
return doGetBean(name, null, null, false);
}
doGetBean()
deGetBean()
- beanName 解析转换
- 检测 手动注册Bean
- 双亲容器检测
- 依赖初始化（递归）
- 创建Bean
createBean()
protected <T> T doGetBean(
final String name, final Class<T> requiredType, final Object[] args, boolean typeCheckOnly)
throws BeansException {
// 反正就是获取到真正beanName
// 处理两个情况，1. 将别名转化成真的beanName；2. 把FactoryBean的前缀"&"给去了
final String beanName = transformedBeanName(name);
Object bean;
// Eagerly check singleton cache for manually registered singletons.
// 检测已经注册的Bean，保证不重复创建
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 + "'");
}
}
// 这个方法还是有点逻辑的
// 如果目前获得的sharedInstance 不是FactoryBean，那bean就赋值成sharedInstance，直接返回
// 如果是FactoryBean就返回FactoryBean创建的实例，
// 这个也是FactoryBean的知识点，我在我的另一篇文章也讲过了，
bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
}
else {
// Fail if we're already creating this bean instance:
// 直接翻译：创建过了此 beanName 的 prototype 类型的 bean，那么抛异常
// We're assumably within a circular reference.
// 往往是因为陷入了循环引用
if (isPrototypeCurrentlyInCreation(beanName)) {
throw new BeanCurrentlyInCreationException(beanName);
}
// Check if bean definition exists in this factory.
// 检查下这个BeanDefinition是否存在
BeanFactory parentBeanFactory = getParentBeanFactory();
if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {
// Not found -> check parent.
// 当前容器没有这个BeanDefinition，去parent 容器去找
String nameToLookup = originalBeanName(name);
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 {
// 这个 getMergedLocalBeanDefinition 前面讲过哦
final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
checkMergedBeanDefinition(mbd, beanName, args);
// 先初始化依赖的所有 Bean，这个很好理解。
// 注意，这里的依赖指的是 depends-on 中定义的依赖
String[] dependsOn = mbd.getDependsOn();
if (dependsOn != null) {
for (String dep : dependsOn) {
if (isDependent(beanName, dep)) {
// 这里循环依赖概念不要紊乱了
// 这里指的是通过 depends-on 定义造成的循环依赖，
// 我们另外一种类成员式的循环引用Spring是支持的
throw new BeanCreationException(mbd.getResourceDescription(), beanName,
"Circular depends-on relationship between '" + beanName + "' and '" + dep + "'");
}
// 注册依赖关系
// 这么做的原因是Spring在即将进行bean销毁的时候会【首先销毁被依赖的bean】。
// 看SpringBean的初始化和销毁顺序就知道了,依赖关系的保存目的就是这个
// 依赖关系的保存是通过一个ConcurrentHashMap<String, Set>完成的，key是bean的真实名字。
registerDependentBean(dep, beanName);
try {
// 先去初始化被依赖项
// 递归然后反递归回来
getBean(dep);
}
catch (NoSuchBeanDefinitionException ex) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName,
"'" + beanName + "' depends on missing bean '" + dep + "'", ex);
}
}
}
// Create bean instance.
// 如果是 singleton scope 的，创建 singleton 的实例
if (mbd.isSingleton()) {
sharedInstance = getSingleton(beanName, new ObjectFactory<Object>() {
@Override
public Object getObject() throws BeansException {
try {
// 创建Bean的详情，等下拉出来单独说
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;
}
}
});
// 前面讲过了
bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
}
// 如果是 prototype scope 的，创建 prototype 的实例
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);
}
// 如果不是 singleton 和 prototype 的话，需要委托给相应的实现类来处理
// 这里非重点，我们的重点是singleton的创建
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, new ObjectFactory<Object>() {
@Override
public Object getObject() throws BeansException {
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.
// 最后再检查下类型对不对，不对就抛异常了，对的话就返回
if (requiredType != null && bean != null && !requiredType.isInstance(bean)) {
try {
return getTypeConverter().convertIfNecessary(bean, requiredType);
}
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;
}
doCreateBean()
步骤概览：
- 开始是单例的话要先清除缓存；
- 实例化bean，将BeanDefinition转换为BeanWrapper；
- 使用MergedBeanDefinitionPostProcessor，Autowired注解就是通过此方法实现类型的预解析；
- 解决循环依赖问题；
- 填充属性，将属性填充到bean实例中；
- 注册DisposableBean；
- 创建完成并返回
三个关注点：
- createBeanInstance() 实例化
- populateBean(); 属性装配
- initializeBean() 处理Bean初始化之后的各种回调事件
protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final Object[] args)
throws BeanCreationException {
// Instantiate the bean.
// 这个BeanWrapper是创建出来持有对象的
BeanWrapper instanceWrapper = null;
if (mbd.isSingleton()) {
// 如果是singleton，先把缓存中的同名bean消除
instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
}
if (instanceWrapper == null)
// 关键代码，后面拉出来单独讲
instanceWrapper = createBeanInstance(beanName, mbd, args);
}
final Object bean = (instanceWrapper != null ? instanceWrapper.getWrappedInstance() : null);
Class<?> beanType = (instanceWrapper != null ? instanceWrapper.getWrappedClass() : null);
mbd.resolvedTargetType = beanType;
// Allow post-processors to modify the merged bean definition.
// 涉及接口：MergedBeanDefinitionPostProcessor
// 不是关键逻辑，不讲了
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.
// 这里是为了解决循环依赖的，先把初步实例化的Bean实例的引用缓存起来，暴露出去，
// 这个可以结合别的文章学习，面试题常考，我后面可能也会写
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");
}
addSingletonFactory(beanName, new ObjectFactory<Object>() {
@Override
public Object getObject() throws BeansException {
return getEarlyBeanReference(beanName, mbd, bean);
}
});
}
// Initialize the bean instance.
Object exposedObject = bean;
try {
// 时序图中的一步，关键步骤，属性装配，前面的实例只是实例化，没有装配属性
// 和前面的createBeanInstance一样会拉出来讲，继续看下去吧
populateBean(beanName, mbd, instanceWrapper);
if (exposedObject != null) {
// 还记得 init-method 吗？还有 InitializingBean 接口？还有 BeanPostProcessor 接口？
// 这里就是处理 bean 初始化完成后的各种回调
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);
}
}
// 这个逻辑分支我一次性说完吧。
// 如果该beanName对象已经注册单例模式，则从单例中获取，并判断获取到的bean实例(B)与BeanWrapper中的bean实例(A)是同一个实例，如果是，则返回A或者B，如果不是，则递归找出它的依赖bean。
if (earlySingletonExposure) {
Object earlySingletonReference = getSingleton(beanName, false);
// earlySingletonReference只有在检测到有循环依赖的情况下才会不为空
if (earlySingletonReference != null) {
if (exposedObject == bean) {
// 两个是同一个引用，bean初始化完成
exposedObject = earlySingletonReference;
}
else if (!this.allowRawInjectionDespiteWrapping && hasDependentBean(beanName)) {
String[] dependentBeans = getDependentBeans(beanName);
Set<String> actualDependentBeans = new LinkedHashSet<String>(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.
// 注册DisposableBean；
try {
registerDisposableBeanIfNecessary(beanName, bean, mbd);
}
catch (BeanDefinitionValidationException ex) {
throw new BeanCreationException(
mbd.getResourceDescription(), beanName, "Invalid destruction signature", ex);
}
return exposedObject;
}
上一步的三个关注点，分开来讲。
1. createBeanInstance()
protected BeanWrapper createBeanInstance(String beanName, RootBeanDefinition mbd, Object[] args) {
// Make sure bean class is actually resolved at this point.
// 解析出 Class
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());
}
// 如果工厂方法不为空，则是用工厂方法初始化
if (mbd.getFactoryMethodName() != null) {
// 相关知识点看另一篇文章关于FactoryBean的
return instantiateUsingFactoryMethod(beanName, mbd, args);
}
// Shortcut when re-creating the same bean...
// 如果不是第一次创建，比如第二次创建 prototype bean。
// 这种情况下，我们可以从第一次创建知道，采用无参构造函数，还是构造函数依赖注入 来完成实例化
// 所以注释说叫shortcut
boolean resolved = false;
boolean autowireNecessary = false;
if (args == null) {
synchronized (mbd.constructorArgumentLock) {
if (mbd.resolvedConstructorOrFactoryMethod != null) {
// 有已经解析过的构造方法
resolved = true;
autowireNecessary = mbd.constructorArgumentsResolved;
}
}
}
// 如果已经解析过则使用解析好的构造方法不需要再次锁定
if (resolved) {
if (autowireNecessary) {
// 构造方法自动注入
return autowireConstructor(beanName, mbd, null, null);
}
else {
// 默认构造方法
return instantiateBean(beanName, mbd);
}
}
// Need to determine the constructor...
// 判断是否采用有参构造函数
// 构造器自动装配
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.
// 使用无参构造器
return instantiateBean(beanName, mbd);
}
2. populateBean(); 属性装配
入口方法: AbstractAutowireCapableBeanFactory#populateBean，
它的作用是: 根据autowire类型进行autowire by name，by type或者是直接进行设置
protected void populateBean(String beanName, RootBeanDefinition mbd, BeanWrapper bw) {
PropertyValues pvs = mbd.getPropertyValues();
if (bw == null) {
if (!pvs.isEmpty()) {
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.
// 这里看注解是一个扩展点
// InstantiationAwareBeanPostProcessor 的实现类可以在这里对 bean 进行状态修改
// 不是个常用的扩展点，这里不讲了
boolean continueWithPropertyPopulation = true;
if (!mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors()) {
for (BeanPostProcessor bp : getBeanPostProcessors()) {
if (bp instanceof InstantiationAwareBeanPostProcessor) {
InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) bp;
// 如果返回 false，代表不需要进行后续的属性设值，也不需要再经过其他的 BeanPostProcessor 的处理
if (!ibp.postProcessAfterInstantiation(bw.getWrappedInstance(), beanName)) {
continueWithPropertyPopulation = false;
break;
}
}
}
}
if (!continueWithPropertyPopulation) {
return;
}
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.
// 通过名字找到所有属性值，如果是 bean 依赖，先初始化依赖的 bean。记录依赖关系
if (mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_BY_NAME) {
autowireByName(beanName, mbd, bw, newPvs);
}
// Add property values based on autowire by type if applicable.
// 通过类型装配
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) {
PropertyDescriptor[] filteredPds = filterPropertyDescriptorsForDependencyCheck(bw, mbd.allowCaching);
if (hasInstAwareBpps) {
for (BeanPostProcessor bp : getBeanPostProcessors()) {
if (bp instanceof InstantiationAwareBeanPostProcessor) {
// InstantiationAwareBeanPostProcessor.postProcessPropertyValues方法
// 代表能对属性值进行修改的能力
// 其中一个很有用实现类提一下，AutowiredAnnotationBeanPostProcessor
// 对采用@Autowired和@Value设值的就是这个BeanPostProcessor干的。
// 不展开讲了，不然要讲不完了
InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) bp;
pvs = ibp.postProcessPropertyValues(pvs, filteredPds, bw.getWrappedInstance(), beanName);
if (pvs == null) {
return;
}
}
}
}
if (needsDepCheck) {
checkDependencies(beanName, mbd, filteredPds, pvs);
}
}
// 这里才是设置bean实例的属性值
applyPropertyValues(beanName, mbd, bw, pvs);
}
3. initializeBean() 处理Bean初始化之后的各种回调事件
看这个方法的javadoc 描述
Initialize the given bean instance, applying factory callbacks as well as init methods and bean post processors.
protected Object initializeBean(final String beanName, final Object bean, RootBeanDefinition mbd) {
if (System.getSecurityManager() != null) {
AccessController.doPrivileged(new PrivilegedAction<Object>() {
@Override
public Object run() {
invokeAwareMethods(beanName, bean);
return null;
}
}, getAccessControlContext());
}
else {
// 涉及到的回调接口点进去一目了然，代码都是自解释的
// BeanNameAware、BeanClassLoaderAware或BeanFactoryAware
invokeAwareMethods(beanName, bean);
}
Object wrappedBean = bean;
if (mbd == null || !mbd.isSynthetic()) {
// BeanPostProcessor 的 postProcessBeforeInitialization 回调
wrappedBean = applyBeanPostProcessorsBeforeInitialization(wrappedBean, beanName);
}
try {
// init-methods
// 或者是实现了InitializingBean接口，会调用afterPropertiesSet() 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()) {
// BeanPostProcessor 的 postProcessAfterInitialization 回调
wrappedBean = applyBeanPostProcessorsAfterInitialization(wrappedBean, beanName);
}
return wrappedBean;
}
大家发现没有，BeanPostProcessor 的两个回调都发生在这边，只不过中间处理了 init-method。这和我原来的认知有点不一样了？因为Spring的源码中代码的命名很多时候是自解释的，很多时候我看英文就知道这些方法的意图，但在这里，为什么BeanPostProcessor的前置处理方法postProcessBeforeInitialization()也是在属性设置完成后调用的？
后面查询了去看英文的解释，
BeanPostProcessor is used to interact with newly created bean instances before and/or after their initialization method is invoked by the Spring container. You can use BeanPostProcessor to execute custom logic before and/or after bean’s initialization method is invoked by the Spring container.
BeanPostProcessor的意图就是在初始化方法的前后做定制化操作。仔细想想好像这个命名也没毛病。如果真的是在SpringBean实例化之前调用应该叫BeforeInstantiation。源码中的initialization指的就是上面的invokeInitMethods操作。
结语
以上，就是关于Spring IoC 依赖注入的主要内容。
Spring IoC 容器这里还有一些其他的知识点，有关于SpringIoC 特性的。比如，Spring bean的生命周期、FactoryBean、BeanPostProcessor，都是在使用Spring IoC 容器经常遇到的特性。在了解了IoC容器的整体运行原理以后，你应该能够对这些特性进行一些分析，将你使用这些特性的方法和源码结合起来，融会贯通。
至此，Spring IoC关于依赖注入的源码“大致”解读完毕，还是那句话，上面的源码解析，肯定不会是完备的，只是提取了我认为重要的东西。
如有疏漏，敬请谅解和自己查阅相关资料学习。如果错误，敬请指正！
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