Spring Bean如何初始化的

做Java都有很多年了,一直有一个疑惑: Spring 如何初始化bean,怎么调用反射实例化对象的,自己动手来解除这个疑惑。
过去我认为spring bean对象实例化一直都是由BeanPostProcessor接口实现类去做的,我就是不知道具体那个实现类,下面就去验证下这个猜想。

三级缓存

为什么面试官特别喜欢问创建bean的三级缓存,主要是因为bean创建都是伴随着三级缓存之间的转换完成的,对象不同状态分别存在不同缓存中,下面我会在分析代码时,顺便支持对象如何在缓存中流转的。
先了解下spring 三级缓存。

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   /** 一级缓存 用于存放完全可以使用单例bean,也就是初始化完成并且注入所有依赖 */
private final Map<String, Object> singletonObjects = new ConcurrentHashMap<>(256);

/** 二级缓存 过早暴露单例对象,此时bean刚刚完成初始化,未完成属性注入和执行 init 方法 */
private final Map<String, Object> earlySingletonObjects = new ConcurrentHashMap<>(16);

/** 三级缓存 装载创建bean的工厂对象 */
private final Map<String, ObjectFactory<?>> singletonFactories = new HashMap<>(16)

三级缓存主要作用: 创建对象ObjectFactory首先放入三级换缓存中,当调用getObject 创建实例时,会将创建好对象加入二级缓存中,并且删除三级中缓存,当对象已经完成初始化方法和属性注入,再将缓存添加到一级缓存中,并且删除二级缓存。

doGetBean

从源头开始找,所有spring bean 初始化都是由AbstractBeanFactory.doGetBean方法实现的。下面我将源码减除臃肿部分,贴出来。

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protected <T> T doGetBean(
String name, @Nullable Class<T> requiredType, @Nullable Object[] args, boolean typeCheckOnly)
throws BeansException {
//name 前缀处理 beanFactory beanName 带有&开头
String beanName = transformedBeanName(name);
Object beanInstance;
//从三级缓存去取bean,三级中都没有则返回null,说明对象还没有创建
Object sharedInstance = getSingleton(beanName);
if (sharedInstance != null && args == null) { //如果缓存中bean 是FactoryBean实例,要通过接口获取到实际bean
beanInstance = getObjectForBeanInstance(sharedInstance, name, beanName, null);
}
else {
//判断bean对象标记是否正在创建中,如果正在创建中则不应该继续下去,出现依赖循环就会出现这个错误
if (isPrototypeCurrentlyInCreation(beanName)) {
throw new BeanCurrentlyInCreationException(beanName);
}
BeanFactory parentBeanFactory = getParentBeanFactory();
// 检查父容器是否存在,尝试从父容器中获取
if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {
String nameToLookup = originalBeanName(name);
if (parentBeanFactory instanceof AbstractBeanFactory) {
return ((AbstractBeanFactory) parentBeanFactory).doGetBean(
nameToLookup, requiredType, args, typeCheckOnly);
}
else if (args != null) {
return (T) parentBeanFactory.getBean(nameToLookup, args);
}
else if (requiredType != null) {
return parentBeanFactory.getBean(nameToLookup, requiredType);
}
else {
return (T) parentBeanFactory.getBean(nameToLookup);
}
}

if (!typeCheckOnly) { //缓存中标记beanName 正在被创建
markBeanAsCreated(beanName);
}
StartupStep beanCreation = this.applicationStartup.start("spring.beans.instantiate")
.tag("beanName", name);
try {
if (requiredType != null) {
beanCreation.tag("beanType", requiredType::toString);
}
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) { //bean 中@DependsOn 信息,用于标记bean之间初始化顺序,优先创建@DependsOn 中bean
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);
}
}
}

//创建单例对象
if (mbd.isSingleton()) { //重点就在这里实例化对象 ,getSingleton 就是在这里将创建完成对象加入到一级缓存中
sharedInstance = getSingleton(beanName, () -> {
try {
return createBean(beanName, mbd, args);
}
catch (BeansException ex)
destroySingleton(beanName);
throw ex;
}
});
//如果生成bean 是FactoryBean ,再获取真正的对象
beanInstance = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
}
//作用域 = prototype,因为不会放入缓存中,每次获取都要重新创建
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);
}
beanInstance = getObjectForBeanInstance(prototypeInstance, name, beanName, mbd);
}

else { // session request 这些作用域,由作用域容器去管理这些对象
String scopeName = mbd.getScope();
if (!StringUtils.hasLength(scopeName)) {
throw new IllegalStateException("No scope name defined for bean ´" + beanName + "'");
}
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);
}
});
beanInstance = getObjectForBeanInstance(scopedInstance, name, beanName, mbd);
}
catch (IllegalStateException ex) {
throw new ScopeNotActiveException(beanName, scopeName, ex);
}
}
}
catch (BeansException ex) {
beanCreation.tag("exception", ex.getClass().toString());
beanCreation.tag("message", String.valueOf(ex.getMessage()));
cleanupAfterBeanCreationFailure(beanName);
throw ex;
}
finally {
beanCreation.end();
}
}
//返回初始化成功的对象,一个对象初始化就这样完成的了
return adaptBeanInstance(name, beanInstance, requiredType);
}

大概总结一下上面代码流程:

  • 先从三级缓存中获取,如果缓存中都没有。再去判断是否存在父容器,从父容器中获取。没有正式进入bean 初始化流程,先根据beanName 获取到RootBeanDefinition,bean类元信息、先处理dependsOn中bean,保证bean依赖的创建顺序,下面会说明org.springframework.context.annotation.@DependsOn这个注解。下一步按照不同scope 进行bean 对象初始化。初始化流程就是这样,我们将目光放在单例bean 如何实例化,集中关注AbstractAutowireCapableBeanFactory.createBean 获取注册一个单例对象

@DependsOn 注解意思是实例化某个对象依赖于某一个实例化,但是不需要持有这个实例对象。比如bean A上 需要依赖bean b才能实例化,但是bean b 不需要作为他的属性,常常用于不同实例实例化顺序标记。

看下getSingleton方法

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public Object getSingleton(String beanName, ObjectFactory<?> singletonFactory) {
synchronized (this.singletonObjects) {
Object singletonObject = this.singletonObjects.get(beanName);
if (singletonObject == null) {
if (this.singletonsCurrentlyInDestruction) { //标记bean 是否在销毁
throw new BeanCreationNotAllowedException(beanName,
"Singleton bean creation not allowed while singletons of this factory are in destruction " +
"(Do not request a bean from a BeanFactory in a destroy method implementation!)");
}
beforeSingletonCreation(beanName);
boolean newSingleton = false;
boolean recordSuppressedExceptions = (this.suppressedExceptions == null);
if (recordSuppressedExceptions) {
this.suppressedExceptions = new LinkedHashSet<>();
}
try {
singletonObject = singletonFactory.getObject();
newSingleton = true;
}
catch (IllegalStateException ex) {
}
catch (BeanCreationException ex) {
throw ex;
}
finally {
if (recordSuppressedExceptions) {
this.suppressedExceptions = null;
}
afterSingletonCreation(beanName);
}
if (newSingleton) {
addSingleton(beanName, singletonObject); //就是在这里删除二三级缓存,提交到一级缓存
}
}
return singletonObject;
}
}
protected void addSingleton(String beanName, Object singletonObject) {
synchronized (this.singletonObjects) {
this.singletonObjects.put(beanName, singletonObject);
this.singletonFactories.remove(beanName);
this.earlySingletonObjects.remove(beanName);
this.registeredSingletons.add(beanName);
}
}

添加到一级缓存则说明bean已经完成实例化,可以正常使用了。下面看下如何进行实例化和属性注入的。

createBean

下面进入AbstractAutowireCapableBeanFactory.createBean

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protected Object createBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args)
throws BeanCreationException {
RootBeanDefinition mbdToUse = mbd;
//克隆一份mbd => mbdToUse
Class<?> resolvedClass = resolveBeanClass(mbd, beanName);
if (resolvedClass != null && !mbd.hasBeanClass() && mbd.getBeanClassName() != null) {
mbdToUse = new RootBeanDefinition(mbd);
mbdToUse.setBeanClass(resolvedClass);
}
// Prepare method overrides.
try {
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.
//通过BeanPostProcessors 增强返回一个代理对象,这个生成AOP的代理对象,使用多个BeanPostProcessors来处理
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 {
// bean 对象实例化就这里实现
Object beanInstance = doCreateBean(beanName, mbdToUse, args);
return beanInstance;
}
catch (BeanCreationException | ImplicitlyAppearedSingletonException ex) {
throw ex;
}
catch (Throwable ex) {
throw new BeanCreationException(
mbdToUse.getResourceDescription(), beanName, "Unexpected exception during bean creation", ex);
}
}

这里逻辑就比较简单了 ,克隆一份RootBeanDefinition用于初始化对象,resolveBeforeInstantiation 主要用于初始化代理对象情况,主要使用BeanPostProcessor子类InstantiationAwareBeanPostProcessor实现方法去实现对象初始化,并且在实例化成功后在调用后置方法进行对象依赖注入,这里可以看见此方法返回对象直接跳出方法栈,这里可以看出单例和代理对象还是有区别的。单例对象初始化就在doCreateBean 实现了

doCreateBean

下面就是AbstractAutowireCapableBeanFactory.doCreateBean非常接近对象如何实例化的了

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protected Object doCreateBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args)
throws BeanCreationException {

// Instantiate the bean.
BeanWrapper instanceWrapper = null;
if (mbd.isSingleton()) {
instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
}
if (instanceWrapper == null) {
instanceWrapper = createBeanInstance(beanName, mbd, args); //这个就是实例化方法
}
Object bean = instanceWrapper.getWrappedInstance();
Class<?> beanType = instanceWrapper.getWrappedClass();
if (beanType != NullBean.class) {
mbd.resolvedTargetType = beanType;
}

// 使用BeanDefinitionPostProcessors 对合并bean进行实例化
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;
}
}

// 这里就需要用到上面说的三级缓存知识了
// even when triggered by lifecycle interfaces like BeanFactoryAware.
boolean earlySingletonExposure = (mbd.isSingleton() && this.allowCircularReferences &&
isSingletonCurrentlyInCreation(beanName)); //是否放入第三级缓存中
if (earlySingletonExposure) {
if (logger.isTraceEnabled()) {
logger.trace("Eagerly caching bean '" + beanName +
"' to allow for resolving potential circular references");
}
addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean)); //将已经实例化的对象加入到第三级缓存 singletonFactories
}

// Initialize the bean instance.
Object exposedObject = bean;
try {
populateBean(beanName, mbd, instanceWrapper); //对属性进入注入,下面会具体分析的
exposedObject = initializeBean(beanName, exposedObject, mbd); //执行初始化方法,或者注入Aware 接口bean
}
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);
}
}

//下面代码省略
//主要就是对设置了DisposableBean 接口销毁钩子方法处理
}

这里代码主要分成三部分

  1. 初始化实例,创建对象完成,并且添加到3级缓存。第3级缓存常常用于存储代理对象,因为有些类需要动态代理方法,需要生成代理对象,会委派给第三级缓存方法ObjectFactroy去实现的,普通对象如果不需要会直接返回。
  2. 对实例化bean进行属性注入
  3. 执行初始化方法,DisposableBean接口加入到disposableBeans容器中

instantiateBean

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protected BeanWrapper createBeanInstance(String beanName, RootBeanDefinition mbd, @Nullable Object[] args) {
// Make sure bean class is actually resolved at this point.
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) {// 有实现Supplier 接口,由instanceSupplier.get() 方法创建实例
return obtainFromSupplier(instanceSupplier, beanName);
}

//factoryName 使用工厂模式创建bean,调用工厂方法去创建,这个支持静态方法和factoryBean.invoke
if (mbd.getFactoryMethodName() != null) {
return instantiateUsingFactoryMethod(beanName, mbd, args);
}

// Shortcut when re-creating the same bean...
boolean resolved = false; //标记构造函数是否需要参数
boolean autowireNecessary = false; //标记构造方法的参数是否使用注入方式
if (args == null) {
synchronized (mbd.constructorArgumentLock) {
if (mbd.resolvedConstructorOrFactoryMethod != null) {
resolved = true;
autowireNecessary = mbd.constructorArgumentsResolved;
}
}
}
if (resolved) {
if (autowireNecessary) {
//使用构造函数注入方式实例化
return autowireConstructor(beanName, mbd, null, null);
}
else {
//实例化对象
return instantiateBean(beanName, mbd);
}
}

// 获取构造函数参数
Constructor<?>[] ctors = determineConstructorsFromBeanPostProcessors(beanClass, beanName);
if (ctors != null || mbd.getResolvedAutowireMode() == AUTOWIRE_CONSTRUCTOR ||
mbd.hasConstructorArgumentValues() || !ObjectUtils.isEmpty(args)) {
return autowireConstructor(beanName, mbd, ctors, args);
}

// Preferred constructors for default construction?
ctors = mbd.getPreferredConstructors();
if (ctors != null) {
return autowireConstructor(beanName, mbd, ctors, null);
}

// No special handling: simply use no-arg constructor.
return instantiateBean(beanName, mbd);
}

实例化方法instantiateBean最终会调用SimpleInstantiationStrategy.instantiate 进行实例化

instantiate

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public Object instantiate(RootBeanDefinition bd, @Nullable String beanName, BeanFactory owner) {
// Don't override the class with CGLIB if no overrides.
if (!bd.hasMethodOverrides()) {
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(); //获取构造函数
}
bd.resolvedConstructorOrFactoryMethod = constructorToUse;
}
catch (Throwable ex) {
throw new BeanInstantiationException(clazz, "No default constructor found", ex);
}
}
}
return BeanUtils.instantiateClass(constructorToUse); //调用构造函数进行实例化
}
else {
// Must generate CGLIB subclass.
return instantiateWithMethodInjection(bd, beanName, owner);
}
}

instantiateClass

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@Override
public Object instantiate(RootBeanDefinition bd, @Nullable String beanName, BeanFactory owner) {
// Don't override the class with CGLIB if no overrides.
if (!bd.hasMethodOverrides()) {
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);
}
}
}
return BeanUtils.instantiateClass(constructorToUse); //调用构造器进行初始化
}
else {
// Must generate CGLIB subclass.
return instantiateWithMethodInjection(bd, beanName, owner);
}
}

这里要注意下先判断bean是否有方法重写的,没有则使用反射生成的构造器,有就使用gclib方式创建代理对象,具体实现方式就在org.springframework.beans.factory.support.SimpleInstantiationStrategy.instantiate,有兴趣同学可以去学习下。
到此一个简单bean实例化完成了。

注入

下面进入IOC另一个特点,bean注入,先从AbstractAutowireCapableBeanFactory.populateBean方法开始

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protected void populateBean(String beanName, RootBeanDefinition mbd, @Nullable BeanWrapper bw) {
// 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.
//通过InstantiationAwareBeanPostProcessors.postProcessAfterInstantiation 如果返回true,目标实例内部的返回值会被populate,否则populate这个过程会被忽视
//翻译说如果返回true可以执行字段注入 真的6666啊
if (!mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors()) {
for (InstantiationAwareBeanPostProcessor bp : getBeanPostProcessorCache().instantiationAware) {
if (!bp.postProcessAfterInstantiation(bw.getWrappedInstance(), beanName)) {
return;
}
}
}

PropertyValues pvs = (mbd.hasPropertyValues() ? mbd.getPropertyValues() : null);
//获取注入方式分布有4种
int resolvedAutowireMode = mbd.getResolvedAutowireMode();
if (resolvedAutowireMode == AUTOWIRE_BY_NAME || resolvedAutowireMode == AUTOWIRE_BY_TYPE) {
MutablePropertyValues newPvs = new MutablePropertyValues(pvs);
// Add property values based on autowire by name if applicable.
if (resolvedAutowireMode == AUTOWIRE_BY_NAME) {
autowireByName(beanName, mbd, bw, newPvs);
}
// Add property values based on autowire by type if applicable.
if (resolvedAutowireMode == AUTOWIRE_BY_TYPE) {
autowireByType(beanName, mbd, bw, newPvs);
}
pvs = newPvs;
}

boolean hasInstAwareBpps = hasInstantiationAwareBeanPostProcessors();
//依赖方式,模式都是没有类型检查,这种依赖方式一般都是xml 配置用得比较多,没有配置这里都是返回false
boolean needsDepCheck = (mbd.getDependencyCheck() != AbstractBeanDefinition.DEPENDENCY_CHECK_NONE); /

PropertyDescriptor[] filteredPds = null;
if (hasInstAwareBpps) {
if (pvs == null) {
pvs = mbd.getPropertyValues();
}
for (InstantiationAwareBeanPostProcessor bp : getBeanPostProcessorCache().instantiationAware) {
PropertyValues pvsToUse = bp.postProcessProperties(pvs, bw.getWrappedInstance(), beanName); //获取注解标注需要注入方法或者是字段,并且进行注入
if (pvsToUse == null) {
if (filteredPds == null) {
filteredPds = filterPropertyDescriptorsForDependencyCheck(bw, mbd.allowCaching);
}
pvsToUse = bp.postProcessPropertyValues(pvs, filteredPds, bw.getWrappedInstance(), beanName);
if (pvsToUse == null) {
return;
}
}
pvs = pvsToUse;
}
}
if (needsDepCheck) {
if (filteredPds == null) {
filteredPds = filterPropertyDescriptorsForDependencyCheck(bw, mbd.allowCaching);
}
checkDependencies(beanName, mbd, filteredPds, pvs);
}

if (pvs != null) {
applyPropertyValues(beanName, mbd, bw, pvs);
}
}

小知识点:
AutowireCapableBeanFactory.AUTOWIRE_NO 表明不会对当前Bean进行外部类的注入,常规使用@Autowire、@Resource 都是这类型
剩下三种都是通过xml 或者 AutowireCapableBeanFactory.autowire(Class<?> beanClass, int autowireMode, boolean dependencyCheck) 进行设置autowireMode 。

根据上面代码可以知道主流程bean注入都是由InstantiationAwareBeanPostProcessor 进行处理的,简单说明接口方法

方法 描述
postProcessBeforeInitialization 方法是最 先执行的方法,它在目标对象实例化之前调用,该方法的返回值类型是Object,我们可以返回任何类型的值。由于这个时候目标对象还未实例化,所以这个返回值可以用来代替原本该生成的目标对象的实例(比如代理对象)。如果该方法的返回值代替原本该生成的目标对象,后续只有postProcessAfterInitialization方法会调用,其它方法不再调用;否则按照正常的流程走
postProcessAfterInitialization 方法在目标对象实例化之后调用,这个时候对象已经被实例化,但是该实例的属性还未被设置,都是null。因为它的返回值是决定要不要调用postProcessPropertyValues方法的其中一个因素(因为还有一个因素是mbd.getDependencyCheck());如果该方法返回false,并且不需要check,那么postProcessPropertyValues就会被忽略不执行;如果返回true,postProcessPropertyValues就会被执行
postProcessPropertyValues 对bean属性值赋值后调用,对属性值的修改。如果postProcessAfterInstantiation方法返回false,该方法可能不会被调用。可以在该方法内对属性值进行修改
postProcessProperties Bean属性赋值就是调用这个方法的

InstantiationAwareBeanPostProcessor 接口实现类主要分3个

  1. ConfigurationClassPostProcessor:看类名就知道处理@Configuration实例化,并没有属性注入逻辑,不详讲略过。
  2. CommonAnnotationBeanPostProcessor:这个类就是实现bean注入,但是是实现JSR-250 注解、@Resource,@EJB、@WebServiceRef,@WebServiceContext,@PostConstrusct、@PreDestory这些注解实现。
  3. AutowiredAnnotationBeanPostProcessor:实现 @Autowired、@Value注入,并且支持JSR-330’s @Inject,主要分析这个类就可以知道bean 注入的。

AutowiredAnnotationBeanPostProcessor分析

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private final Set<Class<? extends Annotation>> autowiredAnnotationTypes = new LinkedHashSet<>(4);
@SuppressWarnings("unchecked")
public AutowiredAnnotationBeanPostProcessor() {
this.autowiredAnnotationTypes.add(Autowired.class);
this.autowiredAnnotationTypes.add(Value.class);
try {
this.autowiredAnnotationTypes.add((Class<? extends Annotation>)
ClassUtils.forName("javax.inject.Inject", AutowiredAnnotationBeanPostProcessor.class.getClassLoader()));
logger.trace("JSR-330 'javax.inject.Inject' annotation found and supported for autowiring");
}
catch (ClassNotFoundException ex) {
// JSR-330 API not available - simply skip.
}
}

在初始化时就将支持注解加入集合中,再使用扫描器去扫描方法、构造器、字段,如果有这些注解就进行注入。

看下怎么判断是否需要注入的

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@Nullable
private MergedAnnotation<?> findAutowiredAnnotation(AccessibleObject ao) {
MergedAnnotations annotations = MergedAnnotations.from(ao);
for (Class<? extends Annotation> type : this.autowiredAnnotationTypes) {
MergedAnnotation<?> annotation = annotations.get(type);
if (annotation.isPresent()) {
return annotation;
}
}
return null;
}

AccessibleObject 是Method、Field、Constructor 父类。

postProcessProperties 如何实现bean注入的
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public PropertyValues postProcessProperties(PropertyValues pvs, Object bean, String beanName) {
//获取需要注入字段,方法
InjectionMetadata metadata = findAutowiringMetadata(beanName, bean.getClass(), pvs);
try {
metadata.inject(bean, beanName, pvs); //注入
}
catch (BeanCreationException ex) {
throw ex;
}
catch (Throwable ex) {
throw new BeanCreationException(beanName, "Injection of autowired dependencies failed", ex);
}
return pvs;
}
//下面就行获取InjectionMetadata
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());
// 快速从缓存中获取,如果没有加锁去解析,然后在结果放入缓存中
InjectionMetadata metadata = this.injectionMetadataCache.get(cacheKey);
if (InjectionMetadata.needsRefresh(metadata, clazz)) {
synchronized (this.injectionMetadataCache) { //双重检查
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;
}
  • InjectionMetadata 主要是集合bean需要被注入类型,因为已经解析过bean Class信息了,相当于解析结果装起来

看下如何去扫描方法、字段的

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	private InjectionMetadata buildAutowiringMetadata(final Class<?> clazz) {
//从给定注解中判断class 是否携带这个注解
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<>();
//遍历所有Field,找出扫描的注解,特意标注不支持static 修饰field
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;
}
// 获取注解内 required 值
boolean required = determineRequiredStatus(ann);
currElements.add(new AutowiredFieldElement(field, required));
}
});

ReflectionUtils.doWithLocalMethods(targetClass, method -> {
//获取方法上桥接方法,因为泛型类型擦除,要对桥接方法进行安全检查,防止在调用是出现异常
Method bridgedMethod = BridgeMethodResolver.findBridgedMethod(method);
if (!BridgeMethodResolver.isVisibilityBridgeMethodPair(method, bridgedMethod)) {
return;
}
//获取注解
MergedAnnotation<?> ann = findAutowiredAnnotation(bridgedMethod);
//方法安全检查
if (ann != null && method.equals(ClassUtils.getMostSpecificMethod(method, clazz))) {
if (Modifier.isStatic(method.getModifiers())) { //不支持静态方法注入
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));
}
});
// 这样写是为了后面加入排在队列前面,父类属性优先于子类
elements.addAll(0, currElements);
targetClass = targetClass.getSuperclass();
}
while (targetClass != null && targetClass != Object.class); //这里写得很好,向上解析父类,直到是Object 为止

return InjectionMetadata.forElements(elements, clazz);
}

逻辑非常简单,就是根据给定注解去class获取指定的注解,从而获取到需要注入类型,但是几行简单的代码可以看出强大编码能力,学习了👍。
现在需要注入对象已经获取到,看如何注入吧

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public void inject(Object target, @Nullable String beanName, @Nullable PropertyValues pvs) throws Throwable {
Collection<InjectedElement> checkedElements = this.checkedElements;
Collection<InjectedElement> elementsToIterate =
(checkedElements != null ? checkedElements : this.injectedElements);
if (!elementsToIterate.isEmpty()) {
for (InjectedElement element : elementsToIterate) {
element.inject(target, beanName, pvs);
}
}
}

@Override
protected void inject(Object bean, @Nullable String beanName, @Nullable PropertyValues pvs) throws Throwable {
Field field = (Field) this.member;
Object value;
if (this.cached) {
try {
value = resolvedCachedArgument(beanName, this.cachedFieldValue);
}
catch (NoSuchBeanDefinitionException ex) {
// Unexpected removal of target bean for cached argument -> re-resolve
value = resolveFieldValue(field, bean, beanName);
}
}
else {
value = resolveFieldValue(field, bean, beanName);
}
if (value != null) {
ReflectionUtils.makeAccessible(field);
field.set(bean, value);
}
}

private Object resolveFieldValue(Field field, Object bean, @Nullable String beanName) {
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(); //类型转换器
Object value;
try {
value = beanFactory.resolveDependency(desc, beanName, autowiredBeanNames, typeConverter);
}
catch (BeansException ex) {
throw new UnsatisfiedDependencyException(null, beanName, new InjectionPoint(field), ex);
}
synchronized (this) {
if (!this.cached) {
Object cachedFieldValue = null;
if (value != null || this.required) {
cachedFieldValue = desc;
// 将注入关系添加到容器中,方便bean销毁时同步销毁
registerDependentBeans(beanName, autowiredBeanNames);
if (autowiredBeanNames.size() == 1) {
String autowiredBeanName = autowiredBeanNames.iterator().next();
if (beanFactory.containsBean(autowiredBeanName) &&
beanFactory.isTypeMatch(autowiredBeanName, field.getType())) { //这些都是为了缓存起来
cachedFieldValue = new ShortcutDependencyDescriptor(
desc, autowiredBeanName, field.getType());
}
}
}
this.cachedFieldValue = cachedFieldValue;
this.cached = true;
}
}
return value;
}
}

主要核心是如从缓存获取到需要注入类型实例在beanFactory.resolveDependency中 进入DefaultListableBeanFactory看下

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public Object resolveDependency(DependencyDescriptor descriptor, @Nullable String requestingBeanName,
@Nullable Set<String> autowiredBeanNames, @Nullable TypeConverter typeConverter) throws BeansException {

descriptor.initParameterNameDiscovery(getParameterNameDiscoverer());
if (Optional.class == descriptor.getDependencyType()) {
return createOptionalDependency(descriptor, requestingBeanName);
}
else if (ObjectFactory.class == descriptor.getDependencyType() ||
ObjectProvider.class == descriptor.getDependencyType()) {
return new DependencyObjectProvider(descriptor, requestingBeanName);
}
else if (javaxInjectProviderClass == descriptor.getDependencyType()) {
return new Jsr330Factory().createDependencyProvider(descriptor, requestingBeanName);
}
else {
//懒加载 扫描@Lazy注解,返回一个代理对象
Object result = getAutowireCandidateResolver().getLazyResolutionProxyIfNecessary(
descriptor, requestingBeanName);
if (result == null) {
result = doResolveDependency(descriptor, requestingBeanName, autowiredBeanNames, typeConverter);
}
return result;
}
}

@Lazy 使用注解修饰bean 或者Class,在容器初始化化时不会立刻创建,只要需要使用bean才会创建的。
根据类型Optional、ObjectFactory、Provider,还有懒加载情景不同的处理,这些处理本质都是要调用doResolveDependency方法初始化对象,无论那种对象都要 获取原始对象然后再交给这些接口去包装增强。

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public Object doResolveDependency(DependencyDescriptor descriptor, @Nullable String beanName,
@Nullable Set<String> autowiredBeanNames, @Nullable TypeConverter typeConverter) throws BeansException {
//如果这个注入是通过构造器注入,可以从构造器解析缓存中去获取注入信息点
InjectionPoint previousInjectionPoint = ConstructorResolver.setCurrentInjectionPoint(descriptor);
try {
Object shortcut = descriptor.resolveShortcut(this);
if (shortcut != null) {
return shortcut;
}

Class<?> type = descriptor.getDependencyType();
//尝试从注解中获取默认值 @Value 的value
Object value = getAutowireCandidateResolver().getSuggestedValue(descriptor);
if (value != null) {
if (value instanceof String) {
String strVal = resolveEmbeddedValue((String) value);
BeanDefinition bd = (beanName != null && containsBean(beanName) ?
getMergedBeanDefinition(beanName) : null);
value = evaluateBeanDefinitionString(strVal, bd);
}
TypeConverter converter = (typeConverter != null ? typeConverter : getTypeConverter());
try {
return converter.convertIfNecessary(value, type, descriptor.getTypeDescriptor());
}
catch (UnsupportedOperationException ex) {
// A custom TypeConverter which does not support TypeDescriptor resolution...
return (descriptor.getField() != null ?
converter.convertIfNecessary(value, type, descriptor.getField()) :
converter.convertIfNecessary(value, type, descriptor.getMethodParameter()));
}
}
//多种混合类型处理,stream、collection、Map Array 这些
Object multipleBeans = resolveMultipleBeans(descriptor, beanName, autowiredBeanNames, typeConverter);
if (multipleBeans != null) {
return multipleBeans;
}

//根据类型获取容器中bean名,返回map key就是bean名,value 初始从容器中获取对象,如果没有找到就会抛出异常了
Map<String, Object> matchingBeans = findAutowireCandidates(beanName, type, descriptor);
if (matchingBeans.isEmpty()) {
if (isRequired(descriptor)) {
raiseNoMatchingBeanFound(type, descriptor.getResolvableType(), descriptor);
}
return null;
}

String autowiredBeanName;
Object instanceCandidate;

if (matchingBeans.size() > 1) { //出现一个类型,不同实例,可以根据@Primary, @Priority、属性名方式去配置
autowiredBeanName = determineAutowireCandidate(matchingBeans, descriptor);
if (autowiredBeanName == null) {
if (isRequired(descriptor) || !indicatesMultipleBeans(type)) { //没有确定,抛出异常
return descriptor.resolveNotUnique(descriptor.getResolvableType(), matchingBeans);
}
else {
return null;
}
}
instanceCandidate = matchingBeans.get(autowiredBeanName);
}
else {
// We have exactly one match.
Map.Entry<String, Object> entry = matchingBeans.entrySet().iterator().next();
autowiredBeanName = entry.getKey();
instanceCandidate = entry.getValue();
}

if (autowiredBeanNames != null) {
autowiredBeanNames.add(autowiredBeanName);
}
if (instanceCandidate instanceof Class) { //这里其实就是从容器中获取实例,如果这时候没有初始化,就走上面初始化流程
instanceCandidate = descriptor.resolveCandidate(autowiredBeanName, type, this);
}
Object result = instanceCandidate;
if (result instanceof NullBean) {
if (isRequired(descriptor)) {
raiseNoMatchingBeanFound(type, descriptor.getResolvableType(), descriptor);
}
result = null;
}
if (!ClassUtils.isAssignableValue(type, result)) {
throw new BeanNotOfRequiredTypeException(autowiredBeanName, type, instanceCandidate.getClass());
}
return result;
}
finally {
ConstructorResolver.setCurrentInjectionPoint(previousInjectionPoint);
}
}

这个方法简单做个总结,先是处理 @Value 情况,然后通过findAutowireCandidates 通过类型去容器中获取实例,如何实例还没有初始化,就会调用上面那个初始化过程,将初始化对象返回。根据注入类型进行相应处理,像stream、Collection,这些混合类型都是直接添加进去。如果出现了一个类型多个bean情况,这时就是就是@Primary、@Priority这些注解来判断或者根据属性名去和beanName匹配,最后将bean对象返回。
这里就简单看完一个bean初始化流程了。

总结

现在知道了Bean实例化是由一个策略模式,使用反射攻击类创建的,和BeanPostProcessor其实并没有太多关系的。像我刚开始学spring时,老师就说@Autowired 和@Resources向比较,基于类型和beanName进行注入的,这样说不完全正确的。他是通过类型去获取bean,如果出现一个类型有多个beanName,才通过bean和属性名进行注入。使用这么多年Spring了,从来没有使用过@DependsOn、@Primary、@Priority、@Lookup如果不看源码还不知道有这个特性呢。看完整个源码,对bean生命周期有了比较清晰 bean实例化-> 属性注入-> 执行初始化方法-> 加入spring容器