本文出处Spring Bean如何初始化的 转载请说明出处做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。
/** 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 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 earlySingletonObjects = new ConcurrentHashMap<>(16);
/** 三级缓存装载创建bean的工厂对象 */
private final Map> 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。protected T doGetBean(
String name, @Nullable Class 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
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 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
- 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。
- 对实例化bean进行属性注入
- 执行初始化方法，DisposableBean接口加入到disposableBeans容器中
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
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>) 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
@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>) 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 |
- 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。
- 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。
- 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。
private final Set> autowiredAnnotationTypes = new LinkedHashSet<>(4);
@SuppressWarnings("unchecked")
public AutowiredAnnotationBeanPostProcessor() {
this.autowiredAnnotationTypes.add(Autowired.class);
this.autowiredAnnotationTypes.add(Value.class);
try {
this.autowiredAnnotationTypes.add((Class)
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
@Nullable
private MergedAnnotation findAutowiredAnnotation(AccessibleObject ao) {
MergedAnnotations annotations = MergedAnnotations.from(ao);
for (Class 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
public PropertyValues postProcessProperties(PropertyValues pvs, Object bean, String beanName) {
//获取需要注入字段，方法
InjectionMetadata metadata = https://www.it610.com/article/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 = https://www.it610.com/article/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
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 elements = new ArrayList<>();
Class targetClass = clazz;
do {
final List 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
public void inject(Object target, @Nullable String beanName, @Nullable PropertyValues pvs) throws Throwable {
Collection checkedElements = this.checkedElements;
Collection 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 = https://www.it610.com/article/resolvedCachedArgument(beanName, this.cachedFieldValue);
}
catch (NoSuchBeanDefinitionException ex) {
// Unexpected removal of target bean for cached argument -> re-resolve
value = https://www.it610.com/article/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 autowiredBeanNames = new LinkedHashSet<>(1);
Assert.state(beanFactory != null, "No BeanFactory available");
TypeConverter typeConverter = beanFactory.getTypeConverter();
//类型转换器
Object value;
try {
value = https://www.it610.com/article/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
public Object resolveDependency(DependencyDescriptor descriptor, @Nullable String requestingBeanName,
@Nullable Set 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。
public Object doResolveDependency(DependencyDescriptor descriptor, @Nullable String beanName,
@Nullable Set 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 = https://www.it610.com/article/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 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 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
【Spring Bean如何初始化的】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
推荐阅读
- Spring 注解开发
- Spring DI依赖注入
- Spring bean的作用域
- Spring入门(一)
- Spring 创建对象的方式
- @Bean与@Component 用在同一个类上，会怎么样（）
- 最全的Spring依赖注入方式，你都会了吗（）
- 数据转pdf（包含echarts图表）
- spring之源码环境构建