Spring源码
参考资料
https://www.bilibili.com/video/BV1Tz4y1a7FM
https://www.bilibili.com/video/BV1iz4y1b75q
bean的生命周期
bean–>推断构造方法（默认是无参构造，或指定的构造方法）–>实例化成普通对象（相当于new bean）
–>进行依赖注入（bean里的属性）–>执行afterPropertiesSet()（InitializingBean的一个回调方法，同@PostConstruct）
–>初始化后是否需要AOP–>AOP–>代理对象（target=普通对象）–>把对象放入Map<beanname,bean对象>单例池
//真正创建Bean的方法
protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final @Nullable Object[] args)
throws BeanCreationException {
// Instantiate the bean.
//封装被创建的Bean对象
BeanWrapper instanceWrapper = null;
if (mbd.isSingleton()) {
instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
}
if (instanceWrapper == null) {
instanceWrapper = createBeanInstance(beanName, mbd, args);
}
final Object bean = instanceWrapper.getWrappedInstance();
//获取实例化对象的类型
Class<?> beanType = instanceWrapper.getWrappedClass();
if (beanType != NullBean.class) {
mbd.resolvedTargetType = beanType;
}
// Allow post-processors to modify the merged bean definition.
//调用PostProcessor后置处理器
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.
//向容器中缓存单例模式的Bean对象，以防循环引用
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");
}
//这里是一个匿名内部类，为了防止循环引用，尽早持有对象的引用
addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));
}
// Initialize the bean instance.
//Bean对象的初始化，依赖注入在此触发
//这个exposedObject在初始化完成之后返回作为依赖注入完成后的Bean
Object exposedObject = bean;
try {
//将Bean实例对象封装，并且Bean定义中配置的属性值赋值给实例对象
populateBean(beanName, mbd, instanceWrapper);
//初始化Bean对象
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) {
//获取指定名称的已注册的单例模式Bean对象
Object earlySingletonReference = getSingleton(beanName, false);
if (earlySingletonReference != null) {
//根据名称获取的已注册的Bean和正在实例化的Bean是同一个
if (exposedObject == bean) {
//当前实例化的Bean初始化完成
exposedObject = earlySingletonReference;
}
//当前Bean依赖其他Bean，并且当发生循环引用时不允许新创建实例对象
else if (!this.allowRawInjectionDespiteWrapping && hasDependentBean(beanName)) {
String[] dependentBeans = getDependentBeans(beanName);
Set<String> actualDependentBeans = new LinkedHashSet<>(dependentBeans.length);
//获取当前Bean所依赖的其他Bean
for (String dependentBean : dependentBeans) {
//对依赖Bean进行类型检查
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.
//注册完成依赖注入的Bean
try {
registerDisposableBeanIfNecessary(beanName, bean, mbd);
}
catch (BeanDefinitionValidationException ex) {
throw new BeanCreationException(
mbd.getResourceDescription(), beanName, "Invalid destruction signature", ex);
}
return exposedObject;
}
//初始容器创建的Bean实例对象，为其添加BeanPostProcessor后置处理器
protected Object initializeBean(final String beanName, final Object bean, @Nullable RootBeanDefinition mbd) {
//JDK的安全机制验证权限
if (System.getSecurityManager() != null) {
//实现PrivilegedAction接口的匿名内部类
AccessController.doPrivileged((PrivilegedAction<Object>) () -> {
invokeAwareMethods(beanName, bean);
return null;
}, getAccessControlContext());
}
else {
//为Bean实例对象包装相关属性，如名称，类加载器，所属容器等信息
invokeAwareMethods(beanName, bean);
}
Object wrappedBean = bean;
//对BeanPostProcessor后置处理器的postProcessBeforeInitialization
//回调方法的调用，为Bean实例初始化前做一些处理
if (mbd == null || !mbd.isSynthetic()) {
wrappedBean = applyBeanPostProcessorsBeforeInitialization(wrappedBean, beanName);
}
//调用Bean实例对象初始化的方法，这个初始化方法是在Spring Bean定义配置
//文件中通过init-method属性指定的
try {
invokeInitMethods(beanName, wrappedBean, mbd);
}
catch (Throwable ex) {
throw new BeanCreationException(
(mbd != null ? mbd.getResourceDescription() : null),
beanName, "Invocation of init method failed", ex);
}
//对BeanPostProcessor后置处理器的postProcessAfterInitialization
//回调方法的调用，为Bean实例初始化之后做一些处理
if (mbd == null || !mbd.isSynthetic()) {
wrappedBean = applyBeanPostProcessorsAfterInitialization(wrappedBean, beanName);
}
return wrappedBean;
}
三级缓存解决循环依赖
循环依赖
在A创建的过程中，A的属性B需要注入属性A
- singletonObjects（一级缓存）
- earlysingletonObjects（二级缓存）
- singletonFactories（三级缓存）
- private final Map<String, Object> singletonObjects = new ConcurrentHashMap(256);
private final Map<String, ObjectFactory<?>> singletonFactories = new HashMap(16);
private final Map<String, Object> earlySingletonObjects = new ConcurrentHashMap(16);
- 创建的A放入 Set singletonsCurrentlyInCreation（只要创建就放入set，表示bean正在初始化）
- 首先是实例化，new 普通对象，addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));为了防止循环引用，尽早持有对象的引用，第二个参数是一个lamda表达式可用来获取普通对象
- 在A填充B时，B需要注入A，判断A正在初始化，此时出现循环依赖
- 先向二级缓存里查找，在多方循环中保证创建的是都是同一个代理对象A
- 再向三级缓存里查找，没有则执行lamda获取普通对象，并且进行AOP，生成代理对象
- 放入bean到二级缓存中
- 填充其他的属性
- 进行Aop的逻辑，若是循环依赖则会提前进行Aop（step5）而不是现在
- 在二级缓存里查找，是不是已经有已经生成好的bean
- 添加到单例池
- Set中removeA，标识初始化完成
