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这篇文章主要讲解了“Spring Ioc中Bean的加载实现方法”，文中的讲解内容简单清晰，易于学习与理解，下面请大家跟着小编的思路慢慢深入，一起来研究和学习“Spring Ioc中Bean的加载实现方法”吧！
代码：
//AbstractAutowireCapableBeanFactory.java
protected Object createBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args)
throws BeanCreationException {
if (logger.isDebugEnabled()) {
logger.debug("Creating instance of bean '" + beanName + "'");
}
RootBeanDefinition mbdToUse = mbd;
// Make sure bean class is actually resolved at this point, and
// clone the bean definition in case of a dynamically resolved Class
// which cannot be stored in the shared merged bean definition.
//判断需要创建的Bean是否可以实例化，即是否可以通过当前的类加载器加载
Class<?> resolvedClass = resolveBeanClass(mbd, beanName);
if (resolvedClass != null && !mbd.hasBeanClass() && mbd.getBeanClassName() != null) {
mbdToUse = new RootBeanDefinition(mbd);
mbdToUse.setBeanClass(resolvedClass);
}
// Prepare method overrides.
//校验和准备Bean中的方法覆盖
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.
//如果Bean配置了初始化前和初始化后的处理器，则试图返回一个需要创建Bean的代理对象
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 {
//创建Bean的入口
Object beanInstance = doCreateBean(beanName, mbdToUse, args);
if (logger.isDebugEnabled()) {
logger.debug("Finished creating instance of bean '" + beanName + "'");
}
return beanInstance;
}
catch (BeanCreationException ex) {
// A previously detected exception with proper bean creation context already...
throw ex;
}
catch (ImplicitlyAppearedSingletonException ex) {
// An IllegalStateException to be communicated up to DefaultSingletonBeanRegistry...
throw ex;
}
catch (Throwable ex) {
throw new BeanCreationException(
mbdToUse.getResourceDescription(), beanName, "Unexpected exception during bean creation", ex);
}
}这段代码分为以下几个步骤：
判断需要创建的Bean是否可以实例化，即是否可以通过当前的类加载器加载
校验和准备Bean中的方法注入
如果Bean配置了初始化前和初始化后的处理器，则试图返回一个需要创建Bean的代理对象
创建Bean
主要是获取bean的class，并设置到BeanDefinition中
主要是处理方法注入
代码：
public void prepareMethodOverrides() throws BeanDefinitionValidationException {
// Check that lookup methods exists.
//检测是否存在方法注入,并循环预处理方法注入
if (hasMethodOverrides()) {
Set<MethodOverride> overrides = getMethodOverrides().getOverrides();
synchronized (overrides) {
//遍历处理
for (MethodOverride mo : overrides) {
prepareMethodOverride(mo);
}
}
}
}prepareMethodOverride(mo)：
protected void prepareMethodOverride(MethodOverride mo) throws BeanDefinitionValidationException {
// 统计注入的方法个数
int count = ClassUtils.getMethodCountForName(getBeanClass(), mo.getMethodName());
if (count == 0) {
throw new BeanDefinitionValidationException(
"Invalid method override: no method with name '" + mo.getMethodName() +
"' on class [" + getBeanClassName() + "]");
}
// 如果为1,则将注入方法标记为未重载
// 注意:当有多个重载方法时,为了确定调用哪个具体的方法,Spring对重载方法的参数解析是很复杂的
// 所以,如果注入方法没有被重载这里就将其标记,省去了对方法参数的解析过程,直接调用即可
else if (count == 1) {
// Mark override as not overloaded, to avoid the overhead of arg type checking.
mo.setOverloaded(false);
}
}代码读到这里，大家可能有疑问，从代码上看明明是处理的方法重载，但是为什么处理的是方法注入呢？而且如果我们在bean里设置几个方法重载的话，hasMethodOverrides()方法返回的是false。如果我们打开 AbstractBeanDefinition 类的 hasMethodOverrides() 方法，就能打消我们之前的疑问。
public boolean hasMethodOverrides() {
return (this.methodOverrides != null && !this.methodOverrides.isEmpty());
}其中methodOverrides是做什么的呢？通过类名AbstractBeanDefinition我们可以发现，该类是BeanDefinition的一个子类，那么它保存的应该是我们解析到的beanDefinition，spring在解析配置文件的时候，如果发现配置了replace-method或者lookup-method那么，就会对应的标签解析，并存入到 AbstractBeanDefinition 的 methodOverrides 属性中，那么当bean实例化的时候，如果检测到了methodOverrides属性不为空，则动态的为当前bean生成代理并使用相应的拦截器对bean做处理，这里大家只要把概念搞清楚即可。
主要是对bean前置后置处理器的处理，给 BeanPostProcessors 后置处理器一个返回代理对象的机会
详细代码：
protected Object resolveBeforeInstantiation(String beanName, RootBeanDefinition mbd) {
Object bean = null;
if (!Boolean.FALSE.equals(mbd.beforeInstantiationResolved)) {
// Make sure bean class is actually resolved at this point.
if (!mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors()) {
Class<?> targetType = determineTargetType(beanName, mbd);
if (targetType != null) {
bean = applyBeanPostProcessorsBeforeInstantiation(targetType, beanName);
if (bean != null) {
bean = applyBeanPostProcessorsAfterInitialization(bean, beanName);
}
}
}
mbd.beforeInstantiationResolved = (bean != null);
}
return bean;
}如果代理对象不为空，则直接返回代理对象，这一步骤有非常重要的作用，Spring 后续实现 AOP 就是基于这个地方判断的。
这个方法核心就在于 applyBeanPostProcessorsBeforeInstantiation()和 applyBeanPostProcessorsAfterInitialization() 两个方法，before 为实例化前的后处理器应用，after 为实例化后的后处理器应用。
doCreateBean()
创建Bean
//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.
//BeanWrapper是对Bean的包装，其接口中所定义的功能很简单包括设置获取被包装的对象，获取被包装bean的属性描述器
BeanWrapper instanceWrapper = null;
if (mbd.isSingleton()) {
//单例模式，删除factoryBean缓存
instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
}
if (instanceWrapper == null) {
//使用合适的实例化策略来创建Bean：工厂方法、构造函数自动注入、简单初始化
instanceWrapper = createBeanInstance(beanName, mbd, args);
}
//从包装类中获取实例化的Bean
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.
//检查是否有后置处理
synchronized (mbd.postProcessingLock) {
if (!mbd.postProcessed) {
try {
//调用PostProcessor后置处理器，修改 BeanDefinition
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.
// 解决单例模式的循环依赖
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;
}代码很长，不过别慌，我们来按步骤分析一波
如果是单例模式，从factoryBeanInstanceCache 缓存中获取BeanWrapper 实例对象并删除缓存
调用 createBeanInstance() 实例化 bean
（主要是将 BeanDefinition 转换为 BeanWrapper）
后置处理
单例模式的循环依赖处理
初始化 bean 实例对象
（属性填充）
依赖检查
注册 DisposableBean
doCreateBean() 完成 bean 的创建和初始化工作，内容太多，比较复杂，这里只列出大致流程，接下来我们将分几篇文章来分别阐述相关内容。
感谢各位的阅读，以上就是“Spring Ioc中Bean的加载实现方法”的内容了，经过本文的学习后，相信大家对Spring Ioc中Bean的加载实现方法这一问题有了更深刻的体会，具体使用情况还需要大家实践验证。这里是亿速云，小编将为大家推送更多相关知识点的文章，欢迎关注！
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