初始化的过程,主要完成的工作是在容器中建立 BeanDefinition 数据映射,并没有看到容器对Bean依赖关系进行注入
假设当前IoC容器已经载入用户定义的Bean信息,依赖注入主要发生在两个阶段
正常情况下,由用户第一次向IoC容器索要Bean时触发
但我们可以在 BeanDefinition 信息中通过控制
lazy-init属性来让容器完成对Bean的预实例化,即在初始化的过程中就完成某些Bean的依赖注入的过程
1 getBean触发的依赖注入
BeanFactory 是最原始的 ioc 容器，有以下方法 1.getBean2.判断是否有 Bean，containsBean3.判断是否单例 isSingleton。
BeanFactory 只是对 ioc 容器最基本行为作了定义，而不关心 Bean 是怎样定义和加载的。如果我们想要知道一个工厂具体产生对象的过程，则要看这个接口的实现类。
在基本的容器接口 BeanFactory 中,有一个 getBean接口,这个接口的实现就是触发依赖注入发生的地方.
为了进一步了解这个依赖注入的过程,我们从 DefaultListableBeanFactory的基类AbstractBeanFactory入手去看看getBean的实现
// 这里是对 BeanFactory 接口的实现，比如getBean接口方法
//这些getBean接口方法最终是通过调用doGetBean来实现的
public <T> T getBean(String name, Class<T> requiredType, Object... args) throws BeansException { return doGetBean(name, requiredType, args, false);
} //这里是实际取得Bean的地方,也就是触发依赖注入发生的地方
@SuppressWarnings("unchecked") protected <T> T doGetBean( final String name, final Class<T> requiredType, final Object[] args, boolean typeCheckOnly)
throws BeansException { final String beanName = transformedBeanName(name);
Object bean; //急切地检查单例模式缓存手动注册的单例
//先从缓存中取得Bean,处理那些已经被创建过的单例Bean,这种Bean不要重复创建
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 + "'");
}
} //这里的getObjectForBeanInstance完成的是FactoryBean的相关处理，以取得FactoryBean的相关处理,以取得FactoryBean的生产结果,BeanFactory和FactoryBean的区别已在前面讲过,这个过程在后面还会详细地分析
bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
} else { // Fail if we're already creating this bean instance:
// We're assumably within a circular reference.
if (isPrototypeCurrentlyInCreation(beanName)) { throw new BeanCurrentlyInCreationException(beanName);
} // 检查IoC容器中的BeanDefinition是否存在,若在当前工厂不存在则去顺着双亲BeanFactory链一直向上找
BeanFactory parentBeanFactory = getParentBeanFactory(); if (parentBeanFactory != null && !containsBeanDefinition(beanName)) { // Not found -> check parent.
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 { //根据Bean的名字取得BeanDefinition
final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
checkMergedBeanDefinition(mbd, beanName, args); // Guarantee initialization of beans that the current bean depends on.
//递归获得当前Bean依赖的所有Bean（如果有的话）
String[] dependsOn = mbd.getDependsOn(); if (dependsOn != null) { 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);
getBean(dep);
}
} //通过调用createBean方法创建Singleton bean实例
if (mbd.isSingleton()) {
sharedInstance = getSingleton(beanName, new ObjectFactory<Object>() { @Override
public Object getObject() throws BeansException { try { 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;
}
}
});
bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
} //这里是创建prototype bean的地方
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);
} 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.
// 这里对创建的Bean进行类型检查,如果没有问题,就返回这个新创建的Bean,这个Bean已经是包含了依赖关系的Bean
if (requiredType != null && bean != null && !requiredType.isAssignableFrom(bean.getClass())) { 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;
}依赖注入就是在这里被触发的.
而依赖注入的发生是在容器中的BeanDefinition数据已经建立好的前提下进行的.
虽然我们可以用最简单的方式来描述容器,那就是视其为一个HashMap,但只能说这个HashMap是容器的最基本的数据结构,而不是IoC容器的全部
关于依赖注入过程会在下面详解
图1.1 依赖注入的过程
getBean是依赖注入的起点,之后会调用AbstractAutowireCapableBeanFactory中的createBean来生产需要的Bean,还对Bean初始化进行了处理,比如实现了在BeanDefinition中的init-method属性定义,Bean后置处理器等.
下面通过createBean代码了解这个过程
@Override
protected Object createBean(String beanName, RootBeanDefinition mbd, 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.
//这里判断需要创建的Bean是否可以被实例化,这个类是否可以通过类加载器来载入
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.
//如果Bean配置了PostProcessor,那么这里返回的是一个Proxy
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 { 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);
}
} //接着到doCreate中去看看Bean是怎样生成的
protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final Object[] args) { // Instantiate the bean.
//用来持有创建出来的Bean对象
BeanWrapper instanceWrapper = null; //如果是单例,则先把缓存中的同名Bean清除
if (mbd.isSingleton()) {
instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
} //这里是创建Bean的地方,由createBeanInstance来完成
if (instanceWrapper == null) { //根据指定bean使用对应的策略创建新的实例,如:工厂方法,构造函数自动注入,简单初始化
instanceWrapper = createBeanInstance(beanName, mbd, args);
}
final Object bean = (instanceWrapper != null ? instanceWrapper.getWrappedInstance() : null);
Class<?> beanType = (instanceWrapper != null ? instanceWrapper.getWrappedClass() : null); // Allow post-processors to modify the merged bean definition.
synchronized (mbd.postProcessingLock) { if (!mbd.postProcessed) {
applyMergedBeanDefinitionPostProcessors(mbd, beanType, beanName);
mbd.postProcessed = true;
}
} // Eagerly cache singletons to be able to resolve circular references
// even when triggered by lifecycle interfaces like BeanFactoryAware.
//是否需要提前曝光:单例&允许循环依赖&当前bean正在创建中,检测循环依赖
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");
} //为避免后期循环依赖,可以在bean初始化完成前将创建实例的ObjectFactory加入工厂
addSingletonFactory(beanName, new ObjectFactory<Object>() {
@Override
public Object getObject() throws BeansException { //对bean再次依赖引用,主要应用SMartInstantialiationAware BeanPostProcessor,
//其中我们熟知的AOP就是在这里将advice动态织入bean中,若无则直接返回bean,不做任何处理
return getEarlyBeanReference(beanName, mbd, bean);
}
});
} // Initialize the bean instance.
//这里是对Bean的初始化,依赖注入往往在这里发生,这个exposedObject在初始化处理完后悔返回作为依赖注入完成后的Bean
Object exposedObject = bean; try { //对bean进行填充,将各个属性值注入,其中可能存在依赖于其他bean的属性,则会递归初始化依赖bean
populateBean(beanName, mbd, instanceWrapper); if (exposedObject != null) { //调用初始化方法,比如init-method
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) { Object earlySingletonReference = getSingleton(beanName, false); // earlySingletonReference 只有在检测到有循环依赖的情况下才会非空
if (earlySingletonReference != null) { if (exposedObject == bean) { //如果exposedObject 没有在初始化方法中被改变,也就是没有被增强
exposedObject = earlySingletonReference;
} else if (!this.allowRawInjectionDespiteWrapping && hasDependentBean(beanName)) { String[] dependentBeans = getDependentBeans(beanName); Set<String> actualDependentBeans = new LinkedHashSet<String>(dependentBeans.length); for (String dependentBean : dependentBeans) { //检测依赖
if (!removeSingletonIfCreatedForTypeCheckOnly(dependentBean)) {
actualDependentBeans.add(dependentBean);
}
} //因为bean创建后其所依赖的bean一定是已经创建的,actualDependentBeans非空则表示当前bean创建后其依赖的bean却没有全部创建完,也就是说存在循环依赖
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.
try { //根据scope注册bean
registerDisposableBeanIfNecessary(beanName, bean, mbd);
} catch (BeanDefinitionValidationException ex) { throw new BeanCreationException(mbd.getResourceDescription(), beanName, "Invalid destruction signature", ex);
} return exposedObject;
}依赖注入其实包括两个主要过程
生产Bean所包含的Java对象
Bean对象生成之后,把这些Bean对象的依赖关系设置好
我们从上可以看到与依赖注入关系特别密切的方法有
createBeanInstance
生成Bean包含的Java对象populateBean.
处理对各种Bean对象的属性进行处理的过程（即依赖关系处理的过程）
先来看 createBeanInstance源码
/**
* Create a new instance for the specified bean, using an appropriate instantiation strategy:
* factory method, constructor autowiring, or simple instantiation.
*/
protected BeanWrapper createBeanInstance(String beanName, RootBeanDefinition mbd, Object[] args) { // 确认需要创建的Bean实例的类可以实例化
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) { return obtainFromSupplier(instanceSupplier, beanName);
} //若工厂方法非空,则使用工厂方法策略对Bean进行实例化
if (mbd.getFactoryMethodName() != null) { return instantiateUsingFactoryMethod(beanName, mbd, args);
} // Shortcut when re-creating the same bean...
boolean resolved = false; boolean autowireNecessary = false; if (args == null) { synchronized (mbd.constructorArgumentLock) { //一个类有多个构造函数,每个构造函数都有不同的参数,所以调用前需要先根据参数锁定构造函数或对应的工厂方法
if (mbd.resolvedConstructorOrFactoryMethod != null) {
resolved = true;
autowireNecessary = mbd.constructorArgumentsResolved;
}
}
} //如果已经解析过则使用解析好的构造函数方法不需要再次锁定
if (resolved) { if (autowireNecessary) { //构造函数自动注入
return autowireConstructor(beanName, mbd, null, null);
} else { //使用默认构造函数构造
return instantiateBean(beanName, mbd);
}
} // 使用构造函数对Bean进行实例化
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.
//使用默认的构造函数对Bean进行实例化
return instantiateBean(beanName, mbd);
} //最常见的实例化过程instantiateBean
protected BeanWrapper instantiateBean(final String beanName, final RootBeanDefinition mbd) { //使用默认的实例化策略对Bean进行实例化,默认的实例化策略是
//CglibSubclassingInstantiationStrategy,也就是使用CGLIB实例化Bean
try {
Object beanInstance; final BeanFactory parent = this; if (System.getSecurityManager() != null) {
beanInstance = AccessController.doPrivileged(new PrivilegedAction<Object>() { @Override
public Object run() { return getInstantiationStrategy().instantiate(mbd, beanName, parent);
}
}, getAccessControlContext());
} else {
beanInstance = getInstantiationStrategy().instantiate(mbd, beanName, parent);
}
BeanWrapper bw = new BeanWrapperImpl(beanInstance);
initBeanWrapper(bw); return bw;
} catch (Throwable ex) { throw new BeanCreationException(
mbd.getResourceDescription(), beanName, "Instantiation of bean failed", ex);
}
}这里使用了CGLIB对Bean进行实例化
CGLIB是一个字节码生成器的类库,它提供了一系列的API来提供生成和转换Java的字节码的功能.
在Spring AOP中也使用CGLIB对Java的字节码进行增强.
在IoC容器中,要了解怎样使用CGLIB来生成Bean对象,需要看一下SimpleInstantiationStrategy类.
它是Spring用来生成Bean对象的默认类,它提供了两种实例化Bean对象的方法
通过BeanUtils,使用Java原生的反射功能
通过CGLIB
public class SimpleInstantiationStrategy implements InstantiationStrategy {@Override
public Object instantiate(RootBeanDefinition bd, String beanName, BeanFactory owner) { // Don't override the class with CGLIB if no overrides.
if (bd.getMethodOverrides().isEmpty()) { //这里取得指定的构造器或者生成对象的工厂方法来对Bean进行实例化
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(new PrivilegedExceptionAction<Constructor<?>>() { @Override
public Constructor<?> run() throws Exception { return clazz.getDeclaredConstructor((Class[]) null);
}
});
} else {
constructorToUse = clazz.getDeclaredConstructor((Class[]) null);
}
bd.resolvedConstructorOrFactoryMethod = constructorToUse;
} catch (Throwable ex) { throw new BeanInstantiationException(clazz, "No default constructor found", ex);
}
}
} //通过BeanUtils进行实例化，这个BeanUtils的实例化通过Constructor来实例化Bean,在BeanUtils中可以看到具体的调用ctor.newInstance(args)
return BeanUtils.instantiateClass(constructorToUse);
} else {
// 使用CGLIB来实例化对象
return instantiateWithMethodInjection(bd, beanName, owner);
}
}
}2 Bean之间依赖关系的处理
依赖关系处理的入口是前面提到的populateBean方法.
由于其中涉及的面太多,在这里就不贴代码了.
简要介绍一下依赖关系处理的流程:
在populateBean方法中,
首先取得在BeanDefinition中设置的property值,然后开始依赖注入的过程
首先处理autowire的注入，可以by Name/Type，之后对属性进行注入
接着需要对Bean Reference进行解析，在对ManageList、ManageSet、ManageMap等进行解析完之后，就已经为依赖注入准备好了条件，这是真正把Bean对象设置到它所依赖的另一个Bean属性中去的地方，其中处理的属性是各种各样的
依赖注入发生在BeanWrapper的setPropertyValues中，具体的完成却是在BeanWrapper的子类BeanWrapperImpl中实现的，它会完成Bean的属性值的注入，其中包括对Array的注入、对List等集合类以及对非集合类的域进行注入
经过一系列的注入，这样就完成了对各种Bean属性的依赖注入过程
在Bean的创建和对象依赖注入的过程中，需要依据BeanDefinition中的信息来递归地完成依赖注入。
从前面的几个递归过程中可以看到，这些递归都是以getBean为入口
一个递归是在上下文中查找需要的Bean和创建Bean的递归调用
另一个递归是在依赖注入时，通过递归调用容器的getBean方法，得到当前Bean的依赖Bean，同时也触发对依赖Bean的创建和注入。
在对Bean的属性进行依赖注入时，解析的过程也是一个递归的过程
这样，根据依赖关系，一层层地完成Bean的创建和注入，直到最后完成当前Bean的创建
有了这个顶层Bean的创建和对它属性依赖注入的完成，意味着和当前Bean相关的整个依赖链的注入也就完成了
在Bean创建和依赖注入完成后,在容器中建立起一系列依靠依赖关系联系起来的Bean，这个Bean已经不再是简单的Java对象了。该Bean系列以及Bean之间的依赖关系建立完成之后，通过IoC的相关接口方法，就可以非常方便地供上层应用使用了。
2. lazy-init属性和预实例化
在前面的refresh中，可看到调用了finishBeanFactoryInitialization来对配置了lazy-init的Bean进行处理
其实在这个方法中,封装了对lazy-init属性的处理,实际的处理是在DefaultListableBeanFactory这个基本容器的preInstantiateSingleton方法中完成的
该方法对单例Bean完成预实例化，这个预实例化的完成巧妙地委托给了容器来实现
如果需要预实例化,那么就直接在这里采用getBean去触发依赖注入
作者：芥末无疆sss
链接：https://www.jianshu.com/p/08c8c7953289
來源：简书
简书著作权归作者所有，任何形式的转载都请联系作者获得授权并注明出处。
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