Spring源码解析 - IoC容器的依赖注入
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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
- 可在
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.getBean
2.判断是否有 Bean，containsBean
3.判断是否单例 isSingleton
BeanFactory 只对 ioc 容器最基本的行为作了定义,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.
为进一步了解该依赖注入的过程,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
public <T> T getBean(String name, Class<T> requiredType, Object... args) throws BeansException {
// 这些 getBean 接口最终都是通过调用 doGetBean 实现
return doGetBean(name, requiredType, args, false);
}
// 实际取得 Bean 的地方,即触发依赖注入
@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);
顺道看看getSingleton()的实现
protected Object getSingleton(String beanName, boolean allowEarlyReference) {
Object singletonObject = this.singletonObjects.get(beanName);
if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {
synchronized (this.singletonObjects) {
singletonObject = this.earlySingletonObjects.get(beanName);
if (singletonObject == null && allowEarlyReference) {
ObjectFactory<?> singletonFactory = this.singletonFactories.get(beanName);
if (singletonFactory != null) {
singletonObject = singletonFactory.getObject();
this.earlySingletonObjects.put(beanName, singletonObject);
this.singletonFactories.remove(beanName);
}
}
}
}
return (singletonObject != NULL_OBJECT ? singletonObject : null);
}
回过头继续看工厂实现
if (sharedInstance != null && args == null) {
...
//以取得 FactoryBean 的相关处理及生产结果
bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
}
来看看该 getObjectForBeanInstance 方法的实现,和FactoryBean有何关系呢
protected Object getObjectForBeanInstance(Object beanInstance, String name, String beanName, RootBeanDefinition mbd) {
// Don't let calling code try to dereference the factory if the bean isn't a factory.
if (BeanFactoryUtils.isFactoryDereference(name) && !(beanInstance instanceof FactoryBean)) {
throw new BeanIsNotAFactoryException(transformedBeanName(name), beanInstance.getClass());
}
// Now we have the bean instance, which may be a normal bean or a FactoryBean.
// If it's a FactoryBean, we use it to create a bean instance, unless the
// caller actually wants a reference to the factory.
if (!(beanInstance instanceof FactoryBean) || BeanFactoryUtils.isFactoryDereference(name)) {
return beanInstance;
}
Object object = null;
if (mbd == null) {
object = getCachedObjectForFactoryBean(beanName);
}
if (object == null) {
// Return bean instance from factory.
FactoryBean<?> factory = (FactoryBean<?>) beanInstance;
// Caches object obtained from FactoryBean if it is a singleton.
if (mbd == null && containsBeanDefinition(beanName)) {
mbd = getMergedLocalBeanDefinition(beanName);
}
boolean synthetic = (mbd != null && mbd.isSynthetic());
object = getObjectFromFactoryBean(factory, beanName, !synthetic);
}
return object;
}
继续回过头
else {
// 若早已创建该 bean 实例则会失败进入到此
// 假设处在引用循环依赖中
if (isPrototypeCurrentlyInCreation(beanName)) {
throw new BeanCurrentlyInCreationException(beanName);
}
// 检查 bean 定义是否存在于该工厂
BeanFactory parentBeanFactory = getParentBeanFactory();
if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {
// 没有找到,即不存在 -> 检查父类
String nameToLookup = originalBeanName(name);
if (args != null) {
// 有更精确的参数 -> 委托给父类
return (T) parentBeanFactory.getBean(nameToLookup, args);
}
else {
// 无参数 -> 委托给标准的 getBean 方法
return parentBeanFactory.getBean(nameToLookup, requiredType);
}
}
if (!typeCheckOnly) {
markBeanAsCreated(beanName);
}
try {
//根据 Bean 名取得 BeanDefinition
final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
checkMergedBeanDefinition(mbd, beanName, args);
//递归获得当前 Bean 依赖的所有Bean,确保它们的初始化
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);
}
}
// 创建 singleton bean 的地方
if (mbd.isSingleton()) {
sharedInstance = getSingleton(beanName, new ObjectFactory<Object>() {
@Override
public Object getObject() throws BeansException {
try {
// 调用 createBean 创建单例 bean实例
return createBean(beanName, mbd, args);
}
catch (BeansException ex) {
// Explicitly remove instance from singleton ***: 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);
}
// 创建 prototype bean 的地方
else if (mbd.isPrototype()) {
// It's a prototype -> create a ne w 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;
}
依赖注入的发生是在容器中的 BeanDefinition 数据已经建立好的前提下进行的.
2 依赖注入过程
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.
来看看createBean
@Override
protected Object createBean(String beanName, RootBeanDefinition mbd, Object[] args) throws BeanCreationException {
...
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 是否可被实例化,该类是否可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();
}
...
try {
//给 BeanPostProcessor一个机会返回的是一个Proxy***而不是目标 bean 的实例
Object bean = resolveBeforeInstantiation(beanName, mbdToUse);
if (bean != null) {
return bean;
}
}
...
try {
Object beanInstance = doCreateBean(beanName, mbdToUse, args);
...
return beanInstance;
}
...
}
doCreateBean 源码解析
接着到 doCreateBean 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 *** 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;
}
小结
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
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）
createBeanInstance 源码解析
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
在Spring AOP中也使用CGLIB对Java的字节码进行增强.
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
- BeanUtils,使用Java原生的反射功能
- 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);
}
}
}
4 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;
由其涉及面广,仅简要介绍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. - 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;
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，解析也是一个递归的过程
根据依赖关系，层层完成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。
5 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
查看10道真题和解析