前言
本文基于springboot版本2.5.1
<parent>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-parent</artifactId>
<version>2.5.1</version>
<relativePath/> <!-- lookup parent from repository -->
</parent>
本文主要聚焦在循环依赖部分，主要用单例bean来进行讲解，其他bean实现的流程不会过多涉及。
1、什么叫循环依赖呢
简单来说就是springboot容器中的多个bean，如A、B两个bean，A有属性B需要注入，B有属性A需要注入，形成相互依赖的情况。
看下代码，就是类似下面这种情况
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Component;
@Component
public class ServiceA {
@Autowired
private ServiceB serviceB;
}
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Component;
@Component
public class ServiceB {
@Autowired
private ServiceA serviceA;
}
上面有两个bean，分别是ServiceA,ServiceB。ServiceA中需要注入ServiceB的实例，ServiceB中需要注入ServiceA的实例，这就是一种典型的循环依赖，其他还有方法参数循环依赖的场景等等，但是它们的内部实现基本是一样的。
2、具体出现循环依赖的代码逻辑
获取bean的方法
在springboot中默认的beanFactory是DefaultListableBeanFactory，在我们获取bean对象的时候，如果bean对象存在就直接返回，如果不存在，就先创建bean对象再返回。
我们先看下我们获取bean的常用方法都有哪些
public <T> T getBean(Class<T> requiredType) throws BeansException public Object getBean(String name) throws BeansException public <T> Map<String, T> getBeansOfType(@Nullable Class<T> type) throws BeansException public Map<String, Object> getBeansWithAnnotation(Class<? extends Annotation> annotationType) public void preInstantiateSingletons() throws BeansException
常用的获取bean的方法主要有上面几个和它们的重载版本，对于第3行、第4行、第5行最终都会调用到第2行的方法来获取bean。而它也会通过调用doGetBean(在AbstractBeanFactory这个类中)来获取bean
public Object getBean(String name) throws BeansException {
return doGetBean(name, null, null, false);
}
第1行的方法也会调用doGetBean来获取bean
public <T> T getBean(String name, @Nullable Class<T> requiredType, @Nullable Object... args)
throws BeansException {
return doGetBean(name, requiredType, args, false);
}
所有最终获取bean的方法都是
protected <T> T doGetBean(
String name, @Nullable Class<T> requiredType, @Nullable Object[] args, boolean typeCheckOnly)
throws BeansException {
这个方法，这个方法是protected的，是不对外提供的。所以我们不能直接调用它，只能通过上面提供的5个方法来获取bean对象。
下面我们从doGetBean这里来看下serviceA创建的过程
protected <T> T doGetBean(
String name, @Nullable Class<T> requiredType, @Nullable Object[] args, boolean typeCheckOnly)
throws BeansException {
//如果bean之前存在，这里返回的shareInstance就是非空，就会从后面的if分支中返回，如果bean之前不存在，就会执行后面的bean创建及注入属性的过程
Object sharedInstance = getSingleton(beanName);
if (sharedInstance != null && args == null) {
......
//如果当前不只是检查，而且是创建bean，这个参数就是false，在这里就会做个bean创建的标记，把beanName 加到alreadyCreated里面去
if (!typeCheckOnly) {
markBeanAsCreated(beanName);
}
//我们当前要创建的bean是单例的，就会走到这里去，下面我们走到里面的调用去看看
// Create bean instance.
if (mbd.isSingleton()) {
sharedInstance = getSingleton(beanName, () -> {
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;
}
});
beanInstance = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
}
}
public Object getSingleton(String beanName, ObjectFactory<?> singletonFactory) {
Assert.notNull(beanName, "Bean name must not be null");
synchronized (this.singletonObjects) {
......
//这里会把当前bean的名字加入到当前正在创建的单例对象集合singletonsCurrentlyInCreation中
beforeSingletonCreation(beanName);
......
try {
//这里就是调用上面的return createBean(beanName, mbd, args);这个方法，我们进这里面去看看
singletonObject = singletonFactory.getObject();
newSingleton = true;
}
......
}
return singletonObject;
}
}
@Override
protected Object createBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args)
throws BeanCreationException {
......
// 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.
//在这里获取要创建的bean的class对象
Class<?> resolvedClass = resolveBeanClass(mbd, beanName);
......
try {
//调用这里来创建，我们再走到这里面去看看
//3个参数分别为
//1、beanName bean对象的名字
//2、mbdToUseRootBeanDefinition对象，可以认为就是bean的元数据信息，包含bean的类对象，bean的类上注解，bean实际位置路径等等
//3、args bean对象的构造方法的实参，这里一般是空的
Object beanInstance = doCreateBean(beanName, mbdToUse, args);
if (logger.isTraceEnabled()) {
logger.trace("Finished creating instance of bean '" + beanName + "'");
}
return beanInstance;
}
......
}
protected Object doCreateBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args)
throws BeanCreationException {
......
//真正创建bean对象是在这里，这里返回的instanceWrapper是bean对象的类实例的包装对象BeanWrapper
if (instanceWrapper == null) {
instanceWrapper = createBeanInstance(beanName, mbd, args);
}
//这里的bean就是实际创建的bean对象的类实例
Object bean = instanceWrapper.getWrappedInstance();
Class<?> beanType = instanceWrapper.getWrappedClass();
if (beanType != NullBean.class) {
mbd.resolvedTargetType = beanType;
}
......
// Eagerly cache singletons to be able to resolve circular references
// even when triggered by lifecycle interfaces like BeanFactoryAware.
//看上面的注释大概也能明白， 大概意思就是早期的单例缓存，为了解决由 BeanFactoryAware等等触发的循环依赖
//mbd.isSingleton() 表示bean是单例的(这个是bean对应的类上的，默认就是单例)，
//this.allowCircularReferences 允许循环引用,这个是beanFactory的成员属性，默认也是true
//isSingletonCurrentlyInCreation(beanName) 表示是否在当前正在创建的bean集合中。beforeSingletonCreation(beanName);我们在前面执行过这句就加到正在创建的bean集合中了
//这里earlySingletonExposure 就是true了，会进到if分支中
boolean earlySingletonExposure = (mbd.isSingleton() && this.allowCircularReferences &&
isSingletonCurrentlyInCreation(beanName));
if (earlySingletonExposure) {
if (logger.isTraceEnabled()) {
logger.trace("Eagerly caching bean '" + beanName +
"' to allow for resolving potential circular references");
}
//这句主要是将将() -> getEarlyBeanReference(beanName, mbd, bean) 这个lambda表达式存储到this.singletonFactories集合中
addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));
}
// Initialize the bean instance.
Object exposedObject = bean;
try {
//在这里就会进行属性填充，完成成员注入等等，也就是在这里serviceA这个bean会注入serviceB这个成员属性,我们走进这个方法去看看
populateBean(beanName, mbd, instanceWrapper);
......
}
......
return exposedObject;
}
protected void populateBean(String beanName, RootBeanDefinition mbd, @Nullable BeanWrapper bw) {
......
if (hasInstAwareBpps) {
if (pvs == null) {
pvs = mbd.getPropertyValues();
}
//真正的属性注入是在这里完成的，aop也是在这里来完成的。这里是获取beanFactory中的InstantiationAwareBeanPostProcessor对bean对象进行增强
//如果属性注入用的是@Resource，就会用CommonAnnotationBeanPostProcessor来完成
//如果属性注入用的是@Autowired,就会用AutowiredAnnotationBeanPostProcessor来完成
//如果是AOP 就会使用InfrastructureAdvisorAutoProxyCreator来生成对应的代理对象
//我们这里使用的是@Autowired,所以会用AutowiredAnnotationBeanPostProcessor来完成注入。我们走到它的postProcessProperties的去看看
for (InstantiationAwareBeanPostProcessor bp : getBeanPostProcessorCache().instantiationAware) {
PropertyValues pvsToUse = bp.postProcessProperties(pvs, bw.getWrappedInstance(), beanName);
......
}
@Override
public PropertyValues postProcessProperties(PropertyValues pvs, Object bean, String beanName) {
//这里主要是获取bean的类属性和方法上的org.springframework.beans.factory.annotation.Autowired,org.springframework.beans.factory.annotation.Value注解来进行注入
InjectionMetadata metadata = findAutowiringMetadata(beanName, bean.getClass(), pvs);
try {
//继续进去看看
metadata.inject(bean, beanName, pvs);
}
......
}
public void inject(Object target, @Nullable String beanName, @Nullable PropertyValues pvs) throws Throwable {
......
//对每一个属性分别进行注入，继续进去
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;
//如果之前缓存过就从缓存取，我们是第一次注入，所以之前没有缓存，不会走这个分支
if (this.cached) {
try {
value = resolvedCachedArgument(beanName, this.cachedFieldValue);
}
catch (NoSuchBeanDefinitionException ex) {
// Unexpected removal of target bean for cached argument -> re-resolve
value = resolveFieldValue(field, bean, beanName);
}
}
else {
//会走这里来解析字段的值，再进去
value = resolveFieldValue(field, bean, beanName);
}
if (value != null) {
ReflectionUtils.makeAccessible(field);
field.set(bean, value);
}
}
@Nullable
private Object resolveFieldValue(Field field, Object bean, @Nullable String beanName) {
//创建字段的包装类DependencyDescriptor
DependencyDescriptor desc = new DependencyDescriptor(field, this.required);
try {
//调用这里完成对应字段值的查找,再进去
value = beanFactory.resolveDependency(desc, beanName, autowiredBeanNames, typeConverter);
}
catch (BeansException ex) {
throw new UnsatisfiedDependencyException(null, beanName, new InjectionPoint(field), ex);
}
synchronized (this) {
//获取到值之后，进行缓存
if (!this.cached) {
......
}
this.cachedFieldValue = cachedFieldValue;
this.cached = true;
}
}
return value;
}
}
public Object resolveDependency(DependencyDescriptor descriptor, @Nullable String requestingBeanName,
@Nullable Set<String> 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 {
//当前的类是一个普通的class，会走到这里面，由于我们的bean没有Lazy注解，所以这里返回时null,走到下面的if分支
Object result = getAutowireCandidateResolver().getLazyResolutionProxyIfNecessary(
descriptor, requestingBeanName);
if (result == null) {
//在这里我们看下这里的入参。
//descriptor是包含了需要注入的字段的信息。
//requestingBeanName是当前正在创建的bean的名字serviceA,
//autowiredBeanNames是当前需要注入的字段的对应的bean的名字的集合，这里只有serviceB
//typeConverter这个是进行注入时做类型转换的，这里我们可以不用关注这个
result = doResolveDependency(descriptor, requestingBeanName, autowiredBeanNames, typeConverter);
}
return result;
}
}
@Nullable
public Object doResolveDependency(DependencyDescriptor descriptor, @Nullable String beanName,
@Nullable Set<String> autowiredBeanNames, @Nullable TypeConverter typeConverter) throws BeansException {
......
if (instanceCandidate instanceof Class) {
//又会调用到这里,我们再进入到DependencyDescriptor的resolveCandidate去看看
//注意：这里的autowiredBeanName是我们需要注入的属性名这里是serviceB
instanceCandidate = descriptor.resolveCandidate(autowiredBeanName, type, this);
}
......
}
public Object resolveCandidate(String beanName, Class<?> requiredType, BeanFactory beanFactory)
throws BeansException {
//看到没，到这里就出现循环调用了，到这里又会重新调用beanFactory.getBean("serviceB")去创建serviceB的bean对象，完成后注入到serivceA对应的Bean上的属性上来，这时代码又会从本节开头的位置开始执行，先创建serviceB对象实例，再去注入serviceB对象的serviceA属性。
//最终会执行到beanFactory.getBean("serviceA")这里
return beanFactory.getBean(beanName);
}
就是下面图的样子
3、解决循环依赖的代码实现
接着上面的beanFactory.getBean("serviceA")这行代码我们继续往下看
这次又会走到这里
protected <T> T doGetBean(
String name, @Nullable Class<T> requiredType, @Nullable Object[] args, boolean typeCheckOnly)
throws BeansException {
//我们第二部分就是从这里开始的，又走回来了，但这次又会有所不同
String beanName = transformedBeanName(name);
Object beanInstance;
// Eagerly check singleton cache for manually registered singletons.
//这次我们这里返回的就不是空了，sharedInstance对象的值就是对应serviceA的bean对象了，这次就会从if分支中返回，而之前我们不会进这里的if分支而是进入else分支导致后面出现了循环依赖的问题，这次我们进到这个方法看看
Object sharedInstance = getSingleton(beanName);
if (sharedInstance != null && args == null) {
if (logger.isTraceEnabled()) {
if (isSingletonCurrentlyInCreation(beanName)) {
logger.trace("Returning eagerly cached instance of singleton bean '" + beanName +
"' that is not fully initialized yet - a consequence of a circular reference");
}
else {
logger.trace("Returning cached instance of singleton bean '" + beanName + "'");
}
}
beanInstance = getObjectForBeanInstance(sharedInstance, name, beanName, null);
}
@Nullable
public Object getSingleton(String beanName) {
//再点进去
return getSingleton(beanName, true);
}
protected Object getSingleton(String beanName, boolean allowEarlyReference) {
// Quick check for existing instance without full singleton lock
Object singletonObject = this.singletonObjects.get(beanName);
//这里由于当前的serviceA bean还没完成创建，所以这里singletonObject返回的是空，
//再看看 isSingletonCurrentlyInCreation(beanName)这里，由于我们在创建serviceA过程中有这么一句beforeSingletonCreation(beanName)(不清楚这句的搜索下本文，上面就有讲到)，所有这个条件是true。这时我们就会进入if分支中
if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {
singletonObject = this.earlySingletonObjects.get(beanName);
//由于我们是第一次进入这里，所以this.earlySingletonObjects.get(beanName)返回的也是null
//我们的入参 allowEarlyReference是true,会继续进到这个if分支中
if (singletonObject == null && allowEarlyReference) {
synchronized (this.singletonObjects) {
// Consistent creation of early reference within full singleton lock
singletonObject = this.singletonObjects.get(beanName);
//这里的singletonObject还是null,继续进到if分支
if (singletonObject == null) {
singletonObject = this.earlySingletonObjects.get(beanName);
if (singletonObject == null) {
//最终会走到这里，在创建serviceA对象之后，属性注入之前，执行了这句 addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean))(不清楚的搜索下本文，上面有说到)，所以这里返回的singletonFactory是个lamdba表达式，getEarlyBeanReference(beanName, mbd, bean))附带了3个参数，第一个beanName是serivceA,mdb是对应serviceA的附带serviceA元数据信息的RootBeanDefinition对象，bean就是创建出来的serviceA对象
ObjectFactory<?> singletonFactory = this.singletonFactories.get(beanName);
if (singletonFactory != null) {
//这里就会调用getEarlyBeanReference(beanName, mbd, bean)对serviceA对象进行一个getEarlyBeanReference增强后返回，返回后放置到earlySingletonObjects中，并从singletonFactories中删除
singletonObject = singletonFactory.getObject();
this.earlySingletonObjects中，并从.put(beanName, singletonObject);
this.singletonFactories.remove(beanName);
}
}
}
}
}
}
return singletonObject;
}
最终在serviceA 这个bean创建完成后，就会从singletonsCurrentlyInCreation移除掉
public Object getSingleton(String beanName, ObjectFactory<?> singletonFactory) {
......
finally {
//在这里从singletonsCurrentlyInCreation中移除掉
afterSingletonCreation(beanName);
}
if (newSingleton) {
//将serviceA bean对象添加到singletonObjects，registeredSingletons中
//从singletonFactories,earlySingletonObjects中移除掉
addSingleton(beanName, singletonObject);
}
}
return singletonObject;
}
}
所以整个获取serviceA的流程就是这样了，
1、首先去创建serviceA这个bean，
由于它有个属性serviceB,在创建完serviceA对象后，就会去进行serviceB的属性注入， 这时由于serviceB之前没有生成，这时又会去创建serviceB这个bean， 先创建serviceB对象，然后再进行serviceA这个属性的注入， 继续去获取serviceA这个bean，第二次进入获取serviceA的流程，这时从之前缓存的lambda表达式中获取到之前创建的serviceA的引用返回。2、总结下关键的代码点
创建bean对象之前调用beforeSingletonCreation(beanName)将bean对象名字添加到singletonsCurrentlyInCreation集合中 创建bean对象对应的类实例后调用addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));添加到singletonFactories中 在循环依赖中第二次调用到创建bean对象时，调用getSingleton(beanName, true)时，从singletonFactories中返回对应的早期bean对象的引用,并添加到earlySingletonObjects中