背景
正在学习Spring源码。微信公众号的推文中有一篇循环依赖相关文章，但是只有大概处理过程，没有具体的流程。借此机会了解一下循环依赖相关源码，希望能加深自己的理解。（然后就和基友走了几个小时源码...）
go
循环依赖就是N个类中循环嵌套引用，如果在日常开发中我们用new 对象的方式发生这种循环依赖的话程序会在运行时一直循环调用，直至内存溢出报错。下面说一下Spring是如果解决循环依赖的。
循环依赖有三种情况
- setter方式原型，prototype
- 构造器参数循环依赖
- setter方式单例，默认方式
setter方式原型，prototype
prototype的Bean在容器初始化时，spring不会去检测它的循环依赖问题，因为只有在getBean的实例化过程中才会有循环依赖检查，prototype类型的Bean不会被在被调用时才触发实例化。也就是说不调用它也不会出现循环依赖的报错。
首先看一哈报错
Exception in thread "main" org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'a' defined in class path resource [spring-config-prototype.xml]: Cannot resolve reference to bean 'b' while setting bean property 'b'; nested exception is org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'b' defined in class path resource [spring-config-prototype.xml]: Cannot resolve reference to bean 'c' while setting bean property 'c'; nested exception is org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'c' defined in class path resource [spring-config-prototype.xml]: Cannot resolve reference to bean 'a' while setting bean property 'a'; nested exception is org.springframework.beans.factory.BeanCurrentlyInCreationException: Error creating bean with name 'a': Requested bean is currently in creation: Is there an unresolvable circular reference?
at org.springframework.beans.factory.support.BeanDefinitionValueResolver.resolveReference(BeanDefinitionValueResolver.java:378)
at org.springframework.beans.factory.support.BeanDefinitionValueResolver.resolveValueIfNecessary(BeanDefinitionValueResolver.java:110)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.applyPropertyValues(AbstractAutowireCapableBeanFactory.java:1681)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.populateBean(AbstractAutowireCapableBeanFactory.java:1433)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:592)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:515)
at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:338)
at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:224)
at org.springframework.beans.factory.support.DefaultListableBeanFactory.resolveNamedBean(DefaultListableBeanFactory.java:1115)
at org.springframework.beans.factory.support.DefaultListableBeanFactory.resolveBean(DefaultListableBeanFactory.java:407)
at org.springframework.beans.factory.support.DefaultListableBeanFactory.getBean(DefaultListableBeanFactory.java:341)
at org.springframework.beans.factory.support.DefaultListableBeanFactory.getBean(DefaultListableBeanFactory.java:335)
at org.springframework.context.support.AbstractApplicationContext.getBean(AbstractApplicationContext.java:1123)
at Test.main(Test.java:35)
Caused by: org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'b' defined in class path resource [spring-config-prototype.xml]: Cannot resolve reference to bean 'c' while setting bean property 'c'; nested exception is org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'c' defined in class path resource [spring-config-prototype.xml]: Cannot resolve reference to bean 'a' while setting bean property 'a'; nested exception is org.springframework.beans.factory.BeanCurrentlyInCreationException: Error creating bean with name 'a': Requested bean is currently in creation: Is there an unresolvable circular reference?
at org.springframework.beans.factory.support.BeanDefinitionValueResolver.resolveReference(BeanDefinitionValueResolver.java:378)
at org.springframework.beans.factory.support.BeanDefinitionValueResolver.resolveValueIfNecessary(BeanDefinitionValueResolver.java:110)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.applyPropertyValues(AbstractAutowireCapableBeanFactory.java:1681)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.populateBean(AbstractAutowireCapableBeanFactory.java:1433)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:592)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:515)
at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:338)
at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:199)
at org.springframework.beans.factory.support.BeanDefinitionValueResolver.resolveReference(BeanDefinitionValueResolver.java:367)
... 13 more
Caused by: org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'c' defined in class path resource [spring-config-prototype.xml]: Cannot resolve reference to bean 'a' while setting bean property 'a'; nested exception is org.springframework.beans.factory.BeanCurrentlyInCreationException: Error creating bean with name 'a': Requested bean is currently in creation: Is there an unresolvable circular reference?
at org.springframework.beans.factory.support.BeanDefinitionValueResolver.resolveReference(BeanDefinitionValueResolver.java:378)
at org.springframework.beans.factory.support.BeanDefinitionValueResolver.resolveValueIfNecessary(BeanDefinitionValueResolver.java:110)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.applyPropertyValues(AbstractAutowireCapableBeanFactory.java:1681)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.populateBean(AbstractAutowireCapableBeanFactory.java:1433)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:592)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:515)
at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:338)
at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:199)
at org.springframework.beans.factory.support.BeanDefinitionValueResolver.resolveReference(BeanDefinitionValueResolver.java:367)
... 21 more
Caused by: org.springframework.beans.factory.BeanCurrentlyInCreationException: Error creating bean with name 'a': Requested bean is currently in creation: Is there an unresolvable circular reference?
at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:264)
at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:199)
at org.springframework.beans.factory.support.BeanDefinitionValueResolver.resolveReference(BeanDefinitionValueResolver.java:367)
... 29 more
org.springframework.beans.factory.support.BeanDefinitionValueResolver.resolveReference(BeanDefinitionValueResolver.java:378
看一下就具体地方的代码
@Nullable
private Object resolveReference(Object argName, RuntimeBeanReference ref) {
try {
Object bean;
//获取引用的Bean名称
String refName = ref.getBeanName();
refName = String.valueOf(doEvaluate(refName));
//如果引用的对象在父类容器中，则从父类容器中获取指定的引用对象
if (ref.isToParent()) {
if (this.beanFactory.getParentBeanFactory() == null) {
throw new BeanCreationException(
this.beanDefinition.getResourceDescription(), this.beanName,
"Can't resolve reference to bean '" + refName +
"' in parent factory: no parent factory available");
}
bean = this.beanFactory.getParentBeanFactory().getBean(refName);
}
//从当前的容器中获取指定的引用Bean对象，如果指定的Bean没有被实例化
//则会递归触发引用Bean的初始化和依赖注入
else {
bean = this.beanFactory.getBean(refName);
//将当前实例化对象的依赖引用对象
this.beanFactory.registerDependentBean(refName, this.beanName);
}
if (bean instanceof NullBean) {
bean = null;
}
return bean;
}
catch (BeansException ex) {
throw new BeanCreationException(
this.beanDefinition.getResourceDescription(), this.beanName,
"Cannot resolve reference to bean '" + ref.getBeanName() + "' while setting " + argName, ex);
}
}
debug后找到bean = this.beanFactory.getBean(refName);
看一下具体方法
@Override
public Object getBean(String name) throws BeansException {
return doGetBean(name, null, null, false);
}
//真正实现向IOC容器获取Bean的功能，也是触发依赖注入功能的地方
protected <T> T doGetBean(final String name, @Nullable final Class<T> requiredType,
@Nullable final Object[] args, boolean typeCheckOnly) throws BeansException {
//根据指定的名称获取被管理Bean的名称，剥离指定名称中对容器的相关依赖
//如果指定的是别名，将别名转换为规范的Bean名称
final String beanName = transformedBeanName(name);
Object bean;
// Eagerly check singleton cache for manually registered singletons.
//先从缓存中取是否已经有被创建过的单态类型的Bean
//对于单例模式的Bean整个IOC容器中只创建一次，不需要重复创建
Object sharedInstance = getSingleton(beanName);
//IOC容器创建单例模式Bean实例对象
if (sharedInstance != null && args == null) {
if (logger.isDebugEnabled()) {
//如果指定名称的Bean在容器中已有单例模式的Bean被创建
//直接返回已经创建的Bean
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 + "'");
}
}
//获取给定Bean的实例对象，主要是完成FactoryBean的相关处理
//注意：BeanFactory是管理容器中Bean的工厂，而FactoryBean是
//创建创建对象的工厂Bean，两者之间有区别
bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
}
else {
// Fail if we're already creating this bean instance:
// We're assumably within a circular reference.
//缓存没有正在创建的单例模式Bean
//缓存中已经有已经创建的原型模式Bean
//但是由于循环引用的问题导致实例化对象失败
if (isPrototypeCurrentlyInCreation(beanName)) {
throw new BeanCurrentlyInCreationException(beanName);
}
// Check if bean definition exists in this factory.
//对IOC容器中是否存在指定名称的BeanDefinition进行检查，首先检查是否
//能在当前的BeanFactory中获取的所需要的Bean，如果不能则委托当前容器
//的父级容器去查找，如果还是找不到则沿着容器的继承体系向父级容器查找
BeanFactory parentBeanFactory = getParentBeanFactory();
//当前容器的父级容器存在，且当前容器中不存在指定名称的Bean
if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {
// Not found -> check parent.
//解析指定Bean名称的原始名称
String nameToLookup = originalBeanName(name);
if (parentBeanFactory instanceof AbstractBeanFactory) {
return ((AbstractBeanFactory) parentBeanFactory).doGetBean(
nameToLookup, requiredType, args, typeCheckOnly);
}
else if (args != null) {
// Delegation to parent with explicit args.
//委派父级容器根据指定名称和显式的参数查找
return (T) parentBeanFactory.getBean(nameToLookup, args);
}
else {
// No args -> delegate to standard getBean method.
//委派父级容器根据指定名称和类型查找
return parentBeanFactory.getBean(nameToLookup, requiredType);
}
}
//创建的Bean是否需要进行类型验证，一般不需要
if (!typeCheckOnly) {
//向容器标记指定的Bean已经被创建
markBeanAsCreated(beanName);
}
try {
//根据指定Bean名称获取其父级的Bean定义
//主要解决Bean继承时子类合并父类公共属性问题
final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
checkMergedBeanDefinition(mbd, beanName, args);
// Guarantee initialization of beans that the current bean depends on.
//获取当前Bean所有依赖Bean的名称
String[] dependsOn = mbd.getDependsOn();
//如果当前Bean有依赖Bean
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 + "'");
}
//递归调用getBean方法，获取当前Bean的依赖Bean
registerDependentBean(dep, beanName);
//把被依赖Bean注册给当前依赖的Bean
getBean(dep);
}
}
// Create bean instance.
//创建单例模式Bean的实例对象
if (mbd.isSingleton()) {
//这里使用了一个匿名内部类，创建Bean实例对象，并且注册给所依赖的对象
sharedInstance = getSingleton(beanName, () -> {
try {
//创建一个指定Bean实例对象，如果有父级继承，则合并子类和父类的定义
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.
//显式地从容器单例模式Bean缓存中清除实例对象
destroySingleton(beanName);
throw ex;
}
});
//获取给定Bean的实例对象
bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
}
//IOC容器创建原型模式Bean实例对象
else if (mbd.isPrototype()) {
// It's a prototype -> create a new instance.
//原型模式(Prototype)是每次都会创建一个新的对象
Object prototypeInstance = null;
try {
//回调beforePrototypeCreation方法，默认的功能是注册当前创建的原型对象
beforePrototypeCreation(beanName);
//创建指定Bean对象实例
prototypeInstance = createBean(beanName, mbd, args);
}
finally {
//回调afterPrototypeCreation方法，默认的功能告诉IOC容器指定Bean的原型对象不再创建
afterPrototypeCreation(beanName);
}
//获取给定Bean的实例对象
bean = getObjectForBeanInstance(prototypeInstance, name, beanName, mbd);
}
//要创建的Bean既不是单例模式，也不是原型模式，则根据Bean定义资源中
//配置的生命周期范围，选择实例化Bean的合适方法，这种在Web应用程序中
//比较常用，如：request、session、application等生命周期
else {
String scopeName = mbd.getScope();
final Scope scope = this.scopes.get(scopeName);
//Bean定义资源中没有配置生命周期范围，则Bean定义不合法
if (scope == null) {
throw new IllegalStateException("No Scope registered for scope name '" + scopeName + "'");
}
try {
//这里又使用了一个匿名内部类，获取一个指定生命周期范围的实例
Object scopedInstance = scope.get(beanName, () -> {
beforePrototypeCreation(beanName);
try {
return createBean(beanName, mbd, args);
}
finally {
afterPrototypeCreation(beanName);
}
});
//获取给定Bean的实例对象
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.
//对创建的Bean实例对象进行类型检查
if (requiredType != null && !requiredType.isInstance(bean)) {
try {
T convertedBean = getTypeConverter().convertIfNecessary(bean, requiredType);
if (convertedBean == null) {
throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
}
return convertedBean;
}
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;
}
循环依赖的判断在创建Bean之前。
能看到有这么一个判断。
if (isPrototypeCurrentlyInCreation(beanName)) {
throw new BeanCurrentlyInCreationException(beanName);
}
/** Names of beans that are currently in creation */
private final ThreadLocal<Object> prototypesCurrentlyInCreation =
new NamedThreadLocal<>("Prototype beans currently in creation");
protected boolean isPrototypeCurrentlyInCreation(String beanName) {
Object curVal = this.prototypesCurrentlyInCreation.get();
return (curVal != null &&
(curVal.equals(beanName) || (curVal instanceof Set && ((Set<?>) curVal).contains(beanName))));
}
大致是，创建Bean时会进行判断，如果在当前线程的ThreadLocal取出的Set中找到同名的beanName则认为出现循环依赖。
判断时从Set中取值，是在哪放进去的呢，又是在哪被移除的呢？
在上面的doGetBean中能找到如下片段
else if (mbd.isPrototype()) {
// It's a prototype -> create a new instance.
//原型模式(Prototype)是每次都会创建一个新的对象
Object prototypeInstance = null;
try {
//回调beforePrototypeCreation方法，默认的功能是注册当前创建的原型对象
beforePrototypeCreation(beanName);
//创建指定Bean对象实例
prototypeInstance = createBean(beanName, mbd, args);
}
finally {
//回调afterPrototypeCreation方法，默认的功能告诉IOC容器指定Bean的原型对象不再创建
afterPrototypeCreation(beanName);
}
//获取给定Bean的实例对象
bean = getObjectForBeanInstance(prototypeInstance, name, beanName, mbd);
}
/**
* Callback before prototype creation.
* <p>The default implementation register the prototype as currently in creation.
* @param beanName the name of the prototype about to be created
* @see #isPrototypeCurrentlyInCreation
*/
@SuppressWarnings("unchecked")
protected void beforePrototypeCreation(String beanName) {
Object curVal = this.prototypesCurrentlyInCreation.get();
if (curVal == null) {
this.prototypesCurrentlyInCreation.set(beanName);
}
else if (curVal instanceof String) {
Set<String> beanNameSet = new HashSet<>(2);
beanNameSet.add((String) curVal);
beanNameSet.add(beanName);
this.prototypesCurrentlyInCreation.set(beanNameSet);
}
else {
Set<String> beanNameSet = (Set<String>) curVal;
beanNameSet.add(beanName);
}
}
/**
* Callback after prototype creation.
* <p>The default implementation marks the prototype as not in creation anymore.
* @param beanName the name of the prototype that has been created
* @see #isPrototypeCurrentlyInCreation
*/
@SuppressWarnings("unchecked")
protected void afterPrototypeCreation(String beanName) {
Object curVal = this.prototypesCurrentlyInCreation.get();
if (curVal instanceof String) {
this.prototypesCurrentlyInCreation.remove();
}
else if (curVal instanceof Set) {
Set<String> beanNameSet = (Set<String>) curVal;
beanNameSet.remove(beanName);
if (beanNameSet.isEmpty()) {
this.prototypesCurrentlyInCreation.remove();
}
}
}
现在就比较清晰了，在每个prototype类型的Bean被创建之前，首先会进行循环依赖的判断，通过判断的每个Bean会暂存在一个Set（只存创建中的Bean，创建完就remove了）中记录。
以A, B, C为例
public class Test {
@Data
@AllArgsConstructor
@NoArgsConstructor
public static class A {
B b;
}
@Data
@AllArgsConstructor
@NoArgsConstructor
public static class B {
C c;
}
@Data
@AllArgsConstructor
@NoArgsConstructor
public static class C {
A a;
}
public static void main(String[] args) {
ClassPathXmlApplicationContext classPathXmlApplicationContext = new ClassPathXmlApplicationContext("spring-config-prototype.xml");
//ClassPathXmlApplicationContext classPathXmlApplicationContext = new ClassPathXmlApplicationContext("spring-config-construct.xml");
//ClassPathXmlApplicationContext classPathXmlApplicationContext = new ClassPathXmlApplicationContext("spring-config-setter.xml");
System.out.println(classPathXmlApplicationContext.getBean(A.class));
}
}
-
A通过了判断记录并进行createBean，发现需要引用B
-
那么就需要创建B，B也通过了，被记录在册，并进行对C的创建
-
C也通过了，被记录并开始创建，可是C需要A，那么试图去创建A
-
当再次试图创建A的时候，发现A已经被记录在册了呀，眉头一皱，发现问题并顺手抛了个异常
虽然Spring初始化的过程是单线程的，但是后续的getBean并不能保证是同一线程，所以把记录的Set存在个自的线程中。
另外两种整体上的流程是一样的，具体走的地方有差异。
构造器参数循环依赖
用constructor的Bean默认是单例，在容器初始化时就会触发Bean实例化，所以启动时就会检测到循环依赖问题。
单例的所以在doGetBean中走Singleton的判断
// Create bean instance.
//mbd这玩意是BeanDefinition，由资源文件解析而来，记录Bean的定义信息的
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;
}
});
bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
}
进入
/**
* Return the (raw) singleton object registered under the given name,
* creating and registering a new one if none registered yet.
* @param beanName the name of the bean
* @param singletonFactory the ObjectFactory to lazily create the singleton
* with, if necessary
* @return the registered singleton object
*/
public Object getSingleton(String beanName, ObjectFactory<?> singletonFactory) {
Assert.notNull(beanName, "Bean name must not be null");
synchronized (this.singletonObjects) {
Object singletonObject = this.singletonObjects.get(beanName);
if (singletonObject == null) {
if (this.singletonsCurrentlyInDestruction) {
throw new BeanCreationNotAllowedException(beanName,
"Singleton bean creation not allowed while singletons of this factory are in destruction " +
"(Do not request a bean from a BeanFactory in a destroy method implementation!)");
}
if (logger.isDebugEnabled()) {
logger.debug("Creating shared instance of singleton bean '" + beanName + "'");
}
//这里
beforeSingletonCreation(beanName);
boolean newSingleton = false;
boolean recordSuppressedExceptions = (this.suppressedExceptions == null);
if (recordSuppressedExceptions) {
this.suppressedExceptions = new LinkedHashSet<>();
}
try {
singletonObject = singletonFactory.getObject();
newSingleton = true;
}
catch (IllegalStateException ex) {
// Has the singleton object implicitly appeared in the meantime ->
// if yes, proceed with it since the exception indicates that state.
singletonObject = this.singletonObjects.get(beanName);
if (singletonObject == null) {
throw ex;
}
}
catch (BeanCreationException ex) {
if (recordSuppressedExceptions) {
for (Exception suppressedException : this.suppressedExceptions) {
ex.addRelatedCause(suppressedException);
}
}
throw ex;
}
finally {
if (recordSuppressedExceptions) {
this.suppressedExceptions = null;
}
afterSingletonCreation(beanName);
}
if (newSingleton) {
addSingleton(beanName, singletonObject);
}
}
return singletonObject;
}
}
通过debug异常在beforeSingletonCreation(beanName);抛出
/** Names of beans that are currently in creation. */
private final Set<String> singletonsCurrentlyInCreation =
Collections.newSetFromMap(new ConcurrentHashMap<>(16));
/**
* Callback before singleton creation.
* <p>The default implementation register the singleton as currently in creation.
* @param beanName the name of the singleton about to be created
* @see #isSingletonCurrentlyInCreation
*/
protected void beforeSingletonCreation(String beanName) {
if (!this.inCreationCheckExclusions.contains(beanName) && !this.singletonsCurrentlyInCreation.add(beanName)) {
throw new BeanCurrentlyInCreationException(beanName);
}
}
emmm熟悉的味道，基本和prototype的流程一样
setter方式单例，默认方式
这种方式和另外两种的区别在于，用这种方式时，不会出现循环依赖的异常，也就是说容器帮我们解决了循环依赖的问题。
这种方式下，Bean还是默认的单例，所以流程应该是和constructor类似的。
通过debug，这两种方式流程的确类似，区别在于doGetBean中
// Eagerly check singleton cache for manually registered singletons.
// 这里constructor返回null，setter则能返回对象
Object sharedInstance = getSingleton(beanName);
if (sharedInstance != null && args == null) {
if (logger.isTraceEnabled()) {
...
}
else {
...
}
进入
/** Cache of singleton factories: bean name to ObjectFactory. */
private final Map<String, ObjectFactory<?>> singletonFactories = new HashMap<>(16);
@Override
@Nullable
public Object getSingleton(String beanName) {
return getSingleton(beanName, true);
}
/**
* Return the (raw) singleton object registered under the given name.
* <p>Checks already instantiated singletons and also allows for an early
* reference to a currently created singleton (resolving a circular reference).
* @param beanName the name of the bean to look for
* @param allowEarlyReference whether early references should be created or not
* @return the registered singleton object, or {@code null} if none found
*/
@Nullable
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) {
// 在出现循环依赖时，这个地方出现了区别
// constructor 的 singletonFactories 是空的，所以取不到值
ObjectFactory<?> singletonFactory = this.singletonFactories.get(beanName);
if (singletonFactory != null) {
singletonObject = singletonFactory.getObject();
this.earlySingletonObjects.put(beanName, singletonObject);
this.singletonFactories.remove(beanName);
}
}
}
}
return singletonObject;
}
找到差异的地方就有头绪了，只要找到setter方式时什么时候在singletonFactories中放入数据的就行
根据调用链，首先在之前的doGetBean中
// Create bean instance.
if (mbd.isSingleton()) {
sharedInstance = getSingleton(beanName, () -> {
try {
//注意这个方法
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);
}
AbstractAutowireCapableBeanFactory
/**
* Central method of this class: creates a bean instance,
* populates the bean instance, applies post-processors, etc.
* @see #doCreateBean
*/
@Override
protected Object createBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args)
throws BeanCreationException {
...
try {
//现在这里
Object beanInstance = doCreateBean(beanName, mbdToUse, args);
if (logger.isTraceEnabled()) {
logger.trace("Finished creating instance of bean '" + beanName + "'");
}
return beanInstance;
}
final Object bean = instanceWrapper.getWrappedInstance();
Class<?> beanType = instanceWrapper.getWrappedClass();
if (beanType != NullBean.class) {
mbd.resolvedTargetType = beanType;
}
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;
}
}
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");
}
//在这边进行add
addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));
}
...
}
AbstractAutowireCapableBeanFactory
/**
* Actually create the specified bean. Pre-creation processing has already happened
* at this point, e.g. checking {@code postProcessBeforeInstantiation} callbacks.
* <p>Differentiates between default bean instantiation, use of a
* factory method, and autowiring a constructor.
* @param beanName the name of the bean
* @param mbd the merged bean definition for the bean
* @param args explicit arguments to use for constructor or factory method invocation
* @return a new instance of the bean
* @throws BeanCreationException if the bean could not be created
* @see #instantiateBean
* @see #instantiateUsingFactoryMethod
* @see #autowireConstructor
*/
protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final @Nullable Object[] args)
throws BeanCreationException {
// Instantiate the bean.
BeanWrapper instanceWrapper = null;
if (mbd.isSingleton()) {
instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
}
if (instanceWrapper == null) {
//接着到这
instanceWrapper = createBeanInstance(beanName, mbd, args);
}
...
}
/**
* Create a new instance for the specified bean, using an appropriate instantiation strategy:
* factory method, constructor autowiring, or simple instantiation.
* @param beanName the name of the bean
* @param mbd the bean definition for the bean
* @param args explicit arguments to use for constructor or factory method invocation
* @return a BeanWrapper for the new instance
* @see #obtainFromSupplier
* @see #instantiateUsingFactoryMethod
* @see #autowireConstructor
* @see #instantiateBean
*/
protected BeanWrapper createBeanInstance(String beanName, RootBeanDefinition mbd, @Nullable Object[] args) {
...
// Candidate constructors for autowiring?
// mbd 这个中存了Bean的定义信息，从中取出bean的构造器，判断，setter不会进入autowireConstructor
Constructor<?>[] ctors = determineConstructorsFromBeanPostProcessors(beanClass, beanName);
if (ctors != null || mbd.getResolvedAutowireMode() == AUTOWIRE_CONSTRUCTOR ||
mbd.hasConstructorArgumentValues() || !ObjectUtils.isEmpty(args)) {
return autowireConstructor(beanName, mbd, ctors, args);
}
// Preferred constructors for default construction?
ctors = mbd.getPreferredConstructors();
if (ctors != null) {
return autowireConstructor(beanName, mbd, ctors, null);
}
// No special handling: simply use no-arg constructor.
return instantiateBean(beanName, mbd);
}
现在就很清楚了，constructor方式会进入autowireConstructor，然后在这里进入A->B->C->A的过程，只存下了在创建过程中的Bean的name，判断出现循环依赖直接抛异常
而setter会在bean实例的过程中会先把单例工厂存下来，在进入A->B->C->A的过程时，当C再次依赖需要A时，容器会从之前存下来的单例工厂中取出A的单例实例（虽然这个A还没有进行具体赋值）。当C得到A后循环依赖也就解决了，可以继续后续步骤了。
整个过程是三级缓存
首先从singletonObjects（一级缓存）中尝试获取，如果获取不到并且对象在创建中，则尝试从earlySingletonObjects(二级缓存)中获取，如果还是获取不到并且允许从singletonFactories通过getObject获取，则通过singletonFactory.getObject()(三级缓存)获取
后记
想了一个很蠢的问题，prototype是没办法。但是为什么setter有办法解决循环依赖，constructor不解决呢？然后发现= =setter可以先从单例工厂创建一个空的暂时用着，constructor的话创都创不出来，解决毛哟。
这边只展示了大概流程思路，有兴趣的同学可以再深入去看一下。
网友评论