Spring5源码阅读－－如何解决循环依赖？
首先要明确的是，循环依赖有以下几种：
- 构造函数的循环依赖。这种依赖显然是解决不了的。
- 非单例Bean的循环依赖。这种依赖也是解决不了的。
- 单例Bean的循环依赖。本文介绍的就是如何解决单例Bean的循环依赖的问题。
先来看一下测试代码，为了省略篇幅去掉setter和getter代码：
//Car.java
package beans;
public class Car {
String name;
Person person;
}
//Person.java
package beans;
public class Person {
Car car;
String name;
int age;
}
//Test.java
import org.springframework.beans.factory.BeanFactory;
import org.springframework.beans.factory.xml.XmlBeanFactory;
import org.springframework.core.io.ClassPathResource;
public class Test {
public static void main(String[] args) {
BeanFactory beanFactory = new XmlBeanFactory(new ClassPathResource("beanFactoryTest.xml")); //此行代码仅仅加载了配置文件，并且将里面的元素解析成BeanDefinition的形式，并添加到beanFactory中。此时并未创建bean。
beanFactory.getBean("car"); //在第一次使用getBean()方法的时候会创建这个bean
}
}
//beanFactoryTest.xml
"1.0" encoding="UTF-8"?>
"http://www.springframework.org/schema/beans"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:util="http://www.springframework.org/schema/util"
xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd http://www.springframework.org/schema/util http://www.springframework.org/schema/util/spring-util.xsd">
"car" class="beans.Car">
"person" ref="person"/>
"person" class="beans.Person">
"age" value="18"/>
"car" ref="car"/>
"name" value="SJ"/>
在这里先简单说明一下，Test类的main()函数中，第一行代码仅仅是加载xml文件里面的内容，生成若干个BeanDefinition类并添加到BeanFactory的beanDefinitionMap中，并未直接根据xml文件来创建定义的bean，而是留到第一次使用getBean()方法的时候才会创建这个bean以及它所依赖的对象，也就是懒加载模式。
那么就从getBean()来入手吧，之前需要注意一下beanFactory中几个与循环依赖相关的比较重要的字段，它们都定义在DefaultSingletonBeanRegistry类中：
/** Cache of singleton objects: bean name --> bean instance */
private final Map
/** Cache of singleton factories: bean name --> ObjectFactory */
private final Map/** Cache of early singleton objects: bean name --> bean instance */
private final Map/** Names of beans that are currently in creation */
private final SetsingletonsCurrentlyInCreation =
Collections.newSetFromMap(new ConcurrentHashMap<>(16));
其中，singletonObjects是用来保存已经注册的bean，里面的bean是已经初始化完成的；其他三个均用来保存正在创建但是还没完成创建的bean（比如正在创建A的同时发现A有一个属性值是B，于是把A摞一边先跑去初始化B了），可以理解为singleton对象的缓存池。但是它们三个也是有区别的，它们保存了不同创建阶段的singleton对象。很明显singletonFactories是一个bean name到bean工厂的映射，通过这个工厂可以得到对应name的bean，至于其它的区别会在下文慢慢讲解，诸位稍安勿躁。
下面开始分析getBean()函数，前面提到过，它才是bean初始化的起点。从现在起，假设所有的BeanDefinition已经被解析生成（本文中就定义了两个bean，Car和Person），但是还没有bean被初始化过，开始执行Test类的第二行。话不多说，放码过来，不过由于篇幅所限，将“无关”代码尽量删除一些：
//AbstractBeanFactory.java
@Override
public Object getBean(String name) throws BeansException {
return doGetBean(name, null, null, false);
}
(1)
protected T doGetBean(final String name, @Nullable final Class requiredType,
@Nullable final Object[] args, boolean typeCheckOnly) throws BeansException {
final String beanName = transformedBeanName(name);
Object bean;
// Eagerly check singleton cache for manually registered singletons.
Object sharedInstance = getSingleton(beanName); //标记１
//如果从缓存中已经拿到了对象（先不管是从哪个缓存中拿到的），那就直接返回。
if (sharedInstance != null && args == null) {
//getObjectForBeanInstance()函数用来处理一个bean实现了FactoryBean接口的情况，在这种情况下，会调用这个bean的getObject()函数，并返回其结果。在本文的语境下，可以理解成这个函数什么都没做
bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
}
//如果缓存中还没有对象，就新建一个
else {
// Fail if we're already creating this bean instance:
// We're assumably within a circular reference.
//如果出现了prototype类型的循环依赖，直接抛出异常
if (isPrototypeCurrentlyInCreation(beanName)) {
throw new BeanCurrentlyInCreationException(beanName);
}
// Check if bean definition exists in this factory.
//如果本BeanFactory中不包含beanName的定义，且有父BeanFactory,那么交给父BeanFactory去创建
BeanFactory parentBeanFactory = getParentBeanFactory();
if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {
// Not found -> check parent.
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.
return (T) parentBeanFactory.getBean(nameToLookup, args);
}
else {
// No args -> delegate to standard getBean method.
return parentBeanFactory.getBean(nameToLookup, requiredType);
}
}
if (!typeCheckOnly) {
markBeanAsCreated(beanName);
}
try {
//下面两行代码从beanName得到对应的BeanDefinition并且转化成RootBeanDefinition类型，再并且如果有父bean的话需要和父bean的属性合并
final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
checkMergedBeanDefinition(mbd, beanName, args);
// Guarantee initialization of beans that the current bean depends on.
//如果这个bean有dependsOn属性。切记，这个和我们今天要讲的循环依赖不是指同一个概念，这个是bean定义的dependsOn属性，我们今天要讲的是属性相互依赖，比如Car类包含了一个Person字段，Person类也包含了一个Car字段。不过两种循环依赖的解决方案都差不多，都是先转去创建依赖的bean，再回来创建自己。
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);
}
}
// Create bean instance.
if (mbd.isSingleton()) {
//如果bean的是单例
//标记2
sharedInstance = getSingleton(beanName, () -> {
try {
return createBean(beanName, mbd, args);
}
catch (BeansException ex) {
}
});
bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
}
else if (mbd.isPrototype()) {
//如果bean的作用域是prototype，blabla...
}
else {
//如果是其它的作用域，blabla...
}
}
// Check if required type matches the type of the actual bean instance.
//这里检查类型是否匹配
return (T) bean;
} 在第一次调用getBean()的时候，singletonObjects、singletonFactories等四个字段都是空的，再看一下方法(1)标记１处的getSingleton()的代码：
//DefaultSingletonBeanRegistry.java
public Object getSingleton(String beanName) {
return getSingleton(beanName, true);
}
(2)
protected Object getSingleton(String beanName, boolean allowEarlyReference) {
Object singletonObject = this.singletonObjects.get(beanName);
//isSingletonCurrentlyInCreation函数检测的是singletonsCurrentlyInCreation字段里有没有对应的bean，如果有，说明这个bean正在被创建
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(); //标记1
this.earlySingletonObjects.put(beanName, singletonObject);
this.singletonFactories.remove(beanName);
}
}
}
}
return singletonObject;
}由于此时几个字段都为空，所以方法(2)直接返回null。再回到方法(1)，下一个比较重要的方法是方法(1)标记2处的代码，此时getSingleton()代码如下：
//DefaultSingletonBeanRegistry.java
(3)
public Object getSingleton(String beanName, ObjectFactory singletonFactory) {
synchronized (this.singletonObjects) {
Object singletonObject = this.singletonObjects.get(beanName);
if (singletonObject == null) {
//下面一行代码把这个bean加入singletonsCurrentlyInCreation字段中
beforeSingletonCreation(beanName);
boolean newSingleton = false;
try {
//调用getObject()方法创建对象！！真正创建对象的过程以及对循环依赖的递归处理就在这个函数中！！所以当这个函数返回的时候，这个bean已经被创建完成，它所依赖的bean也已经被创建完成！！如果暂时有什么不理解的，可以先记住结论，再看看后面的流程。
singletonObject = singletonFactory.getObject();
newSingleton = true;
}
finally {
//创建完成，把bean从singletonsCurrentlyInCreation中移除
afterSingletonCreation(beanName);
}
if (newSingleton) {
//此时才把bean加入singletonObjects字段中，所以singletonObjects字段中都是已经创建完成的bean
addSingleton(beanName, singletonObject);
}
}
return singletonObject;
}
}传递给方法(3)的参数是在方法(1)的标记2处createBean(beanName, mbd, args)，进入这个函数：
protected Object createBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args)
throws BeanCreationException {
//真正开始创建bean的实例
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.
Class resolvedClass = resolveBeanClass(mbd, beanName);
if (resolvedClass != null && !mbd.hasBeanClass() && mbd.getBeanClassName() != null) {
mbdToUse = new RootBeanDefinition(mbd);
mbdToUse.setBeanClass(resolvedClass);
}
// Prepare method overrides.
//处理需要覆盖的方法，对应标签的和属性
try {
mbdToUse.prepareMethodOverrides();
}
try {
// Give BeanPostProcessors a chance to return a proxy instead of the target bean instance.
//此时会调用注册的InstantiationAwareBeanPostProcessor类的postProcessBeforeInstantiation方法来实现一些代理之类的功能。在本文的语境中可以认为bean == null。
Object bean = resolveBeforeInstantiation(beanName, mbdToUse);
if (bean != null) { //如果bean不为null，直接返回，不进行正常的创建流程
return bean;
}
}
try {
//doCreateBean函数返回时代表bean已经创建完毕
Object beanInstance = doCreateBean(beanName, mbdToUse, args);
if (logger.isDebugEnabled()) {
logger.debug("Finished creating instance of bean '" + beanName + "'");
}
return beanInstance;
}
}再来看一下doCreateBean()函数，这个函数先构造的是一个BeanWrapper类的对象，包装了对应的bean，然后再调用BeanWrapper.getWrappedInstance()方法得到被包装的bean：
//AbstractAutowireCapableBeanFactory.java
(4)
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) {
//在这里创建BeanWrapper对象，但是此时并没有注入属性值
instanceWrapper = createBeanInstance(beanName, mbd, args);
}
//得到BeanWrapper包装的bean
final Object bean = instanceWrapper.getWrappedInstance();
Class beanType = instanceWrapper.getWrappedClass();
if (beanType != NullBean.class) {
mbd.resolvedTargetType = beanType;
}
// Allow post-processors to modify the merged bean definition.
//调用MergedBeanDefinitionPostProcessor后处理器的函数
synchronized (mbd.postProcessingLock) {
if (!mbd.postProcessed) {
try {
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.
//判断是否需要提早将bean放入缓存，接下来进行的populateBean()方法可能会产生循环依赖问题，所以需要此时就把未完成的bean放入缓存
boolean earlySingletonExposure = (mbd.isSingleton() && this.allowCircularReferences &&
isSingletonCurrentlyInCreation(beanName));
if (earlySingletonExposure) {
//这里是第一次调用getBean()方法来得到这个bean，把这个bean放入BeanFactory的singletonFactories字段中
addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));
}
// Initialize the bean instance.
Object exposedObject = bean;
try {
//填充bean的属性值，如果有依赖的属性也是一个bean，就先去创建那个bean
populateBean(beanName, mbd, instanceWrapper);
//调用初始化方法，比如init-method，另外如果这个bean实现了一些Aware接口，比如BeanFactoryAware，也在这个方法里面调用了接口的相应函数对bean进行处理
exposedObject = initializeBean(beanName, exposedObject, mbd); //标记1
}
if (earlySingletonExposure) {
Object earlySingletonReference = getSingleton(beanName, false);
if (earlySingletonReference != null) {
if (exposedObject == bean) {
exposedObject = earlySingletonReference;
}
else if (!this.allowRawInjectionDespiteWrapping && hasDependentBean(beanName)) {
String[] dependentBeans = getDependentBeans(beanName);
Set<String> actualDependentBeans = new LinkedHashSet<>(dependentBeans.length);
for (String dependentBean : dependentBeans) {
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.");
}
}
}
}
return exposedObject;
}由于循环依赖问题是在populateBean()函数中完成的，来看一下这个函数的代码：
//AbstractAutowireCapableBeanFactory.java
protected void populateBean(String beanName, RootBeanDefinition mbd, @Nullable BeanWrapper bw) {
// Give any InstantiationAwareBeanPostProcessors the opportunity to modify the
// state of the bean before properties are set. This can be used, for example,
// to support styles of field injection.
boolean continueWithPropertyPopulation = true;
//在属性值注入之前给InstantiationAwareBeanPostProcessor一次机会设置属性值
if (!mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors()) {
for (BeanPostProcessor bp : getBeanPostProcessors()) {
if (bp instanceof InstantiationAwareBeanPostProcessor) {
InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) bp;
//返回值为false会阻止其它的InstantiationAwareBeanPostProcessor再设置属性值，并且会导致不再给这个bean填充属性
if (!ibp.postProcessAfterInstantiation(bw.getWrappedInstance(), beanName)) {
continueWithPropertyPopulation = false;
break;
}
}
}
}
if (!continueWithPropertyPopulation) {
return;
}
//获得BeanDefinition中收集到的属性值
PropertyValues pvs = (mbd.hasPropertyValues() ? mbd.getPropertyValues() : null);
//由于在本例中我们直接使用的xml的标签，所以mbd.getResolvedAutowireMode() ＝ 0，这一段被跳过。不过实际中，autowireByName和autowireByType也会导致bean的递归创建，因为它们内部也会调用getBean()函数来获取另一个bean
if (mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_BY_NAME ||
mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_BY_TYPE) {
MutablePropertyValues newPvs = new MutablePropertyValues(pvs);
// Add property values based on autowire by name if applicable.
if (mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_BY_NAME) {
autowireByName(beanName, mbd, bw, newPvs);
}
// Add property values based on autowire by type if applicable.
if (mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_BY_TYPE) {
autowireByType(beanName, mbd, bw, newPvs);
}
pvs = newPvs;
}
if (pvs != null) {
applyPropertyValues(beanName, mbd, bw, pvs);
}
}是了，设置属性值其实是在applyPropertyValues函数中：
//AbstractAutowireCapableBeanFactory.java
(5)
protected void applyPropertyValues(String beanName, BeanDefinition mbd, BeanWrapper bw, PropertyValues pvs) {
if (pvs.isEmpty()) {
return;
}
MutablePropertyValues mpvs = null;
List original;
if (pvs instanceof MutablePropertyValues) {
mpvs = (MutablePropertyValues) pvs;
if (mpvs.isConverted()) {
// Shortcut: use the pre-converted values as-is.
try {
bw.setPropertyValues(mpvs);
return;
}
}
original = mpvs.getPropertyValueList();
}
else {
original = Arrays.asList(pvs.getPropertyValues());
}
TypeConverter converter = getCustomTypeConverter();
if (converter == null) {
converter = bw;
}
//获取属性值解析器
BeanDefinitionValueResolver valueResolver = new BeanDefinitionValueResolver(this, beanName, mbd, converter);
// Create a deep copy, resolving any references for values.
List deepCopy = new ArrayList<>(original.size());
boolean resolveNecessary = false;
for (PropertyValue pv : original) {
if (pv.isConverted()) {
deepCopy.add(pv);
}
else {
String propertyName = pv.getName();
Object originalValue = pv.getValue();
//在这个函数中把对应ref值的bean解析成一个真实的bean，换名话说，在这个函数里面调用了getBean()方法
Object resolvedValue = valueResolver.resolveValueIfNecessary(pv, originalValue);
Object convertedValue = resolvedValue;
boolean convertible = bw.isWritableProperty(propertyName) &&
!PropertyAccessorUtils.isNestedOrIndexedProperty(propertyName);
if (convertible) {
convertedValue = convertForProperty(resolvedValue, propertyName, bw, converter);
}
// Possibly store converted value in merged bean definition,
// in order to avoid re-conversion for every created bean instance.
if (resolvedValue == originalValue) {
if (convertible) {
pv.setConvertedValue(convertedValue);
}
deepCopy.add(pv);
}
else if (convertible && originalValue instanceof TypedStringValue &&
!((TypedStringValue) originalValue).isDynamic() &&
!(convertedValue instanceof Collection || ObjectUtils.isArray(convertedValue))) {
pv.setConvertedValue(convertedValue);
deepCopy.add(pv);
}
else {
resolveNecessary = true;
deepCopy.add(new PropertyValue(pv, convertedValue));
}
}
}
if (mpvs != null && !resolveNecessary) {
mpvs.setConverted();
}
// Set our (possibly massaged) deep copy.
try {
//设置属性值
bw.setPropertyValues(new MutablePropertyValues(deepCopy));
}
} 其中resolveValueIfNecessary方法中又根据属性值的类型采取了不同的解析逻辑，其中在ref值对应的逻辑是调用resolveReference方法，就是在这个方法中调用了getBean()方法来完成对依赖的处理。至此，总结一下，car对象的初始化还未完成，需要调用getBean(person)方法来得到person对象，然后才能把person对象注入设置成car对象的属性。到目前为止，car对象已经把自己加入到了singletonFactories缓存中（确切地说是把得到自己的方法加入到了缓存中）。
接下来是getBean("person")的过程，和getBean("car")很类似，一路到了方法(5)这里，同样需要设置person对象的属性值，于是接下来又产生了getBean("car")的调用，不过此次是getBean("car")的第二次调用了，来看一下和第一次有什么不同的地方。
在方法(1)的标记1处，此时缓存中已经有了car这个bean，于是在方法(2)的标记1处调用了car对应的工厂函数，在这个函数中得到缓存的car，并且将其加入到另一个缓存earlySingletonObjects中，并将其工厂函数从缓存singletonFactories中删除。然后回到方法(1)的流程，得到缓存中的对象后，就不必再去从头创建，直接进行getObjectForBeanInstance的调用（前面说过这个函数只是用于处理FactoryBean接口的对象），然后再进行一下类型检查就可以直接返回给person了。最后在person的方法(5)中将其做为一个属性设置给person。至此，person的创建完成，返回给car的方法(5)。不过在返回之前，在方法(4)的标记1处，调用了initializeBean()方法，在里面调用了bean的初始化函数和后处理器的方法。之后又在方法(3)中把person从singletonsCurrentlyInCreation和earlySingletonObjects中删除，并且把person加入到singletonObjects缓存中，下次再调用getBean("person")的时候就可以直接从singletonObjects缓存中取得。
把person返回给car的方法(5)之后，也是同样的流程，调用初始化方法和后处理器的方法，然后把car加入永久缓存singletonObjects中，并从临时缓存earlySingletonObjects中删除，至此，两个有循环依赖关系的bean都被初始化完毕。