学了这么久的循环依赖，你竟然还不能解读spring源码！，2024年最新掌握这些知识点再也不怕面试通不过

最新推荐文章于 2024-05-12 03:32:24 发布

原创最新推荐文章于 2024-05-12 03:32:24 发布 · 485 阅读

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#spring #面试 #服务器

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正文

}

public void setB(B b) {
this.b = b;
}

private B b;
}

/**

循环依赖B
@date: 2020/12/24
@author weirx
@version 3.0
*/
public class B {

public A getA() {
return a;
}

public void setA(A a) {
this.a = a;
}

private A a;
}

xml文件：

执行结果，经测试可以解决，得到的对象是循环依赖的:

构造器方式：

/**

循环依赖A
@date: 2020/12/24
@author weirx
@version 3.0
*/
public class A {

private B b;

public A(B b) {
this.b = b;
}
}

/**

循环依赖B
@date: 2020/12/24
@author weirx
@version 3.0
*/
public class B {

private A a;

public B(A a) {
this.a = a;
}
}

xml配置：

结果是不能解决，会得到以下报错信息：

十二月 25, 2020 9:38:00 上午 org.springframework.context.support.AbstractApplicationContext refresh
警告: Exception encountered during context initialization - cancelling refresh attempt: org.springframework.beans.factory.BeanCreationException: Error creating bean with name ‘a’ defined in class path resource [spring-beans.xml]: Cannot resolve reference to bean ‘b’ while setting constructor argument; nested exception is org.springframework.beans.factory.BeanCreationException: Error creating bean with name ‘b’ defined in class path resource [spring-beans.xml]: Cannot resolve reference to bean ‘a’ while setting constructor argument; 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?
Exception in thread “main” org.springframework.beans.factory.BeanCreationException: Error creating bean with name ‘a’ defined in class path resource [spring-beans.xml]: Cannot resolve reference to bean ‘b’ while setting constructor argument; nested exception is org.springframework.beans.factory.BeanCreationException: Error creating bean with name ‘b’ defined in class path resource [spring-beans.xml]: Cannot resolve reference to bean ‘a’ while setting constructor argument; 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:342)
at org.springframework.beans.factory.support.BeanDefinitionValueResolver.resolveValueIfNecessary(BeanDefinitionValueResolver.java:113)
at org.springframework.beans.factory.support.ConstructorResolver.resolveConstructorArguments(ConstructorResolver.java:704)
at org.springframework.beans.factory.support.ConstructorResolver.autowireConstructor(ConstructorResolver.java:196)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.autowireConstructor(AbstractAutowireCapableBeanFactory.java:1356)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBeanInstance(AbstractAutowireCapableBeanFactory.java:1203)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:556)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:516)
at org.springframework.beans.factory.support.AbstractBeanFactory.lambda$doGetBeandoGetBean$0(AbstractBeanFactory.java:324)
at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:234)
at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:322)
at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:202)
at org.springframework.beans.factory.support.BeanDefinitionValueResolver.resolveReference(BeanDefinitionValueResolver.java:330)
… 17 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.DefaultSingletonBeanRegistry.beforeSingletonCreation(DefaultSingletonBeanRegistry.java:355)
at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:227)
at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:322)
at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:202)
at org.springframework.beans.factory.support.BeanDefinitionValueResolver.resolveReference(BeanDefinitionValueResolver.java:330)
… 29 more
Disconnected from the target VM, address: ‘127.0.0.1:60287’, transport: ‘socket’

Process finished with exit code 1

3、源码跟踪

跟踪源码过程的流程图

先扔一张我跟踪源码过程的流程图：

上图中有个关键点，在spring的源码中，通常会看到getBean，doGetBean等方法，在解决循环依赖过程中，重要的流程如下图所示：

代码跟踪开始：

类A和B采用set方式

执行main方法：

/**

main
@date: 2020/12/23
@author weirx
@version 3.0
*/
public class TestSpring {

public static void main(String[] args) {
ClassPathXmlApplicationContext applicationContext
= new ClassPathXmlApplicationContext(“spring-beans.xml”);
}
}

跟踪refresh方法的finishBeanFactoryInitialization方法，直接跟踪器内部的最后一行代码beanFactory.preInstantiateSingletons()：

public void preInstantiateSingletons() throws BeansException {
if (logger.isTraceEnabled()) {
logger.trace("Pre-instantiating singletons in " + this);
}

// 获取bean名称
List beanNames = new ArrayList<>(this.beanDefinitionNames);

// 遍历实例化所有非延时bean
for (String beanName : beanNames) {
//获取bean的定义
RootBeanDefinition bd = getMergedLocalBeanDefinition(beanName);
//bean是否不是抽象&是否单例&是否不是懒加载
if (!bd.isAbstract() && bd.isSingleton() && !bd.isLazyInit()) {
//是否是工厂bean
if (isFactoryBean(beanName)) {
Object bean = getBean(FACTORY_BEAN_PREFIX + beanName);
if (bean instanceof FactoryBean) {
FactoryBean<?> factory = (FactoryBean<?>) bean;
boolean isEagerInit;
if (System.getSecurityManager() != null && factory instanceof SmartFactoryBean) {
isEagerInit = AccessController.doPrivileged(
(PrivilegedAction) ((SmartFactoryBean<?>) factory)::isEagerInit, getAccessControlContext()); } else { isEagerInit = (factory instanceof SmartFactoryBean && ((SmartFactoryBean<?>) factory).isEagerInit());
}
if (isEagerInit) {
getBean(beanName);
}
}
}
else {
//获取bean
getBean(beanName);
}
}
}

// Trigger post-initialization callback for all applicable beans…
for (String beanName : beanNames) {
Object singletonInstance = getSingleton(beanName);
if (singletonInstance instanceof SmartInitializingSingleton) {
SmartInitializingSingleton smartSingleton = (SmartInitializingSingleton) singletonInstance;
if (System.getSecurityManager() != null) {
AccessController.doPrivileged((PrivilegedAction) () -> {
smartSingleton.afterSingletonsInstantiated();
return null;
}, getAccessControlContext());
}
else {
smartSingleton.afterSingletonsInstantiated();
}
}
}
}

getBean方法

上述代码忽略不重要的流程，断点直接走到了getBean方法：

public Object getBean(String name) throws BeansException {
//真正实例化bean的方法
return doGetBean(name, null, null, false);
}

跟踪进入doGetBean，两个关键点，检查已经注册的缓存（getSingleton(beanName)），创建bean实例，在下面的代码中都有注释，需要重点关注:

protected T doGetBean(
String name, @Nullable Class requiredType, @Nullable Object[] args, boolean typeCheckOnly)
throws BeansException {

String beanName = transformedBeanName(name);
Object bean;

// 检查已经注册的缓存
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 + “'”);
}
}
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);
}

// Check if bean definition exists in this factory.
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 if (requiredType != null) {
// No args -> delegate to standard getBean method.
return parentBeanFactory.getBean(nameToLookup, requiredType);
}
else {
return (T) parentBeanFactory.getBean(nameToLookup);
}
}

if (!typeCheckOnly) {
markBeanAsCreated(beanName);
}

try {
RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
checkMergedBeanDefinition(mbd, beanName, args);

// Guarantee initialization of beans that the current bean depends on.
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);
try {
getBean(dep);
}
catch (NoSuchBeanDefinitionException ex) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName,
“'” + beanName + “’ depends on missing bean '” + dep + “'”, ex);
}
}
}

// 创建bean实例
if (mbd.isSingleton()) {
//从缓存获取bean实例，此处使用函数式接口(ObjectFactory是函数式接口，当调用其默认方法，会执行函数内的createBean)
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);
}

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();
if (!StringUtils.hasLength(scopeName)) {
throw new IllegalStateException(“No scope name defined for bean ´” + beanName + “'”);
}
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, () -> {
beforePrototypeCreation(beanName);
try {
return createBean(beanName, mbd, args);
}
finally {
afterPrototypeCreation(beanName);
}
});
bean = getObjectForBeanInstance(scopedInstance, name, beanName, mbd);
}
catch (IllegalStateException ex) {
throw new ScopeNotActiveException(beanName, scopeName, ex);
}
}
}
catch (BeansException ex) {
cleanupAfterBeanCreationFailure(beanName);
throw ex;
}
}

// Check if required type matches the type of the actual bean instance.
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.isTraceEnabled()) {
logger.trace(“Failed to convert bean '” + name + “’ to required type '” +
ClassUtils.getQualifiedName(requiredType) + “'”, ex);
}
throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
}
}
return (T) bean;
}

我们分别去看下这两个关键点做了什么？首先是到缓存获取bean实例getSingleton，此时一定是null，因为还没有进行实例化：

@Nullable
protected Object getSingleton(String beanName, boolean allowEarlyReference) {
//检查一级缓存是否存在当前bean的实例
Object singletonObject = this.singletonObjects.get(beanName);
//判断当前对象在缓存是否为null，且是否是当前正在创建的bean
if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {
synchronized (this.singletonObjects) {
//从二级缓存获取bean实例
singletonObject = this.earlySingletonObjects.get(beanName);

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nCreation(beanName)) {
synchronized (this.singletonObjects) {
//从二级缓存获取bean实例
singletonObject = this.earlySingletonObjects.get(beanName);