spring 如何利用三级缓存解决循环依赖
在java后端服务之间调用时，很容易产生循环依赖，对于spring内部是如何解决循环依赖的，简言之，spring采用缓存机制来解决（三级缓存）。
缓存介绍
| 名称 | 描述 | 级别 |
|---|---|---|
| singletonObjects | 初始化完成的单例对象缓存 | 一级缓存 |
| earlySingletonObjects | 提前曝光的单例对象缓存 | 二级缓存 |
| singletonFactories | 单例对象工厂缓存 | 三级缓存 |
| singletonsCurrentlyInCreation | 当前正在创建中的单例集合 | 判断继续查找二级缓存的条件 |
基本逻辑
主要代码逻辑参考类：
org.springframework.beans.factory.support.DefaultSingletonBeanRegistry
核心代码如下：
/**
* 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) {
// Quick check for existing instance without full singleton lock
Object singletonObject = this.singletonObjects.get(beanName);
if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {
singletonObject = this.earlySingletonObjects.get(beanName);
if (singletonObject == null && allowEarlyReference) {
synchronized (this.singletonObjects) {
// Consistent creation of early reference within full singleton lock
singletonObject = this.singletonObjects.get(beanName);
if (singletonObject == null) {
singletonObject = this.earlySingletonObjects.get(beanName);
if (singletonObject == null) {
ObjectFactory<?> singletonFactory = this.singletonFactories.get(beanName);
if (singletonFactory != null) {
singletonObject = singletonFactory.getObject();
this.earlySingletonObjects.put(beanName, singletonObject);
this.singletonFactories.remove(beanName);
}
}
}
}
}
}
return singletonObject;
}
在调用 getSingleton 方法之前，先调用 beforeSingletonCreation 方法
/**
* 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);
}
}
本地常量缓存，用来记录当前bean是否正在创建中
/** Names of beans that are currently in creation. */
private final Set<String> singletonsCurrentlyInCreation = Collections.newSetFromMap(new ConcurrentHashMap<>(16));
而在同名方法 getSingleton(String beanName, ObjectFactory<?> singletonFactory)中，finally块调用了 afterSingletonCreation(beanName);
/**
* Callback after singleton creation.
* <p>The default implementation marks the singleton as not in creation anymore.
* @param beanName the name of the singleton that has been created
* @see #isSingletonCurrentlyInCreation
*/
protected void afterSingletonCreation(String beanName) {
if (!this.inCreationCheckExclusions.contains(beanName) &&!this.singletonsCurrentlyInCreation.remove(beanName)) {
throw new IllegalStateException("Singleton '" + beanName + "' isn't currently in creation");
}
}
在创建结束后，进行删除。
在bean创建的不同时期，会从缓存中读取bean,在创建bean时，尝试先从一级缓存（singletonObjects）中获取bean，获取为null且判断是正在创建的bean，则继续从二级缓存（earlySingletonObjects）中获取bean，当获取的bean继续为空时，并且允许从singletonFactories通过getObject获取（allowEarlyReference 参数为true时），则通过三级缓存（singletonFactory）继续获取，如果获取到bean，先删除三级缓存中bean，再复制到二级缓存(earlySingletonObjects)中。
缓存之间的转换
接下来的二级缓存到一级缓存的转换则通过 addSingleton 实现，在创建bean时，若一级缓存不存在bean，则直接保存bean到三级缓存。
主要代码如下:
/**
* Add the given singleton object to the singleton cache of this factory.
* <p>To be called for eager registration of singletons.
* @param beanName the name of the bean
* @param singletonObject the singleton object
*/
protected void addSingleton(String beanName, Object singletonObject) {
synchronized (this.singletonObjects) {
this.singletonObjects.put(beanName, singletonObject);
this.singletonFactories.remove(beanName);
this.earlySingletonObjects.remove(beanName);
this.registeredSingletons.add(beanName);
}
}
/**
* Add the given singleton factory for building the specified singleton
* if necessary.
* <p>To be called for eager registration of singletons, e.g. to be able to
* resolve circular references.
* @param beanName the name of the bean
* @param singletonFactory the factory for the singleton object
*/
protected void addSingletonFactory(String beanName, ObjectFactory<?> singletonFactory) {
Assert.notNull(singletonFactory, "Singleton factory must not be null");
synchronized (this.singletonObjects) {
if (!this.singletonObjects.containsKey(beanName)) {
this.singletonFactories.put(beanName, singletonFactory);
this.earlySingletonObjects.remove(beanName);
this.registeredSingletons.add(beanName);
}
}
}
synchronized (this.singletonObjects) {
if (!this.singletonObjects.containsKey(beanName)) {
this.singletonFactories.put(beanName, singletonFactory);
this.earlySingletonObjects.remove(beanName);
this.registeredSingletons.add(beanName);
}
}
spring 实例化bean采用BeanUtils.instantiateClass(constructorToUse);
源码如下：
@Override
public Object instantiate(RootBeanDefinition bd, @Nullable String beanName, BeanFactory owner) {
// Don't override the class with CGLIB if no overrides.
if (!bd.hasMethodOverrides()) {
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(
(PrivilegedExceptionAction<Constructor<?>>) clazz::getDeclaredConstructor);
}
else {
constructorToUse = clazz.getDeclaredConstructor();
}
bd.resolvedConstructorOrFactoryMethod = constructorToUse;
}
catch (Throwable ex) {
throw new BeanInstantiationException(clazz, "No default constructor found", ex);
}
}
}
return BeanUtils.instantiateClass(constructorToUse);
}
else {
// Must generate CGLIB subclass.
return instantiateWithMethodInjection(bd, beanName, owner);
}
}
