Spring循环依赖三级缓存是否可以减少为二级缓存?
阅读本文大概需要 6.5 分钟。
来自:思否segmentfault
public class A {
private B b;
}
public class B {
private A a;
}
/** Cache of singleton objects: bean name to bean instance. */
private final MapsingletonObjects = new ConcurrentHashMap<>(256);
/** Cache of early singleton objects: bean name to bean instance. */
private final MapearlySingletonObjects = new HashMap<>(16);
/** Cache of singleton factories: bean name to ObjectFactory. */
private final Map> singletonFactories = new HashMap<>(16);
一. 在构造Bean对象之后,将对象提前曝光到缓存中,这时候曝光的对象仅仅是构造完成,还没注入属性和初始化。
public abstract class AbstractAutowireCapableBeanFactory extends AbstractBeanFactory
implements AutowireCapableBeanFactory {
protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final @Nullable Object[] args)
throws BeanCreationException {
……
// 是否提前曝光
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");
}
addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));
}
……
}
}
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二. 提前曝光的对象被放入Map> singletonFactories缓存中,这里并不是直接将Bean放入缓存,而是包装成ObjectFactory对象再放入。
public class DefaultSingletonBeanRegistry extends SimpleAliasRegistry implements SingletonBeanRegistry {
protected void addSingletonFactory(String beanName, ObjectFactory> singletonFactory) {
Assert.notNull(singletonFactory, "Singleton factory must not be null");
synchronized (this.singletonObjects) {
// 一级缓存
if (!this.singletonObjects.containsKey(beanName)) {
// 三级缓存
this.singletonFactories.put(beanName, singletonFactory);
// 二级缓存
this.earlySingletonObjects.remove(beanName);
this.registeredSingletons.add(beanName);
}
}
}
}
public interface ObjectFactory<T> {
T getObject() throws BeansException;
}
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三. 为什么要包装一层ObjectFactory对象?
addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));:
public abstract class AbstractAutowireCapableBeanFactory extends AbstractBeanFactory
implements AutowireCapableBeanFactory {
protected Object getEarlyBeanReference(String beanName, RootBeanDefinition mbd, Object bean) {
Object exposedObject = bean;
if (!mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors()) {
for (BeanPostProcessor bp : getBeanPostProcessors()) {
if (bp instanceof SmartInstantiationAwareBeanPostProcessor) {
SmartInstantiationAwareBeanPostProcessor ibp = (SmartInstantiationAwareBeanPostProcessor) bp;
exposedObject = ibp.getEarlyBeanReference(exposedObject, beanName);
}
}
}
return exposedObject;
}
}
public abstract class AbstractAutoProxyCreator extends ProxyProcessorSupport
implements SmartInstantiationAwareBeanPostProcessor, BeanFactoryAware {
private final Map
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四. 注入属性和初始化
public abstract class AbstractAutowireCapableBeanFactory e xtends AbstractBeanFactory
implements AutowireCapableBeanFactory {
protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final @Nullable Object[] args)
throws BeanCreationException {
……
// Initialize the bean instance.
Object exposedObject = bean;
try {
populateBean(beanName, mbd, instanceWrapper);
exposedObject = initializeBean(beanName, exposedObject, mbd);
}
catch (Throwable ex) {
if (ex instanceof BeanCreationException && beanName.equals(((BeanCreationException) ex).getBeanName())) {
throw (BeanCreationException) ex;
}
else {
throw new BeanCreationException(
mbd.getResourceDescription(), beanName, "Initialization of bean failed", ex);
}
}
……
}
}
// 获取要注入的对象
public class DefaultSingletonBeanRegistry extends SimpleAliasRegistry implements SingletonBeanRegistry {
protected Object getSingleton(String beanName, boolean allowEarlyReference) {
// 一级缓存
Object singletonObject = this.singletonObjects.get(beanName);
if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {
synchronized (this.singletonObjects) {
// 二级缓存
singletonObject = this.earlySingletonObjects.get(beanName);
if (singletonObject == null && allowEarlyReference) {
// 三级缓存
ObjectFactory> singletonFactory = this.singletonFactories.get(beanName);
if (singletonFactory != null) {
singletonObject = singletonFactory.getObject();
this.earlySingletonObjects.put(beanName, singletonObject);
this.singletonFactories.remove(beanName);
}
}
}
}
return singletonObject;
}
}
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五. 放入已完成创建的单例缓存
public class DefaultSingletonBeanRegistry extends SimpleAliasRegistry implements SingletonBeanRegistry {
/** Cache of singleton objects: bean name to bean instance. */
private final MapsingletonObjects = new ConcurrentHashMap<>(256);
/** Cache of singleton factories: bean name to ObjectFactory. */
private final Map> singletonFactories = new HashMap<>(16);
/** Cache of early singleton objects: bean name to bean instance. */
private final MapearlySingletonObjects = new HashMap<>(16);
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);
}
}
}
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public abstract class AbstractAutowireCapableBeanFactory extends AbstractBeanFactory
implements AutowireCapableBeanFactory {
protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final @Nullable Object[] args)
throws BeanCreationException {
……
// 是否提前曝光
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");
}
addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));
// 立刻从三级缓存取出放入二级缓存
getSingleton(beanName, true);
}
……
}
}
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