1. 什么是循环依赖?
循环依赖说白了就是在我们的程序中,类A需要依赖于类B,类B又需要依赖于类A,这时候两个类实例就构成了相互依赖,你中有我,我中有你。
@Component
public class A {
@Autowired
private B b;
}
@Component
public class B {
@Autowired
private A a;
}
2. Spring什么情况下可以帮助我们解决循环依赖?
我们知道在spring中,如果我们的bean是单例的,并且不是通过构造方法进行依赖注入,spring大部分情况下是可以帮助我么解决循环依赖问题的,当然也有
可能解决不了。
(具体参考:这个Spring循环依赖的坑,90%以上的人都不知道)。
3. Prototype类型为什么解决不了循环依赖?
在spring的AbstractBeanFactory中的doGetBean方法中有一个判断,如果是Prototype类型并且该实例正在创建中,直接抛出异常。
// Fail if we're already creating this bean instance:
// We're assumably within a circular reference.
if (isPrototypeCurrentlyInCreation(beanName)) {
throw new BeanCurrentlyInCreationException(beanName);
}
/**
* Return whether the specified prototype bean is currently in creation
* (within the current thread).
* @param beanName the name of the bean
*/
protected boolean isPrototypeCurrentlyInCreation(String beanName) {
Object curVal = this.prototypesCurrentlyInCreation.get();
return (curVal != null &&
(curVal.equals(beanName) || (curVal instanceof Set && ((Set<?>) curVal).contains(beanName))));
}
4. 构造方法注入为什么解决不了循环依赖?
思考一个逻辑,spring创建A的时候,发现A的构造函数中需要注入B,然后spring去创建B,这时候发现,B的构造函数中又需要注入A,可是这时候A还在等待B创
建成功后进行注入,A还没创建好呢,这时候就构成了死等待,A等待B创建,B等待A创建,最终的结果就是都进行不下去了,spring检测到后就直接给我们抛异常
了。
5. 自定义BeanPostProcessor
这时候如果我们想要自定义BeanPostProcessor,并且可能需要通过代理原始类,扩展我们的功能,那么这时候,如果我们像下面这样写:
@Component
public class TestBeanPostProcessor implements BeanPostProcessor,MethodInterceptor{
@Override
public Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException {
if(bean instanceof A){
System.out.println("对A创建代理");
Enhancer enhancer = new Enhancer();
enhancer.setSuperclass(A.class);
enhancer.setCallback(this);
return enhancer.create();
}
return bean;
}
@Override
public Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException {
return null;
}
@Override
public Object intercept(Object o, Method method, Object[] objects, MethodProxy methodProxy) throws Throwable {
System.out.println("代理拦截" + method.getName());
Object object = methodProxy.invokeSuper(o, objects);
return object;
}
}
这样你只能收获到无情的报错:
org.springframework.beans.factory.BeanCurrentlyInCreationException: Error creating bean with name 'a': Bean with name
'a' has been injected into other beans [b] 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.
这段错误大概的意思就是,我们已经注入的bean后来被修改了,导致注入的bean和最终实际的bean不是一个,所以报错了,当然,你也可以按照提示,通过
allowEagerInit这个属性进行配置,允许这种不一致的情况发生,只是我不推荐这种做法,很有可能带来莫名其妙的错误。
下面我们来改写一下实现方式
@Component
public class TestBeanPostProcessor implements SmartInstantiationAwareBeanPostProcessor,MethodInterceptor{
@Override
public Object getEarlyBeanReference(Object bean, String beanName) throws BeansException {
if(bean instanceof A){
System.out.println("对A创建代理");
Enhancer enhancer = new Enhancer();
enhancer.setSuperclass(A.class);
enhancer.setCallback(this);
return enhancer.create();
}
return bean;
}
@Override
public Object intercept(Object o, Method method, Object[] objects, MethodProxy methodProxy) throws Throwable {
System.out.println("代理拦截" + method.getName());
Object object = methodProxy.invokeSuper(o, objects);
return object;
}
}
你会发现这种方式成功启动了,并且在B中注入的A实例就是我们代理后的对象。
6. 为什么第二种方式能成功呢?
注意一下这两种实现方式,第一种是实现了BeanPostProcessor,并对postProcessBeforeInitialization方法进行了重写;第二种是实现了
SmartInstantiationAwareBeanPostProcessor,并对getEarlyBeanReference进行了重写。要想明白原因,我们需要知道spring在什么时候回触发
BeanPostProcessor的调用。
之所以第二种能够成功,是因为Spring中提前暴露bean时的一段处理逻辑,这段逻辑位于AbstractAutowireCapableBeanFactory中的doCreateBean方法中。
protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final @Nullable Object[] args)
throws BeanCreationException {
...
// 如果允许循环依赖,并且当前bean正在创建中,那么先放入singletonFactories,singletonFactories属于是一个缓存,spring获取bean
// bean实例的时候,会先从singletonObjects获取,获取不到再从earlySingletonObjects获取,如果还没有,就会从singletonFactories
// 进行获取,因为提前暴露的对象我们已经add到singletonFactories中了,那么从singletonFactories获取对象时就会调用
// () -> getEarlyBeanReference(beanName, mbd, bean),从而就可以提前获取到代理对象了
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");
}
// 这里是重点,里面封装了能够正确注入代理的逻辑
// 重点就是getEarlyBeanReference(beanName, mbd, bean)方法
addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));
}
// Initialize the bean instance.
Object exposedObject = bean;
try {
// 在这个方法里面,会执行AutowiredAnnotationBeanPostProcessor的postProcessProperties,从而进行了@Autowird属性的注入
populateBean(beanName, mbd, instanceWrapper);
// 在这个方法里面,会触发第一种BeanPostProcessor的postProcessBeforeInitialization方法的调用
exposedObject = initializeBean(beanName, exposedObject, mbd);
}catch (Throwable ex) {
...
}
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);
}
}
// 这里就是我们上面报错的地方,因为检测出来了依赖的bean和实际的bean不一致
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;
}
protected Object getEarlyBeanReference(String beanName, RootBeanDefinition mbd, Object bean) {
Object exposedObject = bean;
if (!mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors()) {
for (BeanPostProcessor bp : getBeanPostProcessors()) {
// 从这里就可以发现,对于提前暴露的bean引用,这里会调用SmartInstantiationAwareBeanPostProcessor的
//getEarlyBeanReference的方法,并将该方法返回的对象作为实际暴露的对象,那么就不难理解为什么我们重写了这个方法后,就可以将我们代
//理后的对象正确注入了
if (bp instanceof SmartInstantiationAwareBeanPostProcessor) {
SmartInstantiationAwareBeanPostProcessor ibp = (SmartInstantiationAwareBeanPostProcessor) bp;
exposedObject = ibp.getEarlyBeanReference(exposedObject, beanName);
}
}
}
return exposedObject;
}
现在我们来看一看第一种方式为什么不能成功。通过上面代码的标注,我么知道了第一种方式postProcessBeforeInitialization方法的调用是在initializeBean中,而
我们的@Autowird注解的注入是在initializeBean的前一个方法populateBean中,也就是B中都已经注入了A提前暴露出来的对象后,我们的
postProcessBeforeInitialization方法又将A对象实例给改成了我们的代理对象,从而导致了注入的A的bean对象和最终的A的bean对象不是一个。