Spring中的Bean可以绕过生命周期管理吗？

结论

答案是可以的。

Talk is cheap. Show me the code

第一步：定义一个类实现任意Spring中Bean生命周期相关方法。

package com.xxx.hyl.ignore.lifecycle;import org.springframework.beans.factory.InitializingBean;/**** 演示当前类跳过Spring IoC 容器Bean生命周期管理* @author 君战* */
public class IgnoreLifecycleBean implements InitializingBean {private String createdBy;// 这里只是为了演示，所以只选择实现InitializingBean接口，只要该接口可以跳过，其它的生命周期方法都是一样的。@Overridepublic void afterPropertiesSet() throws Exception {System.out.println(this.getClass().getSimpleName() + " -----> afterPropertiesSet 方法执行");}public String getCreatedBy() {return createdBy;}public void setCreatedBy(String createdBy) {this.createdBy = createdBy;}
}

第二步：编写一个类实现InstantiationAwareBeanPostProcessor接口，并重写其postProcessBeforeInstantiation方法。

package com.xxx.hyl.ignore.lifecycle;import org.springframework.beans.BeansException;
import org.springframework.beans.factory.config.InstantiationAwareBeanPostProcessor;/*** 演示通过实现{@linkplain InstantiationAwareBeanPostProcessor#postProcessBeforeInstantiation(Class,* String)} 方法来让Bean跳过IoC容器生命周期管理** @author 君战 **/
public class CustomizedBeanPostProcessor implements InstantiationAwareBeanPostProcessor {@Overridepublic Object postProcessBeforeInstantiation(Class<?> beanClass, String beanName)throws BeansException {if (IgnoreLifecycleBean.class.equals(beanClass)) {IgnoreLifecycleBean ignoreLifecycleBean = new IgnoreLifecycleBean();ignoreLifecycleBean.setCreatedBy("customize init");return ignoreLifecycleBean;}return null;}
}

第三步：编写一个类来测试

package com.xxx.hyl.ignore.lifecycle;import org.springframework.context.annotation.AnnotationConfigApplicationContext;/*** 演示Bean跳过IoC容器的生命周期管理* @author 君战* **/
public class IgnoreLifecycleBeanDemo {public static void main(String[] args) {AnnotationConfigApplicationContext context = new AnnotationConfigApplicationContext();context.register(IgnoreLifecycleBean.class,CustomizedBeanPostProcessor.class);context.refresh();IgnoreLifecycleBean ignoreLifecycleBean = context.getBean(IgnoreLifecycleBean.class);System.out.println(ignoreLifecycleBean.getCreatedBy());}
}

第四步：查看控制台打印结果。

可以看到，IgnoreLifecycleBean 所实现的InitializingBean的afterPropertiesSet方法并未被调用，并且其getCreatedBy方法所返回的数据和我们在CustomizedBeanPostProcessor的postProcessBeforeInstantiation方法设置的一致。那么接下来就进入底层原理分析。

底层原理分析

在AbstractAutowireCapableBeanFactory的createBean方法中，首先通过resolveBeanClass方法来解析BeanDefinition，获取要实例化的BeanClass。然后调用RootBeanDefinition的prepareMethodOverrides方法，重点是接下来调用的resolveBeforeInstantiation方法，可以看到如果该方法的返回值不为空，则createBean方法结束，而不会再去调用IoC容器中真正实例化Bean的方法-doCreateBean。

// AbstractAutowireCapableBeanFactory#createBean(java.lang.String, org.springframework.beans.factory.support.RootBeanDefinition, java.lang.Object[])
protected Object createBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args)throws BeanCreationException {if (logger.isTraceEnabled()) {logger.trace("Creating instance of bean '" + beanName + "'");}RootBeanDefinition mbdToUse = mbd;Class<?> resolvedClass = resolveBeanClass(mbd, beanName);if (resolvedClass != null && !mbd.hasBeanClass() && mbd.getBeanClassName() != null) {mbdToUse = new RootBeanDefinition(mbd);mbdToUse.setBeanClass(resolvedClass);}try {mbdToUse.prepareMethodOverrides();}catch (BeanDefinitionValidationException ex) {throw new BeanDefinitionStoreException(mbdToUse.getResourceDescription(),beanName, "Validation of method overrides failed", ex);}try {// 调用所有实现了InstantiationAwareBeanPostProcessor接口的postProcessBeforeInstantiation方法// 任何一个InstantiationAwareBeanPostProcessor接口实现类的postProcessBeforeInstantiation方法返回值不为null，那么则结束当前beanName所对应Bean的创建和初始化。Object bean = resolveBeforeInstantiation(beanName, mbdToUse);if (bean != null) {return bean;}}catch (Throwable ex) {throw new BeanCreationException(mbdToUse.getResourceDescription(), beanName,"BeanPostProcessor before instantiation of bean failed", ex);}try {// 这是IoC容器真正实例化Bean的地方Object beanInstance = doCreateBean(beanName, mbdToUse, args);if (logger.isTraceEnabled()) {logger.trace("Finished creating instance of bean '" + beanName + "'");}return beanInstance;}catch (BeanCreationException | ImplicitlyAppearedSingletonException ex) {throw ex;}catch (Throwable ex) {throw new BeanCreationException(mbdToUse.getResourceDescription(), beanName, "Unexpected exception during bean creation", ex);}
}

而讲到这个resolveBeforeInstantiation方法，就不得不提InstantiationAwareBeanPostProcessor接口,该接口继承自Spring IoC容器中大名鼎鼎的BeanPostProcessor接口，并定义了四个接口方法-postProcessBeforeInstantiation方法和postProcessAfterInstantiation方法以及postProcessProperties方法和postProcessPropertyValues方法(该方法已被标记为废弃)。

而前面的resolveBeforeInstantiation方法就是调用IoC容器中所有实现了该接口的实现类的postProce-ssBeforeInstantiation方法，入参为前面解析好的BeanClass和beanName，如果有某一个实现类实现的该方法返回了数据，则直接结束循环，当前Bean的整个实例化过程就结束了。Bean中所有的依赖项、生命周期相关的方法都不会再被处理，因此，很少会有处理器在该方法中返回数据。

// AbstractAutowireCapableBeanFactory#applyBeanPostProcessorsBeforeInstantiation
protected Object resolveBeforeInstantiation(String beanName, RootBeanDefinition mbd) {Object bean = null;if (!Boolean.FALSE.equals(mbd.beforeInstantiationResolved)) {// Make sure bean class is actually resolved at this point.if (!mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors()) {Class<?> targetType = determineTargetType(beanName, mbd);if (targetType != null) {bean = applyBeanPostProcessorsBeforeInstantiation(targetType, beanName);if (bean != null) {bean = applyBeanPostProcessorsAfterInitialization(bean, beanName);}}}mbd.beforeInstantiationResolved = (bean != null);}return bean;
}protected Object applyBeanPostProcessorsBeforeInstantiation(Class<?> beanClass, String beanName) {for (BeanPostProcessor bp : getBeanPostProcessors()) {if (bp instanceof InstantiationAwareBeanPostProcessor) {InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) bp;Object result = ibp.postProcessBeforeInstantiation(beanClass, beanName);if (result != null) {return result;}}}return null;
}

总结

Spring中的Bean是可以绕过IoC容器的Bean生命周期管理的，只需要实现InstantiationAwareBeanPostProcessor接口并重写其postProcessBeforeInstantiation方法来对指定beanClass进行创建，从而绕过IoC容器的生命周期管理。

但请慎用该手段，因为通过这种方式返回的Bean不仅其生命周期不受IoC容器管理，并且其依赖也不会被IoC容器所处理。如果其处于切点表达式的指定范围内，那么也不会被进行增强(动态代理/字节码增强)，其方法上@Transactional注解将失去作用等等。

慎用！慎用！慎用！