如何动态在spring mvc中增加bean

2022-11-22 18:25:02

阅读对象##

搭框架人员，或者其他感兴趣的开发人员

背景##

一般来说在业务代码中，加上 @Component, @Service，@Repository, @Controller等注解就可以实现将bean注册到Spring中了。

但是在写框架，可能有些类会动态生成，怎么动态注册到Spring中呢？

`BeanDefinitionRegistryPostProcessor` 接口##

BeanDefinitionRegistryPostProcessor接口是一个可以修改spring工厂中已定义的bean的接口，该接口有个postProcessBeanDefinitionRegistry方法。

/**

 * Modify the application context's internal bean definition registry after its

 * standard initialization. All regular bean definitions will have been loaded,

 * but no beans will have been instantiated yet. This allows for adding further

 * bean definitions before the next post-processing phase kicks in.

 * [@param](https://my.oschina.net/u/2303379) registry the bean definition registry used by the application context

 * [@throws](https://my.oschina.net/throws) org.springframework.beans.BeansException in case of errors

 */

void postProcessBeanDefinitionRegistry(BeanDefinitionRegistry registry) throws BeansException;

实现##

代码###

public class App implements BeanDefinitionRegistryPostProcessor{

 @Override

 public void postProcessBeanDefinitionRegistry(

		BeanDefinitionRegistry registry) throws BeansException {

	registry.registerBeanDefinition("demoEntityDao", getDefinition(AcAlert.class));

 }

 private BeanDefinition getDefinition(Class<?> cls) {

	DaoInterfaceGenerator g = new DaoInterfaceGenerator(cls);

	Class<?> daoClass = g.generateClass();

	GenericBeanDefinition bd = new GenericBeanDefinition();

	bd.setBeanClass(MapperFactoryBean.class);

	bd.getConstructorArgumentValues().addGenericArgumentValue(daoClass);

	bd.getPropertyValues().add("sqlSessionFactory", new RuntimeBeanReference("sqlSessionFactory"));

	return bd;

 }

}

步骤说明###

实现BeanDefinitionRegistryPostProcessor接口
处理postProcessBeanDefinitionRegistry方法
生成BeanDefinition，上面展示了动态添加一个mybatis Dao的bean注册过程

我遇到的坑##

当动态生成生成mybatis的dao接口时，报这个错误：

Exception in thread "main" org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'demoEntityDao': Post-processing of FactoryBean's singleton object failed; nested exception is java.lang.IllegalArgumentException: interface com.dhcc.ac.domain.AcAlertDao is not visible from class loader

at org.springframework.beans.factory.support.FactoryBeanRegistrySupport.getObjectFromFactoryBean(FactoryBeanRegistrySupport.java:116)

at org.springframework.beans.factory.support.AbstractBeanFactory.getObjectForBeanInstance(AbstractBeanFactory.java:1590)

at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:254)

at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:197)

at org.springframework.context.support.AbstractApplicationContext.getBean(AbstractApplicationContext.java:1054)

at com.dhcc.framework.App.main(App.java:48)

Caused by: java.lang.IllegalArgumentException: interface com.dhcc.ac.domain.AcAlertDao is not visible from class loader

at java.lang.reflect.Proxy$ProxyClassFactory.apply(Proxy.java:581)

at java.lang.reflect.Proxy$ProxyClassFactory.apply(Proxy.java:557)

at java.lang.reflect.WeakCache$Factory.get(WeakCache.java:230)

at java.lang.reflect.WeakCache.get(WeakCache.java:127)

at java.lang.reflect.Proxy.getProxyClass0(Proxy.java:419)

at java.lang.reflect.Proxy.newProxyInstance(Proxy.java:719)

at org.springframework.aop.framework.JdkDynamicAopProxy.getProxy(JdkDynamicAopProxy.java:121)

at org.springframework.aop.framework.ProxyFactory.getProxy(ProxyFactory.java:109)

at org.springframework.aop.framework.AbstractAdvisingBeanPostProcessor.postProcessAfterInitialization(AbstractAdvisingBeanPostProcessor.java:90)

at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.applyBeanPostProcessorsAfterInitialization(AbstractAutowireCapableBeanFactory.java:422)

at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.postProcessObjectFromFactoryBean(AbstractAutowireCapableBeanFactory.java:1723)

at org.springframework.beans.factory.support.FactoryBeanRegistrySupport.getObjectFromFactoryBean(FactoryBeanRegistrySupport.java:113)

... 5 more

错误原因在于动态生成的class加载的classloader，与Proxy.newProxyInstance使用的ClassLoader，不是同一个classloader，所以在Proxy.newProxyInstance找不到这个新生成的类。

解决办法###

动态加载类，一般都是用defineClass动态加载到ClassLoader里去。

但defineClass不是public，所以在这里卡了一段时间。

后来，突然之间找到了解决办法，使用反射调用defineClass就好了呀。

小结##

技术没什么高深的，我后面将要写的一个功能，用到了本篇的技术，这里先介绍一下。

我的微信zouhaibin294148，不知不觉写了10多年代码了

码农公寓