如何动态在spring mvc中增长bean

阅读对象

搭框架人员，或者其余感兴趣的开发人员

背景

通常来讲在业务代码中，加上 @Component, @Service，@Repository, @Controller等注解就能够实现将bean注册到Spring中了。
可是在写框架，可能有些类会动态生成，怎么动态注册到Spring中呢？

BeanDefinitionRegistryPostProcessor 接口

BeanDefinitionRegistryPostProcessor接口是一个能够修改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

1. 实现BeanDefinitionRegistryPostProcessor接口
2. 处理postProcessBeanDefinitionRegistry方法
3. 生成BeanDefinition，上面展现了动态添加一个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

错误缘由在于动态生成的class加载的classloader，与Proxy.newProxyInstance使用的ClassLoader，不是同一个classloader，因此在Proxy.newProxyInstance找不到这个新生成的类。

解决办法

动态加载类，通常都是用defineClass动态加载到ClassLoader里去。
但defineClass不是public，因此在这里卡了一段时间。
后来，忽然之间找到了解决办法，使用反射调用defineClass就行了呀。

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。

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个人微信zouhaibin294148，不知不觉写了10多年代码了