如何動態在spring mvc中增長bean

閱讀對象

搭框架人員，或者其餘感興趣的開發人員

背景

通常來講在業務代碼中，加上 @Component, @Service，@Repository, @Controller等註解就能夠實現將bean註冊到Spring中了。
可是在寫框架，可能有些類會動態生成，怎麼動態註冊到Spring中呢？

BeanDefinitionRegistryPostProcessor 接口

BeanDefinitionRegistryPostProcessor接口是一個能夠修改spring工廠中已定義的bean的接口，該接口有個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;

實現

代碼

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;
 }
}

步驟說明

1. 實現BeanDefinitionRegistryPostProcessor接口
2. 處理postProcessBeanDefinitionRegistry方法
3. 生成BeanDefinition，上面展現了動態添加一個mybatis Dao的bean註冊過程

我遇到的坑

當動態生成生成mybatis的dao接口時，報這個錯誤：

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

錯誤緣由在於動態生成的class加載的classloader，與Proxy.newProxyInstance使用的ClassLoader，不是同一個classloader，因此在Proxy.newProxyInstance找不到這個新生成的類。

解決辦法

動態加載類，通常都是用defineClass動態加載到ClassLoader裏去。
但defineClass不是public，因此在這裏卡了一段時間。
後來，忽然之間找到了解決辦法，使用反射調用defineClass就行了呀。

小結

技術沒什麼高深的，我後面將要寫的一個功能，用到了本篇的技術，這裏先介紹一下。

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個人微信zouhaibin294148，不知不覺寫了10多年代碼了