在Spring 3.1中实例化bean时出错

Question

我有以下课程：

public abstract class AbstractBusinessModule {

}

public class MS3BusinessModule extends AbstractBusinessModule 
{
    public MS3BusinessModule(SomeOtherClass value)
    {

    }
}

以下bean声明：

<bean id="ms3BusinessModule" class="com.hba.MS3BusinessModule" >
    <constructor-arg index="0">
        <ref bean="someOtherBeanID"/>
    </constructor-arg>
    <aop:scoped-proxy />
</bean>

启动我的应用程序后，我收到以下错误：

Exception in thread "main" org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'ms3BusinessModule' defined in BeanDefinition defined in class path resource [spring.xml]: Initialization of bean failed; nested exception is org.springframework.aop.framework.AopConfigException: Could not generate CGLIB subclass of class [class com.hba.MS3BusinessModule]: Common causes of this problem include using a final class or a non-visible class; nested exception is java.lang.IllegalArgumentException: Superclass has no null constructors but no arguments were given
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:527)
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:456)
    at org.springframework.beans.factory.support.AbstractBeanFactory$1.getObject(AbstractBeanFactory.java:293)
    at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:222)
    at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:290)
    at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:192)
    at org.springframework.beans.factory.support.DefaultListableBeanFactory.preInstantiateSingletons(DefaultListableBeanFactory.java:567)
    at org.springframework.context.support.AbstractApplicationContext.finishBeanFactoryInitialization(AbstractApplicationContext.java:895)
    at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:425)
    at org.springframework.context.support.ClassPathXmlApplicationContext.<init>(ClassPathXmlApplicationContext.java:139)
    at org.springframework.context.support.ClassPathXmlApplicationContext.<init>(ClassPathXmlApplicationContext.java:83)
    at com.hba.EhCacheTest.main(EhCacheTest.java:16)
Caused by: org.springframework.aop.framework.AopConfigException: Could not generate CGLIB subclass of class [class com.hba.MS3BusinessModule]: Common causes of this problem include using a final class or a non-visible class; nested exception is java.lang.IllegalArgumentException: Superclass has no null constructors but no arguments were given
    at org.springframework.aop.framework.Cglib2AopProxy.getProxy(Cglib2AopProxy.java:212)
    at org.springframework.aop.framework.ProxyFactory.getProxy(ProxyFactory.java:112)
    at org.springframework.aop.scope.ScopedProxyFactoryBean.setBeanFactory(ScopedProxyFactoryBean.java:109)
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.invokeAwareMethods(AbstractAutowireCapableBeanFactory.java:1439)
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.initializeBean(AbstractAutowireCapableBeanFactory.java:1408)
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:519)
    ... 11 more
Caused by: java.lang.IllegalArgumentException: Superclass has no null constructors but no arguments were given
    at net.sf.cglib.proxy.Enhancer.emitConstructors(Enhancer.java:721)
    at net.sf.cglib.proxy.Enhancer.generateClass(Enhancer.java:499)
    at net.sf.cglib.transform.TransformingClassGenerator.generateClass(TransformingClassGenerator.java:33)
    at net.sf.cglib.core.DefaultGeneratorStrategy.generate(DefaultGeneratorStrategy.java:25)
    at net.sf.cglib.core.AbstractClassGenerator.create(AbstractClassGenerator.java:216)
    at net.sf.cglib.proxy.Enhancer.createHelper(Enhancer.java:377)
    at net.sf.cglib.proxy.Enhancer.create(Enhancer.java:285)
    at org.springframework.aop.framework.Cglib2AopProxy.getProxy(Cglib2AopProxy.java:200)
    ... 16 more

怎么可能出错？

如果我从bean声明中删除<aop:scoped-proxy/>它可以工作。

更新：如果我在MS3BusinessModule放置一个默认构造MS3BusinessModule ，它可以工作。 我不明白为什么需要默认构造函数。 请有人解释一下。

Answer 1

如果我在MS3BusinessModule中放置一个默认构造函数，它就可以工作。 我不明白为什么需要默认构造函数。 请有人解释一下。

<aop:scoped-proxy/>工作方式是以不同的名称隐藏“真正的”bean并创建一个CGLIB代理类，它是真实bean类的子类，并将所有方法调用委托给正确的实例目标bean的。 所以你在这里有两种不同的对象：

• n每个会话/请求的com.hba.MS3BusinessModule实例/无论范围是什么，和
• 动态生成的代理类的一个单例实例。

使用带有参数的构造函数构造n个目标bean，并将<constructor-arg>值传递给它，但代理类需要一个no-arg超类构造函数来调用（当然可以声明为protected而不是public ） 。 代理机构将永远不会真正调用任何代理实例的超类方法的所有调用转到目标实例（S）代替，但代理类需要为了扩展目标bean类代理是一个instanceof的正确的类型。

另一种解决方法是提供M3BusinessModule实现的接口 ， M3BusinessModule其他bean对此的所有引用都使用接口类型而不是具体的类类型。 这将允许Spring使用java.lang.reflect代理而不是CGLIB代理，实现接口而无需扩展具体类。