Caused by: org.springframework.beans.factory.BeanNotOfRequiredTypeException: Bean named 'dao' is exp

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本文解析了Spring框架中使用注解实现依赖注入时遇到的问题，重点介绍了如何正确配置和服务层与数据访问层的交互，同时深入探讨了Java动态代理机制在Spring中的应用。

异常信息:

org.springframework.beans.factory.UnsatisfiedDependencyException: Error creating bean with name 'service': Unsatisfied dependency expressed through field 'dao'; nested exception is org.springframework.beans.factory.BeanNotOfRequiredTypeException: Bean named 'dao' is expected to be of type 'ifox.DAOImpl' but was actually of type 'com.sun.proxy.$Proxy14'

at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor$AutowiredFieldElement.inject(AutowiredAnnotationBeanPostProcessor.java:588)
at org.springframework.beans.factory.annotation.InjectionMetadata.inject(InjectionMetadata.java:88)
at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor.postProcessPropertyValues(AutowiredAnnotationBeanPostProcessor.java:366)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.populateBean(AbstractAutowireCapableBeanFactory.java:1264)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:553)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:483)
at org.springframework.beans.factory.support.AbstractBeanFactory$1.getObject(AbstractBeanFactory.java:306)
at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:230)
at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:302)
at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:197)
at org.springframework.beans.factory.support.DefaultListableBeanFactory.preInstantiateSingletons(DefaultListableBeanFactory.java:761)
at org.springframework.context.support.AbstractApplicationContext.finishBeanFactoryInitialization(AbstractApplicationContext.java:867)
at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:543)
at org.springframework.context.support.ClassPathXmlApplicationContext.<init>(ClassPathXmlApplicationContext.java:139)
at org.springframework.context.support.ClassPathXmlApplicationContext.<init>(ClassPathXmlApplicationContext.java:83)
at ifox.TestDb.<init>(TestDb.java:20)
at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:62)
at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
at java.lang.reflect.Constructor.newInstance(Constructor.java:423)
at org.junit.runners.BlockJUnit4ClassRunner.createTest(BlockJUnit4ClassRunner.java:217)
at org.junit.runners.BlockJUnit4ClassRunner$1.runReflectiveCall(BlockJUnit4ClassRunner.java:266)
at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
at org.junit.runners.BlockJUnit4ClassRunner.methodBlock(BlockJUnit4ClassRunner.java:263)
at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:78)
at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:57)
at org.junit.runners.ParentRunner$3.run(ParentRunner.java:290)
at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:71)
at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:288)
at org.junit.runners.ParentRunner.access$000(ParentRunner.java:58)
at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:268)
at org.junit.runners.ParentRunner.run(ParentRunner.java:363)
at org.junit.runner.JUnitCore.run(JUnitCore.java:137)
at com.intellij.junit4.JUnit4IdeaTestRunner.startRunnerWithArgs(JUnit4IdeaTestRunner.java:117)
at com.intellij.junit4.JUnit4IdeaTestRunner.startRunnerWithArgs(JUnit4IdeaTestRunner.java:42)
at com.intellij.rt.execution.junit.JUnitStarter.prepareStreamsAndStart(JUnitStarter.java:262)
at com.intellij.rt.execution.junit.JUnitStarter.main(JUnitStarter.java:84)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:498)
at com.intellij.rt.execution.application.AppMain.main(AppMain.java:147)
Caused by: org.springframework.beans.factory.BeanNotOfRequiredTypeException: Bean named 'dao' is expected to be of type 'ifox.DAOImpl' but was actually of type 'com.sun.proxy.$Proxy14'
at org.springframework.beans.factory.support.DefaultListableBeanFactory.checkBeanNotOfRequiredType(DefaultListableBeanFactory.java:1510)
at org.springframework.beans.factory.support.DefaultListableBeanFactory.raiseNoMatchingBeanFound(DefaultListableBeanFactory.java:1489)
at org.springframework.beans.factory.support.DefaultListableBeanFactory.doResolveDependency(DefaultListableBeanFactory.java:1104)
at org.springframework.beans.factory.support.DefaultListableBeanFactory.resolveDependency(DefaultListableBeanFactory.java:1066)
at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor$AutowiredFieldElement.inject(AutowiredAnnotationBeanPostProcessor.java:585)
... 41 more

今天在看spring注解的时候遇到这个问题可以说弄的自己很难受，我先贴上我的部分错误源码

DAO接口

package ifox;

/**
 * Created by exphuhong on 17-8-14.
 */
public interface DAO
{
    public Db findDbById(int id);

    public void saveBook(Db db);
}

DAO实现类

import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Repository;

import javax.annotation.Resource;

/**
 * Created by exphuhong on 17-8-14.
 */
@Repository("dao")
public class DAOImpl implements DAO{

    @Autowired
    private SessionFactory sessionFactory;

    //获取和当前线程绑定的Seesion
    private Session getSession()
    {
        return sessionFactory.getCurrentSession();
    }

    public Db findDbById(int id)
    {
        String hql = "from Db  WHERE id = ?";
        return (Db) getSession().createQuery(hql).setInteger(0, id).uniqueResult();
    }
    public void saveBook(Db db)
    {
        getSession().save(db);
    }
}

Service接口

package ifox;

/**
 * Created by exphuhong on 17-8-14.
 */
public interface Service {
        public Db findDbById(int id);
        public void saveBook(Db db);

}

Service实现类

package ifox;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.beans.factory.annotation.Qualifier;


/**
 * Created by exphuhong on 17-8-14.
 */
@org.springframework.stereotype.Service("service")
public class ServiceImpl implements Service {

    @Autowired
    @Qualifier("dao")
    private DAO dao;   //错误用法  private DAOImpl dao;  原理在最下面

    public Db findDbById(int id)
    {
        return dao.findDbById(id);
    }
    public void saveBook(Db db)
    {
        dao.saveBook(db);

    }
}

测试类

package ifox;

import org.junit.Test;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;
import org.springframework.stereotype.Component;

import javax.annotation.Resource;

/**
 * Created by exphuhong on 17-8-14.
 */
public class TestDb {

    private ApplicationContext context=null;
    private Service service=null;

    {
        context= new ClassPathXmlApplicationContext("applicationContext.xml");
        service = (Service) context.getBean("service");
    }

    @Test
    public void test1() {
        System.out.println(context.getBean("dao"));
    }

    @Test
    public void test2()
    {
        Db db= (Db) service.findDbById(5);
        System.out.println(db);
    }

    @Test
    public void test3()
    {
        service.saveBook(new Db("张飞",21,"1"));
    }
}

解释：

读者首先需要去了解java的反射机制以及java动态代理。spring注解是实现java注解接口的。注解的实现和java的一样，都是通过反射。通过反射实现动态代理机制，动态代理需要java.lang.reflect.InvocationHandler接口和java.lang.reflect.Prox类的支持。Prox类是专门完成代理的操作类，可以通过此类为一个或多个接口动态生成实现类。

所以这里在获取通过注解的注入的bean时（通过反射将该类（接口的实现类）对应的bean注入到IOC容器中）需要定义成接口类型了只有这样才叫动态代理。