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步骤 1 : 先运行,看到效果,再学习   
步骤 2 : 模仿和排错   
步骤 3 : 页面截图   
步骤 4 : Product   
步骤 5 : ProductDAO   
步骤 6 : ProductService   
步骤 7 : ProductController   
步骤 8 : listProduct.html+editProduct.html   
步骤 9 : 功能讲解   
步骤 10 : 自己做一遍   

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问答区域    
2018-12-21 为什么会有这个错误
何时优秀



我把你的代码贴上去也是报错。为什么??
package com.wj.tmall.service;

import com.wj.tmall.dao.ProductDAO;
import com.wj.tmall.pojo.Category;
import com.wj.tmall.pojo.Product;
import com.wj.tmall.util.Page4Navigator;
import org.springframework.beans.factory.annotation.Autowired;

import org.springframework.data.domain.Page;
import org.springframework.data.domain.PageRequest;
import org.springframework.data.domain.Pageable;
import org.springframework.data.domain.Sort;
import org.springframework.stereotype.Service;

@Service
public class ProductService {
        @Autowired
        ProductDAO productDAO;
        @Autowired
        CategoryService categoryService;

        public void add(Product product){
             productDAO.save(product);
        }
        public void delete(int id){
            productDAO.delete(id);
        }
        //编辑
        public Product get(int id){
            return productDAO.findOne(id);
        }
        //修改
        public void update(Product product){
             productDAO.save(product);
        }

        /*分页查询
        方法(放上需要的参数)
          1.先找到是那个id分类下的产品(分类id=cid)
          2.排序
          3.调用dao里面的findbycategory分页查询方法
          4.调用分页类里面Page4Navigator的方法。
          5.返回到页面     */
        public Page4Navigator<Product> list(int cid,int start,int size,int navigatePages){
            Category category=categoryService.get(cid);
            Sort s=new Sort(Sort.Direction.DESC,"id");
            Pageable pageable=new PageRequest(start,size,s):
            Page<Product> pageFromJPA =productDAO.findByCategory(category,pageable);

            return  new Page4Navigator<>(pageFromJPA,navigatePages);
        }




}
Error starting ApplicationContext. To display the auto-configuration report re-run your application with 'debug' enabled.
2018-12-21 14:57:16.608 ERROR 16992 --- [  restartedMain] o.s.boot.SpringApplication               : Application startup failed

org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'requestMappingHandlerAdapter' defined in class path resource [org/springframework/boot/autoconfigure/web/WebMvcAutoConfiguration$EnableWebMvcConfiguration.class]: Bean instantiation via factory method failed; nested exception is org.springframework.beans.BeanInstantiationException: Failed to instantiate [org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter]: Factory method 'requestMappingHandlerAdapter' threw exception; nested exception is org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'mvcConversionService' defined in class path resource [org/springframework/boot/autoconfigure/web/WebMvcAutoConfiguration$EnableWebMvcConfiguration.class]: Bean instantiation via factory method failed; nested exception is org.springframework.beans.BeanInstantiationException: Failed to instantiate [org.springframework.format.support.FormattingConversionService]: Factory method 'mvcConversionService' threw exception; nested exception is org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'productDAO': Invocation of init method failed; nested exception is java.lang.IllegalArgumentException: Not a managed type: class java.lang.Object
	at org.springframework.beans.factory.support.ConstructorResolver.instantiateUsingFactoryMethod(ConstructorResolver.java:599) ~[spring-beans-4.3.13.RELEASE.jar:4.3.13.RELEASE]
	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.instantiateUsingFactoryMethod(AbstractAutowireCapableBeanFactory.java:1173) ~[spring-beans-4.3.13.RELEASE.jar:4.3.13.RELEASE]
	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBeanInstance(AbstractAutowireCapableBeanFactory.java:1067) ~[spring-beans-4.3.13.RELEASE.jar:4.3.13.RELEASE]
	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:513) ~[spring-beans-4.3.13.RELEASE.jar:4.3.13.RELEASE]
	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:483) ~[spring-beans-4.3.13.RELEASE.jar:4.3.13.RELEASE]
	at org.springframework.beans.factory.support.AbstractBeanFactory$1.getObject(AbstractBeanFactory.java:306) ~[spring-beans-4.3.13.RELEASE.jar:4.3.13.RELEASE]
	at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:230) ~[spring-beans-4.3.13.RELEASE.jar:4.3.13.RELEASE]
	at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:302) ~[spring-beans-4.3.13.RELEASE.jar:4.3.13.RELEASE]
	at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:197) ~[spring-beans-4.3.13.RELEASE.jar:4.3.13.RELEASE]
	at org.springframework.beans.factory.support.DefaultListableBeanFactory.preInstantiateSingletons(DefaultListableBeanFactory.java:761) ~[spring-beans-4.3.13.RELEASE.jar:4.3.13.RELEASE]
	at org.springframework.context.support.AbstractApplicationContext.finishBeanFactoryInitialization(AbstractApplicationContext.java:867) ~[spring-context-4.3.13.RELEASE.jar:4.3.13.RELEASE]
	at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:543) ~[spring-context-4.3.13.RELEASE.jar:4.3.13.RELEASE]
	at org.springframework.boot.context.embedded.EmbeddedWebApplicationContext.refresh(EmbeddedWebApplicationContext.java:122) ~[spring-boot-1.5.9.RELEASE.jar:1.5.9.RELEASE]
	at org.springframework.boot.SpringApplication.refresh(SpringApplication.java:693) [spring-boot-1.5.9.RELEASE.jar:1.5.9.RELEASE]
	at org.springframework.boot.SpringApplication.refreshContext(SpringApplication.java:360) [spring-boot-1.5.9.RELEASE.jar:1.5.9.RELEASE]
	at org.springframework.boot.SpringApplication.run(SpringApplication.java:303) [spring-boot-1.5.9.RELEASE.jar:1.5.9.RELEASE]
	at org.springframework.boot.SpringApplication.run(SpringApplication.java:1118) [spring-boot-1.5.9.RELEASE.jar:1.5.9.RELEASE]
	at org.springframework.boot.SpringApplication.run(SpringApplication.java:1107) [spring-boot-1.5.9.RELEASE.jar:1.5.9.RELEASE]
	at com.wj.tmall.Application.main(Application.java:10) [classes/:na]
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[na:1.8.0_121]
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) ~[na:1.8.0_121]
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[na:1.8.0_121]
	at java.lang.reflect.Method.invoke(Method.java:498) ~[na:1.8.0_121]
	at org.springframework.boot.devtools.restart.RestartLauncher.run(RestartLauncher.java:49) [spring-boot-devtools-1.5.9.RELEASE.jar:1.5.9.RELEASE]
Caused by: org.springframework.beans.BeanInstantiationException: Failed to instantiate [org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter]: Factory method 'requestMappingHandlerAdapter' threw exception; nested exception is org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'mvcConversionService' defined in class path resource [org/springframework/boot/autoconfigure/web/WebMvcAutoConfiguration$EnableWebMvcConfiguration.class]: Bean instantiation via factory method failed; nested exception is org.springframework.beans.BeanInstantiationException: Failed to instantiate [org.springframework.format.support.FormattingConversionService]: Factory method 'mvcConversionService' threw exception; nested exception is org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'productDAO': Invocation of init method failed; nested exception is java.lang.IllegalArgumentException: Not a managed type: class java.lang.Object
	at org.springframework.beans.factory.support.SimpleInstantiationStrategy.instantiate(SimpleInstantiationStrategy.java:189) ~[spring-beans-4.3.13.RELEASE.jar:4.3.13.RELEASE]
	at org.springframework.beans.factory.support.ConstructorResolver.instantiateUsingFactoryMethod(ConstructorResolver.java:588) ~[spring-beans-4.3.13.RELEASE.jar:4.3.13.RELEASE]
	... 23 common frames omitted
Caused by: org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'mvcConversionService' defined in class path resource [org/springframework/boot/autoconfigure/web/WebMvcAutoConfiguration$EnableWebMvcConfiguration.class]: Bean instantiation via factory method failed; nested exception is org.springframework.beans.BeanInstantiationException: Failed to instantiate [org.springframework.format.support.FormattingConversionService]: Factory method 'mvcConversionService' threw exception; nested exception is org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'productDAO': Invocation of init method failed; nested exception is java.lang.IllegalArgumentException: Not a managed type: class java.lang.Object
	at org.springframework.beans.factory.support.ConstructorResolver.instantiateUsingFactoryMethod(ConstructorResolver.java:599) ~[spring-beans-4.3.13.RELEASE.jar:4.3.13.RELEASE]
	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.instantiateUsingFactoryMethod(AbstractAutowireCapableBeanFactory.java:1173) ~[spring-beans-4.3.13.RELEASE.jar:4.3.13.RELEASE]
	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBeanInstance(AbstractAutowireCapableBeanFactory.java:1067) ~[spring-beans-4.3.13.RELEASE.jar:4.3.13.RELEASE]
	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:513) ~[spring-beans-4.3.13.RELEASE.jar:4.3.13.RELEASE]
	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:483) ~[spring-beans-4.3.13.RELEASE.jar:4.3.13.RELEASE]
	at org.springframework.beans.factory.support.AbstractBeanFactory$1.getObject(AbstractBeanFactory.java:306) ~[spring-beans-4.3.13.RELEASE.jar:4.3.13.RELEASE]
	at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:230) ~[spring-beans-4.3.13.RELEASE.jar:4.3.13.RELEASE]
	at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:302) ~[spring-beans-4.3.13.RELEASE.jar:4.3.13.RELEASE]
	at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:197) ~[spring-beans-4.3.13.RELEASE.jar:4.3.13.RELEASE]
	at org.springframework.context.annotation.ConfigurationClassEnhancer$BeanMethodInterceptor.obtainBeanInstanceFromFactory(ConfigurationClassEnhancer.java:389) ~[spring-context-4.3.13.RELEASE.jar:4.3.13.RELEASE]
	at org.springframework.context.annotation.ConfigurationClassEnhancer$BeanMethodInterceptor.intercept(ConfigurationClassEnhancer.java:361) ~[spring-context-4.3.13.RELEASE.jar:4.3.13.RELEASE]
	at org.springframework.boot.autoconfigure.web.WebMvcAutoConfiguration$EnableWebMvcConfiguration$$EnhancerBySpringCGLIB$$e03022f5.mvcConversionService(<generated>) ~[spring-boot-autoconfigure-1.5.9.RELEASE.jar:1.5.9.RELEASE]
	at org.springframework.web.servlet.config.annotation.WebMvcConfigurationSupport.getConfigurableWebBindingInitializer(WebMvcConfigurationSupport.java:570) ~[spring-webmvc-4.3.13.RELEASE.jar:4.3.13.RELEASE]
	at org.springframework.boot.autoconfigure.web.WebMvcAutoConfiguration$EnableWebMvcConfiguration.getConfigurableWebBindingInitializer(WebMvcAutoConfiguration.java:432) ~[spring-boot-autoconfigure-1.5.9.RELEASE.jar:1.5.9.RELEASE]
	at org.springframework.web.servlet.config.annotation.WebMvcConfigurationSupport.requestMappingHandlerAdapter(WebMvcConfigurationSupport.java:530) ~[spring-webmvc-4.3.13.RELEASE.jar:4.3.13.RELEASE]
	at org.springframework.boot.autoconfigure.web.WebMvcAutoConfiguration$EnableWebMvcConfiguration.requestMappingHandlerAdapter(WebMvcAutoConfiguration.java:384) ~[spring-boot-autoconfigure-1.5.9.RELEASE.jar:1.5.9.RELEASE]
	at org.springframework.boot.autoconfigure.web.WebMvcAutoConfiguration$EnableWebMvcConfiguration$$EnhancerBySpringCGLIB$$e03022f5.CGLIB$requestMappingHandlerAdapter$3(<generated>) ~[spring-boot-autoconfigure-1.5.9.RELEASE.jar:1.5.9.RELEASE]
	at org.springframework.boot.autoconfigure.web.WebMvcAutoConfiguration$EnableWebMvcConfiguration$$EnhancerBySpringCGLIB$$e03022f5$$FastClassBySpringCGLIB$$9400e56c.invoke(<generated>) ~[spring-boot-autoconfigure-1.5.9.RELEASE.jar:1.5.9.RELEASE]
	at org.springframework.cglib.proxy.MethodProxy.invokeSuper(MethodProxy.java:228) ~[spring-core-4.3.13.RELEASE.jar:4.3.13.RELEASE]
	at org.springframework.context.annotation.ConfigurationClassEnhancer$BeanMethodInterceptor.intercept(ConfigurationClassEnhancer.java:358) ~[spring-context-4.3.13.RELEASE.jar:4.3.13.RELEASE]
	at org.springframework.boot.autoconfigure.web.WebMvcAutoConfiguration$EnableWebMvcConfiguration$$EnhancerBySpringCGLIB$$e03022f5.requestMappingHandlerAdapter(<generated>) ~[spring-boot-autoconfigure-1.5.9.RELEASE.jar:1.5.9.RELEASE]
	at sun.reflect.GeneratedMethodAccessor454.invoke(Unknown Source) ~[na:na]
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[na:1.8.0_121]
	at java.lang.reflect.Method.invoke(Method.java:498) ~[na:1.8.0_121]
	at org.springframework.beans.factory.support.SimpleInstantiationStrategy.instantiate(SimpleInstantiationStrategy.java:162) ~[spring-beans-4.3.13.RELEASE.jar:4.3.13.RELEASE]
	... 24 common frames omitted
Caused by: org.springframework.beans.BeanInstantiationException: Failed to instantiate [org.springframework.format.support.FormattingConversionService]: Factory method 'mvcConversionService' threw exception; nested exception is org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'productDAO': Invocation of init method failed; nested exception is java.lang.IllegalArgumentException: Not a managed type: class java.lang.Object
	at org.springframework.beans.factory.support.SimpleInstantiationStrategy.instantiate(SimpleInstantiationStrategy.java:189) ~[spring-beans-4.3.13.RELEASE.jar:4.3.13.RELEASE]
	at org.springframework.beans.factory.support.ConstructorResolver.instantiateUsingFactoryMethod(ConstructorResolver.java:588) ~[spring-beans-4.3.13.RELEASE.jar:4.3.13.RELEASE]
	... 48 common frames omitted
Caused by: org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'productDAO': Invocation of init method failed; nested exception is java.lang.IllegalArgumentException: Not a managed type: class java.lang.Object
	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.initializeBean(AbstractAutowireCapableBeanFactory.java:1628) ~[spring-beans-4.3.13.RELEASE.jar:4.3.13.RELEASE]
	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:555) ~[spring-beans-4.3.13.RELEASE.jar:4.3.13.RELEASE]
	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:483) ~[spring-beans-4.3.13.RELEASE.jar:4.3.13.RELEASE]
	at org.springframework.beans.factory.support.AbstractBeanFactory$1.getObject(AbstractBeanFactory.java:306) ~[spring-beans-4.3.13.RELEASE.jar:4.3.13.RELEASE]
	at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:230) ~[spring-beans-4.3.13.RELEASE.jar:4.3.13.RELEASE]
	at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:302) ~[spring-beans-4.3.13.RELEASE.jar:4.3.13.RELEASE]
	at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:202) ~[spring-beans-4.3.13.RELEASE.jar:4.3.13.RELEASE]
	at org.springframework.context.support.AbstractApplicationContext.getBean(AbstractApplicationContext.java:1086) ~[spring-context-4.3.13.RELEASE.jar:4.3.13.RELEASE]
	at org.springframework.data.repository.support.Repositories.cacheRepositoryFactory(Repositories.java:95) ~[spring-data-commons-1.13.9.RELEASE.jar:na]
	at org.springframework.data.repository.support.Repositories.populateRepositoryFactoryInformation(Repositories.java:88) ~[spring-data-commons-1.13.9.RELEASE.jar:na]
	at org.springframework.data.repository.support.Repositories.<init>(Repositories.java:81) ~[spring-data-commons-1.13.9.RELEASE.jar:na]
	at org.springframework.data.repository.support.DomainClassConverter.setApplicationContext(DomainClassConverter.java:98) ~[spring-data-commons-1.13.9.RELEASE.jar:na]
	at org.springframework.data.web.config.SpringDataWebConfiguration.addFormatters(SpringDataWebConfiguration.java:95) ~[spring-data-commons-1.13.9.RELEASE.jar:na]
	at org.springframework.web.servlet.config.annotation.WebMvcConfigurerComposite.addFormatters(WebMvcConfigurerComposite.java:80) ~[spring-webmvc-4.3.13.RELEASE.jar:4.3.13.RELEASE]
	at org.springframework.web.servlet.config.annotation.DelegatingWebMvcConfiguration.addFormatters(DelegatingWebMvcConfiguration.java:77) ~[spring-webmvc-4.3.13.RELEASE.jar:4.3.13.RELEASE]
	at org.springframework.web.servlet.config.annotation.WebMvcConfigurationSupport.mvcConversionService(WebMvcConfigurationSupport.java:598) ~[spring-webmvc-4.3.13.RELEASE.jar:4.3.13.RELEASE]
	at org.springframework.boot.autoconfigure.web.WebMvcAutoConfiguration$EnableWebMvcConfiguration$$EnhancerBySpringCGLIB$$e03022f5.CGLIB$mvcConversionService$32(<generated>) ~[spring-boot-autoconfigure-1.5.9.RELEASE.jar:1.5.9.RELEASE]
	at org.springframework.boot.autoconfigure.web.WebMvcAutoConfiguration$EnableWebMvcConfiguration$$EnhancerBySpringCGLIB$$e03022f5$$FastClassBySpringCGLIB$$9400e56c.invoke(<generated>) ~[spring-boot-autoconfigure-1.5.9.RELEASE.jar:1.5.9.RELEASE]
	at org.springframework.cglib.proxy.MethodProxy.invokeSuper(MethodProxy.java:228) ~[spring-core-4.3.13.RELEASE.jar:4.3.13.RELEASE]
	at org.springframework.context.annotation.ConfigurationClassEnhancer$BeanMethodInterceptor.intercept(ConfigurationClassEnhancer.java:358) ~[spring-context-4.3.13.RELEASE.jar:4.3.13.RELEASE]
	at org.springframework.boot.autoconfigure.web.WebMvcAutoConfiguration$EnableWebMvcConfiguration$$EnhancerBySpringCGLIB$$e03022f5.mvcConversionService(<generated>) ~[spring-boot-autoconfigure-1.5.9.RELEASE.jar:1.5.9.RELEASE]
	at sun.reflect.GeneratedMethodAccessor459.invoke(Unknown Source) ~[na:na]
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[na:1.8.0_121]
	at java.lang.reflect.Method.invoke(Method.java:498) ~[na:1.8.0_121]
	at org.springframework.beans.factory.support.SimpleInstantiationStrategy.instantiate(SimpleInstantiationStrategy.java:162) ~[spring-beans-4.3.13.RELEASE.jar:4.3.13.RELEASE]
	... 49 common frames omitted
Caused by: java.lang.IllegalArgumentException: Not a managed type: class java.lang.Object
	at org.hibernate.jpa.internal.metamodel.MetamodelImpl.managedType(MetamodelImpl.java:210) ~[hibernate-entitymanager-5.0.12.Final.jar:5.0.12.Final]
	at org.springframework.data.jpa.repository.support.JpaMetamodelEntityInformation.<init>(JpaMetamodelEntityInformation.java:70) ~[spring-data-jpa-1.11.9.RELEASE.jar:na]
	at org.springframework.data.jpa.repository.support.JpaEntityInformationSupport.getEntityInformation(JpaEntityInformationSupport.java:68) ~[spring-data-jpa-1.11.9.RELEASE.jar:na]
	at org.springframework.data.jpa.repository.support.JpaRepositoryFactory.getEntityInformation(JpaRepositoryFactory.java:173) ~[spring-data-jpa-1.11.9.RELEASE.jar:na]
	at org.springframework.data.jpa.repository.support.JpaRepositoryFactory.getTargetRepository(JpaRepositoryFactory.java:106) ~[spring-data-jpa-1.11.9.RELEASE.jar:na]
	at org.springframework.data.jpa.repository.support.JpaRepositoryFactory.getTargetRepository(JpaRepositoryFactory.java:88) ~[spring-data-jpa-1.11.9.RELEASE.jar:na]
	at org.springframework.data.repository.core.support.RepositoryFactorySupport.getRepository(RepositoryFactorySupport.java:200) ~[spring-data-commons-1.13.9.RELEASE.jar:na]
	at org.springframework.data.repository.core.support.RepositoryFactoryBeanSupport.initAndReturn(RepositoryFactoryBeanSupport.java:277) ~[spring-data-commons-1.13.9.RELEASE.jar:na]
	at org.springframework.data.repository.core.support.RepositoryFactoryBeanSupport.afterPropertiesSet(RepositoryFactoryBeanSupport.java:263) ~[spring-data-commons-1.13.9.RELEASE.jar:na]
	at org.springframework.data.jpa.repository.support.JpaRepositoryFactoryBean.afterPropertiesSet(JpaRepositoryFactoryBean.java:101) ~[spring-data-jpa-1.11.9.RELEASE.jar:na]
	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.invokeInitMethods(AbstractAutowireCapableBeanFactory.java:1687) ~[spring-beans-4.3.13.RELEASE.jar:4.3.13.RELEASE]
	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.initializeBean(AbstractAutowireCapableBeanFactory.java:1624) ~[spring-beans-4.3.13.RELEASE.jar:4.3.13.RELEASE]
	... 73 common frames omitted






答案 或者 代码至少填写一项, 如果是自己有问题,请重新提问,否则站长有可能看不到





2018-11-28 @onetomany
Interstellar_
站长,我有个疑惑。既然有“根据某个分类获取所有产品的需求”,为什么不在Category类中使用@onetomany来住解List<Product>对象,而是通过ProductDAO的findByCategory来获取某分类的所有产品




1 个答案

how2j 答案时间:2018-11-29
因为并不是每一次获取分类对象,都需要获取其所有产品数据。 倘若做成onetomany,那么每次都会去取,会增加不必要的性能开销。 而且随着系统数据越来越多,会越来越慢




答案 或者 代码至少填写一项, 如果是自己有问题,请重新提问,否则站长有可能看不到




2018-11-12 @Document注解的作用?




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