Mybatis懒加载时,springMVC返回JSON异常 Could not write JSON: No serializer found for class

Mybatis懒加载时,springMVC返回JSON异常 Could not write JSON: No serializer found for class

错误信息如下：

　错误信息： com.fasterxml.jackson.databind.JsonMappingException: 
 No serializer found for class org.apache.ibatis.executor.loader.javassist.JavassistProxyFactory$EnhancedResultObjectProxyImpl and
  no properties discovered to create BeanSerializer (to avoid exception,
   disable SerializationFeature.FAIL_ON_EMPTYBEANS) ) (through reference
    chain: java.util.HashMap[“XXX”]->java.util.ArrayList[0]->com.moguichun.model.XXX$$_jvst552_0[“handler”]) 　　

错误原因

在mybatis集成springmvc时,假如mybatis采用了懒加载,而springmvc中利用@ResponseBody注解返回实体类的JSON时会抛异常

错误分析

分析的代码如下：
首先是controller中返回的是responsebody:
CategoryController.java

    //将查询的结果放入map传给前端，以json的格式
    @ResponseBody
	@RequestMapping(value="/categoryList", method=RequestMethod.GET)
	public Map<String,Object> categoryList(){
		Map<String,Object> map = new HashMap<>();
		List<MainCategory> categoryList = this.categoryService.categoryList();
		
		if(null!=categoryList  && categoryList.size() > 0) {//控制层做简单判断，并将信息放入map中
			map.put("code", 1001);
			map.put("message", "查询成功");
			map.put("categoryList", categoryList);
		}else {
			map.put("code", 1002);
			map.put("message", "查询失败");
		}
		return map;
	}
	

service中的代码:
CategoryServiceImpl.java

 	//返回目录list
	@Override
	public List<MainCategory> categoryList() {
		List<MainCategory> categoryList = this.categoryDao.selectAllMainCategory();
		return categoryList;
	}

调用的mapper.xml中使用了懒加载:
mybatis-config.xml

<configuration>
 	<settings>	
 		<!-- 开启延迟加载lazyLoadingEnabled\aggressiveLazyLoading -->
 		<setting name="lazyLoadingEnabled" value="true"/>
 		<setting name="aggressiveLazyLoading" value="false"/>	
 	</settings>
  </configuration>

ICategoryDao .java

@Mapper
public interface ICategoryDao {
	/**
	 * 查询一级目录，包含二级目录
	 * @return
	 */
	public List<MainCategory> selectAllMainCategory();
	
}

ICategoryDao .xml

 <mapper namespace="com.zym.dao.ICategoryDao">
 
 	<resultMap type="com.zym.entities.MainCategory" id="myCategory">
 		<id column="main_category_id" property="mainCategoryId"/>
 		<collection property="subCategoryList" column="main_category_id" select="com.zym.dao.ICategoryDao.selectSubCategoryByMainCategoryId" ></collection>
 	</resultMap>
 	
 	<!-- 查询一级目录 List<MainCategory> selectAllMainCategory() -->
 	<select id="selectAllMainCategory" resultMap="myCategory">
		SELECT `main_category_id`,`category_name` FROM `main_category`
 	</select>
  
 	<!-- 查询二级目录 List<MainCategory> selectSubCategoryByMainCategoryId(Integer mainCategoryId) -->
 	<select id="selectSubCategoryByMainCategoryId" resultType="com.zym.entities.SubCategory">
 		SELECT `sub_category_id`,`category_name` FROM `sub_category` WHERE `main_category_id` = #{mainCategoryId}
 	</select>
 </mapper>

这个时候测试就会上面的异常.
经过调试后发现,MyBatis执行了DAO层的方法后,返回的实体类MainCategory被做了代理,如下:
可以看到,被代理后的MainCategory类多了一个 handler的属性 ,之后Jackson在对该代理类做序列化时,由于找不到对应的getter,抛出来异常

解决方法

1、在被代理的MainCategory类上加上 @JsonIgnoreProperties(value = “handler”) 注解,让Jackson序列化时忽略handler属性,这样Jackson序列化时，就不会去调用handler属性的get/set方法

@JsonIgnoreProperties(value = { "handler" })
public class MainCategory implements Serializable{
	private Integer mainCategoryId;
	private String categoryName;
	private List<SubCategory> subCategoryList;//二级目录集合

2、关闭该查询的懒加载 fetchType=“eager”

 	<resultMap type="com.zym.entities.MainCategory" id="myCategory">
 		<id column="main_category_id" property="mainCategoryId"/>
 		<collection property="subCategoryList" column="main_category_id" fetchType="eager" select="com.zym.dao.ICategoryDao.selectSubCategoryByMainCategoryId" ></collection>
 	</resultMap>

3、配置json转换器属性SerializationFeature.FAIL_ON_EMPTY_BEANS为false