mybatis查询结果返回至实体类

近期，利用mybatis做一个简单查询，先看主要代码：

Service层：

package com.example1.service;

import java.util.List;

import org.apache.log4j.Logger;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Service;

import com.example1.dao.OrderMapper;
import com.example1.pojo.Order;

@Service
public class OrderService {
	
	private Logger log = Logger.getLogger(OrderService.class);

	@Autowired
	private OrderMapper orderMapper;
	
	public void findOrders() {
		List<Order> orders = orderMapper.findOrders();
		for (Order order : orders) {
			log.info(order.getId() + " : " + order.getName());
		}
	}
}

dao层：

package com.example1.dao;

import java.util.List;

import com.example1.pojo.Order;

public interface OrderMapper {

	public List<Order> findOrders();
}

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
"http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.example1.dao.OrderMapper">

	<select id="findOrders" parameterType="String" resultType="java.util.Map">
		SELECT * FROM t_order 
	</select>
</mapper>

Order类：

package com.example1.pojo;

public class Order {

	private String id;
	private String name;
	public String getId() {
		return id;
	}
	public void setId(String id) {
		this.id = id;
	}
	public String getName() {
		return name;
	}
	public void setName(String name) {
		this.name = name;
	}
	@Override
	public String toString() {
		return "Order [id=" + id + ", name=" + name + "]";
	}
}

配置妥当，执行，结果报错了：

java.lang.ClassCastException: java.util.HashMap cannot be cast to com.example1.pojo.Order
	at com.example1.service.OrderService.findOrders(OrderService.java:26)
	at com.example1.service.OrderService$$FastClassBySpringCGLIB$$b6b2c61a.invoke(<generated>)
	at org.springframework.cglib.proxy.MethodProxy.invoke(MethodProxy.java:204)
	at org.springframework.aop.framework.CglibAopProxy$CglibMethodInvocation.invokeJoinpoint(CglibAopProxy.java:717)
	at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:157)
	at org.springframework.transaction.interceptor.TransactionInterceptor$1.proceedWithInvocation(TransactionInterceptor.java:99)
	at org.springframework.transaction.interceptor.TransactionAspectSupport.invokeWithinTransaction(TransactionAspectSupport.java:281)
	at org.springframework.transaction.interceptor.TransactionInterceptor.invoke(TransactionInterceptor.java:96)
	at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:179)
	at org.springframework.aop.interceptor.ExposeInvocationInterceptor.invoke(ExposeInvocationInterceptor.java:92)
	at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:179)
	at org.springframework.aop.framework.CglibAopProxy$DynamicAdvisedInterceptor.intercept(CglibAopProxy.java:653)
	at com.example1.service.OrderService$$EnhancerBySpringCGLIB$$e285c4bb.findOrders(<generated>)
	at com.example1.controller.OrderController.findOrders(OrderController.java:22)
	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 org.springframework.web.method.support.InvocableHandlerMethod.invoke(InvocableHandlerMethod.java:219)
	at org.springframework.web.method.support.InvocableHandlerMethod.invokeForRequest(InvocableHandlerMethod.java:132)
	at org.springframework.web.servlet.mvc.method.annotation.ServletInvocableHandlerMethod.invokeAndHandle(ServletInvocableHandlerMethod.java:100)
	at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.invokeHandlerMethod(RequestMappingHandlerAdapter.java:604)
	at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.handleInternal(RequestMappingHandlerAdapter.java:565)
	at org.springframework.web.servlet.mvc.method.AbstractHandlerMethodAdapter.handle(AbstractHandlerMethodAdapter.java:80)
	at org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:923)
	at org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:852)
	at org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:882)
	at org.springframework.web.servlet.FrameworkServlet.doGet(FrameworkServlet.java:778)
	at javax.servlet.http.HttpServlet.service(HttpServlet.java:617)
	at javax.servlet.http.HttpServlet.service(HttpServlet.java:723)
	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:290)
	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
	at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:233)
	at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:191)
	at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:127)
	at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:103)
	at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:109)
	at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:293)
	at org.apache.coyote.http11.Http11AprProcessor.process(Http11AprProcessor.java:879)
	at org.apache.coyote.http11.Http11AprProtocol$Http11ConnectionHandler.process(Http11AprProtocol.java:617)
	at org.apache.tomcat.util.net.AprEndpoint$Worker.run(AprEndpoint.java:1778)
	at java.lang.Thread.run(Thread.java:745)

可以看出，java.util.HashMap不能直接存储到具体的实体类。

解决办法：

一、dao层通过resultType指定返回类型

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
"http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.example1.dao.OrderMapper">

	<select id="findOrders" parameterType="String" resultType="com.example1.pojo.Order">
		SELECT * FROM t_order 
	</select>
</mapper>

二、查询结果返回list，然后遍历list，将map转换成相应的java对象

修改后的Service层：

package com.example1.service;

import java.util.List;
import java.util.Map;

import org.apache.log4j.Logger;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Service;

import com.alibaba.fastjson.JSONArray;
import com.alibaba.fastjson.JSONObject;
import com.example1.dao.OrderMapper;
import com.example1.pojo.Order;

@Service
public class OrderService {
	
	private Logger log = Logger.getLogger(OrderService.class);

	@Autowired
	private OrderMapper orderMapper;
	
	public void findOrders() {
		List<Map> orders = orderMapper.findOrders();
		for (Map orderMap : orders) {
			String orderJsonStr = JSONObject.toJSONString(orderMap);
			Order order = (Order) JSONArray.parseObject(orderJsonStr,Order.class);
			log.info(order.getId() + " : " + order.getName());
		}
	}
}

dao层：

package com.example1.dao;

import java.util.List;
import java.util.Map;

public interface OrderMapper {

	public List<Map> findOrders();
}

其中，关于map转java对象的方法有好多种，具体可以参考本人另一篇文章：

https://blog.csdn.net/liuyunyihao/article/details/86597002

以上两种方法均实际测试通过。