Mybatis foreach标签使用不当导致异常的原因浅析

异常产生场景及异常信息

上周，由于在Mybatis的Mapper接口方法中使用实现了Map.Entry接口的泛型类，同时此方法对应的sql语句也使用了foreach标签，导致出现了异常。如下为异常信息：

org.apache.ibatis.exceptions.PersistenceException:
### Error updating database. Cause: org.apache.ibatis.reflection.ReflectionException: There is no getter for property named 'key' in 'class java.lang.Integer'
### The error may involve org.guojing.test.spring.server.GoodsRoomnight30daysMapper.insertBatch-Inline
### The error occurred while setting parameters
### SQL: REPLACE INTO goods_roomnight_30days (goods_id, checkin_room_night_30days) VALUES (?, ?) , (?, ?), (?, ?), (?, ?)
### Cause: org.apache.ibatis.reflection.ReflectionException: There is no getter for property named 'key' in 'class java.lang.Integer'
 at org.apache.ibatis.exceptions.ExceptionFactory.wrapException(ExceptionFactory.java:30)
 at org.apache.ibatis.session.defaults.DefaultSqlSession.update(DefaultSqlSession.java:200)
 at org.apache.ibatis.session.defaults.DefaultSqlSession.insert(DefaultSqlSession.java:185)
 at org.apache.ibatis.binding.MapperMethod.execute(MapperMethod.java:57)
 at org.apache.ibatis.binding.MapperProxy.invoke(MapperProxy.java:53)
 at com.sun.proxy.$Proxy4.insertBatch(Unknown Source)
 at org.guojing.test.spring.server.GoodsRoomnight30daysTest.test(GoodsRoomnight30daysTest.java:47)
 at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
 at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
 at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
 at java.lang.reflect.Method.invoke(Method.java:606)
 at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:47)
 at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
 at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:44)
 at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)
 at org.junit.internal.runners.statements.RunBefores.evaluate(RunBefores.java:26)
 at org.junit.internal.runners.statements.RunAfters.evaluate(RunAfters.java:27)
 at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:271)
 at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:70)
 at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:50)
 at org.junit.runners.ParentRunner$3.run(ParentRunner.java:238)
 at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:63)
 at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:236)
 at org.junit.runners.ParentRunner.access$000(ParentRunner.java:53)
 at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:229)
 at org.junit.runners.ParentRunner.run(ParentRunner.java:309)
 at org.junit.runner.JUnitCore.run(JUnitCore.java:160)
 at com.intellij.junit4.JUnit4IdeaTestRunner.startRunnerWithArgs(JUnit4IdeaTestRunner.java:68)
 at com.intellij.rt.execution.junit.IdeaTestRunner$Repeater.startRunnerWithArgs(IdeaTestRunner.java:51)
 at com.intellij.rt.execution.junit.JUnitStarter.prepareStreamsAndStart(JUnitStarter.java:237)
 at com.intellij.rt.execution.junit.JUnitStarter.main(JUnitStarter.java:70)
Caused by: org.apache.ibatis.reflection.ReflectionException: There is no getter for property named 'key' in 'class java.lang.Integer'
 at org.apache.ibatis.reflection.Reflector.getGetInvoker(Reflector.java:409)
 at org.apache.ibatis.reflection.MetaClass.getGetInvoker(MetaClass.java:164)
 at org.apache.ibatis.reflection.wrapper.BeanWrapper.getBeanProperty(BeanWrapper.java:162)
 at org.apache.ibatis.reflection.wrapper.BeanWrapper.get(BeanWrapper.java:49)
 at org.apache.ibatis.reflection.MetaObject.getValue(MetaObject.java:122)
 at org.apache.ibatis.reflection.MetaObject.getValue(MetaObject.java:119)
 at org.apache.ibatis.mapping.BoundSql.getAdditionalParameter(BoundSql.java:75)
 at org.apache.ibatis.scripting.defaults.DefaultParameterHandler.setParameters(DefaultParameterHandler.java:72)
 at org.apache.ibatis.executor.statement.PreparedStatementHandler.parameterize(PreparedStatementHandler.java:93)
 at org.apache.ibatis.executor.statement.RoutingStatementHandler.parameterize(RoutingStatementHandler.java:64)
 at org.apache.ibatis.executor.SimpleExecutor.prepareStatement(SimpleExecutor.java:86)
 at org.apache.ibatis.executor.SimpleExecutor.doUpdate(SimpleExecutor.java:49)
 at org.apache.ibatis.executor.BaseExecutor.update(BaseExecutor.java:117)
 at org.apache.ibatis.session.defaults.DefaultSqlSession.update(DefaultSqlSession.java:198)
 ... 29 more

由于本人对Mybatis还不是非常的了解，导致用了很长的时间去debug找具体的异常原因。

接下来介绍我将重现异常并分析导致异常的原因。

异常重现

为了重现上面的异常，写了demo。相关代码如下：

数据库表结构：

CREATE TABLE `goods_roomnight_30days` (
 `goods_id` bigint(20) NOT NULL,
 `checkin_room_night_30days` int(11) NOT NULL DEFAULT '0' COMMENT '近30天消费间夜',
 PRIMARY KEY (`goods_id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COMMENT='goods的近30天消费间夜'

KeyValue.java 参数类：

public class KeyValue<K, V> implements Map.Entry<K, V> {
 private K key;
 private V value;
 public KeyValue() {
 }
 public KeyValue(K key, V value) {
 this.key = key;
 this.value = value;
 }
 @Override
 public K getKey() {
 return key;
 }
 @Override
 public V getValue() {
 return value;
 }
 @Override
 public V setValue(V value) {
 V oldValue = this.value;
 this.value = value;
 return oldValue;
 }
 public JSONObject toJSONObject() {
 return ReportJSONObject.newObject().append(String.valueOf(key), value);
 }
 @Override
 public String toString() {
 return toJSONObject().toJSONString();
 }
}

DAO类GoodsRoomnight30daysMapper.java

public interface GoodsRoomnight30daysMapper {
 int deleteByExample(GoodsRoomnight30daysExample example);
 List<GoodsRoomnight30days> selectByExample(GoodsRoomnight30daysExample example);
 <K, V> int insertBatch(List<KeyValue<K, V>> records);
}

mybatis-config.xml文件：

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE configuration PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
 "http://mybatis.org/dtd/mybatis-3-config.dtd">
<configuration>
 <settings>
 <setting name="cacheEnabled" value="false"/>
 </settings>
 <!-- 和spring整合后 environments配置将废除,交给spring管理-->
 <environments default="development">
 <environment id="development">
  <!-- 使用jdbc事务管理-->
  <transactionManager type="JDBC" />
  <!-- 数据库连接池,整合后一般使用第三方的连接池-->
  <dataSource type="POOLED">
  <property name="driver" value="com.mysql.jdbc.Driver" />
  <property name="url" value="jdbc:mysql://localhost:3306/hotel_report?characterEncoding=utf-8" />
  <property name="username" value="test_user" />
  <property name="password" value="user123" />
  </dataSource>
 </environment>
 </environments>
 <mappers>
 <mapper resource="mybatis/GoodsRoomnight30daysMapper.xml"/>
 </mappers>
</configuration>

GoodsRoomnight30daysMapper.xml文件的主要内容：

<insert id="insertBatch" parameterType="list">
 REPLACE INTO goods_roomnight_30days (goods_id, checkin_room_night_30days)
 VALUES
 <foreach collection="list" index="index" item="item" separator=",">
  (#{item.key}, #{item.value})
  <!--<choose>-->
  <!--<when test="item.value == null">(#{item.key}, 0)</when>-->
  <!--<when test="item.value != null">(#{item.key}, #{item.value})</when>-->
  <!--</choose>-->
 </foreach>
 </insert>

以上为重现此异常的主要代码，完整代码可在Github查看：https://github.com/misterzhou/java-demo/tree/master/test-spring/spring-server

重现异常的测试代码 GoodsRoomnight30daysTest.java：

package org.guojing.test.spring.server;
import org.apache.ibatis.io.Resources;
import org.apache.ibatis.session.SqlSession;
import org.apache.ibatis.session.SqlSessionFactory;
import org.apache.ibatis.session.SqlSessionFactoryBuilder;
import org.guojing.spring.commons.KeyValue;
import org.junit.After;
import org.junit.Before;
import org.junit.Test;
import java.io.IOException;
import java.io.InputStream;
import java.util.ArrayList;
import java.util.List;
/**
 * Created at: 2016-12-24
 *
 * @author guojing
 */
public class GoodsRoomnight30daysTest {
 SqlSessionFactory sqlSessionFactory;
 SqlSession sqlSession;
 GoodsRoomnight30daysMapper goodsRoomnight30daysMapper;
 @Before
 public void init() throws IOException {
 String resource = "mybatis/mybatis-config.xml";
 InputStream inputStream = Resources.getResourceAsStream(resource);
 sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
 sqlSession = sqlSessionFactory.openSession(true);
 goodsRoomnight30daysMapper = sqlSession.getMapper(GoodsRoomnight30daysMapper.class);
 }
 @Test
 public void test() {
 List<KeyValue<Long, Integer>> records = new ArrayList<>();
 records.add(new KeyValue<Long, Integer>(1725L, 5));
 records.add(new KeyValue<Long, Integer>(1728L, 3));
 records.add(new KeyValue<Long, Integer>(1730L, null));
 records.add(new KeyValue<Long, Integer>(1758L, null));
 int deleted = goodsRoomnight30daysMapper.deleteByExample(new GoodsRoomnight30daysExample());
 System.out.println("----- deleted row size: " + deleted);
 int row = goodsRoomnight30daysMapper.insertBatch(records);
 System.out.println("----- affected row: " + row);
 List<GoodsRoomnight30days> result = goodsRoomnight30daysMapper.selectByExample(new GoodsRoomnight30daysExample());
 for (GoodsRoomnight30days item : result) {
  System.out.println(item.toString());
 }
 }
 @After
 public void after() {
 if (sqlSession != null) {
  sqlSession.close();
 }
 }
}

卖个关子，大家先不要往下看，想想导致异常的原因（熟练使用foreach标签的同学应该能看出端倪）。

查找异常过程及异常分析

在项目中，由于DAO方法的参数类和返回结果类经常会包含一个键和键对应的值，为了避免重复定义类，我定义了一个实现了Map.Entry接口的KeyValue泛型类，具体请查看上节。

GoodsRoomnight30daysMapper.insertBatch()方法就使用了此泛型类。在运行之后就抛出了本文开始提到的异常信息。

看到异常信息后，就把重点放到了是不是Mybatis对泛型的支持不够好上。问了下同事(@胜男)，同事在自己的机器上试了下，发现没有异常。这就奇怪了。仔细看了下代码，发现不同之处就是我的KeyValue泛型类实现了Map.Entry接口。 此时还不知道mybatis官网对于foreach标签的说明：

可以将任何可迭代对象（如列表、集合等）和任何的字典或者数组对象传递给foreach作为集合参数。当使用可迭代对象或者数组时，index是当前迭代的次数，item的值是本次迭代获取的元素。当使用字典（或者Map.Entry对象的集合）时，index是键，item是值。

接下来就是通过debug看看，异常产生的具体原因。于是就先用实现了Map.Entry接口的KeyValue类的代码进行debug，通过异常日志可以看到异常是在 DefaultSqlSession.java:200 行抛出，那么将断点打到 DefaultSqlSession.java:197行，然后一步步向下执行，当执行到 ForEachSqlNode.java:73行时，终于焕然大悟。先看下debug调用链图：

再看看具体的代码 ForEachSqlNode.java:73(此类就是foreach标签对象的Node类):

此时具体的异常原因就很明显了，此处的 o 对象的所属的类就是KeyValue类，由于KeyValue类实现了Map.Entry接口，所以 o instance Map.Entry 为true，mybatis就把key值赋给了foreach的index属性，而把value值赋给了item属性，此处也就是把值为5的Integer对象赋给了item属性。所以 GoodsRoomnight30daysMapper.xml 中id为 insertBatch 的select标签的item属性对应的对象也就没有 item.key 和 item.value 属性，这就是最终导致异常的原因。

<insert id="insertBatch" parameterType="list">
 REPLACE INTO goods_roomnight_30days (goods_id, checkin_room_night_30days)
 VALUES
 <foreach collection="list" index="index" item="item" separator=",">
 (#{item.key}, #{item.value})
 </foreach>
</insert>

以上所述是小编给大家介绍的Mybatis foreach标签使用不当导致异常的原因浅析，希望对大家有所帮助，如果大家有任何疑问请给我留言，小编会及时回复大家的。在此也非常感谢大家对我们网站的支持！