为什么会出现实体类没有序列化异常

@Test
   public void testGoodsOneToMany() throws Exception {
       SqlSession sqlSession = null;
       try {
           sqlSession = MybatisUtils.OpenSession();
           List<Goods> list = sqlSession.selectList("goods.selectOneToMany");
           for (Goods g : list) {
               System.out.println(g.getGoodsId() + ":" + g.getTitle() + ":"+ g.getGoodsDetails().size());
           }
       } catch (Exception e) {
           throw e;
       }finally {
           if (sqlSession != null) {
               MybatisUtils.CloseSession(sqlSession);
           }
       }


   }
}

package com.imooc.mybatis.entity;

import java.io.Serializable;
import java.util.List;

public class Goods  {
   private Integer goodsId;
   private String title;
   private String subTitle;
   private Float originalCost;
   private Float currentPrice;
   private Float discount;
   private Integer isFreeDelivery;
   private Integer categoryId;
   private List<GoodsDetail> goodsDetails;

   public Integer getGoodsId() {
       return goodsId;
   }

   public void setGoodsId(Integer goodsId) {
       this.goodsId = goodsId;
   }

   public String getTitle() {
       return title;
   }

   public void setTitle(String title) {
       this.title = title;
   }

   public String getSubTitle() {
       return subTitle;
   }

   public void setSubTitle(String subTitle) {
       this.subTitle = subTitle;
   }

   public Float getOriginalCost() {
       return originalCost;
   }

   public void setOriginalCost(Float originalCost) {
       this.originalCost = originalCost;
   }

   public Float getCurrentPrice() {
       return currentPrice;
   }

   public void setCurrentPrice(Float currentPrice) {
       this.currentPrice = currentPrice;
   }

   public Float getDiscount() {
       return discount;
   }

   public void setDiscount(Float discount) {
       this.discount = discount;
   }

   public Integer getIsFreeDelivery() {
       return isFreeDelivery;
   }

   public void setIsFreeDelivery(Integer isFreeDelivery) {
       this.isFreeDelivery = isFreeDelivery;
   }

   public Integer getCategoryId() {
       return categoryId;
   }

   public void setCategoryId(Integer categoryId) {
       this.categoryId = categoryId;
   }

   public List<GoodsDetail> getGoodsDetails() {
       return goodsDetails;
   }

   public void setGoodsDetails(List<GoodsDetail> goodsDetails) {
       this.goodsDetails = goodsDetails;
   }
}

package com.imooc.mybatis.entity;

public class GoodsDetail {
   private Integer gdId;
   private Integer goodsId;
   private String gdPicUrl;
   private Integer gdOrder;

   public Integer getGdId() {
       return gdId;
   }

   public void setGdId(Integer gdId) {
       this.gdId = gdId;
   }

   public Integer getGoodsId() {
       return goodsId;
   }

   public void setGoodsId(Integer goodsId) {
       this.goodsId = goodsId;
   }

   public String getGdPicUrl() {
       return gdPicUrl;
   }

   public void setGdPicUrl(String gdPicUrl) {
       this.gdPicUrl = gdPicUrl;
   }

   public Integer getGdOrder() {
       return gdOrder;
   }

   public void setGdOrder(Integer gdOrder) {
       this.gdOrder = gdOrder;
   }
}

<?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="goodsDetail">
   <select id="selectById" parameterType="Integer" resultType="com.imooc.mybatis.entity.GoodsDetail">
       select * from t_goods_detail where goods_id=#{value }
   </select>

</mapper>

<?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="goods">
   <cache eviction="LRU" flushInterval="600000" size="512" readOnly="ture"/>

  <resultMap id="rmGoods1" type="com.imooc.mybatis.entity.Goods">
       <id property="goodsId" column="goods_id"></id>
       <collection property="goodsDetails" select="goodsDetail.selectById" column="goods_id"></collection>
   </resultMap>
   <select id="selectOneToMany" resultMap="rmGoods1">
       select * from t_goods  order by  goods_id limit 0,1
   </select>

</mapper>

[main] 11:02:36.0036 DEBUG org.apache.ibatis.logging.LogFactory - Logging initialized using 'class org.apache.ibatis.logging.slf4j.Slf4jImpl' adapter.

[main] 11:02:36.0036 DEBUG o.a.i.d.pooled.PooledDataSource - PooledDataSource forcefully closed/removed all connections.

[main] 11:02:36.0036 DEBUG o.a.i.d.pooled.PooledDataSource - PooledDataSource forcefully closed/removed all connections.

[main] 11:02:36.0036 DEBUG o.a.i.d.pooled.PooledDataSource - PooledDataSource forcefully closed/removed all connections.

[main] 11:02:36.0036 DEBUG o.a.i.d.pooled.PooledDataSource - PooledDataSource forcefully closed/removed all connections.

[main] 11:02:37.0037 DEBUG goods - Cache Hit Ratio [goods]: 0.0

[main] 11:02:37.0037 DEBUG o.a.i.t.jdbc.JdbcTransaction - Opening JDBC Connection

Tue Nov 05 11:02:37 CST 2019 WARN: Establishing SSL connection without server's identity verification is not recommended. According to MySQL 5.5.45+, 5.6.26+ and 5.7.6+ requirements SSL connection must be established by default if explicit option isn't set. For compliance with existing applications not using SSL the verifyServerCertificate property is set to 'false'. You need either to explicitly disable SSL by setting useSSL=false, or set useSSL=true and provide truststore for server certificate verification.

[main] 11:02:38.0038 DEBUG o.a.i.d.pooled.PooledDataSource - Created connection 22004208.

[main] 11:02:38.0038 DEBUG o.a.i.t.jdbc.JdbcTransaction - Setting autocommit to false on JDBC Connection [com.mysql.jdbc.JDBC4Connection@14fc1f0]

[main] 11:02:38.0038 DEBUG goods.selectOneToMany - ==>  Preparing: select * from t_goods order by goods_id limit 0,1 

[main] 11:02:38.0038 DEBUG goods.selectOneToMany - ==> Parameters: 

[main] 11:02:38.0038 DEBUG goodsDetail.selectById - ====>  Preparing: select * from t_goods_detail where goods_id=? 

[main] 11:02:38.0038 DEBUG goodsDetail.selectById - ====> Parameters: 741(Integer)

[main] 11:02:38.0038 DEBUG goodsDetail.selectById - <====      Total: 6

[main] 11:02:38.0038 DEBUG goods.selectOneToMany - <==      Total: 1

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[main] 11:02:38.0038 DEBUG o.a.i.t.jdbc.JdbcTransaction - Resetting autocommit to true on JDBC Connection [com.mysql.jdbc.JDBC4Connection@14fc1f0]

[main] 11:02:38.0038 DEBUG o.a.i.t.jdbc.JdbcTransaction - Closing JDBC Connection [com.mysql.jdbc.JDBC4Connection@14fc1f0]

[main] 11:02:38.0038 DEBUG o.a.i.d.pooled.PooledDataSource - Returned connection 22004208 to pool.

org.apache.ibatis.cache.CacheException: Error serializing object.  Cause: java.io.NotSerializableException: com.imooc.mybatis.entity.Goods

at org.apache.ibatis.cache.decorators.SerializedCache.serialize(SerializedCache.java:100)

at org.apache.ibatis.cache.decorators.SerializedCache.putObject(SerializedCache.java:56)

at org.apache.ibatis.cache.decorators.LoggingCache.putObject(LoggingCache.java:51)

at org.apache.ibatis.cache.decorators.SynchronizedCache.putObject(SynchronizedCache.java:45)

at org.apache.ibatis.cache.decorators.TransactionalCache.flushPendingEntries(TransactionalCache.java:122)

at org.apache.ibatis.cache.decorators.TransactionalCache.commit(TransactionalCache.java:105)

at org.apache.ibatis.cache.TransactionalCacheManager.commit(TransactionalCacheManager.java:44)

at org.apache.ibatis.executor.CachingExecutor.close(CachingExecutor.java:61)

at org.apache.ibatis.session.defaults.DefaultSqlSession.close(DefaultSqlSession.java:263)

at com.imooc.mybatis.utils.MybatisUtils.CloseSession(MybatisUtils.java:48)

at com.imooc.mybatis.MybatisTestor.testGoodsOneToMany(MybatisTestor.java:388)

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.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:50)

at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)

at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:47)

at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)

at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:325)

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:68)

at com.intellij.rt.execution.junit.IdeaTestRunner$Repeater.startRunnerWithArgs(IdeaTestRunner.java:47)

at com.intellij.rt.execution.junit.JUnitStarter.prepareStreamsAndStart(JUnitStarter.java:242)

at com.intellij.rt.execution.junit.JUnitStarter.main(JUnitStarter.java:70)

Caused by: java.io.NotSerializableException: com.imooc.mybatis.entity.Goods

at java.io.ObjectOutputStream.writeObject0(ObjectOutputStream.java:1184)

at java.io.ObjectOutputStream.writeObject(ObjectOutputStream.java:348)

at java.util.ArrayList.writeObject(ArrayList.java:766)

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 java.io.ObjectStreamClass.invokeWriteObject(ObjectStreamClass.java:1140)

at java.io.ObjectOutputStream.writeSerialData(ObjectOutputStream.java:1496)

at java.io.ObjectOutputStream.writeOrdinaryObject(ObjectOutputStream.java:1432)

at java.io.ObjectOutputStream.writeObject0(ObjectOutputStream.java:1178)

at java.io.ObjectOutputStream.writeObject(ObjectOutputStream.java:348)

at org.apache.ibatis.cache.decorators.SerializedCache.serialize(SerializedCache.java:96)

... 32 more