在项目开发中使用嵌套查询可以有三种方式
1.指定<collection>标签的select，可以实现延迟加载，而且sql 也不会那么复杂，易读
<collection property="articles" column="article_id" fetchType="eager"
ofType="com.jv.dynamic.bean.Article" select="queryArticleById">
</collection>2.指定<collection>标签的resultMap属性，可以提高resultMap的重用率
<collection property="articles" ofType="com.jv.dynamic.bean.Article">
<id property="id" column="article_id"/>
<result property="title" column="title"/>
<result property="content" column="content"/>
<result property="blogId" column="bid"/>
</collection>3.直接在<collection>标签中写数据库字段与Java属性的映射关系
<collection property="articles" ofType="com.jv.dynamic.bean.Article" resultMap="articleMap">
</collection>
<resultMap type="com.jv.dynamic.bean.Article" id="articleMap">
<id property="id" column="article_id"/>
<result property="title" column="title"/>
<result property="content" column="content"/>
<result property="blogId" column="bid"/>
</resultMap>我在项目中使用了第一种方式，数据读出来了，但是在springmvc返回的时候序列化报错
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_EMPTY_BEANS) (through reference chain: java.util.ArrayList[0]->com.jv.dynamic.bean.Blog_$$_jvstd75_0["handler"])
at com.fasterxml.jackson.databind.JsonMappingException.from(JsonMappingException.java:275)
at com.fasterxml.jackson.databind.SerializerProvider.mappingException(SerializerProvider.java:1109)
at com.fasterxml.jackson.databind.SerializerProvider.reportMappingProblem(SerializerProvider.java:1134)
at com.fasterxml.jackson.databind.ser.impl.UnknownSerializer.failForEmpty(UnknownSerializer.java:69)
at com.fasterxml.jackson.databind.ser.impl.UnknownSerializer.serialize(UnknownSerializer.java:32)
at com.fasterxml.jackson.databind.ser.BeanPropertyWriter.serializeAsField(BeanPropertyWriter.java:693)
at com.fasterxml.jackson.databind.ser.std.BeanSerializerBase.serializeFields(BeanSerializerBase.java:690)
at com.fasterxml.jackson.databind.ser.BeanSerializer.serialize(BeanSerializer.java:155)
at com.fasterxml.jackson.databind.ser.impl.IndexedListSerializer.serializeContents(IndexedListSerializer.java:119)
at com.fasterxml.jackson.databind.ser.impl.IndexedListSerializer.serialize(IndexedListSerializer.java:79)
at com.fasterxml.jackson.databind.ser.impl.IndexedListSerializer.serialize(IndexedListSerializer.java:18)
at com.fasterxml.jackson.databind.ser.DefaultSerializerProvider.serializeValue(DefaultSerializerProvider.java:292)
at com.fasterxml.jackson.databind.ObjectMapper._configAndWriteValue(ObjectMapper.java:3672)
at com.fasterxml.jackson.databind.ObjectMapper.writeValueAsString(ObjectMapper.java:3048)
at com.jv.test.TestBlog.test1(TestBlog.java:46)
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:483)
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.eclipse.jdt.internal.junit4.runner.JUnit4TestReference.run(JUnit4TestReference.java:86)
at org.eclipse.jdt.internal.junit.runner.TestExecution.run(TestExecution.java:38)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:538)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:760)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.run(RemoteTestRunner.java:460)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.main(RemoteTestRunner.java:206)
在网上搜了很多，说这种错误可以使用@JsonIgnore @JsonIgnoreProperties等方式解决，可试完所有我能找到的解决方法，依然不行。。。怎么办喃，慢慢debug，一步一步缩小发生错误的范围，最终新建了一个普通java工程用jackson来一点点序列化，突然发现打印出来的嵌套查询结果只有一行，但是真正的查询结果是有6行的，如果对Mybatis有一定了解的同学估计猜到原因了吧。。。对。。。就是我选择的嵌套查询方式默认是延迟加载的，我设置<collection property="articles" column="article_id" fetchType="eager" ofType="com.jv.dynamic.bean.Article" select="queryArticleById"></collection>，再测试，奇迹发生了。
所以在项目开发的时候，如果要使用第一种嵌套查询方式，那么对你的对象就需要认真考虑，如果不涉及到序列化，可以大胆使用它的延迟加载（fetchType="lazy"），否则就一致性加载完成（fetchType="eager"）