Mybatis高级特性学习（二）

主要内容：延迟加载

1. 什么是延迟加载

resultMap可以通过association,collection实现高级映射，association和collection具备延迟加载功能。

延迟加载：先从单表查询，需要时在从关联表去关联查询。大大提高数据库性能，因为查询单表要比关联查询多张表速度要快。

开启延迟加载：SqlMapConfig中配置settings全局变量

<settings>
<!-- 打开延迟加载的开关 -->
<setting name="lazzyLoadingEnabled" value="true"></setting>
<!-- 将积极加载改为消极加载 按需加载 -->
<setting name="aggressiveLazyLoading" value="false"></setting>
</settings>

2. 使用association来实现延迟加载

1）定义Mapper.xml

需要定义两个mapper方法对应的statement

<!-- 延迟加载测试 -->
<resultMap type="com.cupdata.zicon.po.FilmCategory" id="delayLoadMap">
<id column="film_id" property="film_id"></id>
<result column="category_id" property="category_id"></result>
<result column="last_update" property="last_update"></result>

<!-- association :用于映射关联查询单个对象信息 -->
<!-- select:延迟加载所需要执行的SQL语句的id column:两个表关联查询的列 关联查询的SQL -->
<<span style="color:#ff0000;">association </span>property="category" javaType="com.cupdata.zicon.po.Category"
<span style="color:#ff0000;">select</span>="findCategoryById" <span style="color:#ff0000;">column</span>="category_id">
<!-- 对category进行延迟加载 -->
</association>
</resultMap>
<select id="<span style="color:#ff0000;">delayLoad</span>" resultMap="delayLoadMap">
select * from film_category
</select>

<select id="findCategoryById" parameterType="int"
resultType="com.cupdata.zicon.po.Category">
select * from category where category_id=#{value}
</select>

2) 定义Mapper.java

public interface FilmCategoryMapper {

public List<FilmCategoryVo> findFilmCategory();

public List<FilmCategory> findFilmCategoryMap();

public List<FilmCategory> delayLoad();
}

3)测试

/**
* 延迟加载测试
*/
@Test
public void testDelayLoad() {
SqlSession sqlSession = sqlSessionFactory.openSession();
FilmCategoryMapper mapper = sqlSession
.getMapper(FilmCategoryMapper.class);
//单表查询列表
List<FilmCategory> list = mapper.delayLoad();
//遍历列表
for(FilmCategory fc:list){
//执行findCategory
Category category = fc.getCategory();
System.out.println(category.getName());
}
//		System.out.println(list.get(0).getCategory().getName());
sqlSession.close();
}

测试时候报错如下：

org.apache.ibatis.exceptions.PersistenceException:
### Error querying database.  Cause: java.lang.IllegalStateException: Cannot enable lazy loading because CGLIB is not available. Add CGLIB to your classpath.
### The error may exist in mapper/FilmCategory.xml
### The error may involve com.cupdata.zicon.dao.mapper.FilmCategoryMapper.delayLoad
### The error occurred while handling results
### SQL: select * from film_category
### Cause: java.lang.IllegalStateException: Cannot enable lazy loading because CGLIB is not available. Add CGLIB to your classpath.
at org.apache.ibatis.exceptions.ExceptionFactory.wrapException(ExceptionFactory.java:26)
at org.apache.ibatis.session.defaults.DefaultSqlSession.selectList(DefaultSqlSession.java:111)
at org.apache.ibatis.session.defaults.DefaultSqlSession.selectList(DefaultSqlSession.java:102)
at org.apache.ibatis.binding.MapperMethod.executeForMany(MapperMethod.java:119)
at org.apache.ibatis.binding.MapperMethod.execute(MapperMethod.java:63)
at org.apache.ibatis.binding.MapperProxy.invoke(MapperProxy.java:52)
at com.sun.proxy.$Proxy6.delayLoad(Unknown Source)
at com.cupdata.zicon.dao.mapper.FilmCategoryMapperTest.testDelayLoad(FilmCategoryMapperTest.java:59)
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: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.internal.runners.statements.RunBefores.evaluate(RunBefores.java:26)
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:50)
at org.eclipse.jdt.internal.junit.runner.TestExecution.run(TestExecution.java:38)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:459)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:675)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.run(RemoteTestRunner.java:382)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.main(RemoteTestRunner.java:192)
Caused by: java.lang.IllegalStateException: Cannot enable lazy loading because CGLIB is not available. Add CGLIB to your classpath.
at org.apache.ibatis.executor.loader.cglib.CglibProxyFactory.<init>(CglibProxyFactory.java:56)
at org.apache.ibatis.session.Configuration.getProxyFactory(Configuration.java:300)
at org.apache.ibatis.executor.resultset.DefaultResultSetHandler.createResultObject(DefaultResultSetHandler.java:523)
at org.apache.ibatis.executor.resultset.DefaultResultSetHandler.getRowValue(DefaultResultSetHandler.java:334)
at org.apache.ibatis.executor.resultset.DefaultResultSetHandler.handleRowValuesForSimpleResultMap(DefaultResultSetHandler.java:294)
at org.apache.ibatis.executor.resultset.DefaultResultSetHandler.handleRowValues(DefaultResultSetHandler.java:269)
at org.apache.ibatis.executor.resultset.DefaultResultSetHandler.handleResultSet(DefaultResultSetHandler.java:239)
at org.apache.ibatis.executor.resultset.DefaultResultSetHandler.handleResultSets(DefaultResultSetHandler.java:153)
at org.apache.ibatis.executor.statement.PreparedStatementHandler.query(PreparedStatementHandler.java:60)
at org.apache.ibatis.executor.statement.RoutingStatementHandler.query(RoutingStatementHandler.java:73)
at org.apache.ibatis.executor.SimpleExecutor.doQuery(SimpleExecutor.java:60)
at org.apache.ibatis.executor.BaseExecutor.queryFromDatabase(BaseExecutor.java:267)
at org.apache.ibatis.executor.BaseExecutor.query(BaseExecutor.java:137)
at org.apache.ibatis.executor.CachingExecutor.query(CachingExecutor.java:96)
at org.apache.ibatis.executor.CachingExecutor.query(CachingExecutor.java:77)
at org.apache.ibatis.session.defaults.DefaultSqlSession.selectList(DefaultSqlSession.java:108)
... 30 more
Caused by: java.lang.ClassNotFoundException: Cannot find class: net.sf.cglib.proxy.Enhancer
at org.apache.ibatis.io.ClassLoaderWrapper.classForName(ClassLoaderWrapper.java:190)
at org.apache.ibatis.io.ClassLoaderWrapper.classForName(ClassLoaderWrapper.java:89)
at org.apache.ibatis.io.Resources.classForName(Resources.java:256)
at org.apache.ibatis.executor.loader.cglib.CglibProxyFactory.<init>(CglibProxyFactory.java:54)
... 45 more

原因：缺少cglib的包，maven中配置好dependency