一、前言
ResultMap可实现高级映射(使用association、collection实现一对一及一对多映射)，且association、collection都具备延迟加载功能。延迟加载：先从单表查询，需要时再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。
二、案例
结合前面《老司机学习MyBatis之如何使用association解决一对一关联查询》和《老司机学习MyBatis之如何使用collection解决一对多关联查询》章节，拷贝这两个章节的相关代码
在mybatis-config.xml全局配置文件中加入如下配置，就能实现延迟加载的效果
<settings>
<!-- 打开延迟加载的开关 -->
<setting name="lazyLoadingEnabled" value="true" />
<!-- 将积极加载改为消息加载即按需加载 -->
<setting name="aggressiveLazyLoading" value="false"/>
</settings>
没有配置之前，修改testGetUserByAssocicationStep方法
@Test
public void testGetUserByAssocicationStep() throws IOException {
SqlSessionFactory sqlSessionFactory = getSqlSessionFactory();
SqlSession openSession = sqlSessionFactory.openSession();
try {
UserMapper mapper = openSession.getMapper(UserMapper.class);
User user = mapper.getUserByIdStep(1);
System.out.println(user.getUserName());
//System.out.println(user.getRole());
} finally {
openSession.close();
}
}
控制台打印如下
2017-08-12 14:50:16,331 [main] [com.queen.mybatis.mapper.UserMapper.getUserByIdStep]-[DEBUG] ==> Preparing: select id, loginId, userName, roleId, note from t_user where id=? 2017-08-12 14:50:16,382 [main] [com.queen.mybatis.mapper.UserMapper.getUserByIdStep]-[DEBUG] ==> Parameters: 1(Integer) 2017-08-12 14:50:16,435 [main] [com.queen.mybatis.mapper.RoleMapper.getRoleById]-[DEBUG] ====> Preparing: select id, roleName from t_role where id=? 2017-08-12 14:50:16,436 [main] [com.queen.mybatis.mapper.RoleMapper.getRoleById]-[DEBUG] ====> Parameters: 1(Integer) 2017-08-12 14:50:16,438 [main] [com.queen.mybatis.mapper.RoleMapper.getRoleById]-[DEBUG] <==== Total: 1 2017-08-12 14:50:16,439 [main] [com.queen.mybatis.mapper.UserMapper.getUserByIdStep]-[DEBUG] <== Total: 1 奎恩
如上打印结果，我们只是获取了用户的姓名，但是SQL语句中却去查询了角色信息
加上配置后，我们再次测试一下，打印结果会有何不同呢？
2017-08-12 14:56:33,840 [main] [com.queen.mybatis.mapper.UserMapper.getUserByIdStep]-[DEBUG] ==> Preparing: select id, loginId, userName, roleId, note from t_user where id=? 2017-08-12 14:56:33,915 [main] [com.queen.mybatis.mapper.UserMapper.getUserByIdStep]-[DEBUG] ==> Parameters: 1(Integer) 2017-08-12 14:56:34,201 [main] [com.queen.mybatis.mapper.UserMapper.getUserByIdStep]-[DEBUG] <== Total: 1 奎恩
如上，控制台只打印了一条SQL，这条SQL只查询出了用户表信息，没有查出角色的相关信息。这样就实现了延迟加载。当我们需要角色的信息时，才会去数据库查询数据。
使用collection实现延迟加载，同理。需要大家自己动手实践一下就知道了。
注意：本工程之前没有配置cglib包，可能会报错，需要手工添加cglib的maven依赖，具体请参考《Mybatis异常“Cannot enable lazy loading because CGLIB is not available. Add CGLIB to your classpath.”》，报错信息如下：
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 UserMapper.xml ### The error may involve com.queen.mybatis.mapper.UserMapper.getUserByIdStep ### The error occurred while handling results ### SQL: select id, loginId, userName, roleId, note from t_user where id=? ### 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.session.defaults.DefaultSqlSession.selectOne(DefaultSqlSession.java:66) at org.apache.ibatis.binding.MapperMethod.execute(MapperMethod.java:68) at org.apache.ibatis.binding.MapperProxy.invoke(MapperProxy.java:52) at com.sun.proxy.$Proxy2.getUserByIdStep(Unknown Source) at com.queen.mybatis.MyBatisTest.testGetUserByAssocicationStep(MyBatisTest.java:50) 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:45) at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:15) at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:42) at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:20) at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:263) at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:68) at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:47) at org.junit.runners.ParentRunner$3.run(ParentRunner.java:231) at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:60) at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:229) at org.junit.runners.ParentRunner.access$000(ParentRunner.java:50) at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:222) at org.junit.runners.ParentRunner.run(ParentRunner.java:300) 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) ... 29 more
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