下面的例子是Student类关联一个Teacher对象，在访问Student对象时，不立即加载其关联的Teacher对象，而是等到访问Teacher对象的属性时，才加载Teacher对象。
源代码下载：http://download.csdn.net/detail/u014569459/7363097
1.Student.java
package dao.domain;
public class Student {
public int id;
public String name;
public int teacher_id;
public Teacher teacher;
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public int getTeacher_id() {
return teacher_id;
}
public void setTeacher_id(int teacher_id) {
this.teacher_id = teacher_id;
}
public Teacher getTeacher() {
return teacher;
}
public void setTeacher(Teacher teacher) {
this.teacher = teacher;
}
}
2.Teacher.java
package dao.domain;
public class Teacher {
public int id;
public String name;
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
}
3.StudentDAO.java
package dao;
import dao.domain.Student;
public interface StudentDAO {
public Student getStudentByID(int id);
}
4.StudentDAOImpl.java
package dao.impl;
import java.io.IOException;
import java.io.Reader;
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 dao.StudentDAO;
import dao.domain.Student;
public class StudentDaoImpl implements StudentDAO {
private static SqlSession session = null;
private static StudentDAO mapper = null;
static{
String resouce = "config/ibatisConfiguration.xml";
Reader reader = null;
try {
reader = Resources.getResourceAsReader(resouce);
} catch (IOException e) {
e.printStackTrace();
}
SqlSessionFactory factory = new SqlSessionFactoryBuilder().build(reader);
session = factory.openSession();
mapper = session.getMapper(StudentDAO.class);
try {
reader.close();
} catch (IOException e) {
e.printStackTrace();
}
}
@Override
public Student getStudentByID(int id) {
return mapper.getStudentByID(id);
}
}
5.TeacherMapper.xml
<?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="dao.TeacherDAO">
<select id="getTeacherByID" parameterType="int" resultType="Teacher">
select id,name
from p_teacher
where
id = #{id}
</select>
</mapper>
6.StudentMapper.xml
<?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="dao.StudentDAO">
<resultMap id="StudentMap" type="Student">
<id property="id" column="id" />
<result property="name" column="name" />
<association property="teacher" column="teacher_id"
select="dao.TeacherDAO.getTeacherByID" />
</resultMap>
<select id="getStudentByID" resultMap="StudentMap"
parameterType="int">
select *
from p_stu
where
id = #{id}
</select>
</mapper>
7.ibatisConfiguration.xml
<?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="lazyLoadingEnabled" value="true" /> <setting name="aggressiveLazyLoading" value="true" /> </settings> <typeAliases> <typeAlias alias="Student" type="dao.domain.Student" /> <typeAlias alias="Teacher" type="dao.domain.Teacher" /> </typeAliases> <environments default="development"> <environment id="development"> <transactionManager type="JDBC" /> <dataSource type="POOLED"> <property name="driver" value="com.mysql.jdbc.Driver" /> <property name="url" value="jdbc:mysql://localhost:3307/test?characterEncoding=UTF-8" /> <property name="username" value="root" /> <property name="password" value="root" /> </dataSource> </environment> </environments> <mappers> <mapper resource="config/StudentMapper.xml" /> <mapper resource="config/TeacherMapper.xml" /> </mappers> </configuration>
8.测试类
import dao.StudentDAO;
import dao.domain.Student;
import dao.impl.StudentDaoImpl;
public class Test {
public static void main(String[] args) {
StudentDAO dao = new StudentDaoImpl();
Student article = dao.getStudentByID(1);
System.out.println(article.getTeacher().getName());
}
}
9.mybatis延迟加载说明：
1）在mybatis配置文件中，配置如下两个配置项：
<settings> <setting name="lazyLoadingEnabled" value="true" /> <setting name="aggressiveLazyLoading" value="true" /> </settings>
2）查询语句样例，下面通过association将Student对象一个属性与一个查询语句关联起来：
<resultMap id="StudentMap" type="Student"> <id property="id" column="id" /> <result property="name" column="name" /> <association property="teacher" column="teacher_id" select="dao.TeacherDAO.getTeacherByID" /> </resultMap>
这样，在查询出StudentMap这个结果集时，对于teacher字段就会采取延迟加载了，等到访问teacher对象属性时，才会去加载关联的teacher对象。
10.逐步深入延迟加载细节
1）在Test类中如下行打上断点，然后进行Debug
System.out.println(article.getTeacher().getName());
可以看到article对象为（每次测试，结果会不同）：dao.domain.Student$$EnhancerByCGLIB$$aa39207a@adc40c
这就是一个cglib自动生成的代理对象。
2）那这个代理对象什么时候生成的呢？ 经过反复不停的debug跟踪，得到下面这样一个调用栈。
Thread [main] (Suspended (breakpoint at line 61 in CglibProxyFactory)) CglibProxyFactory.createProxy(Object, ResultLoaderMap, Configuration, ObjectFactory, List<Class<?>>, List<Object>) line: 61 DefaultResultSetHandler.createResultObject(ResultSetWrapper, ResultMap, ResultLoaderMap, String) line: 512 DefaultResultSetHandler.getRowValue(ResultSetWrapper, ResultMap) line: 331 DefaultResultSetHandler.handleRowValuesForSimpleResultMap(ResultSetWrapper, ResultMap, ResultHandler, RowBounds, ResultMapping) line: 291 DefaultResultSetHandler.handleRowValues(ResultSetWrapper, ResultMap, ResultHandler, RowBounds, ResultMapping) line: 266 DefaultResultSetHandler.handleResultSet(ResultSetWrapper, ResultMap, List<Object>, ResultMapping) line: 236 DefaultResultSetHandler.handleResultSets(Statement) line: 150 PreparedStatementHandler.query(Statement, ResultHandler) line: 60 RoutingStatementHandler.query(Statement, ResultHandler) line: 73 SimpleExecutor.doQuery(MappedStatement, Object, RowBounds, ResultHandler, BoundSql) line: 60 SimpleExecutor(BaseExecutor).queryFromDatabase(MappedStatement, Object, RowBounds, ResultHandler, CacheKey, BoundSql) line: 267 SimpleExecutor(BaseExecutor).query(MappedStatement, Object, RowBounds, ResultHandler, CacheKey, BoundSql) line: 137 CachingExecutor.query(MappedStatement, Object, RowBounds, ResultHandler, CacheKey, BoundSql) line: 96 CachingExecutor.query(MappedStatement, Object, RowBounds, ResultHandler) line: 77 DefaultSqlSession.selectList(String, Object, RowBounds) line: 108 DefaultSqlSession.selectList(String, Object) line: 102 DefaultSqlSession.selectOne(String, Object) line: 66 MapperMethod.execute(SqlSession, Object[]) line: 68 MapperProxy<T>.invoke(Object, Method, Object[]) line: 52 $Proxy0.getStudentByID(int) line: not available StudentDaoImpl.getStudentByID(int) line: 40 Test.main(String[]) line: 12
可以看到在查询Student对象时，DefaultResultSetHandler(orgapacheibatisexecutor esultset)类会对数据库查询的每一行结果进行封装处理。
通过下面的代码可以看得更清楚，当结果对象中存在嵌套查询时，当前对象就会被替换为一个代理对象。
private Object createResultObject(ResultSetWrapper rsw, ResultMap resultMap, ResultLoaderMap lazyLoader, String columnPrefix) throws SQLException {
final List<Class<?>> constructorArgTypes = new ArrayList<Class<?>>();
final List<Object> constructorArgs = new ArrayList<Object>();
final Object resultObject = createResultObject(rsw, resultMap, constructorArgTypes, constructorArgs, columnPrefix);
if (resultObject != null && !typeHandlerRegistry.hasTypeHandler(resultMap.getType())) {
final List<ResultMapping> propertyMappings = resultMap.getPropertyResultMappings();
for (ResultMapping propertyMapping : propertyMappings) {
if (propertyMapping.getNestedQueryId() != null && propertyMapping.isLazy()) { // issue gcode #109 && issue #149
return configuration.getProxyFactory().createProxy(resultObject, lazyLoader, configuration, objectFactory, constructorArgTypes, constructorArgs);
}
}
}
return resultObject;
}
3）那当访问代理对象时，会发生什么事情呢？
如调用System.out.println(article.getTeacher().getName())时，得到如下的调用栈信息
Thread [main] (Suspended (breakpoint at line 143 in CglibProxyFactory$EnhancedResultObjectProxyImpl)) owns: ResultLoaderMap (id=38) CglibProxyFactory$EnhancedResultObjectProxyImpl.intercept(Object, Method, Object[], MethodProxy) line: 143 Student$$EnhancerByCGLIB$$e1afda28.getTeacher() line: not available Test.main(String[]) line: 10
代码执行到了如下地方：
if (lazyLoader.size() > 0 && !FINALIZE_METHOD.equals(methodName)) {
if (aggressive || lazyLoadTriggerMethods.contains(methodName)) {
lazyLoader.loadAll();
} else if (PropertyNamer.isProperty(methodName)) {
final String property = PropertyNamer.methodToProperty(methodName);
if (lazyLoader.hasLoader(property)) {
lazyLoader.load(property);
}
}
}
通过调用lazyLoader.loadAll来对需要加载的对象进行加载.
继续顺藤摸瓜，得到如下的调用栈：
Thread [main] (Suspended) owns: ResultLoaderMap (id=38) BeanWrapper.set(PropertyTokenizer, Object) line: 57 MetaObject.setValue(String, Object) line: 133 ResultLoaderMap$LoadPair.load(Object) line: 207 ResultLoaderMap$LoadPair.load() line: 172 ResultLoaderMap.load(String) line: 80 ResultLoaderMap.loadAll() line: 90 CglibProxyFactory$EnhancedResultObjectProxyImpl.intercept(Object, Method, Object[], MethodProxy) line: 143 Student$$EnhancerByCGLIB$$e1afda28.getTeacher() line: not available Test.main(String[]) line: 10
如下是BeanWrapper中setValue方法：
public void set(PropertyTokenizer prop, Object value) {
if (prop.getIndex() != null) {
Object collection = resolveCollection(prop, object);
setCollectionValue(prop, collection, value);
} else {
setBeanProperty(prop, object, value);
}
}
通过setBeanProperty方法，就完成向Student对象设置Teacher属性的过程（上面代码中的object就是Student对象）。
下面通过一张图，展示一下代理对象与实际对象之间的关系，“Student$$EnhancerByCGLIB$$aa39207a@adc40c”为代理对象，是Student类的子类，当访问到Student的getTeacher方法时，其通过关联的EnhancedResultObjectProxyImpl类来加载teacher对象，具体关联见下图，其中BeanWrapper中object就是代理对象本身，通过对object的设置，将teacher属性设置为从数据库查询出来的对象，完成延迟加载。
至此，mybatis的整个延迟加载过程就分析完了。
参考链接：
mybatis下载主页：https://github.com/mybatis/mybatis-3