mybatis源代码分析：深入了解mybatis延迟加载机制

下文从mybatis(3.2.7)延迟加载样例讲起，逐步深入其实现机制。

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	

查看原图

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