1.编写一个小demo
导入mybatis源码，demo目录结构如下：
源码的pom文件中需要加入以下依赖：
<dependency>
<groupId>mysql</groupId>
<artifactId>mysql-connector-java</artifactId>
<version>5.1.38</version>
</dependency>
UserMapper.java
public interface UserMapper {
User selectById(String id);
}
User.java
public class User {
private String id;
private String name;
public User(String id, String name) {
this.id = id;
this.name = name;
}
public String getId() {
return id;
}
public void setId(String id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
@Override
public String toString() {
return "User{" +
"id='" + id + '\'' +
", name='" + name + '\'' +
'}';
}
}
UserMapper.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">
<!-- 名称空间namespace必须唯一 -->
<mapper namespace="demo.mapper.UserMapper"> //namespace指定映射的接口文件
<!--resultMap标签：映射实体与表，非主键属性标签 type属性:实体全路径名，可在mybatis.xml文件中统一配置 id属性:为实体与表的映射取一个唯一的编号-->
<resultMap type="user" id="userMap">
<!--id标签:映射主键属性 result标签：映射非主键属性 property属性:实体属性名 column属性：表的字段名-->
<id property="id" column="id"/>
<result property="name" column="name"/>
</resultMap>
<select id="selectById" resultMap="userMap" parameterType="demo.pojo.User">
SELECT * FROM USER WHERE id=#{id};
</select>
</mapper>
db.properties
mysql.driver=com.mysql.jdbc.Driver
mysql.url=jdbc:mysql://localhost:3306/mybatisdb
mysql.username=root
mysql.password=12345678
mybatis-config.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>
<properties resource="db.properties"></properties> //定义属性文件
<!-- 设置实体类的类型别名 -->
<typeAliases>
<typeAlias type="demo.pojo.User" alias="user"/>
</typeAliases>
<!-- 设置一个默认的连接环境信息，支持多数据源 -->
<environments default="mysql_env">
<!-- 连接环境信息，取一个唯一的编号 -->
<environment id="mysql_env">
<!-- mybatis使用的jdbc事务管理方式 -->
<transactionManager type="jdbc">
</transactionManager>
<!-- mybatis使用连接池方式来获取链接 -->
<dataSource type="pooled">
<!-- 配置与数据库交互的四个属性 -->
<property name="driver" value="${mysql.driver}"/>
<property name="url" value="${mysql.url}"/>
<property name="username" value="${mysql.username}"/>
<property name="password" value="${mysql.password}"/>
</dataSource>
</environment>
</environments>
<mappers>
<mapper resource="sqlMapper/UserMapper.xml"/> //定义映射XML文件
</mappers>
</configuration>
MybatisDemo.java
public class MybatisDemo {
SqlSessionFactory sqlSessionFactory;
@Before
public void init() throws IOException {
String resource = "mybatis-config.xml";
InputStream inputStream = Resources.getResourceAsStream(resource);
sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
inputStream.close();
}
@Test
public void testAutoMapping(){
SqlSession sqlSession=sqlSessionFactory.openSession();
UserMapper mapper = sqlSession.getMapper(UserMapper.class);
User user = mapper.selectById("1");
System.out.println(user);
}
}
运行结果为：
User{id='1', name='xiaoming'}
2.源码的分析
主要分三个阶段:
(1)初始化阶段
主要是对mybatis-config.xml配置文件以及UserMapper的解析，代码跟踪如下:
sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
--> return build(parser.parse());
--> parseConfiguration(parser.evalNode("/configuration"));
private void parseConfiguration(XNode root) {
try {
//issue #117 read properties first
propertiesElement(root.evalNode("properties"));
Properties settings = settingsAsProperties(root.evalNode("settings"));
loadCustomVfs(settings);
loadCustomLogImpl(settings);
typeAliasesElement(root.evalNode("typeAliases"));
pluginElement(root.evalNode("plugins"));
objectFactoryElement(root.evalNode("objectFactory"));
objectWrapperFactoryElement(root.evalNode("objectWrapperFactory"));
reflectorFactoryElement(root.evalNode("reflectorFactory"));
settingsElement(settings);
// read it after objectFactory and objectWrapperFactory issue #631
environmentsElement(root.evalNode("environments"));
databaseIdProviderElement(root.evalNode("databaseIdProvider"));
typeHandlerElement(root.evalNode("typeHandlers"));
mapperElement(root.evalNode("mappers"));
} catch (Exception e) {
throw new BuilderException("Error parsing SQL Mapper Configuration. Cause: " + e, e);
}
}
跟踪一下 environmentsElement(root.evalNode("environments"));
environmentsElement(root.evalNode("environments"));
-->TransactionFactory txFactory = transactionManagerElement(child.evalNode("transactionManager")); //构建 transaction
-->DataSourceFactory dsFactory = dataSourceElement(child.evalNode("dataSource")); //构建dataSource
-->configuration.setEnvironment(environmentBuilder.build()); //设置环境变量
跟踪一下 mapperElement(root.evalNode("mappers"));
mapperElement(root.evalNode("mappers"));
--> XMLMapperBuilder mapperParser = new XMLMapperBuilder(inputStream, configuration, resource, configuration.getSqlFragments());
--> mapperParser.parse(); //解析mappers标签下的文件
--> configurationElement(parser.evalNode("/mapper"));
--> buildStatementFromContext(context.evalNodes("select|insert|update|delete"));
--> buildStatementFromContext(list, null);
--> statementParser.parseStatementNode();
-->builderAssistant.addMappedStatement(id, sqlSource, statementType, sqlCommandType,fetchSize, timeout, parameterMap, parameterTypeClass, resultMap, resultTypeClass,resultSetTypeEnum, flushCache, useCache, resultOrdered,keyGenerator, keyProperty, keyColumn, databaseId, langDriver, resultSets);
--> configuration.addMappedStatement(statement);
可以看出，所有的相关的额配置信息都存储到了Configuration这个类的实例里
(2)代理阶段
主要是这两句代码
SqlSession sqlSession=sqlSessionFactory.openSession(); //定义sqlSession
UserMapper mapper = sqlSession.getMapper(UserMapper.class); //关键语句
SqlSession中主要是定义操作数据库的一些方法，如CRUD，重点看一下getMapper方法：
UserMapper mapper = sqlSession.getMapper(UserMapper.class);
--> return configuration.getMapper(type, this);
--> return mapperRegistry.getMapper(type, sqlSession);
--> return mapperProxyFactory.newInstance(sqlSession); //从Mapper代理工厂创建mapper
--> final MapperProxy<T> mapperProxy = new MapperProxy<>(sqlSession, mapperInterface, methodCache);//定义了一个Mapper的动态代理，调用Mapper接口时会调用里面的的invoke方法
--> return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[] { mapperInterface }, mapperProxy);//返回代理创建的对象
这里很关键，完成了mapper接口与实际操作sql语句的映射关系。
(3)数据读取阶段
执行User user = mapper.selectById("1")时，会调用MapperProxy类下的invoke方法，过程如下：
return mapperMethod.execute(sqlSession, args);
-->switch (command.getType())//根据command字段类型执行相应的操作
--> result = sqlSession.selectOne(command.getName(), param);//执行sqlSession的selectOne方法
-->List<T> list = this.selectList(statement, parameter);
--> return executor.query(ms, wrapCollection(parameter), rowBounds, Executor.NO_RESULT_HANDLER);
--> return query(ms, parameter, rowBounds, resultHandler, key, boundSql);
--> list = queryFromDatabase(ms, parameter, rowBounds, resultHandler, key, boundSql);//从数据库查询
--> list = doQuery(ms, parameter, rowBounds, resultHandler, boundSql);
-->stmt = prepareStatement(handler, ms.getStatementLog());
--> Connection connection = getConnection(statementLog); //这里执行的jdbc的操作
--> stmt = handler.prepare(connection, transaction.getTimeout());//这里执行的jdbc的操作
-->return handler.queryCursor(stmt); //返回游标
-->String sql = boundSql.getSql();//获取sql
-->statement.execute(sql);//执行sql
