手擼MyBatis（一）原始碼解析
阿新 • • 發佈：2020-12-17
技術標籤：企業級框架mybatisjdbcjavamysql原始碼
概述
MyBatis是大家最熟悉的ORM框架，大家基本都會用，那如果面試官問到MyBatis的實現原理該如何回答呢？本文將帶大家過一下MyBatis的原始碼，好對MyBatis有一個更深刻的認識。
MyBatis的基本操作
先帶大家過一下MyBatis的使用過程，這裡是沒有整合Spring，就是純粹的MyBatis。
1）建立表：
drop table if exists user; CREATE TABLE `user` ( `id` int(11) NOT NULL AUTO_INCREMENT, `username` varchar(32) NOT NULL COMMENT '使用者名稱稱', `birthday` date DEFAULT NULL COMMENT '生日', `sex` char(1) DEFAULT NULL COMMENT '性別', `address` varchar(256) DEFAULT NULL COMMENT '地址', PRIMARY KEY (`id`) ) ENGINE=InnoDB AUTO_INCREMENT=27 DEFAULT CHARSET=utf8; drop table if exists orders; CREATE TABLE `orders` ( `id` int(11) NOT NULL AUTO_INCREMENT, `user_id` int(11) NOT NULL COMMENT '下單使用者id', `number` varchar(32) NOT NULL COMMENT '訂單號', `createtime` datetime NOT NULL COMMENT '建立訂單時間', `note` varchar(100) DEFAULT NULL COMMENT '備註', PRIMARY KEY (`id`), KEY `FK_orders_1` (`user_id`), CONSTRAINT `FK_orders_id` FOREIGN KEY (`user_id`) REFERENCES `user` (`id`) ON DELETE NO ACTION ON UPDATE NO ACTION ) ENGINE=InnoDB AUTO_INCREMENT=6 DEFAULT CHARSET=utf8; insert into `user`(`id`,`username`,`birthday`,`sex`,`address`) values (1,'王五',NULL,'2',NULL),(10,'張三','2014-07-10','1','北京市'),(16,'張小明',NULL,'1','河南鄭州'),(22,'陳小明',NULL,'1','河南鄭州'),(24,'張三丰',NULL,'1','河南鄭州'),(25,'陳小明',NULL,'1','河南鄭州'),(26,'王五',NULL,NULL,NULL); insert into `orders`(`id`,`user_id`,`number`,`createtime`,`note`) values (3,1,'1000010','2015-02-04 13:22:35',NULL),(4,1,'1000011','2015-02-03 13:22:41',NULL),(5,10,'1000012','2015-02-12 16:13:23',NULL);
2）新增Maven依賴：
<dependency> <groupId>junit</groupId> <artifactId>junit</artifactId> <version>4.11</version> <scope>test</scope> </dependency> <!-- https://mvnrepository.com/artifact/org.mybatis/mybatis --> <dependency> <groupId>org.mybatis</groupId> <artifactId>mybatis</artifactId> <version>3.4.1</version> </dependency> <!-- https://mvnrepository.com/artifact/mysql/mysql-connector-java --> <dependency> <groupId>mysql</groupId> <artifactId>mysql-connector-java</artifactId> <version>5.1.38</version> </dependency>
3）jdbc配置檔案：
jdbc.properties
driver=com.mysql.jdbc.Driver
url=jdbc:mysql://localhost:3306/test_db?useUnicode=true&characterEncoding=UTF-8&useSSL=true
username=root
password=123456
4）MyBatis配置檔案：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="jdbc.properties"></properties> <environments default="develop"> <environment id="develop"> <transactionManager type="JDBC"></transactionManager> <dataSource type="POOLED"> <property name="driver" value="${driver}"/> <property name="url" value="${url}"/> <property name="username" value="${username}"/> <property name="password" value="${password}"/> </dataSource> </environment> </environments> <mappers> <!--<package name="com.qf.mbs.mapper"/>--> <mapper resource="com/qf/mbs/mapper/UserDAO.xml"/> <mapper resource="com/qf/mbs/mapper/OrderDAO.xml"/> </mappers> </configuration>
5）新增DAO介面：
public interface UserDAO {
List<User> findAll();
User findById(Integer id);
void addUser(User user);
void updateUser(User user);
void deleteUser(Integer id);
}
public interface OrderDAO {
List<Order> findAll();
List<Order> findByMap(Map<String,String> where);
}
6）新增對映檔案：
<?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="com.qf.mbs.dao.UserDAO">
<insert id="addUser" parameterType="com.qf.mbs.po.User">
<selectKey keyProperty="id" resultType="int" order="AFTER">
SELECT LAST_INSERT_ID()
</selectKey>
INSERT INTO USER (USERNAME,BIRTHDAY,SEX,ADDRESS) VALUES (#{username},#{birthday},#{sex},#{address})
</insert>
<update id="updateUser" parameterType="com.qf.mbs.po.User">
UPDATE USER set username=#{username},birthday=#{birthday},sex=#{sex},address=#{address} where id=#{id}
</update>
<delete id="deleteUser" parameterType="java.lang.Integer">
DELETE FROM USER where id=#{id}
</delete>
<select id="findById" parameterType="java.lang.Integer" resultType="com.qf.mbs.po.User">
SELECT * FROM USER where id=#{id}
</select>
<select id="findAll" resultType="com.qf.mbs.po.User">
select * from user
</select>
</mapper>
<?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="com.qf.mbs.dao.OrderDAO">
<resultMap id="OrderMap" type="com.qf.mbs.po.Order">
<id property="id" column="id"/>
<result property="number" column="number"/>
<result property="createtime" column="createtime"/>
<result property="note" column="note"/>
<association property="user" javaType="com.qf.mbs.po.User">
<id property="id" column="id"/>
<result property="username" column="username"/>
<result property="birthday" column="birthday"/>
<result property="sex" column="sex"/>
<result property="address" column="address"/>
</association>
</resultMap>
<select id="findAll" resultMap="OrderMap">
select * from orders o inner join user u on o.user_id = u.id
</select>
<select id="findByMap" resultMap="OrderMap">
select * from orders o inner join user u on o.user_id = u.id
<where>
<if test="username != null">
AND username = #{username}
</if>
<if test="number != null">
AND number = #{number}
</if>
</where>
</select>
</mapper>
7） 單元測試：
public class Test1 {
SqlSessionFactory factory = null;
{
try {
factory = new SqlSessionFactoryBuilder().build(Resources.getResourceAsStream("config.xml"));
} catch (IOException e) {
e.printStackTrace();
}
}
@Test
public void testAddUser() {
SqlSession sqlSession = factory.openSession();
UserDAO userDAO = sqlSession.getMapper(UserDAO.class);
userDAO.addUser(new User(1,"張大四","1999-9-9","男","wuhan"));
sqlSession.commit();
sqlSession.close();
}
@Test
public void testFindAll(){
SqlSession sqlSession = factory.openSession();
OrderDAO orderDAO = sqlSession.getMapper(OrderDAO.class);
List<Order> orders = orderDAO.findAll();
System.out.println(orders);
sqlSession.close();
}
@Test
public void testFindMap(){
SqlSession sqlSession = factory.openSession();
OrderDAO orderDAO = sqlSession.getMapper(OrderDAO.class);
Map<String,String> where = new HashMap<>();
where.put("username","王五");
where.put("number","1000010");
List<Order> orders = orderDAO.findByMap(where);
System.out.println(orders);
sqlSession.close();
}
}
原始碼分析
接下來，我們從基本的MyBatis操作，看看MyBatis是如何完成一個數據庫操作的。
首先，建立SqlSessionFactoryBuilder物件，讀取配置檔案，然後建立DefaultSqlSessionFactory物件。
原始碼如下：
public SqlSessionFactory build(Reader reader, String environment, Properties properties) {
try {
//通過XMLConfigBuilder解析配置檔案，封裝為Configuration物件
XMLConfigBuilder parser = new XMLConfigBuilder(reader, environment, properties);
//建立DefaultSessionFactory物件
return build(parser.parse());
} catch (Exception e) {
throw ExceptionFactory.wrapException("Error building SqlSession.", e);
} finally {
ErrorContext.instance().reset();
try {
reader.close();
} catch (IOException e) {
// Intentionally ignore. Prefer previous error.
}
}
}
//build方法
public SqlSessionFactory build(Configuration config) {
return new DefaultSqlSessionFactory(config);
}
獲得SqlSessionFactory物件之後，通過openSession獲得SsqlSession。
原始碼如下：
private SqlSession openSessionFromDataSource(ExecutorType execType,
TransactionIsolationLevel level, boolean autoCommit) {
Transaction tx = null;
try {
//獲得配置的環境
final Environment environment = configuration.getEnvironment();
final TransactionFactory transactionFactory =
getTransactionFactoryFromEnvironment(environment);
tx = transactionFactory.newTransaction(environment.getDataSource(), level,
autoCommit);
//建立一個執行器去執行SQL命令
final Executor executor = configuration.newExecutor(tx, execType);
//返回DefaultSqlSession物件
return new DefaultSqlSession(configuration, executor, autoCommit);
} catch (Exception e) {
closeTransaction(tx); // may have fetched a connection so lets call close()
throw ExceptionFactory.wrapException("Error opening session. Cause: " + e, e);
} finally {
ErrorContext.instance().reset();
}
}
我們操作資料庫時需要先定義一個DAO的介面，然後通過SqlSession的getMapper獲得該介面的實現，那麼這個介面是如何實現的呢？
通過SqlSession從Configuration中獲取。
原始碼如下：
DefaultSqlSession:
public <T> T getMapper(Class<T> type) {
return configuration.<T>getMapper(type, this);
}
Configuration:
public <T> T getMapper(Class<T> type, SqlSession sqlSession) {
return mapperRegistry.getMapper(type, sqlSession);
}
MapperRegistry：
public <T> T getMapper(Class<T> type, SqlSession sqlSession) {
//獲得一個代理工廠
final MapperProxyFactory<T> mapperProxyFactory = (MapperProxyFactory<T>) knownMappers.get(type);
if (mapperProxyFactory == null) {
throw new BindingException("Type " + type + " is not known to the MapperRegistry.");
}
try {
//通過代理工廠建立代理
return mapperProxyFactory.newInstance(sqlSession);
} catch (Exception e) {
throw new BindingException("Error getting mapper instance. Cause: " + e, e);
}
}
MapperProxyFactory：
public T newInstance(SqlSession sqlSession) {
final MapperProxy<T> mapperProxy = new MapperProxy<T>(sqlSession, mapperInterface, methodCache);
return newInstance(mapperProxy);
}
protected T newInstance(MapperProxy<T> mapperProxy) {
//最後就是使用動態代理實現了DAO介面
return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[] { mapperInterface }, mapperProxy);
}
然後動態代理是如何執行SQL命令的呢，看程式碼：
MapperProxy:
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
if (Object.class.equals(method.getDeclaringClass())) {
try {
return method.invoke(this, args);
} catch (Throwable t) {
throw ExceptionUtil.unwrapThrowable(t);
}
}
final MapperMethod mapperMethod = cachedMapperMethod(method);
//主要是這裡執行對映的方法
return mapperMethod.execute(sqlSession, args);
}
MapperMethod:
public Object execute(SqlSession sqlSession, Object[] args) {
Object result;
//這邊就是判斷SQL的型別，然後分別執行增刪改查的操作
if (SqlCommandType.INSERT == command.getType()) {
Object param = method.convertArgsToSqlCommandParam(args);
result = rowCountResult(sqlSession.insert(command.getName(), param));
} else if (SqlCommandType.UPDATE == command.getType()) {
Object param = method.convertArgsToSqlCommandParam(args);
result = rowCountResult(sqlSession.update(command.getName(), param));
} else if (SqlCommandType.DELETE == command.getType()) {
Object param = method.convertArgsToSqlCommandParam(args);
result = rowCountResult(sqlSession.delete(command.getName(), param));
} else if (SqlCommandType.SELECT == command.getType()) {
if (method.returnsVoid() && method.hasResultHandler()) {
executeWithResultHandler(sqlSession, args);
result = null;
} else if (method.returnsMany()) {
result = executeForMany(sqlSession, args);
} else if (method.returnsMap()) {
result = executeForMap(sqlSession, args);
} else {
Object param = method.convertArgsToSqlCommandParam(args);
result = sqlSession.selectOne(command.getName(), param);
}
} else {
throw new BindingException("Unknown execution method for: " + command.getName());
}
if (result == null && method.getReturnType().isPrimitive() && !method.returnsVoid()) {
throw new BindingException("Mapper method '" + command.getName()
+ " attempted to return null from a method with a primitive return type (" + method.getReturnType() + ").");
}
return result;
}
看一下selectList的實現：
public <E> List<E> selectList(String statement, Object parameter, RowBounds rowBounds) {
try {
MappedStatement ms = configuration.getMappedStatement(statement);
//交給執行器區執行SQL
return executor.query(ms, wrapCollection(parameter),
rowBounds, Executor.NO_RESULT_HANDLER);
} catch (Exception e) {
throw ExceptionFactory.wrapException("Error querying database. Cause: " + e, e);
} finally {
ErrorContext.instance().reset();
}
}
Executor有多種實現，看看SimpleExecutor:
public <E> List<E> doQuery(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) throws SQLException {
Statement stmt = null;
try {
Configuration configuration = ms.getConfiguration();
StatementHandler handler = configuration.newStatementHandler(wrapper, ms, parameter, rowBounds, resultHandler, boundSql);
stmt = prepareStatement(handler, ms.getStatementLog());
//用JDBC的Statement去執行命令了
return handler.<E>query(stmt, resultHandler);
} finally {
closeStatement(stmt);
}
}
query方法的實現：
public <E> List<E> query(Statement statement, ResultHandler resultHandler) throws SQLException {
//獲得PreparedStatement，執行命令
PreparedStatement ps = (PreparedStatement) statement;
ps.execute();
//結果由ResultSetHandler 處理
return resultSetHandler.<E> handleResultSets(ps);
}
總結
最後我們梳理一下MyBatis整個執行過程：
- 通過SQLSessionFactoryBuilder建立SQLSessionFactory時，將核心配置檔案中configuration節點的內容，解析到SQLSessionFactory中
- 通過SQLSessionFactory獲得SqlSession時，返回DefaultSqlSession
- 呼叫SQLSessionFactory的getMapper方法時，返回了Mapper介面的動態代理物件，代理物件在invoke方法中實現了增刪改查操作
- 具體的增刪改查操作通過JDBC的Statement介面實現