MyBatis源码分析-SQL语句执行的完整流程

时间:2023-03-08 16:46:06

  MyBatis 是支持定制化 SQL、存储过程以及高级映射的优秀的持久层框架。MyBatis 避免了几乎所有的 JDBC 代码和手动设置参数以及获取结果集。MyBatis 可以对配置和原生Map使用简单的 XML 或注解,将接口和 Java 的 POJOs(Plain Old Java Objects,普通的 Java对象)映射成数据库中的记录。如何新建MyBatis源码工程请点击MyBatis源码分析-IDEA新建MyBatis源码工程

  MyBatis框架主要完成的是以下2件事情:

  1. 根据JDBC规范建立与数据库的连接。
  2. 通过反射打通Java对象与数据库参数交互之间相互转换的关系。

  MyBatis框架是一种典型的交互式框架,先准备好交互的必要条件,然后构建一个交互的环境,在交互环境中划分会话,在会话中与数据库进行交互数据。

1 MyBatis主要的类

  • Configuration        MyBatis所有的配置信息都维持在Configuration对象之中。
  • SqlSession            作为MyBatis工作的主要顶层API,表示和数据库交互的会话,完成必要数据库增删改查功能
  • Executor               MyBatis执行器,是MyBatis 调度的核心,负责SQL语句的生成和查询缓存的维护
  • StatementHandler 封装了JDBC Statement操作,负责对JDBC statement 的操作,如设置参数、将Statement结果集转换成List集合。
  • ParameterHandler  负责对用户传递的参数转换成JDBC Statement 所需要的参数,
  • ResultSetHandler   负责将JDBC返回的ResultSet结果集对象转换成List类型的集合;
  • TypeHandler          负责java数据类型和jdbc数据类型之间的映射和转换
  • MappedStatement  MappedStatement维护了一条<select|update|delete|insert>节点的封装,
  • SqlSource              负责根据用户传递的parameterObject,动态地生成SQL语句,将信息封装到BoundSql对象中,并返回
  • BoundSql              表示动态生成的SQL语句以及相应的参数信息

  以上几个类在SQL操作中都会涉及,在SQL操作中重点关注下SQL参数什么时候写入和结果集怎么转换为Java对象,这两个过程正好对应的类是PreparedStatementHandler和ResultSetHandler类。

MyBatis源码分析-SQL语句执行的完整流程

(图片来自《深入理解mybatis原理》 MyBatis的架构设计以及实例分析)

2 SQL执行流程

  MyBatis主要设计目的还是为了让我们在执行SQL时对输入输出的数据的管理更加方便,所以方便的让我们写出SQL和方便的获取SQL的执行结果是MyBatis的核心竞争力。下面就用一个例子来从源码角度看一下SQL的完整执行流程。

新建配置文件conf.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="cacheEnabled" value="true"/>
<setting name="lazyLoadingEnabled" value="false"/>
<!--setting name="logImpl" value="STDOUT_LOGGING"/--> <!-- 日志 -->
</settings> <typeAliases>
<typeAlias type="com.luoxn28.dao.User" alias="User"/>
</typeAliases> <environments default="development">
<environment id="development">
<transactionManager type="JDBC" /> <!-- 声明使用那种事务管理机制 JDBC/MANAGED -->
<!-- 配置数据库连接信息 -->
<dataSource type="POOLED">
<property name="driver" value="com.mysql.jdbc.Driver" />
<property name="url" value="jdbc:mysql://192.168.1.150:3306/xxx" />
<property name="username" value="xxx" />
<property name="password" value="xxx" />
</dataSource>
</environment>
</environments> <mappers>
<mapper resource="userMapper.xml"/>
</mappers> </configuration>

conf.xml

首先建立数据表,这里就以user表为例 :

DROP TABLE IF EXISTS user;
CREATE TABLE user (
id INT NOT NULL PRIMARY KEY AUTO_INCREMENT,
name VARCHAR(32) NOT NULL,
password VARCHAR(32) NOT NULL,
sex int,
email VARCHAR(32),
phone VARCHAR(16),
admin VARCHAR(16)
);

然后新建与数据表对应的类User:

/**
* User - 用户类
*/
public class User { public static final int MAN = 0; // 男生
public static final int WOMAN = 1; // 女生
public static final int OTHER = 2; // 其他 private int id; // 用户id
private String name; // 用户名
private String password; // 用户密码
private int sex; // 用户性别
private String email; // 用户邮箱
private String phone; // 用户手机
private String admin; // 用户是否是管理员,"admin"表示是管理员,其他为普通用户 public User() { } public User(String name, String password, int sex, String email, String phone) {
this.name = name;
this.password = password;
this.sex = sex;
this.email = email;
this.phone = phone;
this.admin = "";
}
public User(String name, String password, String sex, String email, String phone) {
this.name = name;
this.password = password;
setSex(sex); // this.sex = sex;
this.email = email;
this.phone = phone;
this.admin = "";
} 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 String getPassword() {
return password;
} public void setPassword(String password) {
this.password = password;
} public int getSex() {
return sex;
} public void setSex(int sex) {
this.sex = sex;
} public void setSex(String sexStr) {
int sex = Integer.valueOf(sexStr);
switch (Integer.valueOf(sexStr)) {
case 0: {
this.sex = MAN;
break;
}
case 1: {
this.sex = WOMAN;
break;
}
default: {
this.sex = OTHER;
break;
}
}
} public String getEmail() {
return email;
} public void setEmail(String email) {
this.email = email;
} public String getPhone() {
return phone;
} public void setPhone(String phone) {
this.phone = phone;
} public String getAdmin() {
return admin;
} public void setAdmin(String admin) {
this.admin = admin;
} @Override
public String toString() {
return "User{" +
"id=" + id +
", name='" + name + '\'' +
", password='" + password + '\'' +
", sex=" + sex +
", email='" + email + '\'' +
", phone='" + phone + '\'' +
", admin='" + admin + '\'' +
'}';
} }

User

再新建usre表的配置文件:

<?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.luoxn28.dao.UserDao"> <select id="getById" parameterType="int" resultType="User">
SELECT * FROM user WHERE id=#{id}; <!-- #{xxx} xxx为类中的数据域名称 -->
</select> <select id="getAll" resultType="com.luoxn28.dao.User">
SELECT * FROM user;
</select>
</mapper>

userMapper.xml

最后新建测试类:

/**
* MyBatis测试类
*/
public class TestMain { public static void main(String[] args) throws IOException {
String resouce = "conf.xml";
InputStream is = Resources.getResourceAsStream(resouce); // 构建sqlSession工厂
SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(is);
// 获取sqlSession
SqlSession session = sqlSessionFactory.openSession(); User user; try {
/**
* 第一种方式: 直接执行已映射的 SQL 语句
*/
String statement = "com.luoxn28.dao.UserDao.getById";
user = session.selectOne(statement, 1);
System.out.println(user);
}
finally {
session.close();
} /**
* 第二种方式: 执行更清晰和类型安全的代码
*/
// UserDao userDao = session.getMapper(UserDao.class);
// user = userDao.getById(1);
// System.out.println(user);
} }

  由于我们分析的是SQL的执行流程,那就重点关注下 user = session.selectOne(statement, 1); 这行代码~ 注意,传进去的参数是1。

  session是DefaultSqlSession类型的,因为sqlSessionFactory默认生成的SqlSession是DefaultSqlSession类型。selectOne()会调用selectList()。

// DefaultSqlSession类
public <E> List<E> selectList(String statement, Object parameter, RowBounds rowBounds) {
try {
MappedStatement ms = configuration.getMappedStatement(statement);
// CURD操作是交给Excetor去处理的
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();
}
}

  在DefaultSqlSession.selectList中的各种CURD操作都是通多Executor进行的,这里executor的类型是CachingExecutor,接着跳转到其中的query方法中。

// CachingExecutor 类
public <E> List<E> query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler) throws SQLException {
BoundSql boundSql = ms.getBoundSql(parameterObject); // 获取绑定的sql命令,比如"SELECT * FROM xxx"
CacheKey key = createCacheKey(ms, parameterObject, rowBounds, boundSql);
return query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
}

  getBoundSql为了获取绑定的sql命令,在创建完cacheKey之后,就进入到CachingExecutor 类中的另一个query方法中。

// CachingExecutor 类
@Override
public <E> List<E> query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql)
throws SQLException {
Cache cache = ms.getCache();
if (cache != null) {
flushCacheIfRequired(ms);
if (ms.isUseCache() && resultHandler == null) {
ensureNoOutParams(ms, parameterObject, boundSql);
@SuppressWarnings("unchecked")
List<E> list = (List<E>) tcm.getObject(cache, key);
if (list == null) {
list = delegate.<E> query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
tcm.putObject(cache, key, list); // issue #578 and #116
}
return list;
}
}
return delegate.<E> query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
}

  这里真正执行query操作的是SimplyExecutor代理来完成的,接着就进入到了SimplyExecutor的父类BaseExecutor的query方法中。

// SimplyExecutor的父类BaseExecutor类
@SuppressWarnings("unchecked")
@Override
public <E> List<E> query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException {
ErrorContext.instance().resource(ms.getResource()).activity("executing a query").object(ms.getId());
if (closed) {
throw new ExecutorException("Executor was closed.");
}
if (queryStack == 0 && ms.isFlushCacheRequired()) {
clearLocalCache();
}
List<E> list;
try {
queryStack++;
/**
* localCache是一级缓存,如果找不到就调用queryFromDatabase从数据库中查找
*/
list = resultHandler == null ? (List<E>) localCache.getObject(key) : null;
if (list != null) {
handleLocallyCachedOutputParameters(ms, key, parameter, boundSql);
} else {
list = queryFromDatabase(ms, parameter, rowBounds, resultHandler, key, boundSql);
}
} finally {
queryStack--;
}
if (queryStack == 0) {
for (DeferredLoad deferredLoad : deferredLoads) {
deferredLoad.load();
}
// issue #601
deferredLoads.clear();
if (configuration.getLocalCacheScope() == LocalCacheScope.STATEMENT) {
// issue #482
clearLocalCache();
}
}
return list;
}

  因为是第一次SQL查询操作,所以会调用queryFromDatabase方法来执行查询。

// SimplyExecutor的父类BaseExecutor类
private <E> List<E> queryFromDatabase(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException {
List<E> list;
localCache.putObject(key, EXECUTION_PLACEHOLDER);
try {
list = doQuery(ms, parameter, rowBounds, resultHandler, boundSql);
} finally {
localCache.removeObject(key);
}
localCache.putObject(key, list);
if (ms.getStatementType() == StatementType.CALLABLE) {
localOutputParameterCache.putObject(key, parameter);
}
return list;
}

  从数据库中查询数据,进入到SimplyExecutor中进行操作。

// SimplyExecutor类
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);
// 子流程1:SQL查询参数的设置
stmt = prepareStatement(handler, ms.getStatementLog());
// StatementHandler封装了Statement
// 子流程2:SQL查询操作和结果集的封装
return handler.<E>query(stmt);
} finally {
closeStatement(stmt);
}
}

  注意,在prepareStatement方法中会进行SQL查询参数的设置,也就是咱们最开始传递进来的参数,其值为1。handler.<E>query(stmt)方法中会进行实际的SQL查询操作和结果集的封装(封装成Java对象)。当流程走到这里时,程序已经压栈有一定深度了,因为接下来程序分析会兵分两路,一方面深入到SQL查询及结果集的设置子流程1中,然后再深入到SQL查询操作和结果集的封装子流程2,因为还会回到这里,所以就来一张调用栈的特写吧:

MyBatis源码分析-SQL语句执行的完整流程

子流程1:SQL查询参数的设置

// SimplyExecutor类
private Statement prepareStatement(StatementHandler handler, Log statementLog) throws SQLException {
Statement stmt;
// 获取一个Connection
Connection connection = getConnection(statementLog);
stmt = handler.prepare(connection, transaction.getTimeout());
handler.parameterize(stmt); // 设置SQL查询中的参数值
return stmt;
}

  通过getConnection方法来获取一个Connection,调用prepare方法来获取一个Statement(这里的handler类型是RoutingStatementHandler,RoutingStatementHandler的prepare方法调用的是PrepareStatementHandler的prepare方法,因为PrepareStatementHandler并没有覆盖其父类的prepare方法,其实最后调用的是BaseStatementHandler中的prepare方法。是不是绕晕了,那就再看一遍吧 :) )。调用parameterize方法来设置SQL的参数值(这里最后调用的是PrepareStatementHandler中的parameterize方法,而PrepareStatementHandler.parameterize方法调用的是DefaultParameterHandler中的setParameters方法)。

// PrepareStatementHandler类
@Override
public void parameterize(Statement statement) throws SQLException {
parameterHandler.setParameters((PreparedStatement) statement);
}
// DefaultParameterHandler类
@Override
public void setParameters(PreparedStatement ps) {
ErrorContext.instance().activity("setting parameters").object(mappedStatement.getParameterMap().getId());
List<ParameterMapping> parameterMappings = boundSql.getParameterMappings();
if (parameterMappings != null) {
for (int i = 0; i < parameterMappings.size(); i++) {
ParameterMapping parameterMapping = parameterMappings.get(i);
if (parameterMapping.getMode() != ParameterMode.OUT) {
Object value;
String propertyName = parameterMapping.getProperty();
if (boundSql.hasAdditionalParameter(propertyName)) { // issue #448 ask first for additional params
value = boundSql.getAdditionalParameter(propertyName);
} else if (parameterObject == null) {
value = null;
} else if (typeHandlerRegistry.hasTypeHandler(parameterObject.getClass())) {
value = parameterObject;
} else {
MetaObject metaObject = configuration.newMetaObject(parameterObject);
value = metaObject.getValue(propertyName);
}
TypeHandler typeHandler = parameterMapping.getTypeHandler();
JdbcType jdbcType = parameterMapping.getJdbcType();
if (value == null && jdbcType == null) {
jdbcType = configuration.getJdbcTypeForNull();
}
try {
typeHandler.setParameter(ps, i + 1, value, jdbcType);
} catch (TypeException e) {
throw new TypeException("Could not set parameters for mapping: " + parameterMapping + ". Cause: " + e, e);
} catch (SQLException e) {
throw new TypeException("Could not set parameters for mapping: " + parameterMapping + ". Cause: " + e, e);
}
}
}
}
}

  到这里为止,已经给Statement设置了最初传递进去的参数(值为1)了,那么接着分析流程2:

流程2:SQL查询及结果集的设置

// RoutingStatementHandler类
@Override
public <E> List<E> query(Statement statement) throws SQLException {
return delegate.<E>query(statement);
}
// RoutingStatementHandler类
@Override
public <E> List<E> query(Statement statement) throws SQLException {
// 这里就到了熟悉的PreparedStatement了
PreparedStatement ps = (PreparedStatement) statement;
// 执行SQL查询操作
ps.execute();
// 结果交给ResultHandler来处理
return resultSetHandler.<E> handleResultSets(ps);
}
// DefaultResultSetHandler类(封装返回值,将查询结果封装成Object对象)
@Override
public List<Object> handleResultSets(Statement stmt) throws SQLException {
ErrorContext.instance().activity("handling results").object(mappedStatement.getId()); final List<Object> multipleResults = new ArrayList<Object>(); int resultSetCount = 0;
ResultSetWrapper rsw = getFirstResultSet(stmt); List<ResultMap> resultMaps = mappedStatement.getResultMaps();
int resultMapCount = resultMaps.size();
validateResultMapsCount(rsw, resultMapCount);
while (rsw != null && resultMapCount > resultSetCount) {
ResultMap resultMap = resultMaps.get(resultSetCount);
handleResultSet(rsw, resultMap, multipleResults, null);
rsw = getNextResultSet(stmt);
cleanUpAfterHandlingResultSet();
resultSetCount++;
} String[] resultSets = mappedStatement.getResultSets();
if (resultSets != null) {
while (rsw != null && resultSetCount < resultSets.length) {
ResultMapping parentMapping = nextResultMaps.get(resultSets[resultSetCount]);
if (parentMapping != null) {
String nestedResultMapId = parentMapping.getNestedResultMapId();
ResultMap resultMap = configuration.getResultMap(nestedResultMapId);
handleResultSet(rsw, resultMap, null, parentMapping);
}
rsw = getNextResultSet(stmt);
cleanUpAfterHandlingResultSet();
resultSetCount++;
}
} return collapseSingleResultList(multipleResults);
}

  ResultSetWrapper是ResultSet的包装类,调用getFirstResultSet方法获取第一个ResultSet,同时获取数据库的MetaData数据,包括数据表列名、列的类型、类序号等,这些信息都存储在ResultSetWrapper类中了。然后调用handleResultSet方法来来进行结果集的封装。

// DefaultResultSetHandler类
private void handleResultSet(ResultSetWrapper rsw, ResultMap resultMap, List<Object> multipleResults, ResultMapping parentMapping) throws SQLException {
try {
if (parentMapping != null) {
handleRowValues(rsw, resultMap, null, RowBounds.DEFAULT, parentMapping);
} else {
if (resultHandler == null) {
DefaultResultHandler defaultResultHandler = new DefaultResultHandler(objectFactory);
handleRowValues(rsw, resultMap, defaultResultHandler, rowBounds, null);
multipleResults.add(defaultResultHandler.getResultList());
} else {
handleRowValues(rsw, resultMap, resultHandler, rowBounds, null);
}
}
} finally {
// issue #228 (close resultsets)
closeResultSet(rsw.getResultSet());
}
}

  这里调用handleRowValues方法来进行值的设置:

// DefaultResultSetHandler类
public void handleRowValues(ResultSetWrapper rsw, ResultMap resultMap, ResultHandler<?> resultHandler, RowBounds rowBounds, ResultMapping parentMapping) throws SQLException {
if (resultMap.hasNestedResultMaps()) {
ensureNoRowBounds();
checkResultHandler();
handleRowValuesForNestedResultMap(rsw, resultMap, resultHandler, rowBounds, parentMapping);
} else {
// 封装数据
handleRowValuesForSimpleResultMap(rsw, resultMap, resultHandler, rowBounds, parentMapping);
}
}
// DefaultResultSetHandler类
// 封装数据
private void handleRowValuesForSimpleResultMap(ResultSetWrapper rsw, ResultMap resultMap, ResultHandler<?> resultHandler, RowBounds rowBounds, ResultMapping parentMapping)
throws SQLException {
DefaultResultContext<Object> resultContext = new DefaultResultContext<Object>();
skipRows(rsw.getResultSet(), rowBounds);
while (shouldProcessMoreRows(resultContext, rowBounds) && rsw.getResultSet().next()) {
ResultMap discriminatedResultMap = resolveDiscriminatedResultMap(rsw.getResultSet(), resultMap, null);
Object rowValue = getRowValue(rsw, discriminatedResultMap);
storeObject(resultHandler, resultContext, rowValue, parentMapping, rsw.getResultSet());
}
}
// DefaultResultSetHandler类
private Object getRowValue(ResultSetWrapper rsw, ResultMap resultMap) throws SQLException {
final ResultLoaderMap lazyLoader = new ResultLoaderMap();
// createResultObject为新创建的对象,数据表对应的类
Object resultObject = createResultObject(rsw, resultMap, lazyLoader, null);
if (resultObject != null && !hasTypeHandlerForResultObject(rsw, resultMap.getType())) {
final MetaObject metaObject = configuration.newMetaObject(resultObject);
boolean foundValues = !resultMap.getConstructorResultMappings().isEmpty();
if (shouldApplyAutomaticMappings(resultMap, false)) {
// 这里把数据填充进去,metaObject中包含了resultObject信息
foundValues = applyAutomaticMappings(rsw, resultMap, metaObject, null) || foundValues;
}
foundValues = applyPropertyMappings(rsw, resultMap, metaObject, lazyLoader, null) || foundValues;
foundValues = lazyLoader.size() > 0 || foundValues;
resultObject = foundValues ? resultObject : null;
return resultObject;
}
return resultObject;
}
// DefaultResultSetHandler类(把ResultSet中查询结果填充到JavaBean中)
private boolean applyAutomaticMappings(ResultSetWrapper rsw, ResultMap resultMap, MetaObject metaObject, String columnPrefix) throws SQLException {
List<UnMappedColumnAutoMapping> autoMapping = createAutomaticMappings(rsw, resultMap, metaObject, columnPrefix);
boolean foundValues = false;
if (autoMapping.size() > 0) {
// 这里进行for循环调用,因为user表中总共有7项,所以也就调用7次
for (UnMappedColumnAutoMapping mapping : autoMapping) {
// 这里将esultSet中查询结果转换为对应的实际类型
final Object value = mapping.typeHandler.getResult(rsw.getResultSet(), mapping.column);
if (value != null) {
foundValues = true;
}
if (value != null || (configuration.isCallSettersOnNulls() && !mapping.primitive)) {
// gcode issue #377, call setter on nulls (value is not 'found')
metaObject.setValue(mapping.property, value);
}
}
}
return foundValues;
}

  mapping.typeHandler.getResult会获取查询结果值的实际类型,比如我们user表中id字段为int类型,那么它就对应Java中的Integer类型,然后通过调用statement.getInt("id")来获取其int值,其类型为Integer。metaObject.setValue方法会把获取到的Integer值设置到Java类中的对应字段。

// MetaObject类
public void setValue(String name, Object value) {
PropertyTokenizer prop = new PropertyTokenizer(name);
if (prop.hasNext()) {
MetaObject metaValue = metaObjectForProperty(prop.getIndexedName());
if (metaValue == SystemMetaObject.NULL_META_OBJECT) {
if (value == null && prop.getChildren() != null) {
// don't instantiate child path if value is null
return;
} else {
metaValue = objectWrapper.instantiatePropertyValue(name, prop, objectFactory);
}
}
metaValue.setValue(prop.getChildren(), value);
} else {
objectWrapper.set(prop, value);
}
}

  metaValue.setValue方法最后会调用到Java类中对应数据域的set方法,这样也就完成了SQL查询结果集的Java类封装过程。最后贴一张调用栈到达Java类的set方法中的快照:

MyBatis源码分析-SQL语句执行的完整流程

参考:

  1、MyBatis源码

  2、《深入分析Java Web技术内幕》的iBatis章节

  3、《深入理解mybatis原理》 MyBatis的架构设计以及实例分析

  4、luoxn28/tuiku

  5、Java JDBC基础学习小结