MyBatis源码分析-SQL语句执行的完整流程

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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：

根据JDBC规范建立与数据库的连接。
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。

(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"/>
 
</settings> <typeAliases>
<typeAlias type="com.luoxn28.dao.User" alias="User"/>
</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://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}; 
</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：

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);
}
}