源码剖析-初始化
代码语言：javascript复制//获得核心配置文件
InputStream resourceAsStream = Resources.getResourceAsStream("sqlMapConfig.xml");
//获得session工厂对象,正是初始化⼯作的开始。
SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(resourceAsStream);
//获得session回话对象
SqlSession sqlSession = sqlSessionFactory.openSession();
//执行操作 参数：namespace id
List<User> userList = sqlSession.selectList("com.zjq.mapper.UserMapper.findAll");
//打印数据
System.out.println(userList);
//释放资源
sqlSession.close();源码分析
代码语言：javascript复制//1.开始进入的build方法
public SqlSessionFactory build(InputStream inputStream) {
//调用了build的重载方法
return build(inputStream, null, null);
}
//2.build的重载方法
public SqlSessionFactory build(InputStream inputStream, String environment, Properties properties) {
try {
// XMLConfigBuilder是XML配置构建器，专⻔解析mybatis配置⽂件的类
XMLConfigBuilder parser = new XMLConfigBuilder(inputStream, environment, properties);
//这⾥⼜调⽤了⼀个重载⽅法。parser.parse()的返回值是Configuration对象
return build(parser.parse());
} catch (Exception e) {
throw ExceptionFactory.wrapException("Error building SqlSession.", e);
} finally {
ErrorContext.instance().reset();
try {
inputStream.close();
} catch (IOException e) {
// Intentionally ignore. Prefer previous error.
}
}
}MyBatis在初始化的时候，会将MyBatis的配置信息全部加载到内存中，使用org.apache.ibatis.session.Configuration 实例来维护。 下⾯进⼊对配置⽂件解析部分：
Configuration
⾸先对Configuration对象进⾏介绍：
Configuration对象的结构和xml配置⽂件的对象⼏乎相同。 回顾⼀下xml中的配置标签有哪些： properties (属性)，settings (设置)，typeAliases (类型别名)，typeHandlers (类型处理器)，objectFactory (对象⼯⼚)，mappers (映射器)等 Configuration也有对应的对象属性来封装它们 也就是说，初始化配置⽂件信息的本质就是创建Configuration对象，将解析的xml数据封装到Configuration内部属性中。
继续跟踪上述的parse()方法
代码语言：javascript复制/**
*3.解析XML封装到Configuration对象中
**/
public Configuration parse() {
//若已解析，抛出BuilderException异常
if (parsed) {
throw new BuilderException("Each XMLConfigBuilder can only be used once.");
}
//标记已解析
parsed = true;
// 解析 XML中的 configuration 节点
parseConfiguration(parser.evalNode("/configuration"));
return configuration;
}
/**
*4.解析XML
**/
private void parseConfiguration(XNode root) {
try {
//issue #117 read properties first
// 解析 <properties/> 标签
propertiesElement(root.evalNode("properties"));
// 解析 <settings/> 标签
Properties settings = settingsAsProperties(root.evalNode("settings"));
//加载⾃定义的VFS实现类
loadCustomVfs(settings);
// 解析 <typeAliases/> 标签
typeAliasesElement(root.evalNode("typeAliases"));
// 解析 <plugins/> 标签
pluginElement(root.evalNode("plugins"));
// 解析 <objectFactory/> 标签
objectFactoryElement(root.evalNode("objectFactory"));
// 解析 <objectWrapperFactory/> 标签
objectWrapperFactoryElement(root.evalNode("objectWrapperFactory"));
// 解析 <reflectorFactory/> 标签
reflectorFactoryElement(root.evalNode("reflectorFactory"));
// 赋值 <settings/> ⾄ Configuration 属性
settingsElement(settings);
// read it after objectFactory and objectWrapperFactory issue #631
// 解析 <environments/> 标签
environmentsElement(root.evalNode("environments"));
// 解析 <databaseIdProvider/> 标签
databaseIdProviderElement(root.evalNode("databaseIdProvider"));
// 解析 <typeHandlers/> 标签
typeHandlerElement(root.evalNode("typeHandlers"));
// 解析 <mappers/> 标签
mapperElement(root.evalNode("mappers"));
} catch (Exception e) {
throw new BuilderException("Error parsing SQL Mapper Configuration. Cause: " e, e);
}
}MappedStatement
作⽤：MappedStatement与Mapper配置⽂件中的⼀个select/update/insert/delete节点相对应。mapper中配置的标签都被封装到了此对象中，主要⽤途是描述⼀条SQL语句。 初始化过程：回顾刚开 始介绍的加载配置⽂件的过程中，会对mybatis-config.xm l中的各个标签都进⾏ 解析，其中有mappers 标签⽤来引⼊mapper.xml⽂件或者配置mapper接⼝的⽬录。
代码语言：javascript复制<select id="getUser" resultType="user" >
select * from user where id=#{id}
</select>这样的⼀个select标签会在初始化配置⽂件时被解析封装成⼀个MappedStatement对象，然后存储在Configuration对象的mappedStatements属性中，mappedStatements 是⼀个HashMap，存储时key=全限定类名 ⽅法名，value =对应的MappedStatement对象。 在Configuration中对应的属性为
代码语言：javascript复制protected final Map<String, MappedStatement> mappedStatements = new StrictMap<MappedStatement>("Mapped Statements collection");mappers在 XMLConfigBuilder 中的处理：
代码语言：javascript复制private void parseConfiguration(XNode root) {
try {
//省略其他标签的处理
mapperElement(root.evalNode("mappers"));
} catch (Exception e) {
throw new BuilderException("Error parsing SQL Mapper Configuration.Cause:" e, e);
}
}到此对xml配置⽂件的解析就结束了，回到步骤2.中调⽤的重载build⽅法
代码语言：javascript复制// 5.调⽤的重载⽅法
public SqlSessionFactory build(Configuration config) {
//创建了 DefaultSqlSessionFactory 对象，传⼊ Configuration 对象。
return new DefaultSqlSessionFactory(config);
}源码剖析-执行SQL流程
SqlSession
SqlSession是⼀个接⼝，它有两个实现类：DefaultSqlSession (默认)和SqlSessionManager (弃⽤，不做介绍) SqlSession是MyBatis中⽤于和数据库交互的顶层类，通常将它与ThreadLocal绑定，⼀个会话使⽤⼀个SqlSession,并且在使⽤完毕后需要close。
代码语言：javascript复制public class DefaultSqlSession implements SqlSession {
private final Configuration configuration;
private final Executor executor;
}SqlSession中的两个最重要的参数，configuration与初始化时的相同，Executor为执⾏器 Executor： Executor也是⼀个接⼝，他有三个常⽤的实现类： BatchExecutor (重⽤语句并执⾏批量更新) ReuseExecutor (重⽤预处理语句 prepared statements) SimpleExecutor (普通的执⾏器，默认) 继续分析，初始化完毕后，我们就要执⾏SQL 了
代码语言：javascript复制//获得session回话对象
SqlSession sqlSession = sqlSessionFactory.openSession();
//执行操作 参数：namespace id
List<User> userList = sqlSession.selectList("userMapper.findAll");获得 sqlSession
SqlSessionFactory
代码语言：javascript复制public class DefaultSqlSessionFactory implements SqlSessionFactory {
//6.进入openSession方法
@Override
public SqlSession openSession() {
//getDefaultExecutorType()传递的是SimpleExecutor
return openSessionFromDataSource(configuration.getDefaultExecutorType(), null, false);
}
//7.进入openSessionFromDataSource，从数据源创建会话
//ExecutorType为Executor的类型，TransactionIsolationLevel为事务隔离级别，autoCommit是否开启事务
//openSession的多个重载⽅法可以指定获得的SqlSession的Executor类型和事务的处理
private SqlSession openSessionFromDataSource(ExecutorType execType, TransactionIsolationLevel level, boolean autoCommit) {
Transaction tx = null;
try {
final Environment environment = configuration.getEnvironment();
final TransactionFactory transactionFactory = getTransactionFactoryFromEnvironment(environment);
tx = transactionFactory.newTransaction(environment.getDataSource(), level, autoCommit);
//根据参数创建指定类型的Executor
final Executor executor = configuration.newExecutor(tx, execType);
//返回DefaultSqlSession
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();
}
}
}执⾏ sqlsession 中的 api
DefaultSqlSession
代码语言：javascript复制public class DefaultSqlSession implements SqlSession {
//8.进⼊selectList⽅法，多个重载⽅法。
public <E > List < E > selectList(String statement) {
return this.selectList(statement, null);
}
public <E > List < E > selectList(String statement, Object parameter)
{
return this.selectList(statement, parameter, RowBounds.DEFAULT);
}
public <E > List < E > selectList(String statement, Object
parameter, RowBounds rowBounds) {
try {
//根据传⼊的全限定名 ⽅法名从映射的Map中取出MappedStatement对象
MappedStatement ms = configuration.getMappedStatement(statement);
//调⽤Executor中的⽅法处理
//RowBounds是⽤来逻辑分⻚
// wrapCollection(parameter)是⽤来装饰集合或者数组参数
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();
}
}
}源码剖析-Executor
继续源码中的步骤，进⼊executor.query()
BaseExecutor
代码语言：javascript复制public abstract class BaseExecutor implements Executor {
//9.此⽅法在SimpleExecutor的⽗类BaseExecutor中实现
@Override
public <E> List<E> query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler) throws SQLException {
//根据传⼊的参数动态获得SQL语句，最后返回⽤BoundSql对象表示
BoundSql boundSql = ms.getBoundSql(parameter);
//为本次查询创建缓存的Key
CacheKey key = createCacheKey(ms, parameter, rowBounds, boundSql);
//进入重载方法
return query(ms, parameter, rowBounds, resultHandler, key, boundSql);
}
//10.进⼊query的重载⽅法中
@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 ;
list = resultHandler == null ? (List<E>) localCache.getObject(key) : null;
if (list != null) {
handleLocallyCachedOutputParameters(ms, key, parameter, boundSql);
} else {
//如果缓存中没有本次查找的值，那么从数据库中查询
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;
}
//11.从数据库查询
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 {
//执行查询
list = doQuery(ms, parameter, rowBounds, resultHandler, boundSql);
} finally {
localCache.removeObject(key);
}
//将查询结果放⼊缓存
localCache.putObject(key, list);
if (ms.getStatementType() == StatementType.CALLABLE) {
localOutputParameterCache.putObject(key, parameter);
}
return list;
}
}在SimpleExecutor实现对应的查询
SimpleExecutor
代码语言：javascript复制public class SimpleExecutor extends BaseExecutor {
//12.SimpleExecutor中实现⽗类的doQuery抽象⽅法
@Override
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();
//传⼊参数创建StatementHanlder对象来执⾏查询
StatementHandler handler = configuration.newStatementHandler(wrapper, ms, parameter, rowBounds, resultHandler, boundSql);
//创建jdbc中的statement对象
stmt = prepareStatement(handler, ms.getStatementLog());
// StatementHandler 进⾏处理
return handler.<E>query(stmt, resultHandler);
} finally {
closeStatement(stmt);
}
}
//13.创建Statement的⽅法
private Statement prepareStatement(StatementHandler handler, Log statementLog) throws SQLException {
Statement stmt;
//这条代码中的getConnection⽅法经过重重调⽤最后会调⽤openConnection⽅法，从连接池中获得连接。
Connection connection = getConnection(statementLog);
stmt = handler.prepare(connection, transaction.getTimeout());
//进⼊到 StatementHandler 的 parameterize(statement)⽅法
handler.parameterize(stmt);
return stmt;
}
}public class JdbcTransaction implements Transaction {
//14.从连接池中获取连接的方法
protected void openConnection() throws SQLException {
if (log.isDebugEnabled()) {
log.debug("Opening JDBC Connection");
}
//从连接池获得连接
connection = dataSource.getConnection();
if (level != null) {
connection.setTransactionIsolation(level.getLevel());
}
setDesiredAutoCommit(autoCommmit);
}
}上述的Executor.query()⽅法⼏经转折，最后会创建⼀个StatementHandler对象，然后将必要的参数传递给 StatementHandler，使⽤StatementHandler来完成对数据库的查询，最终返回List结果集。 从上⾯的代码中我们可以看出，Executor的功能和作用是：
- 根据传递的参数，完成SQL语句的动态解析，⽣成BoundSql对象，供StatementHandler使⽤；
- 为查询创建缓存，以提⾼性能
- 创建JDBC的Statement连接对象，传递给 StatementHandler对象，返回List查询结果。
源码剖析-StatementHandler
StatementHandler对象主要完成两个⼯作：
- 对于JDBC的PreparedStatement类型的对象，创建的过程中，我们使⽤的是SQL语句字符串会包含若⼲个？占位符，我们其后再对占位符进⾏设值。StatementHandler通过parameterize(statement)⽅法对 Statement 进⾏设值；
- StatementHandler 通过 List query(Statement statement, ResultHandler resultHandler)⽅法来完成执⾏Statement，和将Statement对象返回的resultSet封装成List；
进⼊到 StatementHandler 的 parameterize(statement)⽅法的实现：
代码语言：javascript复制public interface StatementHandler {
void parameterize(Statement statement)
throws SQLException;
}
//15.StatementHandler 的 parameterize(statement)⽅法实现
public class PreparedStatementHandler extends BaseStatementHandler {
@Override
public void parameterize(Statement statement) throws SQLException {
//使⽤ParameterHandler对象来完成对Statement的设值
parameterHandler.setParameters((PreparedStatement) statement);
}
}ParameterHandler
代码语言：javascript复制public class DefaultParameterHandler implements ParameterHandler {
/** 16.ParameterHandler 类的 setParameters(PreparedStatement ps) 实现
* 对某⼀个Statement进⾏设置参数
* */
@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);
}
// 每⼀个 Mapping都有⼀个 TypeHandler，根据 TypeHandler 来对preparedStatement 进 ⾏设置参数
TypeHandler typeHandler = parameterMapping.getTypeHandler();
JdbcType jdbcType = parameterMapping.getJdbcType();
if (value == null && jdbcType == null) {
jdbcType = configuration.getJdbcTypeForNull();
}
try {
//设置参数
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);
}
}
}
}
}
}从上述的代码可以看到,StatementHandler的parameterize(Statement)⽅法调⽤了ParameterHandler的setParameters(statement)⽅法，ParameterHandler的setParameters(Statement )⽅法负责根据我们输⼊的参数，对statement对象的?占位符处进⾏赋值。进⼊到StatementHandler 的 List query(Statement statement, ResultHandler resultHandler)⽅法的实现：
StatementHandler
代码语言：javascript复制public interface StatementHandler {
<E> List<E> query(Statement statement, ResultHandler resultHandler)
throws SQLException;
}
public class PreparedStatementHandler extends BaseStatementHandler {
//17.进入query方法
@Override
public <E> List<E> query(Statement statement, ResultHandler resultHandler) throws SQLException {
// 1.调⽤preparedStatemnt.execute()⽅法，然后将resultSet交给ResultSetHandler处理
PreparedStatement ps = (PreparedStatement) statement;
ps.execute();
//2.使⽤ ResultHandler 来处理 ResultSet
return resultSetHandler.<E> handleResultSets(ps);
}
}从上述代码我们可以看出，StatementHandler 的List query(Statement statement, ResultHandler resultHandler)⽅法的实现，是调⽤了 ResultSetHandler 的 handleResultSets(Statement)⽅法。 ResultSetHandler 的 handleResultSets(Statement)⽅法会将 Statement 语句执⾏后⽣成的 resultSet结 果集转换成List结果集。
ResultSetHandler
代码语言：javascript复制public interface ResultSetHandler {
<E> List<E> handleResultSets(Statement stmt) throws SQLException;
}
public class DefaultResultSetHandler implements ResultSetHandler {
//18.进入handleResultSets方法
@Override
public List<Object> handleResultSets(Statement stmt) throws SQLException {
ErrorContext.instance().activity("handling results").object(mappedStatement.getId());
//多ResultSet的结果集合，每个ResultSet对应⼀个Object对象。⽽实际上，每 个 Object 是List<Object> 对象。
//在不考虑存储过程的多ResultSet的情况，普通的查询，实际就⼀个ResultSet，也 就是说，multipleResults最多就⼀个元素。
final List<Object> multipleResults = new ArrayList<Object>();
int resultSetCount = 0;
//获得⾸个ResultSet对象，并封装成ResultSetWrapper对象
ResultSetWrapper rsw = getFirstResultSet(stmt);
//获得ResultMap数组
//在不考虑存储过程的多ResultSet的情况，普通的查询，实际就⼀个ResultSet，也 就是说，resultMaps就⼀个元素。
List<ResultMap> resultMaps = mappedStatement.getResultMaps();
int resultMapCount = resultMaps.size();
// 校验
validateResultMapsCount(rsw, resultMapCount);
while (rsw != null && resultMapCount > resultSetCount) {
//获得ResultMap对象
ResultMap resultMap = resultMaps.get(resultSetCount);
//处理ResultSet，将结果添加到multipleResults中
handleResultSet(rsw, resultMap, multipleResults, null);
//获得下⼀个ResultSet对象，并封装成ResultSetWrapper对象
rsw = getNextResultSet(stmt);
//清理
cleanUpAfterHandlingResultSet();
// resultSetCount
resultSetCount ;
}
//因为'mappedStatement.resultSets'只在存储过程中使⽤，暂时不考虑
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 ;
}
}
//如果是multipleResults单元素，则取⾸元素返回
return collapseSingleResultList(multipleResults);
}
}
