详解MyBatis中Executor执行SQL语句的过程

<settings>
    <setting name="cacheEnabled" value="false"/>
</settings>

public interface BookMapper {
    Book selectBookById(int id);
}

<?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.mybatis.learn.dao.BookMapper">
    <resultMap id="bookResultMap" type="com.mybatis.learn.entity.Book">
        <result property="bookName" column="b_name"/>
        <result property="bookPrice" column="b_price"/>
    </resultMap>
    <select id="selectBookById" resultMap="bookResultMap">
        SELECT
            b.id, 
            b.b_name, 
            b.b_price
        FROM book b
        WHERE b.id=#{id}
    </select>
</mapper>

public class MybatisTest {
    public static void main(String[] args) throws Exception {
        String resource = "mybatis-config.xml";
        SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder()
                .build(Resources.getResourceAsStream(resource));
        // 获取SqlSession
        SqlSession sqlSession = sqlSessionFactory.openSession();
        // 获取映射接口的动态代理对象
        BookMapper bookMapper = sqlSession.getMapper(BookMapper.class);
        // 执行一次查询操作
        System.out.println(bookMapper.selectBookById(1));
    }
}

public Object execute(SqlSession sqlSession, Object[] args) {
    Object result;
    switch (command.getType()) {
        // ......
        case SELECT:
            // 根据实际执行的方法的返回值的情况进入不同的逻辑分支
            if (method.returnsVoid() && method.hasResultHandler()) {
                // 无返回值情况
                executeWithResultHandler(sqlSession, args);
                result = null;
            } else if (method.returnsMany()) {
                // 返回值为集合的情况
                result = executeForMany(sqlSession, args);
            } else if (method.returnsMap()) {
                // 返回值为map的情况
                result = executeForMap(sqlSession, args);
            } else if (method.returnsCursor()) {
                // 返回值为迭代器的情况
                result = executeForCursor(sqlSession, args);
            } else {
                // 上述情况之外的情况
                // 将方法的入参转换为Sql语句的参数
                Object param = method.convertArgsToSqlCommandParam(args);
                // 调用DefaultSqlSession的selectOne()方法执行查询操作
                result = sqlSession.selectOne(command.getName(), param);
                if (method.returnsOptional()
                        && (result == null || !method.getReturnType().equals(result.getClass()))) {
                    result = Optional.ofNullable(result);
                }
            }
            break;
        // ......
    }
    // ......
    return result;
}

@Override
public <T> T selectOne(String statement) {
    return this.selectOne(statement, null);
}
@Override
public <T> T selectOne(String statement, Object parameter) {
    // 查询操作会由selectList()完成
    List<T> list = this.selectList(statement, parameter);
    if (list.size() == 1) {
        // 查询结果只有一个时，返回查询结果
        return list.get(0);
    } else if (list.size() > 1) {
        // 查询结果大于一个时，报错
        throw new TooManyResultsException(
            "Expected one result (or null) to be returned by selectOne(), but found: " + list.size());
    } else {
        return null;
    }
}

@Override
public <E> List<E> selectList(String statement, Object parameter, RowBounds rowBounds) {
    try {
        // 从Configuration中的mappedStatements缓存中获取MappedStatement
        MappedStatement ms = configuration.getMappedStatement(statement);
        // 调用Executor的query()方法执行查询操作
        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();
    }
}

@Override
public <E> List<E> query(MappedStatement ms, Object parameter, RowBounds rowBounds, 
                       ResultHandler resultHandler) throws SQLException {
    // 获取Sql语句
    BoundSql boundSql = ms.getBoundSql(parameter);
    // 生成CacheKey
    CacheKey key = createCacheKey(ms, parameter, rowBounds, boundSql);
    // 调用重载的query()方法
    return query(ms, parameter, rowBounds, resultHandler, key, boundSql);
}

@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++;
        // 先从一级缓存中根据CacheKey命中查询结果
        list = resultHandler == null ? (List<E>) localCache.getObject(key) : null;
        if (list != null) {
            // 成功命中，则返回缓存中的查询结果
            handleLocallyCachedOutputParameters(ms, key, parameter, boundSql);
        } else {
            // 未命中，则直接查数据库
            list = queryFromDatabase(ms, parameter, rowBounds, resultHandler, key, boundSql);
        }
    } finally {
        queryStack--;
    }
    if (queryStack == 0) {
        for (BaseExecutor.DeferredLoad deferredLoad : deferredLoads) {
            deferredLoad.load();
        }
        deferredLoads.clear();
        if (configuration.getLocalCacheScope() == LocalCacheScope.STATEMENT) {
            clearLocalCache();
        }
    }
    return list;
}

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 {
        // 调用doQuery()进行查询操作
        list = doQuery(ms, parameter, rowBounds, resultHandler, boundSql);
    } finally {
        localCache.removeObject(key);
    }
    // 将查询结果添加到一级缓存中
    localCache.putObject(key, list);
    if (ms.getStatementType() == StatementType.CALLABLE) {
        localOutputParameterCache.putObject(key, parameter);
    }
    // 返回查询结果
    return list;
}

@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();
        // 创建RoutingStatementHandler
        StatementHandler handler = configuration.newStatementHandler(
                wrapper, ms, parameter, rowBounds, resultHandler, boundSql);
        // 实例化Statement
        stmt = prepareStatement(handler, ms.getStatementLog());
        // 执行查询
        return handler.query(stmt, resultHandler);
    } finally {
        closeStatement(stmt);
    }
}

public StatementHandler newStatementHandler(Executor executor, MappedStatement mappedStatement, 
            Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) {
    // 创建RoutingStatementHandler
    StatementHandler statementHandler = new RoutingStatementHandler(
                executor, mappedStatement, parameterObject, rowBounds, resultHandler, boundSql);
    // 为RoutingStatementHandler植入插件逻辑
    statementHandler = (StatementHandler) interceptorChain.pluginAll(statementHandler);
    return statementHandler;
}

private Statement prepareStatement(StatementHandler handler, Log statementLog) 
                throws SQLException {
    Statement stmt;
    // 获取到Connection对象并为Connection对象生成动态代理对象
    Connection connection = getConnection(statementLog);
    // 通过Connection对象的动态代理对象实例化Statement
    stmt = handler.prepare(connection, transaction.getTimeout());
    handler.parameterize(stmt);
    return stmt;
}

@Override
public <E> List<E> query(Statement statement, ResultHandler resultHandler) 
            throws SQLException {
    PreparedStatement ps = (PreparedStatement) statement;
    // 调用到JDBC的逻辑了
    ps.execute();
    // 调用ResultSetHandler处理查询结果
    return resultSetHandler.handleResultSets(ps);
}