Mybatis源码-Executor的执行过程

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所以，映射接口的动态代理对象实际执行方法时，执行的请求最终会由

正文

本节将以一个实际的查询例子，以单步跟踪并结合源码的方法，对

public interface BookMapper {

    Book selectBookById(int id);

}

给定映射文件如下所示。

        SELECT
        b.id, b.b_name, b.b_price
        FROM
        book b
        WHERE
        b.id=#{id}
    

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));
    }

}

基于上述的映射接口，映射文件和执行代码，最终执行查询操作时，会调用到

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;
}

已知映射接口中的每个方法都会对应一个

@Override
public  T selectOne(String statement) {
    return this.selectOne(statement, null);
}

@Override
public  T selectOne(String statement, Object parameter) {
    // 查询操作会由selectList()完成
    List 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  List 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();
    }
}

在

@Override
public  List 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);
}

继续看

@Override
public  List 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 list;
    try {
        queryStack++;
        // 先从一级缓存中根据CacheKey命中查询结果
        list = resultHandler == null ? (List) 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;
}

上述的

private  List queryFromDatabase(MappedStatement ms, Object parameter, RowBounds rowBounds, 
                    ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException {
    List 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;
}

上述

@Override
public  List 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);
    }
}

上述的

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;
}

继续分析

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  List 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);
}

如上所示，在

PreparedStatementHandler

query()

JDBC

ResultSetHandler

至此，对

Mybatis

总结

Mybatis

Executor

SqlSession

SqlSession

Executor

SimpleExecutor

CachingExecutor

Mybatis

MapperMethod

execute()

execute()

SqlSession

insert()

update()

delete()

select()

MapperMethod

MapperMethod

SQL

SqlSession

insert()

update()

delete()

select()

SqlSession

Executor

SimpleExecutor

CachingExecutor

Executor

StatementHandler

StatementHandler

ParameterHandler

ResultSetHandler

StatementHandler

StatementHandler

Statement

StatementHandler

Statement

ResultSetHandler

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