MyBatis 原始碼深度解析 (一)：核心架構與執行流程總覽

MyBatis 核心類別架構

SqlSessionFactoryBuilder
        ↓ build()
SqlSessionFactory (介面)
        ↓ openSession()
DefaultSqlSessionFactory → DefaultSqlSession (核心實作)
        ↓ 內部持有
Configuration (全域配置)
        ↓ 包含
MapperRegistry → MapperProxyFactory → MapperProxy (動態代理)
        ↓
Executor (執行器介面)
        ↓ 三種實作
BaseExecutor (抽象基類)
    ├── SimpleExecutor    (簡單執行)
    ├── ReuseExecutor     (重用 PreparedStatement)
    └── BatchExecutor     (批次執行)
        ↓
StatementHandler (語句處理器)
    ├── SimpleStatementHandler
    ├── PreparedStatementHandler
    └── CallableStatementHandler
        ↓
ParameterHandler / ResultSetHandler

核心流程：SqlSession 查詢執行

1. 入口：DefaultSqlSession.selectList()

public  List selectList(String statement, Object parameter, RowBounds rowBounds) {
    try {
        MappedStatement ms = configuration.getMappedStatement(statement);
        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();
    }
}

2. 委派給 Executor

// BaseExecutor.query()
public  List query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler handler) {
    // 1. 建立緩存鍵
    CacheKey key = createCacheKey(ms, parameter, rowBounds);
    
    // 2. 緩存查詢 (CachingExecutor 會攔截)
    List list = resultHandler == null ? (List) localCache.getObject(key) : null;
    if (list != null) return list;
    
    // 3. 實際查詢
    list = queryFromDatabase(ms, parameter, rowBounds, handler, key, boundSql);
    
    // 4. 存入本地緩存
    localCache.putObject(key, list);
    return list;
}

// 抽象方法留給子類實作
protected abstract  List doQuery(MappedStatement ms, Object parameter, RowBounds rowBounds, 
                                       ResultHandler handler, BoundSql boundSql);

3. SimpleExecutor 實作

public  List doQuery(MappedStatement ms, Object parameter, RowBounds rowBounds, 
                          ResultHandler handler, BoundSql boundSql) {
    Statement stmt = null;
    try {
        Configuration config = ms.getConfiguration();
        // 1. 建立 StatementHandler
        StatementHandler handler = config.newStatementHandler(wrapper, ms, parameter, rowBounds, resultHandler, boundSql);
        // 2. 準備 Statement (預編譯、設定參數)
        stmt = handler.prepare(connection, transaction.getTimeout());
        // 3. 執行查詢並處理結果集
        return handler.query(stmt, resultHandler);
    } finally {
        closeStatement(stmt);
    }
}

4. StatementHandler 責任鏈

// PreparedStatementHandler.query()
public  List query(Statement statement, ResultHandler resultHandler) throws SQLException {
    PreparedStatement ps = (PreparedStatement) statement;
    ps.execute();  // 執行 SQL
    return resultSetHandler.handleResultSets(ps);  // 委派結果集處理
}

// ResultSetHandler.handleResultSets()
public List