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根據前文Mybatis源碼分析-SqlSessionTemplate的簡單分析，對於SqlSession的CURD操作都需要經過Executor介面的update/query方法，本文將分析下BaseExecutor如何解析執行sql語句

BaseExecutor-抽象類

其是Executor介面的實現類但為抽象類，另外一個則為具體實現類為CachingExecutor，主要是通過對象適配的設計模式在原來的executor上再附上緩存的屬性，有興趣的可自行查閱。先從構造函數看一發

protected BaseExecutor(Configuration configuration, Transaction transaction) {
//事務對象
this.transaction = transaction;
this.deferredLoads = new ConcurrentLinkedQueue<DeferredLoad>;
this.localCache = new PerpetualCache("LocalCache");
this.localOutputParameterCache = new PerpetualCache("LocalOutputParameterCache");
//表明exector的狀態
this.closed = false;
//主文件屬性，主要獲取MappedStatement對象
this.configuration = configuration;
this.wrapper = this;
}

BaseExecutor#update-SqlSession之insert/update/delete入口

具體源碼如下

@Override
public int update(MappedStatement ms, Object parameter) throws SQLException {
ErrorContext.instance.resource(ms.getResource).activity("executing an update").object(ms.getId);
if (closed) {
throw new ExecutorException("Executor was closed.");
}
clearLocalCache;
//供子類複寫執行CUD操作
return doUpdate(ms, parameter);
}

BaseExecutor#query-SqlSession之select入口

具體源碼如下

@Override
public <E> List<E> query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler) throws SQLException {
//獲取綁定的sql，並將參數對象與sql語句的#{}一一對應
BoundSql boundSql = ms.getBoundSql(parameter);
//獲取cacheKey供緩存，包含完整的語句、參數等
CacheKey key = createCacheKey(ms, parameter, rowBounds, boundSql);
//比原先多傳入CacheKey和BoundSql參數
return query(ms, parameter, rowBounds, resultHandler, key, boundSql);
}

具體的查詢處理邏輯如下

@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.");
}
//是否清除本地緩存
if (queryStack == 0 && ms.isFlushCacheRequired) {
clearLocalCache;
}
List<E> list;
try {
queryStack++;
//如果查詢的語句已存在本地緩存中，則直接從本地獲取，反之從資料庫中讀取內容
list = resultHandler == null ? (List<E>) localCache.getObject(key) : null;
if (list != null) {
//此處嘗試對Callable類型的表達式進行處理，主要是針對mode=out類型的參數
//此參數主要是通過map來定義，直接從map中獲取
handleLocallyCachedOutputParameters(ms, key, parameter, boundSql);
} else {
//從資料庫中獲取並進行緩存處理，其也會調用子類需複寫的doQuery方法
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;
}

由上可知，BaseExecutor對CRUD操作均轉化為對子類的doUpdate/doQuery方法的調用，並一般都會相應的結果進行緩存以免頻繁請求資料庫導致性能下降。本文則從SimpleExecutor子類來進行分析

分別看下SimpleExecutor複寫的doUpdate和doQuery方法，具體源碼如下 doUpdate

@Override
public int doUpdate(MappedStatement ms, Object parameter) throws SQLException {
Configuration configuration = ms.getConfiguration;
//創建StatementHandler來處理update
StatementHandler handler = configuration.newStatementHandler(this, ms, parameter, RowBounds.DEFAULT, null, null);
//創建表達式對象Statement
Statement stmt = prepareStatement(handler, ms.getStatementLog);
return handler.update(stmt);
}

doQuery

@Override
public <E> List<E> doQuery(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) throws SQLException {
Configuration configuration = ms.getConfiguration;
//創建StatementHandler來處理query
StatementHandler handler = configuration.newStatementHandler(wrapper, ms, parameter, rowBounds, resultHandler, boundSql);
//創建表達式對象Statement
Statement stmt = prepareStatement(handler, ms.getStatementLog);
return handler.<E>query(stmt, resultHandler);
}

doUpdate/doQuery代碼的執行邏輯一致，均是先創建StatementHandler對象，然後通過prepareStatement方法創建表達式對象，供前者調用處理update/query方法

SimpleExecutor#prepareStatement-創建預表達式對象

邏輯如下

//handler對象對應的為RoutingStatementHandler對象，其實也是個適配管理類
//可根據MappedStatement的statementType來確定表達式處理handler類，後續講解
private Statement prepareStatement(StatementHandler handler, Log statementLog) throws SQLException {
Statement stmt;
//獲取連接對象，如果該日誌等級為debug，則會列印相應的處理日誌，採用代理實現
Connection connection = getConnection(statementLog);
//創建真實的Statement對象，比如SimpleStatement/PrepareStatement/CallableStatement
stmt = handler.prepare(connection, transaction.getTimeout);
//設置返回參數
handler.parameterize(stmt);
return stmt;
}

小結

1. BaseExecutor抽象類提供了對CRUD操作的入口，並帶有緩存效應，子類只需要複寫doUpdate和doQuery抽象方法即可

2. SimpleExecutor-簡單的處理實現類，即基本每次對相同的sql語句都會創建新的Statement對象；ReuseExecutor-復用處理實現類，即對相同的sql語句會緩存Statement對象；BatchExecutor-批處理實現類

3. 最終獲取Statement對象以及執行sql語句的解釋權在於StatementHandler介面，詳情看下節內容

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