SQL 語句的執行涉及多個元件 ，其中比較重要的是 Executor、StatementHandler、ParameterHandler 和 ResultSetHandler 。
- Executor 主要負責維護一級快取和二級快取，並提供事務管理的相關操作，它會將資料庫相關操作委託給StatementHandler完成。
- StatementHandler 首先通過 ParameterHandler 完成 SQL 語句的實參繫結，然後通過
java.sql.Statement物件執行 SQL 語句並得到結果集，最後通過 ResultSetHandler 完成結果集的對映，得到結果物件並返回。
SqlSession物件表示MyBaits框架與資料庫建立的會話，我們可以通過SqlSession例項完成對資料庫的增刪改查操作。SqlSession是一個介面，它的預設實現為DefaultSqlSession。
下面以selectOne為例
// DefaultSqlSession.java
public class DefaultSqlSession implements SqlSession {
private final Configuration configuration;
private final Executor executor;
private final boolean autoCommit;
private boolean dirty;
private List<Cursor<?>> cursorList;
@Override
public <T> T selectOne(String statement, Object parameter) {
// Popular vote was to return null on 0 results and throw exception on too many.
List<T> list = this.<T>selectList(statement, parameter);
if (list.size() == 1) {
return list.get(0);
} else if (list.size() > 1) {
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) {
return this.selectList(statement, parameter, RowBounds.DEFAULT);
}
@Override
public <E> List<E> selectList(String statement, Object parameter, RowBounds rowBounds) {
try {
// 根據 statement 獲取對應的 MappedStatement
// statement = Mapper類路徑.方法名
// 如com.example.demo.UserMapper.getUserByUserName
MappedStatement ms = configuration.getMappedStatement(statement);
// 這裡的 executor 使用的是 CachingExecutor
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();
}
}
}// CachingExecutor.java
public class CachingExecutor implements Executor {
@Override
public <E> List<E> query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler) throws SQLException {
BoundSql boundSql = ms.getBoundSql(parameterObject);
// 建立 CacheKey 物件
CacheKey key = createCacheKey(ms, parameterObject, rowBounds, boundSql);
// 查詢
return query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
}
@Override
public <E> List<E> query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql)
throws SQLException {
Cache cache = ms.getCache();
if (cache != null) {
// 這裡判斷是否需要清空快取
flushCacheIfRequired(ms);
if (ms.isUseCache() && resultHandler == null) {
// 這裡是儲存過程相關的
ensureNoOutParams(ms, parameterObject, boundSql);
// 從二級快取中獲取結果
@SuppressWarnings("unchecked")
List<E> list = (List<E>) tcm.getObject(cache, key);
if (list == null) {
// 如果不存在，則從資料庫中查詢
// 這裡的 delegate 為 SimpleExecutor，SimpleExecutor繼承了BaseExecutor
// 這裡的 query 方法是 BaseExecutor 類中的
list = delegate.<E> query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
// 快取結果到二級快取中
tcm.putObject(cache, key, list); // issue #578 and #116
}
return list;
}
}
// 不使用快取，則從資料庫中查詢
// 這裡的 delegate 為 SimpleExecutor，SimpleExecutor繼承了BaseExecutor
// 這裡的 query 方法是 BaseExecutor 類中的
return delegate.<E> query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
}
}// BaseExecutor.java
public abstract class BaseExecutor implements Executor {
@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.");
}
// 清空本地快取
if (queryStack == 0 && ms.isFlushCacheRequired()) {
clearLocalCache();
}
// 用來儲存結果列表
List<E> list;
try {
queryStack++;
// 從一級快取中查詢結果
list = resultHandler == null ? (List<E>) localCache.getObject(key) : null;
if (list != null) {
// 儲存過程相關
handleLocallyCachedOutputParameters(ms, key, parameter, boundSql);
} else {
// 獲取不到則從資料庫中查詢
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;
}
private <E> List<E> queryFromDatabase(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException {
List<E> list;
// 在快取中新增佔位物件。這裡的佔位符和延遲載入有關係。
localCache.putObject(key, EXECUTION_PLACEHOLDER);
try {
// 執行讀操作
// 這裡的 doQuery 由子類SimpleExecutor實現
list = doQuery(ms, parameter, rowBounds, resultHandler, boundSql);
} finally {
// 從快取中移除佔位物件
localCache.removeObject(key);
}
// 將結果新增到快取中
localCache.putObject(key, list);
// 儲存過程相關
if (ms.getStatementType() == StatementType.CALLABLE) {
localOutputParameterCache.putObject(key, parameter);
}
// 返回查詢結果
return list;
}
}// SimpleExecutor.java
public class SimpleExecutor extends BaseExecutor {
@Override
public <E> List<E> doQuery(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) throws SQLException {
Statement stmt = null;
try {
// 獲取 Configuration
Configuration configuration = ms.getConfiguration();
// 建立 StatementHandler 物件
StatementHandler handler = configuration.newStatementHandler(wrapper, ms, parameter, rowBounds, resultHandler, boundSql);
// 獲取 Statement 物件
stmt = prepareStatement(handler, ms.getStatementLog());
// 執行查詢操作
// 這裡的 handler 為 RoutingStatementHandler
return handler.<E>query(stmt, resultHandler);
} finally {
// 關閉 StatementHandler 物件
closeStatement(stmt);
}
}
private Statement prepareStatement(StatementHandler handler, Log statementLog) throws SQLException {
Statement stmt;
// 獲得 Connection 物件
Connection connection = getConnection(statementLog);
// 建立 Statement 或 PrepareStatement 物件
stmt = handler.prepare(connection, transaction.getTimeout());
// 設定 SQL 上的引數，例如 PrepareStatement 物件上的佔位符設定上使用者傳遞過來的值
// 這裡的 handler 為 RoutingStatementHandler
handler.parameterize(stmt);
return stmt;
}
}// RoutingStatementHandler.java
public class RoutingStatementHandler implements StatementHandler {
// 完成SQL語句的實參繫結
@Override
public void parameterize(Statement statement) throws SQLException {
// 這裡的 delegate 為 PreparedStatementHandler
delegate.parameterize(statement);
}
@Override
public <E> List<E> query(Statement statement, ResultHandler resultHandler) throws SQLException {
// 這裡的 delegate 為 PreparedStatementHandler
return delegate.<E>query(statement, resultHandler);
}
}// PreparedStatementHandler.java
public class PreparedStatementHandler extends BaseStatementHandler {
// 完成SQL語句的實參繫結
@Override
public void parameterize(Statement statement) throws SQLException {
// 這裡的 parameterHandler 為
parameterHandler.setParameters((PreparedStatement) statement);
}
@Override
public <E> List<E> query(Statement statement, ResultHandler resultHandler) throws SQLException {
PreparedStatement ps = (PreparedStatement) statement;
ps.execute();
// 這裡的 resultSetHandler 為 DefaultResultSetHandler
// handleResultSets方法的功能是將查詢結果封裝成使用者設定的返回型別
return resultSetHandler.<E> handleResultSets(ps);
}
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