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Mybatis源码分析(6)—— 从JDBC看Mybatis的设计

程序员文章站 2022-06-16 08:09:54
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Java数据库连接,(Java Database Connectivity,简称JDBC)是Java语言中用来规范客户端程序如何来访问数据库的应用程序接口,提供了诸如查询和更新数据库中数据的方法。

六步流程:

  • 加载驱动(5.x驱动包不需要这步了)
  • 建立连接
  • 创建Statement
  • 执行SQL语句
  • 获取结果集
  • 关闭资源

这里只取后面几步分析下,基本上都是从Executor开始。DefaultSqlSession被每个Mapper持有,将各种基于SQL的操作转移到调用Executor的query和update方法。

Executor的类图如下:

Mybatis源码分析(6)—— 从JDBC看Mybatis的设计

以ReuseExecutor的doQuery方法为例:

public <E> List<E> doQuery(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) throws SQLException {
    Configuration configuration = ms.getConfiguration();
    StatementHandler handler = configuration.newStatementHandler(this, ms, parameter, rowBounds, resultHandler, boundSql);
    Statement stmt = prepareStatement(handler, ms.getStatementLog());
    return handler.<E>query(stmt, resultHandler);
}

建立连接

上面ReuseExecutor的doQuery方法调用其prepareStatement方法:

private Statement prepareStatement(StatementHandler handler, Log statementLog) throws SQLException {
    Statement stmt;
    BoundSql boundSql = handler.getBoundSql();
    String sql = boundSql.getSql();
    if (hasStatementFor(sql)) {
      stmt = getStatement(sql);
    } else {
      Connection connection = getConnection(statementLog);
      stmt = handler.prepare(connection);
      putStatement(sql, stmt);
    }
    handler.parameterize(stmt);
    return stmt;
}

然后调用getConnection方法获取连接:

protected Connection getConnection(Log statementLog) throws SQLException {
    Connection connection = transaction.getConnection();
    if (statementLog.isDebugEnabled() || connectionLog.isDebugEnabled()) {
      return ConnectionLogger.newInstance(connection, statementLog);
    } else {
      return connection;
    }
}

这个连接是从Transaction对象中获取的。

当我们和Spring一起使用时,当然由Spring统一管理,回顾DefaultSqlSessionFactory类中的openSessionFromDataSource方法,这个连接就是找Spring要的(DataSourceUtils#getConnection),要过来放到了SpringManagedTransaction中

创建Statement

  • Statement createStatement()

创建Statement 对象,Statement接口提供基本执行SQL语句的能力。

  • PreparedStatement prepareStatement(String sql)

创建PreparedStatement对象,PreparedStatement接口继承了Statement接口,提供SQL语句接受输入参数的能力。

  • CallableStatement prepareCall(String sql)

创建CallableStatement对象,CallableStatement接口继承了PreparedStatement接口,提供执行存储过程的能力。

针对这三种情况,Mybatis提供了三个StatementHandler实现:
Mybatis源码分析(6)—— 从JDBC看Mybatis的设计

并创建了一个RoutingStatementHandler作为路由(代理类),根据MappedStatement中的statementType来确定创建哪个作为实际的StatementHandler实现。对外提供一致接口,实际调用交给委托类。

还是接着看上面的prepareStatement方法,它获取连接后就接着调用StatementHandler的prepare方法:

private Statement prepareStatement(StatementHandler handler, Log statementLog) throws SQLException {
    Statement stmt;
    BoundSql boundSql = handler.getBoundSql();
    String sql = boundSql.getSql();
    if (hasStatementFor(sql)) {
      stmt = getStatement(sql);
    } else {
      Connection connection = getConnection(statementLog);
      stmt = handler.prepare(connection);
      putStatement(sql, stmt);
    }
    handler.parameterize(stmt);
    return stmt;
}

这个来自BaseStatementHandler的prepare方法又是调用子类的instantiateStatement方法来实例化Statement:

protected Statement instantiateStatement(Connection connection) throws SQLException {
    String sql = boundSql.getSql();
    if (mappedStatement.getKeyGenerator() instanceof Jdbc3KeyGenerator) {
      String[] keyColumnNames = mappedStatement.getKeyColumns();
      if (keyColumnNames == null) {
        return connection.prepareStatement(sql, PreparedStatement.RETURN_GENERATED_KEYS);
      } else {
        return connection.prepareStatement(sql, keyColumnNames);
      }
    } else if (mappedStatement.getResultSetType() != null) {
      return connection.prepareStatement(sql, mappedStatement.getResultSetType().getValue(), ResultSet.CONCUR_READ_ONLY);
    } else {
      return connection.prepareStatement(sql);
    }
}

这里最终调用connection的prepareStatement方法,回到了我们传统的操作。

执行SQL语句

在Executor的实现类中,doUpdate和doQuery方法都会调用下面两个方法:

handler.update(stmt);

handler.query(stmt, resultHandler);

SimpleStatementHandler的query方法:

public <E> List<E> query(Statement statement, ResultHandler resultHandler)
      throws SQLException {
    String sql = boundSql.getSql();
    statement.execute(sql);
    return resultSetHandler.<E>handleResultSets(statement);
}

PreparedStatementHandler的query方法:

public <E> List<E> query(Statement statement, ResultHandler resultHandler) throws SQLException {
    PreparedStatement ps = (PreparedStatement) statement;
    ps.execute();
    return resultSetHandler.<E> handleResultSets(ps);
}

CallableStatementHandler的query方法:

public <E> List<E> query(Statement statement, ResultHandler resultHandler)
      throws SQLException {
    CallableStatement cs = (CallableStatement) statement;
    cs.execute();
    List<E> resultList = resultSetHandler.<E>handleResultSets(cs);
    resultSetHandler.handleOutputParameters(cs);
    return resultList;
}

这里都是直接调的Statement或PreparedStatement的execute方法。

返回结果集

上面的三种StatementHandler,最终都会处理结果集,CallableStatementHandler有点特殊,其它两种都是调用ResultSetHandler的handleResultSets方法来处理结果集。ResultSetHandler在基类BaseStatementHandler的构造函数中通过configuration对象来创建:

this.resultSetHandler = configuration.newResultSetHandler(executor, mappedStatement, rowBounds, parameterHandler, resultHandler, boundSql);

具体的实现有两种,FastResultSetHandler和NestedResultSetHandler,且NestedResultSetHandler继承了FastResultSetHandler,重写了handleRowValues方法。

在FastResultSetHandler的handleResultSets方法中:

public List<Object> handleResultSets(Statement stmt) throws SQLException {
    final List<Object> multipleResults = new ArrayList<Object>();
    final List<ResultMap> resultMaps = mappedStatement.getResultMaps();
    int resultMapCount = resultMaps.size();
    int resultSetCount = 0;
    ResultSet rs = stmt.getResultSet();

    while (rs == null) {
      // move forward to get the first resultset in case the driver
      // doesn't return the resultset as the first result (HSQLDB 2.1)
      if (stmt.getMoreResults()) {
        rs = stmt.getResultSet();
      } else {
        if (stmt.getUpdateCount() == -1) {
          // no more results.  Must be no resultset
          break;
        }
      }
    }
    // 验证是否定义了resultType或者resultMap
    validateResultMapsCount(rs, resultMapCount);
    while (rs != null && resultMapCount > resultSetCount) {
      final ResultMap resultMap = resultMaps.get(resultSetCount);
      ResultColumnCache resultColumnCache = new ResultColumnCache(rs.getMetaData(), configuration);
      handleResultSet(rs, resultMap, multipleResults, resultColumnCache);
      rs = getNextResultSet(stmt);
      cleanUpAfterHandlingResultSet();
      resultSetCount++;
    }
    return collapseSingleResultList(multipleResults);
}

处理结果行:

protected void handleRowValues(ResultSet rs, ResultMap resultMap, ResultHandler resultHandler, RowBounds rowBounds, ResultColumnCache resultColumnCache) throws SQLException {
    final DefaultResultContext resultContext = new DefaultResultContext();
    skipRows(rs, rowBounds);
    while (shouldProcessMoreRows(rs, resultContext, rowBounds)) {
      final ResultMap discriminatedResultMap = resolveDiscriminatedResultMap(rs, resultMap, null);
      Object rowValue = getRowValue(rs, discriminatedResultMap, null, resultColumnCache);
      callResultHandler(resultHandler, resultContext, rowValue);
    }
}

skipRows就是跳过行,基于查询结果的分页使用,其中会直接调用ResultSet的absolute方法:

rs.absolute(rowBounds.getOffset());

而shouldProcessMoreRows方法就是判断是否应该获取更多的行:

protected boolean shouldProcessMoreRows(ResultSet rs, ResultContext context, RowBounds rowBounds) throws SQLException {
    return !context.isStopped() && rs.next() && context.getResultCount() < rowBounds.getLimit();
}

也对应了分页的limit字段,这种分页可以说是伪分页,查出来再分页。所以我们一般使用插件的形式来实现分页,基于sql的动态替换。

具体针对行的处理在getRowValue方法中:

protected Object getRowValue(ResultSet rs, ResultMap resultMap, CacheKey rowKey, ResultColumnCache resultColumnCache) throws SQLException {
    final ResultLoaderMap lazyLoader = instantiateResultLoaderMap();
    Object resultObject = createResultObject(rs, resultMap, lazyLoader, null, resultColumnCache);
    if (resultObject != null && !typeHandlerRegistry.hasTypeHandler(resultMap.getType())) {
      final MetaObject metaObject = configuration.newMetaObject(resultObject);
      boolean foundValues = resultMap.getConstructorResultMappings().size() > 0;
      if (shouldApplyAutomaticMappings(resultMap, !AutoMappingBehavior.NONE.equals(configuration.getAutoMappingBehavior()))) {
        final List<String> unmappedColumnNames = resultColumnCache.getUnmappedColumnNames(resultMap, null);
        foundValues = applyAutomaticMappings(rs, unmappedColumnNames, metaObject, null, resultColumnCache) || foundValues;
      }
      final List<String> mappedColumnNames = resultColumnCache.getMappedColumnNames(resultMap, null);
      foundValues = applyPropertyMappings(rs, resultMap, mappedColumnNames, metaObject, lazyLoader, null) || foundValues;
      foundValues = (lazyLoader != null && lazyLoader.size() > 0) || foundValues;
      resultObject = foundValues ? resultObject : null;
      return resultObject;
    }
    return resultObject;
}

根据ResultMap提供的相关参数(返回的类型等),构造一个空的对象,然后根据属性名到ResultSet中获取结果,再通过MetaObject给set进对象里。

具体的取值是通过TypeHandler:

final Object value = typeHandler.getResult(rs, columnName);

每种类型的都对应一个TypeHandler的实现,以Integer类型的为例(IntegerTypeHandler):

@Override
public Integer getNullableResult(ResultSet rs, String columnName)
  throws SQLException {
    return rs.getInt(columnName);
}

框架就是这样,底层就是最基本的原理和最简单的接口,而这些接口通常就是所谓的标准。它所做的事就是解放我们的双手,把能省的都给省了,让我们专注于业务代码的编写。

但我们也应该知其所以然。一是出于好奇心,这是程序员的本能;二是为了学习其优秀的思想和设计理念,以后可以为我所用,这可称之为野心吧。