Mybatis源碼分析4--StatementHandler

MyBatis sql執(zhí)行過程如下圖所示：

MyBatis sql執(zhí)行過程

SqlSession將執(zhí)行過程委托給Executor，Executor又將執(zhí)行過程交給StatementHandler具體執(zhí)行。

下面我們對StatementHandler的設計進行具體分析。

StatementHandler類繼承結構

  • SimpleStatementHandler：用于處理Statement對象的數(shù)據庫操作

  • PreparedStatementHandler：用于處理PreparedStatement對象的數(shù)據庫操作

  • CallableStatementHandler：用于處理CallableStatement對象的數(shù)據庫操作（存儲過程）

  • RoutingStatementHandler：用于創(chuàng)建上面三種Handler的策略類

BaseStatementHandler

在BaseStatementHandler中定義了生成Statement對象的基本算法結構，而具體生成Statement的類型和算法，則由子類自行決定：

public Statement prepare(Connection connection, Integer transactionTimeout) throws SQLException {
    ErrorContext.instance().sql(boundSql.getSql());
    Statement statement = null;
    try {
      //由子類自行決定要生成的Statement類型和算法
      statement = instantiateStatement(connection);
      setStatementTimeout(statement, transactionTimeout);
      setFetchSize(statement);
      return statement;
    } catch (SQLException e) {
      closeStatement(statement);
      throw e;
    } catch (Exception e) {
      closeStatement(statement);
      throw new ExecutorException("Error preparing statement.  Cause: " + e, e);
    }
  }

SimpleStatementHandler

SimpleStatementHandler是利用Statement對象進行數(shù)據庫操作的相關算法定義。由instantiateStatement方法生成一個Statement對象。

protected Statement instantiateStatement(Connection connection) throws SQLException {
    if (mappedStatement.getResultSetType() != null) {
      return connection.createStatement(mappedStatement.getResultSetType().getValue(), ResultSet.CONCUR_READ_ONLY);
    } else {
      return connection.createStatement();
    }
  }

因為Statement不支持？形式的參數(shù)占位符，所以其parameterize方法時空的：

public void parameterize(Statement statement) throws SQLException {
    // N/A
  }

PreparedStatementHandler

PreparedStatementHandler定義了生成一個PrepareStatement對象，和對PrepareStatement對象進行參數(shù)填充的方法。

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);
    }
  }

public void parameterize(Statement statement) throws SQLException {
    //將參數(shù)填充委托給parameterHandler進行處理
    parameterHandler.setParameters((PreparedStatement) statement);
  }

CallableStatementHandler

CallableStatementHandler定義了生成一個CallableStatement對象，和對CallableStatement對象進行參數(shù)填充的方法。

protected Statement instantiateStatement(Connection connection) throws SQLException {
    String sql = boundSql.getSql();
    if (mappedStatement.getResultSetType() != null) {
      return connection.prepareCall(sql, mappedStatement.getResultSetType().getValue(), ResultSet.CONCUR_READ_ONLY);
    } else {
      return connection.prepareCall(sql);
    }
  }

ParameterHandler

ParameterHandler負責對Statement的參數(shù)進行填充和處理。MyBatis提供了一個默認實現(xiàn)DefaultParameterHandler。

public void setParameters(PreparedStatement ps) {
    ErrorContext.instance().activity("setting parameters").object(mappedStatement.getParameterMap().getId());
    List<ParameterMapping> parameterMappings = boundSql.getParameterMappings();
    if (parameterMappings != null) {
      for (int i = 0; i < parameterMappings.size(); i++) {
        ParameterMapping parameterMapping = parameterMappings.get(i);
        if (parameterMapping.getMode() != ParameterMode.OUT) {
          Object value;
          String propertyName = parameterMapping.getProperty();
          if (boundSql.hasAdditionalParameter(propertyName)) { // issue #448 ask first for additional params
            value = boundSql.getAdditionalParameter(propertyName);
          } else if (parameterObject == null) {
            value = null;
          } else if (typeHandlerRegistry.hasTypeHandler(parameterObject.getClass())) {
            value = parameterObject;
          } else {
            MetaObject metaObject = configuration.newMetaObject(parameterObject);
            value = metaObject.getValue(propertyName);
          }
          TypeHandler typeHandler = parameterMapping.getTypeHandler();
          JdbcType jdbcType = parameterMapping.getJdbcType();
          if (value == null && jdbcType == null) {
            jdbcType = configuration.getJdbcTypeForNull();
          }
          try {
            typeHandler.setParameter(ps, i + 1, value, jdbcType);
          } catch (TypeException e) {
            throw new TypeException("Could not set parameters for mapping: " + parameterMapping + ". Cause: " + e, e);
          } catch (SQLException e) {
            throw new TypeException("Could not set parameters for mapping: " + parameterMapping + ". Cause: " + e, e);
          }
        }
      }
    }
  }

StatementHandler的創(chuàng)建過程

Executor每次執(zhí)行select & update，都會創(chuàng)建一個StatementHandler對象。

public int doUpdate(MappedStatement ms, Object parameter) throws SQLException {
    Statement stmt = null;
    try {
      Configuration configuration = ms.getConfiguration();
      //新建一個StatementHandler
      StatementHandler handler = configuration.newStatementHandler(this, ms, parameter, RowBounds.DEFAULT, null, null);
      stmt = prepareStatement(handler, ms.getStatementLog());
      return handler.update(stmt);
    } finally {
      closeStatement(stmt);
    }
  }

public StatementHandler newStatementHandler(Executor executor, MappedStatement mappedStatement, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) {
    //根據配置的類型創(chuàng)建不同的StatementHandler
    StatementHandler statementHandler = new RoutingStatementHandler(executor, mappedStatement, parameterObject, rowBounds, resultHandler, boundSql);
    statementHandler = (StatementHandler) interceptorChain.pluginAll(statementHandler);
    return statementHandler;
  }

public RoutingStatementHandler(Executor executor, MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) {

    switch (ms.getStatementType()) {
      case STATEMENT:
        delegate = new SimpleStatementHandler(executor, ms, parameter, rowBounds, resultHandler, boundSql);
        break;
      case PREPARED:
        delegate = new PreparedStatementHandler(executor, ms, parameter, rowBounds, resultHandler, boundSql);
        break;
      case CALLABLE:
        delegate = new CallableStatementHandler(executor, ms, parameter, rowBounds, resultHandler, boundSql);
        break;
      default:
        throw new ExecutorException("Unknown statement type: " + ms.getStatementType());
    }

  }

StatementType參數(shù)可以在SQL Mapper文件中進行配置：

<select id="select" resultType="mybatis.bean.User" statementType="STATEMENT">
        select * from user_info where id = #{id}
    </select>

StatementType有三種不同的類型：

public enum StatementType {
  STATEMENT, PREPARED, CALLABLE
}

默認為PREPARED。

總結

  • SimpleStatementHandler
Statement stm = conn.createStatement()
return stm.execute(sql);
  • PreparedStatementHandler
PreparedStatement pstm = conn.prepareStatement(sql);
pstm.setString(1, "Hello");
return pstm.execute();
  • CallableStatementHandler
CallableStatement cs = conn.prepareCall("{call pr_add(?,?,?)}");
cs.registerOutParameter(3, Types.INTEGER);
cs.setInt(1, 10);
cs.setString(2, "Hello");
cs.execute();
return cs.getInt(3);
  • RoutingStatementHandler

根據mapper文件中配置的type類型創(chuàng)建相應的上述StatementHandler類型

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