【MyBatis源码分析】insert方法、update方法、delete方法处理流程（下篇）

Configuration的newStatementHandler分析

SimpleExecutor的doUpdate方法上文有分析过：

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

这两天重看第5行的newStatementHandler方法的时候，发现方法上文在这个方法中分析地太简略了，这里过一遍一下Configuration的newStatementHandler方法，方法的实现为：

public StatementHandler newStatementHandler(Executor executor, MappedStatement mappedStatement, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) {
    StatementHandler statementHandler = new RoutingStatementHandler(executor, mappedStatement, parameterObject, rowBounds, resultHandler, boundSql);
    statementHandler = (StatementHandler) interceptorChain.pluginAll(statementHandler);
    return statementHandler;
}

第3行的代码是加入插件没什么好看的，看下第2行的代码，StatementHandler接口真正实例化出来的是RoutingStatementHandler，构造方法实现为：

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());
    }

}

RoutingStatementHandler同样是装饰器模式的实现，实现了StatementHandler接口并持有StatementHandler接口引用delegate。这里StatementType的为PREPARED，因此执行的第7行的判断，实例化出PreparedStatementHandler，实例化的过程为：

protected BaseStatementHandler(Executor executor, MappedStatement mappedStatement, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) {
    this.configuration = mappedStatement.getConfiguration();
    this.executor = executor;
    this.mappedStatement = mappedStatement;
    this.rowBounds = rowBounds;

    this.typeHandlerRegistry = configuration.getTypeHandlerRegistry();
    this.objectFactory = configuration.getObjectFactory();

    if (boundSql == null) { // issue #435, get the key before calculating the statement
      generateKeys(parameterObject);
      boundSql = mappedStatement.getBoundSql(parameterObject);
    }

    this.boundSql = boundSql;

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

这里的重点是BoundSql，它可以通过MappedStatement获取到，BoundSql中存储了几个重要的内容：

1. 参数对象本身
2. 参数列表
3. 待执行的SQL语句

有些基于MyBatis二次开发的框架通常都会拿到BoundSql中的SQL语句进行修改并重新设置进BoundSql中。

生成Statement

上文已经写了生成Connection的流程，本文继续看，首先还是再贴一下SimpleExecutor的prepareStatement方法：

private Statement prepareStatement(StatementHandler handler, Log statementLog) throws SQLException {
    Statement stmt;
    Connection connection = getConnection(statementLog);
    stmt = handler.prepare(connection, transaction.getTimeout());
    handler.parameterize(stmt);
    return stmt;
}

接着就是第4行的代码，生成Statement了，第4行的代码实现为：

public Statement prepare(Connection connection, Integer transactionTimeout) throws SQLException {
    return delegate.prepare(connection, transactionTimeout);
}

delegate上文是装饰器模式中的被装饰角色，其接口类型为StatementHandler，真实类型为PreparedStatementHandler，这个在最开头的部分已经分析过了。看一下prepare方法实现：

public Statement prepare(Connection connection, Integer transactionTimeout) throws SQLException {
    ErrorContext.instance().sql(boundSql.getSql());
    Statement statement = null;
    try {
      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);
    }
}

第6行的代码设置的是查询超时时间、第7行的代码设置的是接收的数据大小，就不跟进去看了，接着看下第6行的instantiateStatement方法实现：

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);
    }
}

第2行，从boundSql中获取真正的SQL语句，第一部分已经分析过了。拿到SQL语句之后，执行第3行与第5行的判断，这里就是我们熟悉的通过Connection拿Statement的代码了，通过prepareStatement方法获取到PreparedStatement，其真实类型为com.mysql.jdbc.JDBC4PreparedStatement，是PreparedStatement的子类。

Statement参数设置

获取了Statement后，下一步就是设置参数了，看一下设置参数的代码，还是回到SimpleExecutor的prepareStatement方法：

private Statement prepareStatement(StatementHandler handler, Log statementLog) throws SQLException {
    Statement stmt;
    Connection connection = getConnection(statementLog);
    stmt = handler.prepare(connection, transaction.getTimeout());
    handler.parameterize(stmt);
    return stmt;
}

跟第5行的代码：

public void parameterize(Statement statement) throws SQLException {
    parameterHandler.setParameters((PreparedStatement) statement);
}

继续跟第2行的代码：

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);
          }
        }
      }
    }
}

最终执行的是第26行的代码，从26行的代码我们可以知道，参数设置到最后都是通过参数的TypeHandler来执行的，JDBC为我们预定义了很多TypeHandler，比如int值的TypeHandler就是IntegerTypeHandler，当然我们也可以定义自己的TypeHandler，通常来说继承BaseTypeHandler就可以了。

但是在此之前，会获取到Statement（setParameters方法形参）、占位符位置号（for循环的遍历参数i）、参数值（通过属性名获取）与jdbcType（配置在配置文件中，默认为null），最终执行TypeHandler的setParameters方法，这是BaseTypeHandler中的一个方法：

public void setParameter(PreparedStatement ps, int i, T parameter, JdbcType jdbcType) throws SQLException {
    if (parameter == null) {
      if (jdbcType == null) {
        throw new TypeException("JDBC requires that the JdbcType must be specified for all nullable parameters.");
      }
      try {
        ps.setNull(i, jdbcType.TYPE_CODE);
      } catch (SQLException e) {
        throw new TypeException("Error setting null for parameter #" + i + " with JdbcType " + jdbcType + " . " +
                "Try setting a different JdbcType for this parameter or a different jdbcTypeForNull configuration property. " +
                "Cause: " + e, e);
      }
    } else {
      try {
        setNonNullParameter(ps, i, parameter, jdbcType);
      } catch (Exception e) {
        throw new TypeException("Error setting non null for parameter #" + i + " with JdbcType " + jdbcType + " . " +
                "Try setting a different JdbcType for this parameter or a different configuration property. " +
                "Cause: " + e, e);
      }
    }
}

这里的参数不为null，走13行的else，执行setNonNullParameter方法，这是IntegerTypeHandler中的一个方法：

public void setNonNullParameter(PreparedStatement ps, int i, Integer parameter, JdbcType jdbcType)
      throws SQLException {
    ps.setInt(i, parameter);
}

这里的代码就比较熟悉了，PreparedStatement的setInt方法。

执行更新操作并处理结果

最后一步，执行更新操作并对结果进行处理，回到SimpleExecuto的doUpdate方法：

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

第6行已经准备好了Statement，第7行执行update操作并对结果进行处理并返回：

public int update(Statement statement) throws SQLException {
    return delegate.update(statement);
}

这里的委托delegate前面已经说过了，其真实类型是PreparedStatementHandler，update方法的实现为：

public int update(Statement statement) throws SQLException {
    PreparedStatement ps = (PreparedStatement) statement;
    ps.execute();
    int rows = ps.getUpdateCount();
    Object parameterObject = boundSql.getParameterObject();
    KeyGenerator keyGenerator = mappedStatement.getKeyGenerator();
    keyGenerator.processAfter(executor, mappedStatement, ps, parameterObject);
    return rows;
}

第3行的execute方法是PreparedStatement中的方法，execute方法执行操作，然后第4行通过getUpdateCount()方法获取本次操作更新了几条数据，作为最终的值返回给用户。

第5行的代码通过BoundSql获取参数对象，这里是MailDO对象，因为我们知道在插入场景下，开发者是有这种需求的，需要返回插入的主键id，此时会将主键id设置到MailDO中。

第6行的代码通过MappedStatement获取KeyGenerator，一个主键生成器。

第7行的代码做了一个操作完毕的后置处理：

public void processAfter(Executor executor, MappedStatement ms, Statement stmt, Object parameter) {
    processBatch(ms, stmt, getParameters(parameter));
}

首先将对象包装成集合类型，然后跟第2行的代码processBatch方法：

public void processBatch(MappedStatement ms, Statement stmt, Collection<Object> parameters) {
    ResultSet rs = null;
    try {
      rs = stmt.getGeneratedKeys();
      final Configuration configuration = ms.getConfiguration();
      final TypeHandlerRegistry typeHandlerRegistry = configuration.getTypeHandlerRegistry();
      final String[] keyProperties = ms.getKeyProperties();
      final ResultSetMetaData rsmd = rs.getMetaData();
      TypeHandler<?>[] typeHandlers = null;
      if (keyProperties != null && rsmd.getColumnCount() >= keyProperties.length) {
        for (Object parameter : parameters) {
          // there should be one row for each statement (also one for each parameter)
          if (!rs.next()) {
            break;
          }
          final MetaObject metaParam = configuration.newMetaObject(parameter);
          if (typeHandlers == null) {
            typeHandlers = getTypeHandlers(typeHandlerRegistry, metaParam, keyProperties, rsmd);
          }
          populateKeys(rs, metaParam, keyProperties, typeHandlers);
        }
      }
    } catch (Exception e) {
      throw new ExecutorException("Error getting generated key or setting result to parameter object. Cause: " + e, e);
    } finally {
      if (rs != null) {
        try {
          rs.close();
        } catch (Exception e) {
          // ignore
        }
      }
    }
}

简单说这里就是遍历集合，通过JDBC4PreparedStatement的getGeneratedKeys获取ResultSet，然后从ResultSet中使用getLong方法获取生成的主键，设置到MailDO中。完成整个操作。

最后，本文演示的是insert数据的update方法流程，前文已经说过insert、update、delete在MyBatis中都是一样的，因此update、delete也是一样的操作，这里就不再赘述了。