剛回答了一個問題這樣一個問題，mybatis不能正常返回主鍵增加值 下面通過源碼分析一下selectKey都具體實現；關於Mybatis 基於註解Mapper源碼分析 可以看一下具體解析過程。

 @Insert("insert into table2 (name) values(#{name})")
    @SelectKey(statement="call identity()", keyProperty="nameId", before=false, resultType=int.class)
    int insertTable2(Name name);

KeyGenerator接口

/**
 * key生成器接口
 */
public interface KeyGenerator {
  //在執行主SQL前執行selectKey
  void processBefore(Executor executor, MappedStatement ms, Statement stmt, Object parameter);

  //在主SQL執行後執行selectkey
  void processAfter(Executor executor, MappedStatement ms, Statement stmt, Object parameter);

}

before具體執行時機　　

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
//  //執行before
      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);
  }

  protected void generateKeys(Object parameter) {
    KeyGenerator keyGenerator = mappedStatement.getKeyGenerator();
    ErrorContext.instance().store();
    keyGenerator.processBefore(executor, mappedStatement, null, parameter);
    ErrorContext.instance().recall();
  }

  @Override
  public void processBefore(Executor executor, MappedStatement ms, Statement stmt, Object parameter) {
    //是否執行前執行，如果不是就不執行
    if (executeBefore) {
      processGeneratedKeys(executor, ms, parameter);
    }
  }

　　after執行時機

@Override
  public void processAfter(Executor executor, MappedStatement ms, Statement stmt, Object parameter) {
    //如果executeBefore配置為false則執行
    if (!executeBefore) {
      processGeneratedKeys(executor, ms, parameter);
    }
  }

具體執行源碼

 private void processGeneratedKeys(Executor executor, MappedStatement ms, Object parameter) {
    try {
      //如果parameter不為null同時keyStatement不為null且keyStatement 指定了keyProperties
      if (parameter != null && keyStatement != null && keyStatement.getKeyProperties() != null) {
        //獲取keyProperties
        String[] keyProperties = keyStatement.getKeyProperties();
        //獲取配置信息
        final Configuration configuration = ms.getConfiguration();
        //獲取參數對象元數據
        final MetaObject metaParam = configuration.newMetaObject(parameter);
        //其實已經判斷過了
        if (keyProperties != null) {
          //新建keyExecutor
          Executor keyExecutor = configuration.newExecutor(executor.getTransaction(), ExecutorType.SIMPLE);
          //執行查詢
          List<Object> values = keyExecutor.query(keyStatement, parameter, RowBounds.DEFAULT, Executor.NO_RESULT_HANDLER);
          //如果查詢結果為0個則拋出異常
          if (values.size() == 0) {
            throw new ExecutorException("SelectKey returned no data.");            
          } else if (values.size() > 1) {//查詢的結果個數多余1個則拋出異常
            throw new ExecutorException("SelectKey returned more than one value.");
          } else {//只返了一個結果值
            MetaObject metaResult = configuration.newMetaObject(values.get(0));
            //如果keyProperty個數只有1個
            if (keyProperties.length == 1) {
              //如果查詢結果對象存在這個屬性的getter方法
              if (metaResult.hasGetter(keyProperties[0])) {
                //將屬性值設置到param中
                setValue(metaParam, keyProperties[0], metaResult.getValue(keyProperties[0]));
              } else {
                //如果沒有getter方法就將當前值設置到屬性中
                setValue(metaParam, keyProperties[0], values.get(0));
              }
            } else {//處理指定多個key屬性場景
              handleMultipleProperties(keyProperties, metaParam, metaResult);
            }
          }
        }
      }
    } catch (ExecutorException e) {
      throw e;
    } catch (Exception e) {
      throw new ExecutorException("Error selecting key or setting result to parameter object. Cause: " + e, e);
    }
  }

  private void handleMultipleProperties(String[] keyProperties,
      MetaObject metaParam, MetaObject metaResult) {
    //獲取所有key  column
    String[] keyColumns = keyStatement.getKeyColumns();
      //如果key column不存在
    if (keyColumns == null || keyColumns.length == 0) {
      //沒有指定key column則直接使用配置到 key property
      for (String keyProperty : keyProperties) {
        setValue(metaParam, keyProperty, metaResult.getValue(keyProperty));
      }
    } else {
      //存在key column 但是數量不一致
      if (keyColumns.length != keyProperties.length) {
        throw new ExecutorException("If SelectKey has key columns, the number must match the number of key properties.");
      }
      //數量一致，要求keyColumn 和keyProperty一一對應
      for (int i = 0; i < keyProperties.length; i++) {
        setValue(metaParam, keyProperties[i], metaResult.getValue(keyColumns[i]));
      }
    }
  }

Mybatis中selectKey源碼分析