Mybatis深入剖析 - 各种KeyGenerator的说明

KeyGenerator接口有三个实现类，SelectkeyGenerator,Jdbc3KeyGenerator,NoKeyGenerator

其中SelectkeyGenerator对应于生成主键操作，即针对insert类型的sql语句，配置selectKey的策略，自动为insert语句的参数补全id

Jdbc3KeyGenerator和NoKeyGenerator对应于insert写库，是否取回表的自增Id，它对应的Configuration的useGeneratedKeys全局配置，或者Mapper中insert操作的useGeneratedKeys属性，可以看到如果配置了，则生成Jdbc3KeyGenerator对象，没有配置则生成NoKeyGenerator对象，其中NoKeyGenerator的方法都为空，不产生任何效果

  keyGenerator = context.getBooleanAttribute("useGeneratedKeys",
          configuration.isUseGeneratedKeys() && SqlCommandType.INSERT.equals(sqlCommandType))
          ? new Jdbc3KeyGenerator(context.getStringAttribute("keyColumn", null)) : new NoKeyGenerator();

public class NoNoKeyGenerator implements KeyGenerator {

  public void processBefore(Executor executor, MappedStatement ms, Statement stmt, Object parameter) {
  }

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

}

SelectKeyGenerator

SelectKey在Mybatis中是为了解决Insert数据时不支持主键自动生成的问题。在实际应用中最好代码设置主键，或者简单使用数据库自增主键。

SelectKey需要注意order属性，像Mysql一类支持自动增长类型的数据库中，order需要设置为after才会取到正确的值。

像Oracle这样取序列的情况，需要设置为before，否则会报错。

另外在用Spring管理事务时，SelectKey和插入在同一事务当中，因而Mysql这样的情况由于数据未插入到数据库中，所以是得不到自动增长的Key。取消事务管理就不会有问题。

下面是一个xml和注解的例子，SelectKey很简单，两个例子就够了：

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1. <insert id="insert" parameterType="map">  
2.     insert into table1 (name) values (#{name})  
3.     <selectKey resultType="java.lang.Integer" keyProperty="id">  
4.       CALL IDENTITY()  
5.     </selectKey>  
6.   </insert>  

上面xml的传入参数是map，selectKey会将结果放到入参数map中。用POJO的情况一样，但是有一点需要注意的是，keyProperty对应的字段在POJO中必须有相应的setter方法，setter的参数类型还要一致，否则会报错。

在statementHandler的数据库操作前后，例如准备parameter时执行processBefore，update之后执行processAfter,SelectKeyGenerator根据mapper里配置的order判断是否执行获取id操作

  public void processBefore(Executor executor, MappedStatement ms, Statement stmt, Object parameter) {
    if (executeBefore) {
      processGeneratedKeys(executor, ms, stmt, parameter);
    }
  }

  public void processAfter(Executor executor, MappedStatement ms, Statement stmt, Object parameter) {
    if (!executeBefore) {
      processGeneratedKeys(executor, ms, stmt, parameter);
    }
  }

private void processGeneratedKeys(Executor executor, MappedStatement ms, Statement stmt, Object parameter) {
    try {
      final Configuration configuration = ms.getConfiguration();
      if (parameter != null) {
        String keyStatementName = ms.getId() + SELECT_KEY_SUFFIX;
        if (configuration.hasStatement(keyStatementName)) {

          if (keyStatement != null) {
            String keyProperty = keyStatement.getKeyProperty();
            final MetaObject metaParam = configuration.newMetaObject(parameter);
            if (keyProperty != null && metaParam.hasSetter(keyProperty)) {
              // Do not close keyExecutor.
              // The transaction will be closed by parent executor.
              Executor keyExecutor = configuration.newExecutor(executor.getTransaction(), ExecutorType.SIMPLE);
              List values = keyExecutor.query(keyStatement, parameter, RowBounds.DEFAULT, Executor.NO_RESULT_HANDLER);
              if (values.size() > 1) {
                throw new ExecutorException("Select statement for SelectKeyGenerator returned more than one value.");
              }
              metaParam.setValue(keyProperty, values.get(0));
            }
          }
        }
      }
    } catch (Exception e) {
      throw new ExecutorException("Error selecting key or setting result to parameter object. Cause: " + e, e);
    }
  }

Jdbc3KeyGenerator

 statment中useGeneratedKeys属性对应的类，该配置的作用是调用java.sql.Statement.getGeneratedKeys（）获取自动生成的主键创建此Statement对象执行的结果。如果此Statement对象没有产生任何键，则返回空的ResultSet对象。

可以看到该类作用于插入操作结束后，同样是借助keyProperty配置，将取回的id回写到传入的param对象中

public void processBefore(Executor executor, MappedStatement ms, Statement stmt, Object parameter) {
  }

  public void processAfter(Executor executor, MappedStatement ms, Statement stmt, Object parameter) {
    try {
      final Configuration configuration = ms.getConfiguration();
      final TypeHandlerRegistry typeHandlerRegistry = configuration.getTypeHandlerRegistry();
      if (parameter != null) {
        String keyProperty = ms.getKeyProperty();
        final MetaObject metaParam = configuration.newMetaObject(parameter);
        if (keyProperty != null && metaParam.hasSetter(keyProperty)) {
          Class<?> keyPropertyType = metaParam.getSetterType(keyProperty);
          TypeHandler th = typeHandlerRegistry.getTypeHandler(keyPropertyType);
          if (th != null) {
            ResultSet rs = stmt.getGeneratedKeys();
            try {
              ResultSetMetaData rsmd = rs.getMetaData();
              int colCount = rsmd.getColumnCount();
              if (colCount > 0) {
                String colName;
                if (keyColumnName != null && keyColumnName.length() > 0) {
                  colName = keyColumnName;
                } else {
                  colName = rsmd.getColumnName(1);
                }
              
                while (rs.next()) {
                  Object value = th.getResult(rs, colName);
                  metaParam.setValue(keyProperty, value);
                }
              }
            } finally {
              try {
                if (rs != null) rs.close();
              } catch (Exception e) {
                //ignore
              }
            }
          }
        }
      }
    } catch (Exception e) {
      throw new ExecutorException("Error getting generated key or setting result to parameter object. Cause: " + e, e);
    }
  }