Mybatis获取自增主键的两种实现原理小结

<insert id="insertUser" useGeneratedKeys="true" keyProperty="id">
  INSERT INTO user (username, age) VALUES (#{username}, #{age})
</insert>

<insert id="insertUser">
  <!-- order="AFTER" 表示插入后执行查询 -->
  <selectKey keyProperty="id" resultType="java.lang.Long" order="AFTER">
    SELECT LAST_INSERT_ID AS id -- MySQL 专用函数，返回当前会话最后生成的自增主键
  </selectKey>
  INSERT INTO user (username, age) VALUES (#{username}, #{age})
</insert>

    <insert id="insertUser">
      <!-- order="BEFORE" 表示插入前先查询序列 -->
      <selectKey keyProperty="id" resultType="java.lang.Long" order="BEFORE">
        SELECT USER_SEQ.NEXTVAL FROM DUAL -- 查询序列的下一个值
      </selectKey>
      INSERT INTO user (id, username, age) VALUES (#{id}, #{username}, #{age})
    </insert>

public interface KeyGenerator {
  // 插入操作执行前调用（如 Oracle 序列查询）
  void processBefore(Executor executor, MappedStatement ms, Statement stmt, Object parameter);
  
  // 插入操作执行后调用（如 MySQL 自增主键获取）
  void processAfter(Executor executor, MappedStatement ms, Statement stmt, Object parameter);
}

// PreparedStatementHandler.java
@Override
protected Statement prepare(Connection connection, Integer transactionTimeout) throws SQLException {
  PreparedStatement ps;
  String sql = boundSql.getSql();
  
  // 若使用 Jdbc3KeyGenerator，设置 RETURN_GENERATED_KEYS 标志
  if (mappedStatement.getKeyGenerator() instanceof Jdbc3KeyGenerator) {
    ps = connection.prepareStatement(sql, Statement.RETURN_GENERATED_KEYS);
  } else {
    ps = connection.prepareStatement(sql);
  }
  // 设置超时时间等
  return ps;
}

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

public void processBatch(MappedStatement ms, Statement stmt, Collection<Object> parameters) {
  ResultSet rs = null;
  try {
    //核心：使用getGeneratedKeys方法获取主键
    rs = stmt.getGeneratedKeys();
    final Configuration configuration = ms.getConfiguration();
    final TypeHandlerRegistry typeHandlerRegistry = configuration.getTypeHandlerRegistry();
    //获取参数对象主键属性
    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) {
          //获取主键对应的typeHandlers
          typeHandlers = getTypeHandlers(typeHandlerRegistry, metaParam, keyProperties, rsmd);
        }
        //反射设置到参数中
        populateKeys(rs, metaParam, keyProperties, typeHandlers);
      }
    }
  }
  //省略一些异常处理的代码和关闭ResultSet的代码
}

// SelectKeyGenerator.java
@Override
public void processBefore(Executor executor, MappedStatement ms, Statement stmt, Object parameter) {
  if (executeBefore) { // order="BEFORE" 时执行
    processGeneratedKeys(executor, ms, parameter);
  }
}

@Override
public void processAfter(Executor executor, MappedStatement ms, Statement stmt, Object parameter) {
  if (!executeBefore) { // order="AFTER" 时执行
    processGeneratedKeys(executor, ms, parameter);
  }
}

private void processGeneratedKeys(Executor executor, MappedStatement ms, Object parameter) {
    try {
        if (parameter != null && keyStatement != null && keyStatement.getKeyProperties() != null) {
            String[] keyProperties = keyStatement.getKeyProperties();
            final Configuration configuration = ms.getConfiguration();
            final MetaObject metaParam = configuration.newMetaObject(parameter);
            // 创建执行器，执行主键查询操作
            Executor keyExecutor = configuration.newExecutor(executor.getTransaction(), ExecutorType.SIMPLE);
            // 执行查询主键的操作
            List<Object> values = keyExecutor.query(keyStatement, parameter, RowBounds.DEFAULT, Executor.NO_RESULT_HANDLER);
            if (values.size() == 0) {
                throw new ExecutorException("SelectKey returned no data.");
            } else if (values.size() > 1) {
                throw new ExecutorException("SelectKey returned more than one value.");
            } else {
                // 创建 MetaObject 对象，访问查询主键的结果
                MetaObject metaResult = configuration.newMetaObject(values.get(0));
                // 单个主键
                if (keyProperties.length == 1) {
                    // 设置属性到 metaParam 中，相当于设置到 parameter 中
                    if (metaResult.hasGetter(keyProperties[0])) {
                        setValue(metaParam, keyProperties[0], metaResult.getValue(keyProperties[0]));
                    } else {
                        setValue(metaParam, keyProperties[0], values.get(0));
                    }
                // 多个主键
                } else {
                    // 遍历，进行赋值
                    handleMultipleProperties(keyProperties, metaParam, metaResult);
                }
            }
        }
    }
    //省略一些异常处理
}