mybatis SelectKey解析

1.selectKey介绍及作用 

标签有如下属性 

resultType：sql返回的java类型

statementType：STATEMENT|PREPARED|CALLABLE三种默认PREPARED

keyProperty：列名对应的java属性名，可逗号分隔

keyColumn：列名，可逗号分隔

order：BEFORE|AFTER，BEFORE表示里的sql先执行然后再把获取到的值进行设置，AFTER则表示后执行，获取自增主键并设置肯定是需要用AFTER的，毕竟先等主sql插入才能获取到自增Id~

databaseId：数据库Id一般不需要填

mybatis的标签主要可以用来获取自增主键id的值并进行设置，SELECT LAST_INSERT_ID() 该sql的作用返回最近一次插入的id通常用来配合标签来使用 ,但要注意假如用insert同时插入多条sql，其只能返回插入的第一条记录的自增主键id因此是不支持批量插入获取主键值的

2.selectKey测试及解析

测试代码

#mapper
int insert(UserDO userDO);

#mapper.xml
  
     
       select last_insert_id()
     
     insert into user(username, password, nickname)
     values(#{username}, #{password}, #{nickname})
  
  

#java测试代码
public class Test {

  public static void main(String[] args) throws IOException {

    try (InputStream inputStream = Resources.getResourceAsStream("mybatis-config.xml")) {
      // 构建session工厂 DefaultSqlSessionFactory
      SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
      SqlSession sqlSession = sqlSessionFactory.openSession();
      UserMapper userMapper = sqlSession.getMapper(UserMapper.class);
      UserDO userDO = new UserDO();
      userDO.setUsername("monian");
      userDO.setPassword("123");
      userDO.setNickname("monianx");
      userMapper.insert(userDO);
      System.out.println("自增主键userId:" + userDO.getUserId());
    }
  }

}

从输出可以看到成功获取到自增主键userId并已经设置到userDO参数对象中了，下面来看看具体解析

public class PreparedStatementHandler extends BaseStatementHandler {

  public PreparedStatementHandler(Executor executor, MappedStatement mappedStatement, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) {
    super(executor, mappedStatement, parameter, rowBounds, resultHandler, boundSql);
  }

  @Override
  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;
  }
}

当为标签时会调用此update方法，执行完sql后调用 keyGenerator.processAfter(executor, mappedStatement, ps, parameterObject);  而根据标签解析出来的keyGenerator为SelectKeyGenerator，下面具体分析下这个类它是怎么获取主键值并设置的。

public class SelectKeyGenerator implements KeyGenerator {

  public static final String SELECT_KEY_SUFFIX = "!selectKey";
  private final boolean executeBefore;
  private final MappedStatement keyStatement;

  public SelectKeyGenerator(MappedStatement keyStatement, boolean executeBefore) {
    // 主sql前面执行还是后面执行
    this.executeBefore = executeBefore;
    this.keyStatement = keyStatement;
  }

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

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

  // 处理生成的键
  private void processGeneratedKeys(Executor executor, MappedStatement ms, Object parameter) {
    try {
      if (parameter != null && keyStatement != null && keyStatement.getKeyProperties() != null) {
        // 获取需要设置的属性值 如id
        String[] keyProperties = keyStatement.getKeyProperties();
        final Configuration configuration = ms.getConfiguration();
        final MetaObject metaParam = configuration.newMetaObject(parameter);
        // Do not close keyExecutor.
        // The transaction will be closed by parent executor.
        Executor keyExecutor = configuration.newExecutor(executor.getTransaction(), ExecutorType.SIMPLE);
        // 查询sql如 select last_insert_id()获取主键id
        List