在Mybatis專案中，遇到一個需要返回布林值來確定某條記錄是否存在的需求。由於查詢的結果可能是0、1或者>1，就想確認一下，當resultType為boolean型別時，Mybatis是怎麼轉換的。查詢了一下部落格，一些部落格說：當記錄數行數是1時返回true，行數為0時返回false， 如果查詢到多條記錄(大於1)，返回的卻是false。這時我就產生了疑問，自己驗證了一下發現，其實當記錄數大於1時，返回的是true，而不是false。所以，凡事我們都要自己驗證一下，可能版本不同，結果也不同。多動手實踐才是真理。

以下是我的驗證：

資料庫User表資訊：

查詢語句：

  
    <select id="exist" parameterType="Integer" resultType="java.lang.Boolean">
        SELECT
           count(1)
        FROM
           user
        WHERE age = #{age};
    </select>

    <select id="getUserNumbers" parameterType="Integer" resultType="java.lang.Integer">
        SELECT
           count(1)
        FROM
           user
        WHERE age = #{age};
    </select>

    <select id="getUserInfo" parameterType="Integer" resultType="com.qiqi.juint.test.model.vo.UserVO">
        SELECT
        `id` as id,
        `age` as age,
        `name` as name
        FROM
           user
        WHERE age = #{age};
    </select>
 

    @Test
    public void test(){

        boolean exist = userMapper.exist(12);
        System.out.println("exist:  "+exist);

        int userNumbers = userMapper.getUserNumbers(12);
        System.out.println("userNumbers: "+userNumbers);

        List<UserVO> list = userMapper.getUserInfo(12);
        list.forEach(user-> System.out.println(user));

        boolean existAge100 = userMapper.exist(100);
        System.out.println("existAge100:  "+existAge100);
    }
 

結果：

10:36:01.492 [main] DEBUG c.q.juint.test.dao.UserMapper.exist - ==>  Preparing: SELECT count(1) FROM user WHERE age = ?; 
10:36:01.511 [main] DEBUG c.q.juint.test.dao.UserMapper.exist - ==> Parameters: 12(Integer)
10:36:01.527 [main] DEBUG c.q.juint.test.dao.UserMapper.exist - <==      Total: 1
10:36:01.530 [main] DEBUG org.mybatis.spring.SqlSessionUtils - Releasing transactional SqlSession [
 
[email protected]]
exist:  true
10:36:01.530 [main] DEBUG org.mybatis.spring.SqlSessionUtils - Fetched SqlSession [[email protected]] from current transaction
10:36:01.530 [main] DEBUG c.q.j.t.d.UserMapper.getUserNumbers - ==>  Preparing: SELECT count(1) FROM user WHERE age = ?; 
10:36:01.530 [main] DEBUG c.q.j.t.d.UserMapper.getUserNumbers - ==> Parameters: 12(Integer)
10:36:01.531 [main] DEBUG c.q.j.t.d.UserMapper.getUserNumbers - <==      Total: 1
10:36:01.531 [main] DEBUG org.mybatis.spring.SqlSessionUtils - Releasing transactional SqlSession [[email protected]]
userNumbers: 3
10:36:01.531 [main] DEBUG org.mybatis.spring.SqlSessionUtils - Fetched SqlSession [[email protected]] from current transaction
10:36:01.531 [main] DEBUG c.q.j.t.dao.UserMapper.getUserInfo - ==>  Preparing: SELECT `id` as id, `age` as age, `name` as name FROM user WHERE age = ?; 
10:36:01.531 [main] DEBUG c.q.j.t.dao.UserMapper.getUserInfo - ==> Parameters: 12(Integer)
10:36:01.535 [main] DEBUG c.q.j.t.dao.UserMapper.getUserInfo - <==      Total: 3
10:36:01.535 [main] DEBUG org.mybatis.spring.SqlSessionUtils - Releasing transactional SqlSession [[email protected]]
id:2  age:12  name:小花
id:3  age:12  name:小蘭
id:7  age:12  name:大師兄
10:36:01.535 [main] DEBUG org.mybatis.spring.SqlSessionUtils - Fetched SqlSession [[email protected]] from current transaction
10:36:01.535 [main] DEBUG c.q.juint.test.dao.UserMapper.exist - ==>  Preparing: SELECT count(1) FROM user WHERE age = ?; 
10:36:01.536 [main] DEBUG c.q.juint.test.dao.UserMapper.exist - ==> Parameters: 100(Integer)
10:36:01.536 [main] DEBUG c.q.juint.test.dao.UserMapper.exist - <==      Total: 1
10:36:01.537 [main] DEBUG org.mybatis.spring.SqlSessionUtils - Releasing transactional SqlSession [[email protected]]
existAge100:  false
10:36:01.537 [main] DEBUG org.mybatis.spring.SqlSessionUtils - Transaction synchronization deregistering SqlSession [[email protected]]
10:36:01.537 [main] DEBUG org.mybatis.spring.SqlSessionUtils - Transaction synchronization closing SqlSession [[email protected]

總結：

通過結果。我們可知，當記錄數行數是1時返回true，行數為0時返回false， 如果查詢到多條記錄(大於1)，返回true。