How does MyBatis determine if a list is empty or null?

2 years ago
Benjamin Taylor
1 minute

In MyBatis, you can use the tag to determine if a List is empty or null.

The example code is as follows:

<select id="selectUsers" resultType="User">
  SELECT * FROM users
  WHERE id IN
  <foreach item="item" collection="ids" open="(" separator="," close=")">
    #{item}
  </foreach>
  <if test="ids == null or ids.isEmpty()">
    AND 1=0
  </if>
</select>

In the code above, the tag is used to check if ids is null or empty. If it is, the SQL will add the condition AND 1=0 to ensure that the query result is empty.

Additionally, if you need to check if a List is empty or null in Java code, you can also do the check before passing the parameter to the SQL statement in MyBatis.

The sample code is as follows:

public List<User> selectUsers(List<Integer> ids) {
  if (ids == null || ids.isEmpty()) {
    return Collections.emptyList();
  }
  // 调用MyBatis的SQL语句查询
}

In the above code, first check if ids is null or empty, if it is, simply return an empty List to avoid calling MyBatis SQL statements.

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