今天同事和我分享關於BeanPropertyRowMapper回傳List<String>長度大於零但內容為空的現象。
同事發現用NamedParameterJdbcTemplate.query()搭配BeanPropertyRowMapper<String.class>回傳的List不是空的且長度正確但裡面的元素卻是null。
測試環境：
- Spring Boot 2.3.2.RELEASE
- Spring Core 5.2.8.RELEASE
例如資料表EMPLOYEE有資料如下。
+----+------+-----+ | ID | NAME | AGE | +----+------+-----+ | 1 | John | 30 | | 2 | Iris | 31 | | 3 | Dave | 25 | | 4 | Marl | 19 | +----+------+-----+
下面EmployeeDao使用NamedParameterJdbcTemplate查詢資料。
EmployeeDao
package com.abc.demo.entity.dao;
import com.abc.demo.entity.Employee;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.jdbc.core.BeanPropertyRowMapper;
import org.springframework.jdbc.core.namedparam.MapSqlParameterSource;
import org.springframework.jdbc.core.namedparam.NamedParameterJdbcTemplate;
import org.springframework.stereotype.Service;
import java.util.List;
@Service
public class EmployeeDao {
@Autowired
private NamedParameterJdbcTemplate namedParameterJdbcTemplate;
public List<Employee> getEmployeeListByAboveAge(int age) {
String sql = "select * from employee e where age > :age order by id";
MapSqlParameterSource params = new MapSqlParameterSource();
params.addValue("age", age);
return namedParameterJdbcTemplate.query(
sql,
params,
new BeanPropertyRowMapper<>(Employee.class));
}
public List<String> getNameListByAboveAge(int age) {
String sql = "select e.name from employee e where age > :age order by id";
MapSqlParameterSource params = new MapSqlParameterSource();
params.addValue("age", age);
return namedParameterJdbcTemplate.query(
sql,
params,
new BeanPropertyRowMapper<>(String.class));
}
}
依條件getNameListByAboveAge()回傳的List長度為3，但裡面的elements卻都是空字串。原因出在BeanPropertyRowMapper是用泛型類別的屬性名稱去對映資料表的欄位名稱塞值，但BeanPropertyRowMapper<String.class>的String並沒有屬性來對應欄位名稱NAME，又BeanPropertyRowMapper可能利用反射產生裝載查詢結果的物件，也就是產生了String又無法正確對映塞值所以造成上述結果。
測試如下。
EmployeeRepositoryTests
package com.abc.demo.repository;
import com.abc.demo.entity.Employee;
import com.abc.demo.entity.dao.EmployeeDao;
import org.junit.jupiter.api.Assertions;
import org.junit.jupiter.api.Test;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.test.context.SpringBootTest;
import java.util.List;
@SpringBootTest
class EmployeeRepositoryTests {
@Autowired
private EmployeeDao employeeDao;
@Test
void getEmployeeListByAboveAge_returnListLengthIs3WithCorrectName() {
List<Employee> resultList = employeeDao.getEmployeeListByAboveAge(20);
System.out.println(resultList); // [Employee(id=1, name=John, age=30), Employee(id=2, name=Iris, age=31), Employee(id=3, name=Dave, age=25)]
for (Employee employee : resultList) {
System.out.println(employee.getName());
}
Assertions.assertAll(
() -> Assertions.assertEquals(3, resultList.size()),
() -> Assertions.assertEquals("John", resultList.get(0).getName()),
() -> Assertions.assertEquals("Iris", resultList.get(1).getName()),
() -> Assertions.assertEquals("Dave", resultList.get(2).getName())
);
}
@Test
void getNameListByAboveAge_returnListLengthIs3ButElementsAreNull() {
List<String> resultList = employeeDao.getNameListByAboveAge(20);
System.out.println(resultList); // [, , ]
for (String name : resultList) {
System.out.println(name);
}
Assertions.assertAll(
() -> Assertions.assertEquals(3, resultList.size()),
() -> Assertions.assertEquals(0, resultList.get(0).length()),
() -> Assertions.assertEquals(0, resultList.get(1).length()),
() -> Assertions.assertEquals(0, resultList.get(2).length())
);
}
}
測試getEmployeeListByAboveAge_returnListLengthIs3WithCorrectName印出以下。
[Employee(id=1, name=John, age=30), Employee(id=2, name=Iris, age=31), Employee(id=3, name=Dave, age=25)]
John
Iris
Dave
測試getNameListByAboveAge_returnListLengthIs3ButElementsAreNull印出以下。
[, , ]
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