有许多选项可以使用 jdbcTemplate 映射结果，包括您的。
也许这会帮助你更好地理解它：
public List<Action> findAllActions() {
final String selectStatement = "SELECT id,name FROM Actions"; // or your query
try {
return jdbcTemplate.query(selectStatement,(resultSet, rowNum) -> {
int actionId = resultSet.getInt("id");
String actionName = resultSet.getString("name");
Action action = new Action();
action.setId(actionId);
action.setName(actionName);
return action;
});
} catch (EmptyResultDataAccessException e) {
LOGGER.error("Get all actions - empty set", e);
return Collections.emptyList();
}
}
如果没有 lambda 表达式，您可以像这样使用 jdbcTemplate 查询：
public List<Action> findAllActions() {
final String selectStatement = "SELECT id,name FROM Actions"; // or your query
try {
return jdbcTemplate.query(selectStatement, getActionRowMapper());
} catch (EmptyResultDataAccessException e) {
LOGGER.error("Get all actions - empty set", e);
return Collections.emptyList();
}
}
private RowMapper<Action> getActionRowMapper() {
return (resultSet, rowNum) -> {
int actionId = resultSet.getInt("id");
String actionName = resultSet.getString("name");
return action;
};
}
如您所见，jdbcTemplate.query 方法的第二个参数采用RowMapper<Action> 类型，但使用 lambdas 表达式隐藏。除了为每一行映射结果集并返回结果之外，此选项“限制”您进行任何其他操作。最终结果最终将是List 的操作。
第二个选项是使用ResultSetExtractor，它可以让您循环遍历结果集并为您提供更大的灵活性。查询是一样的，只是 jdbcTemplate.query 方法的第二个参数会改变。为此，我会亲自实现 ResultSetExtractor 并覆盖 extractData 方法，或者如果您不需要其他任何东西来映射结果，您也可以执行与上述相同的操作
public List<Group> findGroups() {
final String selectStatement = "SELECT stud.id, stud.name, gr.id, gr.name FROM student stud INNER JOIN group gr ON gr.id=stud.group_id ORDER BY stud.id"; // or your query
try {
return jdbcTemplate.query(selectStatement, new GroupExtractor() );
} catch (EmptyResultDataAccessException e) {
LOGGER.error("Get all groups - empty set", e);
return Collections.emptyList();
}
}
public class GroupExtractor implements ResultSetExtractor<List<Group>> {
@Override
public List<Group> extractData(ResultSet resultSet) {
Map<Group, List<Student>> studentsGroup= new HashMap<>();
List<Group> groups = new ArrayList<>();
try {
while (resultSet.next()) {
int studentId = resultSet.getInt("stud.id");
String studentName = resultSet.getString("stud.name");
int groupId = resultSet.getInt("gr.id");
String groupName = resultSet.getString("gr.name");
Group group = createGroup(groupId, groupName);
Student student = createStudent(studentId, studentName);
studentsGroup.putIfAbsent(group, new ArrayList<>());
studentsGroup.get(group).add(student);
}
studentsGroup.forEach((group,students) ->{
group.setStudents(students);
groups.add(group);
}
return groups;
} catch (SQLException e) {
LOGGER.info("An error occured during extracting data", e);
}
return actions;
}
private Student createStudent(String studentId, String studentName)
{
Student student=new Student();
student.setId(studentId);
student.setName(studentName);
return student;
}
//idem for createGroup
}