Mapping relational DB to a List each containing a List using JdbcTemplate

问题

I am using Spring MVC with JdbcTemplate and a MySQL database.

Say I have the following 2 tables :

table_school

ID    NAME

table_students

ID    NAME    ADDRESS    SCHOOL_ID

I have a School POJO that has the following class variables :

int id, String name, List<Student> students

Is there a way of retrieving a List with each School object containing the appropriate List of Student objects using JdbcTemplate in one query? I know this is easily achievable using Hibernate but I would like to use JdbcTemplate ..

Many thanks !

回答1:

Yes, you can fetch all data in 1 query.

Simple example:

class Student {
    int id;
    String name;
    String addr;

    Student(int id, String name, String addr) {
        this.addr = addr;
        this.id = id;
        this.name = name;
    }
}

class School {
    int id;
    String name;
    List<Student> students = new ArrayList<>();

    School(int id, String name) {
        this.id = id;
        this.name = name;
    }

    void addStudent(Student s) {
        students.add(s);
    }
}

/*
 * helper method that gets school from map or create if not present
 */
private School getSchool(Map<Integer, School> schoolMap, int id, String name) {
    School school = schoolMap.get(id);
    if (school == null) {
        school = new School(id, name);
        schoolMap.put(id, school);
    }
    return school;
}

// RUN QUERY

String sql = 
     " select st.ID, st.NAME, st.ADDRESS. s.id, s.name" +
     " from table_students st" +
     "   inner join table_school s on st.school_id = s.id";

    final Map<Integer, School> schoolMap = new HashMap<>();

    jdbcTemplate.query(sql, new RowCallbackHandler() {

        @Override
        public void processRow(ResultSet rs) throws SQLException {

            int studentId = rs.getInt(1);
            String studentName = rs.getString(2);
            String studentAddr = rs.getString(3);
            int schoolId = rs.getInt(4);
            String schoolName = rs.getString(5);

            Student student = new Student(studentId, studentName, studentAddr);

            getSchool(schoolMap, schoolId, schoolName).addStudent(student);
        }
    });

One final point regarding fetching performance:

If you expect many records to fetch it is nearly always a good idea to increase jdbc fetch size parameter. So before run query set it on your jdbcTemplate:

jdbcTemplate.setFetchSize(200);  // you can experiment with this value

or if you are using spring's JdbcDaoSupport you can use such pattern:

public class MyDao extends JdbcDaoSupport {

    ....

    @Override
    protected void initTemplateConfig() {
        getJdbcTemplate().setFetchSize(200);
    }

}

来源：https://stackoverflow.com/questions/25727099/mapping-relational-db-to-a-listobject-each-containing-a-listobject-using-jdb

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java