"非静态方法不能从静态上下文引用"JPA Java

Question

我从这一行得到"非静态方法无法从静态上下文引用"错误:

createStudent("stu00001", new Date(631152000000)), "m", "WB", new Type_Name("Bob", "", "Smith"));

你如何正确地形成'日期'？我已经看过API并尝试了不同的东西,但我仍然收到错误的日期.

package grade_db;

import bean.Student;
import bean.Type_Name;
import bean.University;
import java.util.Date;
import javax.persistence.EntityManager;
import javax.persistence.EntityManagerFactory;
import javax.persistence.Persistence;
/**
 *
 * @author Sam
 */

public class Main {

EntityManager em;
EntityManagerFactory emf;

    /**
     * @param args the command line arguments
     */
    public static void main(String[] args) {
        // TODO code application logic here

        EntityManagerFactory emf = Persistence.createEntityManagerFactory("db/grades.odb");

        EntityManager em;
        em = emf.createEntityManager();

        createStudent("stu00001", new Date(631152000000)), "m", "WB", new Type_Name("Bob", "", "Smith"));

        em.close();
        emf.close();
    }


    public Student createStudent(String student_id, Date dob, String gender, String nationality, Type_Name name){
    Student stu = new Student();
    stu.setDob(dob);
    stu.setGender(gender);
        stu.setName(name);
    stu.setNationality(nationality);

        stu.setCampus_id("cam00001");
        stu.setCourse_id(null);
        stu.setStudent_id(student_id);

    em.persist(stu);
    return stu;
}
}

Answer 1

问题是你试图createStudent()从静态上下文中调用实例方法main().如果你改变你的createStudent()方法是静态的,你应该好好去:

public static Student createStudent(String student_id, Date dob, String gender, String nationality, Type_Name name) {
    // ... And so on 
}

编辑:OP指出,这一变化在访问变量时,单独给他另一个错误em和emf.要解决这个问题,你还需要制作这些变量static:

static EntityManager em;
static EntityManagerFactory emf;

那时,你班上的一切都是静态的.假设这是一个简单的一次性或示例 - 我很自在地假设这个类被调用Main- 让一切都静止就好了.总之,代码看起来像这样:

package grade_db;

import bean.Student;
import bean.Type_Name;
import bean.University;
import java.util.Date;
import javax.persistence.EntityManager;
import javax.persistence.EntityManagerFactory;
import javax.persistence.Persistence;
/**
 *
 * @author Sam
 */

public class Main {

    static EntityManager em;
    static EntityManagerFactory emf;

    /**
     * @param args the command line arguments
     */
    public static void main(String[] args) {
        // TODO code application logic here

        EntityManagerFactory emf = Persistence.createEntityManagerFactory("db/grades.odb");

        EntityManager em;
        em = emf.createEntityManager();

        createStudent("stu00001", new Date(631152000000)), "m", "WB", new Type_Name("Bob", "", "Smith"));

        em.close();
        emf.close();
    }

    public static Student createStudent(String student_id, Date dob, String gender, String nationality, Type_Name name){
        Student stu = new Student();
        stu.setDob(dob);
        stu.setGender(gender);
        stu.setName(name);
        stu.setNationality(nationality);

        stu.setCampus_id("cam00001");
        stu.setCourse_id(null);
        stu.setStudent_id(student_id);

        em.persist(stu);
        return stu;
    }
}