“不能从静态上下文引用非静态方法". JPA Java [英] "non static method cannot be referenced from a static context" JPA Java

问题描述

我从此行收到无法从静态上下文引用非静态方法"错误:

I'm getting the "non static method cannot be referenced from a static context" error from this line:

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

您如何正确形成日期"?我看过API并尝试了不同的方法，但仍然收到日期错误.

How do you form 'Date' properly? I've had a look on the API's and tried different things but I still get an error for date.

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;
}
}

推荐答案

问题是您正在尝试从main()中的静态上下文调用实例方法createStudent().如果将您的createStudent()方法更改为静态方法，则应该可以进行以下操作:

The problem is that you're trying to call the instance method createStudent() from a static context in main(). If you change your createStudent() method to be static, you should be good to go:

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

编辑:OP指出，访问变量em和emf时，仅此更改就给他带来了另一个错误.要解决此问题，您还需要将这些变量设置为static:

EDIT: OP pointed out that this change alone gives him another error when accessing the variables em and emf. To fix that, you'd need to make those variables static, too:

static EntityManager em;
static EntityManagerFactory emf;

那时，您班级中的所有内容都是静态的.假设这是一个简单的单例或示例(由于该类称为Main，我很容易假设)，则使所有内容静态化就可以了.总的来说，代码看起来像这样:

At that point, everything in your class is static. Assuming this is a simple one-off or example -- which I'm comfortable assuming since the class is called Main -- making everything static is just fine. In all, the code would look like this:

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;
    }
}

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