java - JPA 更新没有效果

Question

我是 JPA 和 Hibernate 的新手。我试图了解如何更新各个实体中的属性值。以我正在编写的代码为例 - :

Semester sem = entitymanager.find(Semester.class, id);

sem.setSemNum(3);

entitymanager.getTransaction().commit();

此代码对数据库没有任何影响，当我查询学期表时，没有值发生更改，日志中也没有生成更新查询。据此tutorial您获得更改值的对象，然后将其提交回来。

我做错了什么？

这是完整的代码:

学期测试.java

public class SemesterTest {

    private static EntityManagerFactory entityManagerFactory;
    public static void main(String[] args) {
            EntityManager entitymanager = null;

            try {
            entityManagerFactory = Persistence.createEntityManagerFactory("com.test.hibernate.jpa");

            entitymanager = entityManagerFactory.createEntityManager();
            entitymanager.getTransaction().begin();

            Course course = new Course();

            course.setCourseId("BCA");
            course.setCourseName("Bachelor of Computer Applications");

            Semester sem1 = new Semester();
            sem1.setSemNum(1);

            Semester sem2 = new Semester();
            sem2.setSemNum(2);

            ArrayList<Semester> semesters = new ArrayList<>();

            semesters.add(sem1);
            semesters.add(sem2);

            course.setSemesters(semesters);

            sem1.setCourse(course);
            sem2.setCourse(course);

            entitymanager.persist(course);
            entitymanager.persist(sem1);
            entitymanager.persist(sem2);

            entitymanager.getTransaction().commit();

            entitymanager.getTransaction().begin();

            Semester id = new Semester();

            id.setCourse(course);
            id.setSemNum(1);

            Semester sem = entitymanager.find(Semester.class, id);

            sem.setSemNum(3);

            entitymanager.getTransaction().commit();
            } 

            catch(Exception e) {
                e.printStackTrace();
                if(entitymanager!=null)
                entitymanager.getTransaction().rollback();;
            } 

            finally {
                if(entitymanager!=null)
                entitymanager.close();

                if(entityManagerFactory!=null)
                entityManagerFactory.close();
            }

        }


}

类(class).java

@Entity
@Table (name = "COURSES")
public class Course implements Serializable{

    private static final long serialVersionUID = 1L;

    private String courseId;
    private String courseName;

    private Collection<Semester> semesters = new ArrayList<>();



    @OneToMany (targetEntity = Semester.class, mappedBy = "course")
    public Collection<Semester> getSemesters() {
        return semesters;
    }
    public void setSemesters(Collection<Semester> semesters) {
        this.semesters = semesters;
    }
    @Id
    @Column (name = "COURSE_ID")
    public String getCourseId() {
        return courseId;
    }
    public void setCourseId(String courseId) {
        this.courseId = courseId;
    }
    @Column (name = "COURSE_NAME", nullable = false, unique = true)
    public String getCourseName() {
        return courseName;
    }
    public void setCourseName(String courseName) {
        this.courseName = courseName;
    }




}

学期.java

@Entity
@Table (name = "SEMESTERS")
public class Semester implements Serializable{

    private static final long serialVersionUID = 1L;

    private int semNum;

    private Course course;

    @EmbeddedId
    private SemesterPK pk;

    @Id
    @ManyToOne
    @JoinColumn (name = "COURSE_ID")
    public Course getCourse() {
        return course;
    }

    public void setCourse(Course course) {
        this.course = course;
    }

    @Id
    @Column (name = "SEM_NUM")
    public int getSemNum() {
        return semNum;
    }

    public void setSemNum(int semNum) {
        this.semNum = semNum;
    }   

}

持久性.xml

<?xml version="1.0" encoding="UTF-8"?>
<persistence version="2.1" xmlns="http://xmlns.jcp.org/xml/ns/persistence" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/persistence http://xmlns.jcp.org/xml/ns/persistence/persistence_2_1.xsd">
    <persistence-unit name="com.test.hibernate.jpa" transaction-type="RESOURCE_LOCAL">
        <description>A Sample Hibernate Application</description>
        <class>test.entity.Course</class>
        <class>test.entity.Semester</class>
        <properties>
            <property name="javax.persistence.jdbc.url" value="jdbc:mysql://localhost:3306/practice"/>
            <property name="javax.persistence.jdbc.user" value=""/>
            <property name="javax.persistence.jdbc.password" value=""/>
            <property name="javax.persistence.jdbc.driver" value="com.mysql.jdbc.Driver"/>

            <property name="hibernate.dialect" value="org.hibernate.dialect.MySQLDialect" />
            <property name="hibernate.show_sql" value="true" />
            <property name="hibernate.hbm2ddl.auto" value="create" />
            <property name="hibernate.cache.provider_class" value="org.hibernate.cache.NoCacheProvider"/>
        </properties>
    </persistence-unit>
</persistence>

Answer 1

ID 是不可变的。你可能无法改变它。做正确的事情并使用单列、自动生成的纯技术 ID。