调用persist方法触发插入查询hiberante

Question

这是我使用身份注释时的学生随身听

            package mappingdemo;

            import javax.persistence.Entity;
            import javax.persistence.GeneratedValue;
            import javax.persistence.GenerationType;
            import javax.persistence.Id;


            @Entity(name="student")

            public class Student {

            @GeneratedValue(strategy=GenerationType.IDENTITY)   
            @Id
            int id;
            String firstName;
            String lastName;

            public int getId() {
                return id;
            }

            public void setId(int id) {
                this.id = id;
            }

            public String getFirstName() {
                return firstName;
            }

            public void setFirstName(String firstName) {
                this.firstName = firstName;
            }

            public String getLastName() {
                return lastName;
            }

            public void setLastName(String lastName) {
                this.lastName = lastName;
            }

            @Override
            public String toString() {
                return "Student [id=" + id + ", firstName=" + firstName + ", lastName=" + lastName + "]";
            }

            }

这是主要班级

    public class SetStudentDetails {

            public static void main(String ar[])
            {
            SessionFactory sfac=null;
            sfac=new Configuration().configure("hibernateconfig/rs.xml").buildSessionFactory();

            Session session = sfac.getCurrentSession();
            session.beginTransaction();

            Student stu=new Student();
            stu.setFirstName("abc23");
            stu.setLastName("xyz");
            System.out.println("-------------"+stu);
            session.persist(stu);

            System.out.println("after saving obj"+stu);
            session.getTransaction().commit();
            }
        }

这是输出：-

        -------------Student [id=0, firstName=abc23, lastName=xyz]
        Hibernate: 
            insert 
            into
                student
                (firstName, lastName) 
            values
                (?, ?)
        after saving objStudent [id=14, firstName=abc23, lastName=xyz]

这里在调用persist（）时为什么要运行插入查询，并且在提交时不会触发任何查询。

如果我在学生pojo中评论了@GeneratedValue（strategy = GenerationType.IDENTITY）并在主类中使用setId，例如-

            package mappingdemo;

            import javax.persistence.Entity;
            import javax.persistence.GeneratedValue;
            import javax.persistence.GenerationType;
            import javax.persistence.Id;


            @Entity(name="student")

            public class Student {

                //@GeneratedValue(strategy=GenerationType.IDENTITY) 
                @Id
                int id;
                String firstName;
                String lastName;

                public int getId() {
                    return id;
                }

                public void setId(int id) {
                    this.id = id;
                }

                public String getFirstName() {
                    return firstName;
                }

                public void setFirstName(String firstName) {
                    this.firstName = firstName;
                }

                public String getLastName() {
                    return lastName;
                }

                public void setLastName(String lastName) {
                    this.lastName = lastName;
                }

                @Override
                public String toString() {
                    return "Student [id=" + id + ", firstName=" + firstName + ", lastName=" + lastName + "]";
                }

            }

这是主要课程-

            package mappingdemo;

            import org.hibernate.Session;
            import org.hibernate.SessionFactory;
            import org.hibernate.Transaction;
            import org.hibernate.cfg.Configuration;

            public class SetStudentDetails {

                public static void main(String ar[])
                {
                    SessionFactory sfac=null;

                    sfac=new Configuration().configure("hibernateconfig/rs.xml")

                            .buildSessionFactory();

                    Session session = sfac.getCurrentSession();
                    session.beginTransaction();

                    Student stu=new Student();
                    stu.setId(17);
                    stu.setFirstName("abc23");
                    stu.setLastName("xyz");
                    System.out.println("-------------"+stu);
                    session.persist(stu);

                    System.out.println("after saving obj"+stu);
                    session.getTransaction().commit();
                }

            }

然后输出将是：-

            -------------Student [id=17, firstName=abc23, lastName=xyz]
            after saving objStudent [id=17, firstName=abc23, lastName=xyz]
            Hibernate: 
                insert 
                into
                    student
                    (firstName, lastName, id) 
                values
                    (?, ?, ?)

这里将运行commit（）插入。“不运行查询”意味着在我不使用@GeneratedValue（strategy = GenerationType.IDENTITY）的情况下，调用持久化（）时没有查询正在运行。