Annotation @ManyToOne(fetch=FetchType.EAGER)为什么会多一条select语句啊？

qq343675979 2010-11-24 04:44:56

课程，学生，成绩三张表
这是成绩表：



package com.wanczy.student.pojo;



import javax.persistence.CascadeType;

import javax.persistence.Column;

import javax.persistence.Entity;

import javax.persistence.FetchType;

import javax.persistence.GeneratedValue;

import javax.persistence.GenerationType;

import javax.persistence.Id;

import javax.persistence.ManyToOne;

import javax.persistence.Table;



@Entity

@Table(name="tb_grade")

public class Grade {

	@Id

	@GeneratedValue(strategy=GenerationType.AUTO)

	@Column(name="grad_id")

	private Integer gradId;

	@ManyToOne(fetch=FetchType.EAGER)

	private Student student;

	@ManyToOne(fetch=FetchType.EAGER)

	private Course course;

	private Float score;

	

	

	public Grade() {

		super();

	}

	private Integer valid;

	public Integer getGradId() {

		return gradId;

	}

	public void setGradId(Integer gradId) {

		this.gradId = gradId;

	}

	public Student getStudent() {

		return student;

	}

	public void setStudent(Student student) {

		this.student = student;

	}

	public Course getCourse() {

		return course;

	}

	public void setCourse(Course course) {

		this.course = course;

	}

	public Float getScore() {

		return score;

	}

	public void setScore(Float score) {

		this.score = score;

	}

	public Integer getValid() {

		return valid;

	}

	public void setValid(Integer valid) {

		this.valid = valid;

	}

}

这是学生表：



package com.wanczy.student.pojo;



import java.util.Date;

import java.util.HashSet;

import java.util.Set;



import javax.persistence.CascadeType;

import javax.persistence.Column;

import javax.persistence.Entity;

import javax.persistence.FetchType;

import javax.persistence.GeneratedValue;

import javax.persistence.GenerationType;

import javax.persistence.Id;

import javax.persistence.OneToMany;

import javax.persistence.Table;

import javax.persistence.Temporal;

import javax.persistence.TemporalType;



@Entity

@Table(name="tb_student")

public class Student {

	

	@Id

	@GeneratedValue(strategy=GenerationType.AUTO)

	@Column(name="stud_id")

	private Integer id;

	@Column(name="stud_name")

	private String studName;

	@Column(name="stud_no")

	private String studNo;

	@Column(name="stud_age")

	private Integer studAge;

	@OneToMany(cascade=CascadeType.ALL,targetEntity=com.wanczy.student.pojo.Grade.class,mappedBy="student",fetch=FetchType.EAGER)

	private Set grades=new HashSet(0);

//	@Temporal(TemporalType.TIMESTAMP)//时间类型

//	private Date date;

	

	public Student() {

		super();

	}

	private Integer valid;

	public Integer getId() {

		return id;

	}

	public void setId(Integer id) {

		this.id = id;

	}

	public String getStudName() {

		return studName;

	}

	public void setStudName(String studName) {

		this.studName = studName;

	}

	public String getStudNo() {

		return studNo;

	}

	public void setStudNo(String studNo) {

		this.studNo = studNo;

	}

	public Integer getStudAge() {

		return studAge;

	}

	public void setStudAge(Integer studAge) {

		this.studAge = studAge;

	}

	public Integer getValid() {

		return valid;

	}

	public void setValid(Integer valid) {

		this.valid = valid;

	}

	public Student(Integer id, String studName, String studNo, Integer studAge,

			Integer valid) {

		super();

		this.id = id;

		this.studName = studName;

		this.studNo = studNo;

		this.studAge = studAge;

		this.valid = valid;

	}

	public Student(String studName, String studNo, Integer studAge,

			Integer valid) {

		super();

		this.studName = studName;

		this.studNo = studNo;

		this.studAge = studAge;

		this.valid = valid;

	}

	public Set getGrades() {

		return grades;

	}

	public void setGrades(Set grades) {

		this.grades = grades;

	}

}

查询课程的时候会多产生一条select 语句



Grade grade=gradeService.findGradeById(3);

		System.out.println(grade.getStudent().getStudName());

这是结果：

Hibernate: 

    select

        grade0_.grad_id as grad1_1_1_,

        grade0_.course_cour_id as course4_1_1_,

        grade0_.score as score1_1_,

        grade0_.student_stud_id as student5_1_1_,

        grade0_.valid as valid1_1_,

        student1_.stud_id as stud1_0_0_,

        student1_.stud_age as stud2_0_0_,

        student1_.stud_name as stud3_0_0_,

        student1_.stud_no as stud4_0_0_,

        student1_.valid as valid0_0_ 

    from

        tb_grade grade0_ 

    left outer join

        tb_student student1_ 

            on grade0_.student_stud_id=student1_.stud_id 

    where

        grade0_.grad_id=?

Hibernate: 

    select

        grades0_.student_stud_id as student5_1_,

        grades0_.grad_id as grad1_1_,

        grades0_.grad_id as grad1_1_0_,

        grades0_.course_cour_id as course4_1_0_,

        grades0_.score as score1_0_,

        grades0_.student_stud_id as student5_1_0_,

        grades0_.valid as valid1_0_ 

    from

        tb_grade grades0_ 

    where

        grades0_.student_stud_id=?