fetch = FetchType.EAGER 作用

如果是EAGER，那么表示取出这条数据时，它关联的数据也同时取出放入内存中

如果是LAZY，那么取出这条数据时，它关联的数据并不取出来，在同一个session中，什么时候要用，就什么时候取(再次访问数据库)。

但是，在session外，就不能再取了。用EAGER时，因为在内存里，所以在session外也可以取。

 

一般只在一边设Eager，JPA接口默认为一对多为Lazy，多对一为Eager，但是Hibernate反向工程生成Entity时，多对一为Lazy，需要手动改为Eager。

 

而两边都设Eager，那么代码中取一条记录时，会发2次SQL。

 

Tgroup：

package com.hibernate.entity;

import java.util.HashSet;

/**
 * Tgroup entity. @author MyEclipse Persistence Tools
 */
@Entity
@Table(name = "tgroup", catalog = "test")
public class Tgroup implements java.io.Serializable {

    // Fields

    /**
     *
     */
    private static final long serialVersionUID = -7208715716759269846L;
    private Integer id;
    private String name;
    private Set<Tuser> tusers = new HashSet<Tuser>(0);

    // Constructors

    /** default constructor */
    public Tgroup() {
    }

    /** full constructor */
    public Tgroup(String name, Set<Tuser> tusers) {
        this.name = name;
        this.tusers = tusers;
    }

    // Property accessors
    @Id
    @GeneratedValue
    @Column(name = "id", unique = true, nullable = false)
    public Integer getId() {
        return this.id;
    }

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

    @Column(name = "name")
    public String getName() {
        return this.name;
    }

    public void setName(String name) {
        this.name = name;
    }

    @OneToMany(cascade = CascadeType.ALL, fetch = FetchType.LAZY, mappedBy = "tgroup")
    public Set<Tuser> getTusers() {
        return this.tusers;
    }

    public void setTusers(Set<Tuser> tusers) {
        this.tusers = tusers;
    }

}

 

 

Tuser：

package com.hibernate.entity;

import javax.persistence.Column;

/**
 * Tuser entity. @author MyEclipse Persistence Tools
 */
@Entity
@Table(name = "tuser", catalog = "test")
public class Tuser implements java.io.Serializable {

    // Fields

    /**
     *
     */
    private static final long serialVersionUID = -7792597282750540598L;
    private Integer id;
    private Tgroup tgroup;
    private String name;

    // Constructors

    /** default constructor */
    public Tuser() {
    }

    /** full constructor */
    public Tuser(Tgroup tgroup, String name) {
        this.tgroup = tgroup;
        this.name = name;
    }

    // Property accessors
    @Id
    @GeneratedValue
    @Column(name = "id", unique = true, nullable = false)
    public Integer getId() {
        return this.id;
    }

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

    @ManyToOne(fetch = FetchType.EAGER)//<span style="color: #ff0000;">注意这行代码</span>







    @JoinColumn(name = "groupid")
    public Tgroup getTgroup() {
        return this.tgroup;
    }

    public void setTgroup(Tgroup tgroup) {
        this.tgroup = tgroup;
    }

    @Column(name = "name")
    public String getName() {
        return this.name;
    }

    public void setName(String name) {
        this.name = name;
    }

}

 

当

@ManyToOne(fetch = FetchType.EAGER)//注意这行代码
@JoinColumn(name = "groupid")
    public Tgroup getTgroup() {
        return this.tgroup;
    }

 时

 

@Test
    public void getUser() {
        Session s = sessionFactory.getCurrentSession();
        s.beginTransaction();
        Tuser user = (Tuser)s.get(Tuser.class, 1);
        System.out.println(user.getName());
        s.getTransaction().commit();
        System.out.println(user.getTgroup().getName());//这行代码OK，当然，也可以把这行代码放在commit()之前
    }

 

如果

@ManyToOne(fetch = FetchType.LAZY)//注意这行代码
@JoinColumn(name = "groupid")
    public Tgroup getTgroup() {
        return this.tgroup;
    }

时，

@Test
    public void getUser() {
        Session s = sessionFactory.getCurrentSession();
        s.beginTransaction();
        Tuser user = (Tuser)s.get(Tuser.class, 1);
        System.out.println(user.getName());
        s.getTransaction().commit();
        System.out.println(user.getTgroup().getName());//这行代码错误，不过，这行代码放在commit()之前，是正确的。
    }