hibernate 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：
Java代码 收藏代码
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：
Java代码 收藏代码
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
Java代码 收藏代码
@ManyToOne(fetch = FetchType.EAGER)//注意这行代码
@JoinColumn(name = “groupid”)
public Tgroup getTgroup() {
return this.tgroup;
}
时

Java代码 收藏代码
@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
Java代码 收藏代码
@ManyToOne(fetch = FetchType.LAZY)//注意这行代码
@JoinColumn(name = “groupid”)
public Tgroup getTgroup() {
return this.tgroup;
}
时，
Java代码 收藏代码
@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。
}