Hibernate应用cascade&fetch

Hibernate应用cascade&fetch

简单说明：cascade和fetch是Hibernate读取和写入数据库对象两个参数，cascade（层叠）管写入（Save、Update...）；fetch管读取（Load、Get...）

一、cascade——它是用来指明做什么操作的时候关联对象绑定在一起：
     a)CascadeType.ALL 表示所有情况
     b)CascadeType.MERGE表示在Save或UPdate的情况
     c)CascadeType.REFRESH=A 里面需要读B改过之后的数据
     d)CascadeType.PERSIST
    e)CascadeType.REMOVE
总之，用得比较多的是CascadeType.All

二、fetch——它是用来指明读取数据时关联对象的情况：
     a)a)FetchType.EAGER   
     b)FetchType.LAZY(默认)

三、实例代码：

团队类：

package com.sean.model.one2many;

import java.util.HashSet;
import java.util.Set;

import javax.persistence.CascadeType;
import javax.persistence.Entity;
import javax.persistence.FetchType;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.OneToMany;
import javax.persistence.Table;

/**
 * 一对多：团队对象
 * 
 * @author Sean 2013-2-26
 * 
 */
@Entity
@Table(name = "p_group")
public class Group {

	private int id;
	private String name;
	private Set<Person> pset = new HashSet<Person>();

	@OneToMany(cascade = (CascadeType.ALL), fetch = FetchType.EAGER)
	public Set<Person> getPset() {
		return pset;
	}

	public void setPset(Set<Person> pset) {
		this.pset = pset;
	}

	@Id
	@GeneratedValue
	public int getId() {
		return id;
	}

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

	public String getName() {
		return name;
	}

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

}

成员类：

package com.sean.model.one2many;

import javax.persistence.CascadeType;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.ManyToOne;


/**
 * 一对多：成员对象
 * 
 * @author Sean 2013-2-26
 * 
 */
@Entity
public class Person {

	private int id;
	private String name;
	private Group group;

	@ManyToOne(cascade = CascadeType.ALL)
	public Group getGroup() {
		return group;
	}

	public void setGroup(Group group) {
		this.group = group;
	}

	@Id
	@GeneratedValue
	public int getId() {
		return id;
	}

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

	public String getName() {
		return name;
	}

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

}

测试方法：

	public void testGroup2Person() {

		Group g = new Group();
		g.setName("淘宝");

		Person p1 = new Person();
		Person p2 = new Person();
		Person p3 = new Person();
		Person p4 = new Person();
		p1.setName("赵一");
		// p1.setGroup(g);
		p2.setName("赵二");
		// p2.setGroup(g);
		p3.setName("赵三");
		// p3.setGroup(g);
		p4.setName("赵四");
		// p4.setGroup(g);

		g.getPset().add(p1);
		g.getPset().add(p2);
		g.getPset().add(p3);
		g.getPset().add(p4);
		SessionFactory factory = new AnnotationConfiguration().configure()
				.buildSessionFactory();
		Session session = factory.openSession();
		session.beginTransaction();
		session.save(g);
		session.getTransaction().commit();
		session.close();
		factory.close();

	}

	public void testLoadGroup() {
		testGroup2Person();
		SessionFactory factory = new AnnotationConfiguration().configure()
				.buildSessionFactory();
		Session session = factory.openSession();
		session.beginTransaction();

		Group group = (Group) session.get(Group.class, 17);

		session.getTransaction().commit();
		session.close();
		factory.close();

		for (Person i : group.getPset()) {
			System.out.println(i.getName());
		}

	}