Hibernate！！关联关系cascade
在写代码的时候，需要设置好关联，不然数据库中会没有相应的数据。
双向关系在程序中药设定双向关联，不然在数据库中会出问题
cascade翻译成中文叫：级联
cascade一般用在级联保存，级联更新，级联删除上
cascade可以取得值：
- ALL 在所有情况下，在持久化操作的增删改查，都会级联到另一个对象
- MERGE 合并的时候
- PERSIST 存储的时候
- REFRESH 刷新
- REMOVE 删除
如果两个对象之间有关联，那么就需要设置cascade这个属性。
@ManyToOne注解的这端，是多端
1.在注释@ManyToOne(cascade=CascadeType.ALL,optional=true)中将属性optional设置为true，这可以使得即使外键为空时仍可以向表中添加数据。
注意：导入包的时候是：import javax.persistence.CascadeType;
例子：
1 package com.hb.model; 2 3 import javax.persistence.Entity; 4 import javax.persistence.GeneratedValue; 5 import javax.persistence.Id; 6 import javax.persistence.Table; 7 8 @Entity 9 @Table(name="t_group") 10 public class Group { 11 private int gid ; 12 private String gname ; 13 14 @Id 15 @GeneratedValue 16 public int getGid() { 17 return gid; 18 } 19 public void setGid(int gid) { 20 this.gid = gid; 21 } 22 public String getGname() { 23 return gname; 24 } 25 public void setGname(String gname) { 26 this.gname = gname; 27 } 28 29 30 }
1 package com.hb.model; 2 3 4 import javax.persistence.CascadeType; 5 import javax.persistence.Entity; 6 import javax.persistence.GeneratedValue; 7 import javax.persistence.Id; 8 import javax.persistence.ManyToOne; 9 import javax.persistence.Table; 10 11 12 @Entity 13 @Table(name="t_user") 14 public class User { 15 private int id ; 16 private String name ; 17 private Group group ; 18 19 @Id 20 @GeneratedValue 21 public int getId() { 22 return id; 23 } 24 public void setId(int id) { 25 this.id = id; 26 } 27 public String getName() { 28 return name; 29 } 30 public void setName(String name) { 31 this.name = name; 32 } 33 34 35 //cascade属性是一个数组，cascade是级联 36 @ManyToOne(cascade={CascadeType.ALL}) 37 public Group getGroup() { 38 return group; 39 } 40 public void setGroup(Group group) { 41 this.group = group; 42 } 43 44 45 46 }
1 package com.hb.model; 2 3 import static org.junit.Assert.*; 4 5 import org.hibernate.Session; 6 import org.hibernate.SessionFactory; 7 import org.hibernate.cfg.AnnotationConfiguration; 8 import org.hibernate.tool.hbm2ddl.SchemaExport; 9 import org.junit.AfterClass; 10 import org.junit.BeforeClass; 11 import org.junit.Test; 12 13 public class hibernateCRUDTest { 14 15 private static SessionFactory sessionFactory ; 16 17 @BeforeClass 18 public static void beforeClass(){ 19 new SchemaExport(new AnnotationConfiguration().configure()).create(true, true); 20 sessionFactory = new AnnotationConfiguration().configure().buildSessionFactory(); 21 22 } 23 24 @AfterClass 25 public static void afterClass(){ 26 sessionFactory.close(); 27 } 28 29 30 @Test 31 public void testSaveUser() { 32 User u = new User(); 33 u.setName("u1"); 34 Group g = new Group(); 35 u.setGroup(g); 36 g.setGname("g1"); 37 Session session = sessionFactory.getCurrentSession(); 38 session.beginTransaction(); 39 //session.save(g); 40 session.save(u); 41 session.getTransaction().commit(); 42 43 44 45 } 46 47 public static void main(String[] args) { 48 beforeClass(); 49 } 50 51 }
1 <?xml version='1.0' encoding='UTF-8'?> 2 <!DOCTYPE hibernate-configuration PUBLIC 3 "-//Hibernate/Hibernate Configuration DTD 3.0//EN" 4 "http://hibernate.sourceforge.net/hibernate-configuration-3.0.dtd"> 5 6 <!-- Generated by MyEclipse Hibernate Tools. --> 7 <hibernate-configuration> 8 9 <session-factory> 10 <property name="dialect"> 11 org.hibernate.dialect.MySQLDialect 12 </property> 13 <property name="connection.url"> 14 jdbc:mysql://localhost:3306/hb 15 </property> 16 <property name="connection.username">root</property> 17 <property name="connection.password">root</property> 18 <property name="connection.driver_class"> 19 com.mysql.jdbc.Driver 20 </property> 21 <property name="myeclipse.connection.profile">Mysql</property> 22 <property name="show_sql">true</property> 23 <property name="format_sql">true</property> 24 25 <property name="current_session_context_class">thread</property> 26 27 <mapping class="com.hb.model.Group" /> 28 <mapping class="com.hb.model.User" /> 29 </session-factory> 30 31 </hibernate-configuration>
在 public class hibernateCRUDTest 类中，测试保存Group
1 @Test 2 public void testSaveGroup() { 3 User u1= new User(); 4 u1.setName("u1"); 5 User u2 = new User(); 6 u2.setName("u2"); 7 Group g = new Group(); 8 g.setGname("g1"); 9 10 g.getUsers().add(u1); 11 g.getUsers().add(u2); 12 13 u1.setGroup(g); 14 u2.setGroup(g); 15 16 Session session = sessionFactory.getCurrentSession(); 17 session.beginTransaction(); 18 session.save(g); 19 session.getTransaction().commit(); 20 }
由上面的 g.getUsers().add(u1); 和 g.getUsers().add(u2) 可知现在是双向关联，所以对应的Group类中需要加入@OneToMany属性
双向关系在程序中要设定双向关联，不然在数据库中会出问题
双向关系在程序中要设定mappedBy="group"，不然在数据库中会出问题
代码如下：
1 package com.hb.model; 2 3 import java.util.HashSet; 4 import java.util.Set; 5 6 import javax.persistence.CascadeType; 7 import javax.persistence.Entity; 8 import javax.persistence.GeneratedValue; 9 import javax.persistence.Id; 10 import javax.persistence.OneToMany; 11 import javax.persistence.Table; 12 13 @Entity 14 @Table(name="t_group") 15 public class Group { 16 private int gid ; 17 private String gname ; 18 private Set<User> users = new HashSet<User>(); 19 20 @OneToMany(mappedBy="group",cascade={CascadeType.ALL}) 21 public Set<User> getUsers() { 22 return users; 23 } 24 public void setUsers(Set<User> users) { 25 this.users = users; 26 } 27 @Id 28 @GeneratedValue 29 public int getGid() { 30 return gid; 31 } 32 public void setGid(int gid) { 33 this.gid = gid; 34 } 35 public String getGname() { 36 return gname; 37 } 38 public void setGname(String gname) { 39 this.gname = gname; 40 } 41 42 43 }