JPA一对多,多对多映射

JPA(Java Persistence API)是Sun官方提出的Java持久化规范。它为Java开发人员提供了一种对象/关系映射工具来管理Java应用中的关系数据。他的出现主要是为了简化现有的持久化开发工作和整合ORM技术
总的来说，JPA包括以下3方面的技术：
ORM映射元数据
Java持久化API
查询语言

采用JPA的实现hibernate开发必须的包
hibernate核心包hibernate-distribution-3.3.1.GA
hibernate3.jar
lib\bytecode\cglib\hibernate-cglib-repack-2.1_3.jar
lib\required\*.jar
Hiberante注解包hibernate-annotations-3.4.0.GA
hibernate-annotations.jar
lib\ejb3-persistence.jar、hibernate-commons-annotations.jar
Hibernate针对JPA的实现包hibernate-entitymanager-3.4.0.GA
hibernate-entitymanager.jar
lib\test\log4j.jar、slf4j-log4j12.jar

JPA的配置文件
JPA规范要求在类路径的META-INF目录下放置persistence.xml，文件的名称是固定的，配置模版如下：
<?xml version="1.0"?>
<persistence xmlns="http://java.sun.com/xml/ns/persistence" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://java.sun.com/xml/ns/persistence http://java.sun.com/xml/ns/persistence/persistence_1_0.xsd" version="1.0">

<persistence-unit name="itcast" transaction-type="RESOURCE_LOCAL">
      <properties>
         <property name="hibernate.dialect" value="org.hibernate.dialect.MySQL5Dialect"/>
         <property name="hibernate.connection.driver_class" value="org.gjt.mm.mysql.Driver"/>
         <property name="hibernate.connection.username" value="root"/>
         <property name="hibernate.connection.password" value="123456"/>
         <property name="hibernate.connection.url"        value="jdbc:mysql://localhost:3306/itcast?useUnicode=true&characterEncoding=UTF-8"/>
         <property name="hibernate.hbm2ddl.auto" value="update"/>
      </properties>
</persistence-unit>
</persistence>

JPA事务
RESOURCE_LOCAL本地事务、JTA(全局事务)

持久化API
EntityManager
persist(Object)     持久化
remove(Object)     删除对象
find(Class entityClass,Object key) 根据主键查询
getReference(Class entityClass,Object key)根据主键查询
flush()     实体与底层同步，执行sql
createQuery() 创建JPQL查询对象
createNativeQuery() 根据普通SQL查询
createNamedQuery() 命名查询@NamedQuerie标注
merge(Object) 将一个detached的实体持久化到EntityManager中
close()    关闭管理器
find()与getReference()区别
find()不支持延迟加载，无记录返回NULL
getReference()支持延迟加载，getReference()不会返回null值,而是抛出EntityNotFoundException

javax.persistence.Query
int executeUpdate() 执行更新、删除、添加
Object getSingleResult() 执行查询(返回一条记录)
List getResultList() 执行查询(返回结果链表)
Query setParameter(int position,object value) 给Query对象设置参数
Query setMaxResults(int maxResult) 给Query对象设置返回数分页查询
Query setFirstResult(int firstResult) 给Query对象设置返回偏移

JPA一对多双向
1-m:多的一方为关系维护端,关系维护端负责外键纪录的更新,关系被维护端没有权力更新外键纪录.
维护端注解
拥有mappedBy注解的实体类为关系被维护端，另外的实体类为关系维护端的。顾名思意，关系的维护端对关系（在多对多为中间关联表）的CRUD做操作。关系的被维护端没有该操作，不能维护关系。

@ManyToOne(cascade = { CascadeType.MERGE, CascadeType.REFRESH }, optional = false)
@JoinColumn(name = "order_id")
被维护端注解
@OneToMany(cascade = { CascadeType.PERSIST, CascadeType.REFRESH,
            CascadeType.MERGE, CascadeType.REMOVE },
                       fetch = FetchType.EAGER,
                       mappedBy = "order")

@Entity
@Table(name = "order_info")
public class OrderInfo {
    private Integer id;
    private String name;
    private Set<OrderItem> items = new HashSet<OrderItem>();
    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    public Integer getId() {
        return id;
    }
    public void setId(Integer id) {
        this.id = id;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    @OneToMany(cascade = { CascadeType.PERSIST, CascadeType.REFRESH,
            CascadeType.MERGE, CascadeType.REMOVE }, fetch = FetchType.EAGER,
                       mappedBy = "order")
    public Set<OrderItem> getItems() {
        return items;
    }
    public void setItems(Set<OrderItem> items) {
        this.items = items;
    }
    public void addOrderItem(OrderItem orderItem) {
        orderItem.setOrder(this);
        this.items.add(orderItem);
    }
}

@Entity
@Table(name = "order_item")
public class OrderItem {
    private Integer Id;
    private String name;
    private OrderInfo order;
    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    public Integer getId() {
        return Id;
    }
    public void setId(Integer id) {
        Id = id;
    }
    @Column(length = 20, nullable = true)
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    @ManyToOne(cascade = { CascadeType.MERGE, CascadeType.REFRESH }, optional = false)
    @JoinColumn(name = "order_id")
    public OrderInfo getOrder() {
        return order;
    }
    public void setOrder(OrderInfo order) {
        this.order = order;
    }
}

测试
public class OneToMany {
    @Test
    public void save() {
        EntityManagerFactory emf = Persistence
                .createEntityManagerFactory("JPAPU");
        EntityManager em = emf.createEntityManager();
        em.getTransaction().begin();
        OrderInfo o = new OrderInfo();
        o.setName("订单一");
        OrderItem oi1 = new OrderItem();
        oi1.setName("产品一");
        o.addOrderItem(oi1);
        OrderItem oi2 = new OrderItem();
        oi2.setName("产品一");
        o.addOrderItem(oi2);
        OrderItem oi3 = new OrderItem();
        oi3.setName("产品一");
        o.addOrderItem(oi3);
        OrderItem oi4 = new OrderItem();
        oi4.setName("产品一");
        o.addOrderItem(oi4);
        em.persist(o);
        // UUID.randomUUID().toString();
        em.getTransaction().commit();
        emf.close();
    }
}

JPA多对多双向
维护端注解
@ManyToMany (cascade = CascadeType.REFRESH)
@JoinTable (//关联表
                   name = "student_teacher" , //关联表名
                   inverseJoinColumns = @JoinColumn (name = "teacher_id" ),//被维护端外键
                   joinColumns = @JoinColumn (name = "student_id" ))//维护端外键
被维护端注解
@ManyToMany(
                   cascade = CascadeType.REFRESH,
                   mappedBy = "teachers",//通过维护端的属性关联
                   fetch = FetchType.LAZY)
@Entity
public class Student {
    private Integer id;
    private String name;
    private Set<Teacher> teachers = new HashSet<Teacher>();
    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    public Integer getId() {
        return id;
    }
    public void setId(Integer id) {
        this.id = id;
    }
    @Column(nullable = false, length = 16)
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    @ManyToMany(cascade = CascadeType.REFRESH)
    @JoinTable(name = "student_teacher", inverseJoinColumns = @JoinColumn(name = "teacher_id"), joinColumns = @JoinColumn(name = "student_id"))
    public Set<Teacher> getTeachers() {
        return teachers;
    }
    public void setTeachers(Set<Teacher> teachers) {
        this.teachers = teachers;
    }
    public void addTeacher(Teacher teacher) {
        this.teachers.add(teacher);
    }
    public void removeTeachers(Teacher teacher) {
        this.teachers.remove(teacher);
    }
}
@Entity
public class Teacher {
    private Integer id;
    private String name;
    private Set<Student> students = new HashSet<Student>();
    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    public Integer getId() {
        return id;
    }
    public void setId(Integer id) {
        this.id = id;
    }
    @Column(nullable = false, length = 16)
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    @ManyToMany(cascade = CascadeType.REFRESH, mappedBy = "teachers", fetch = FetchType.LAZY)
    public Set<Student> getStudents() {
        return students;
    }
    public void setStudents(Set<Student> students) {
        this.students = students;
    }
}

测试
public class ManyToMany {
    @Test
    public void save() {
        EntityManagerFactory emf = Persistence
                .createEntityManagerFactory("JPAPU");
        EntityManager em = emf.createEntityManager();
        em.getTransaction().begin();
        Student student = new Student();
        student.setName("小李");
        Teacher teacher = new Teacher();
        teacher.setName("大李");
        em.persist(student);
        em.persist(teacher);
        em.getTransaction().commit();
        emf.close();
    }

    @Test
    public void bind() {
        EntityManagerFactory emf = Persistence
                .createEntityManagerFactory("JPAPU");
        EntityManager em = emf.createEntityManager();
        em.getTransaction().begin();
        Student student = em.find(Student.class, 1);
        Teacher teacher = em.find(Teacher.class, 1);
        student.addTeacher(teacher);
        em.persist(student);
        em.getTransaction().commit();
        emf.close();
    }

    @Test
    public void unbind() {
        EntityManagerFactory emf = Persistence
                .createEntityManagerFactory("JPAPU");
        EntityManager em = emf.createEntityManager();
        em.getTransaction().begin();
        Student student = em.find(Student.class, 1);
        Teacher teacher = em.find(Teacher.class, 1);
        student.removeTeachers(teacher);
        em.persist(student);
        em.getTransaction().commit();
        emf.close();
    }

    @Test
    public void removeTeacher() {
        EntityManagerFactory emf = Persistence
                .createEntityManagerFactory("JPAPU");
        EntityManager em = emf.createEntityManager();
        em.getTransaction().begin();
        // 关系被维护端删除时，如果中间表存在些纪录的关联信息，则会删除失败
        em.remove(em.getReference(Teacher.class, 1));
        em.getTransaction().commit();
        emf.close();
    }

    @Test
    public void removeStudent() {
        EntityManagerFactory emf = Persistence
                .createEntityManagerFactory("JPAPU");
        EntityManager em = emf.createEntityManager();
        em.getTransaction().begin();
        // 关系维护端删除时，如果中间表存在些纪录的关联信息，则会删除该关联信息
        em.remove(em.getReference(Student.class, 1));
        em.getTransaction().commit();
        emf.close();
    }

}

常见异常
1、异常信息：org.hibernate.hql.ast.QuerySyntaxException: person is not mapped
异常环境：查询
异常原因：查询语句中Person类没有大写

2、java.lang.ClassCastException: [Ljava.lang.Object; cannot be cast to java.lang.String
异常环境：查询、遍历显示
异常原因：转型出错

3、javax.persistence.NonUniqueResultException: result returns more than one elements
异常环境：查询、getSingleResult
异常原因：getSingleResult只能获取一条数据，而查询语句返回的是多条数据

4、 org.hibernate.PropertyValueException: not-null property references a null or transient value: com.sunyard.entities.Person.name
异常环境：数据插入
异常原因：JPA的Entity中一个属性定义为nullable=false，插入数据该字段为null

5、执行添加没反应、没异常
异常原因：没有开启事务、没有提交事务

6、javax.persistence.PersistenceException: org.hibernate.PersistentObjectException: detached entity passed to persist: com.sunyard.entities.Person
异常环境：OneToOne 共享主键关联
异常原因：一对一中，一个提供主键、另一个共享其主键，共享主键的对象可以set 提供主键的对象然后添加到数据库中
方向弄反了后果就是没人提供主键

7、org.hibernate.TransientObjectException: object references an unsaved transient instance - save the transient instance before flushing:
异常环境：多对一添加
异常原因：在多的一端维护，没有添加级联

8、javax.persistence.PersistenceException: [PersistenceUnit: JPA] Unable to configure EntityManagerFactory
异常原因：很多、实体管理器Factory没有成功创建，是注解的问题
9、org.hibernate.MappingException: Unable to find column with logical name: sid in org.hibernate.mapping.
异常环境：添加表做多对一关联映射
异常原因：表字段写反了，name添加表字段名referencedColumnName指向本表字段名

本文来自CSDN博客，转载请标明出处：http://blog.csdn.net/LJHABC1982/archive/2011/06/20/6556580.aspx

JPA一对多,多对多映射

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