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：
http://java.sun.com/xml/ns/persistence/persistence_1_0.xsd\
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 = \ 被维护端注解 @OneToMany(cascade = { CascadeType.PERSIST, CascadeType.REFRESH, CascadeType.MERGE, CascadeType.REMOVE }, fetch = FetchType.EAGER, mappedBy = \ @Entity @Table(name = \ public class OrderInfo { private Integer id; private String name; private Set @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 = \ public Set public void setItems(Set public void addOrderItem(OrderItem orderItem) { orderItem.setOrder(this); this.items.add(orderItem); } } @Entity @Table(name = \ 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 = \ 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(\ 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 = \关联表名 inverseJoinColumns = @JoinColumn (name = \被维护端外键 joinColumns = @JoinColumn (name = \维护端外键 被维护端注解 @ManyToMany( cascade = CascadeType.REFRESH, mappedBy = \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 @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 = \inverseJoinColumns = @JoinColumn(name = \ public Set return teachers; } public void setTeachers(Set 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 @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 = \fetch = FetchType.LAZY) public Set public void setStudents(Set 测试 public class ManyToMany { @Test public void save() { EntityManagerFactory emf = Persistence .createEntityManagerFactory(\ 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(\ 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(\ 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(\ 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(\ 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