jpa学习笔记——3

//根据外键来生成主键 // @TableGenerator(name="ID_GENERATOR", // table="jpa_id_generators", // pkColumnName="PK_NAME", // pkColumnValue="CUSTOMER_ID", // valueColumnName="PK_VALUE", // allocationSize=100) //生成主键的策略 @GeneratedValue(strategy=GenerationType.AUTO) @Id public Integer getId() { return id; } public void setId(Integer id) { this.id = id; } @Column(name="LAST_NAME", length=50, nullable=false) public String getLastName() { return lastName; } public void setLastName(String lastName) { this.lastName = lastName; }

 以上是外键的生成策略

学习了entityManager的一些函数

find()   查询 

getReference()  也是查询，但是跟find不一样，一样的结果不一样的过程。getReference()返回customer的一个代理，真正始用的时候才向数据库发sql，初始化这个对象

persist()  增加对象，与save方法的不同之处，若对象有ID,不能执行insert操作，会抛出异常

remove()  删除对象,类似于delete方法,将对象对应的记录从数据库中移除，但该方法只能移除持久化对象

Customer customer = entityManager.find(Customer.class, 1); entityManager.remove(customer);

后续代码

package com.huawei.jpa.test; import java.util.Date; import javax.persistence.EntityManager; import javax.persistence.EntityManagerFactory; import javax.persistence.EntityTransaction; import javax.persistence.Persistence; import org.junit.After; import org.junit.Before; import org.junit.Test; import com.huawei.test.Customer; public class JPATest { private EntityManagerFactory entityManagerFactory; private EntityManager entityManager; private EntityTransaction transaction; @Before public void init() { entityManagerFactory = Persistence.createEntityManagerFactory("jpa-1"); entityManager = entityManagerFactory.createEntityManager(); transaction = entityManager.getTransaction(); transaction.begin(); } @After public void destory() { transaction.commit(); entityManager.close(); entityManagerFactory.close(); } //类似于delete方法,将对象对应的记录从数据库中移除，但该方法只能移除持久化对象 @Test public void testRemove() { // Customer customer = new Customer(); // customer.setId(2); // entityManager.remove(customer); //errror Customer customer = entityManager.find(Customer.class, 1); entityManager.remove(customer); } //与save方法的不同之处 //若对象有ID,不能执行insert操作，会抛出异常 @Test public void testPersistence() { Customer customer = new Customer(); customer.setAge(1000); customer.setBirth(new Date()); customer.setCreateTime(new Date()); customer.setEmail("huaei@huawei.com"); customer.setLastName("haa"); //customer.setId(9000); entityManager.persist(customer); System.out.println(customer.getId()); System.out.println("===="); } @Test public void getReference() { System.out.println("---------->"); Customer customer = entityManager.getReference(Customer.class, 1); System.out.println(customer); } @Test public void TestFind() { Customer customer = entityManager.find(Customer.class, 1); System.out.println(customer); } }