第二百三十二节 JPA教程 - JPA教程 - JPA ID自动生成器示例、JPA ID生成策略示例

JPA教程 - JPA ID自动生成器示例

我们可以将id字段标记为自动生成的主键列。 数据库将在插入时自动为id字段生成一个值数据到表。

例子

下面的代码来自Person.java。

package cn.w3cschool.common;import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;@Entity
public class Person {@Id@GeneratedValueprivate Long id;private String name;private String surname;public Person() {}public Person(String name, String surname) {this.name = name;this.surname = surname;}public Long getId() {return id;}public void setId(Long id) {this.id = id;}public String getName() {return name;}public void setName(String name) {this.name = name;}public String getSurname() {return surname;}public void setSurname(String surname) {this.surname = surname;}@Overridepublic String toString() {return "Person [id=" + id + ", name=" + name + ", surname=" + surname + "]";}
}

以下代码来自App.java。

从代码中我们可以看到，我们没有设置对象的id值。

package cn.w3cschool.common;import java.util.List;import org.springframework.context.support.ClassPathXmlApplicationContext;public class App {public static void main(String[] args) {ClassPathXmlApplicationContext context = new ClassPathXmlApplicationContext("applicationContext.xml");PersonDaoImpl dao = (PersonDaoImpl) context.getBean("personDao");Person peter = new Person("XML", "HTML");Person nasta = new Person("Java", "SQL");dao.save(peter);dao.save(nasta);List<Person> persons = dao.getAll();for (Person person : persons) {System.out.println(person);}context.close();}
}

下面的代码来自PersonDaoImpl.java。

package cn.w3cschool.common;import java.util.List;import javax.persistence.EntityManager;
import javax.persistence.PersistenceContext;import org.springframework.transaction.annotation.Transactional;@Transactional
public class PersonDaoImpl {@PersistenceContextprivate EntityManager em;public Long save(Person person) {em.persist(person);return person.getId();}public List<Person>getAll() {return em.createQuery("SELECT p FROM Person p", Person.class).getResultList();}}

上面的代码生成以下结果。

JPA教程 - JPA ID生成策略示例

当使用id字段的自动生成值时，我们可以选择生成策略。我们使用的一个常见策略是IDENTITY。

例子

以下代码来自Professor.java。

package cn.w3cschool.common;import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;@Entity
public class Professor {@Id @GeneratedValue(strategy=GenerationType.IDENTITY)private int id;private String name;private long salary;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;}public long getSalary() {return salary;}public void setSalary(long salary) {this.salary = salary;}public String toString() {return "Employee id: " + getId() + " name: " + getName() + " salary: "+ getSalary();}
}

以下代码来自App.java。

package cn.w3cschool.common;import org.springframework.context.support.ClassPathXmlApplicationContext;public class App {public static void main(String[] args) {ClassPathXmlApplicationContext context = new ClassPathXmlApplicationContext("applicationContext.xml");PersonDaoImpl dao = (PersonDaoImpl) context.getBean("personDao");dao.test();context.close();Helper.checkData();}
}

下面的代码来自PersonDaoImpl.java。

package cn.w3cschool.common;import javax.persistence.EntityManager;
import javax.persistence.PersistenceContext;import org.springframework.transaction.annotation.Transactional;@Transactional
public class PersonDaoImpl {public void test() {Professor emp = new Professor();emp.setName("name");emp.setSalary(12345);em.persist(emp);}@PersistenceContextprivate EntityManager em;
}

这里是数据库表转储。

Table Name: PROFESSORRow:Column Name: ID,Column Type: INTEGER:Column Value: 1Column Name: NAME,Column Type: VARCHAR:Column Value: nameColumn Name: SALARY,Column Type: BIGINT:Column Value: 12345

 

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