Spring系列——使用JPA来操作数据

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原文链接：http://www.dubby.cn/detail.html?id=9042

代码地址：https://github.com/dubby1994/spring-demo

JPA全称Java Persistence API，Java持久性API(简称JPA)是类和方法的集合，以数据关系映射持久并存储到数据库，这是由Oracle公司提供方案技术。可以减少开发者自己操作数据库时需要很多臃肿的代码。

1. 我们要做什么

我们会使用JPA来操作数据库，这里使用的时内存数据库，上一篇Spring系列——访问MySQL提到过，如果不配置，默认就是内置的内存数据库H2。

2.你需要什么

MySQL 5.6或者更高版本
大约15分钟
一个最爱的编辑器或者IDE
JDK 1.8 +
Maven 3.0+

3.创建项目

3.1 项目结构

maven项目，结构如下：

└── src    └── main        └── java            └── hello

3.2 项目依赖

pom.xml

<?xml version="1.0" encoding="UTF-8"?><project xmlns="http://maven.apache.org/POM/4.0.0"    xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/maven-v4_0_0.xsd"    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">    <modelVersion>4.0.0</modelVersion>    <groupId>cn.dubby</groupId>    <artifactId>accessing-data-jpa</artifactId>    <version>0.1.0</version>    <parent>        <groupId>org.springframework.boot</groupId>        <artifactId>spring-boot-starter-parent</artifactId>        <version>1.5.8.RELEASE</version>    </parent>    <properties>        <java.version>1.8</java.version>    </properties>    <dependencies>        <dependency>            <groupId>org.springframework.boot</groupId>            <artifactId>spring-boot-starter-data-jpa</artifactId>        </dependency>        <dependency>            <groupId>com.h2database</groupId>            <artifactId>h2</artifactId>        </dependency>        <dependency>            <groupId>org.springframework.boot</groupId>            <artifactId>spring-boot-starter-test</artifactId>            <scope>test</scope>        </dependency>    </dependencies>    <build>        <plugins>            <plugin>                <groupId>org.springframework.boot</groupId>                <artifactId>spring-boot-maven-plugin</artifactId>            </plugin>        </plugins>    </build>    <repositories>        <repository>            <id>spring-releases</id>            <name>Spring Releases</name>            <url>https://repo.spring.io/libs-release</url>        </repository>        <repository>            <id>org.jboss.repository.releases</id>            <name>JBoss Maven Release Repository</name>            <url>https://repository.jboss.org/nexus/content/repositories/releases</url>        </repository>    </repositories>    <pluginRepositories>        <pluginRepository>            <id>spring-releases</id>            <name>Spring Releases</name>            <url>https://repo.spring.io/libs-release</url>        </pluginRepository>    </pluginRepositories></project>

3.3 实体类

JPA是一个实体类和数据库映射的技术，所以这里先写出实体类
src/main/java/hello/Customer.java

package hello;import javax.persistence.Entity;import javax.persistence.GeneratedValue;import javax.persistence.GenerationType;import javax.persistence.Id;@Entitypublic class Customer {    @Id    @GeneratedValue(strategy=GenerationType.AUTO)    private Long id;    private String firstName;    private String lastName;    protected Customer() {}    public Customer(String firstName, String lastName) {        this.firstName = firstName;        this.lastName = lastName;    }    @Override    public String toString() {        return String.format(                "Customer[id=%d, firstName='%s', lastName='%s']",                id, firstName, lastName);    }    public Long getId() {        return id;    }    public String getFirstName() {        return firstName;    }    public String getLastName() {        return lastName;    }}

Customer这个实体类有三个属性，id，firstName和lastName。有两个构造函数，没有参数的构造函数时为了给JPA使用，所以这里设置成protected，我们自己的代码中还是使用有参数的那个构造函数。

Customer使用了@Entity修饰，说明它是一个JPA的实体。没有使用@Table修饰，所以这个实体类对应的表名就是Customer。

Customer的属性id使用@Id修饰，说明这个字段就是id。id还被@GeneratedValue修饰，说明这个id时有数据库自动生成的。

firstName 和lastName这两个字段没有被修饰，说明这两个就是普通的字段对应数据库里两列，列名和字段名一样。

3.4 创建查询

Spring Data JPA专注于使用JPA来操作数据。其中最吸引人的特性就是我们只需要定义接口，Spring会帮我在运行时创建真正的实现。

src/main/java/hello/CustomerRepository.java

package hello;import org.springframework.data.repository.CrudRepository;import java.util.List;public interface CustomerRepository extends CrudRepository<Customer, Long> {    List<Customer> findByLastName(String lastName);}

CustomerRepository继承了CrudRepository，泛型参数是<Customer, Long>，分别代表了，实体类的类型和ID的类型。只要继承了CrudRepository，就拥有了基本的增删改查的功能，类如其名CrudRepository。

org.springframework.data.repository.CrudRepository

package org.springframework.data.repository;import java.io.Serializable;@NoRepositoryBeanpublic interface CrudRepository<T, ID extends Serializable> extends Repository<T, ID> {    <S extends T> S save(S entity);    <S extends T> Iterable<S> save(Iterable<S> entities);    T findOne(ID id);    boolean exists(ID id);    Iterable<T> findAll();    Iterable<T> findAll(Iterable<ID> ids);    long count();    void delete(ID id);    void delete(T entity);    void delete(Iterable<? extends T> entities);    void deleteAll();}

当然这些只是最基本的数据操作，Spring Data JPA也允许你写自己的查询语句。上面代码中的findByLastName()就是自定义的查询语句。

3.5 启动类

src/main/java/hello/Application.java

package hello;import org.slf4j.Logger;import org.slf4j.LoggerFactory;import org.springframework.boot.CommandLineRunner;import org.springframework.boot.SpringApplication;import org.springframework.boot.autoconfigure.SpringBootApplication;import org.springframework.context.annotation.Bean;@SpringBootApplicationpublic class Application {    private static final Logger log = LoggerFactory.getLogger(Application.class);    public static void main(String[] args) {        SpringApplication.run(Application.class);    }    @Bean    public CommandLineRunner demo(CustomerRepository repository) {        return (args) -> {            // save a couple of customers            repository.save(new Customer("Jack", "Bauer"));            repository.save(new Customer("Chloe", "O'Brian"));            repository.save(new Customer("Kim", "Bauer"));            repository.save(new Customer("David", "Palmer"));            repository.save(new Customer("Michelle", "Dessler"));            // fetch all customers            log.info("Customers found with findAll():");            log.info("-------------------------------");            for (Customer customer : repository.findAll()) {                log.info(customer.toString());            }            log.info("");            // fetch an individual customer by ID            Customer customer = repository.findOne(1L);            log.info("Customer found with findOne(1L):");            log.info("--------------------------------");            log.info(customer.toString());            log.info("");            // fetch customers by last name            log.info("Customer found with findByLastName('Bauer'):");            log.info("--------------------------------------------");            for (Customer bauer : repository.findByLastName("Bauer")) {                log.info(bauer.toString());            }            log.info("");        };    }}

3.6 测试

src/main/java/hello/CustomerRepositoryTests.java

package hello;import org.junit.Test;import org.junit.runner.RunWith;import org.springframework.beans.factory.annotation.Autowired;import org.springframework.boot.test.autoconfigure.orm.jpa.DataJpaTest;import org.springframework.boot.test.autoconfigure.orm.jpa.TestEntityManager;import org.springframework.test.context.junit4.SpringRunner;import java.util.List;import static org.assertj.core.api.Assertions.assertThat;@RunWith(SpringRunner.class)@DataJpaTestpublic class CustomerRepositoryTests {    @Autowired    private TestEntityManager entityManager;    @Autowired    private CustomerRepository customers;    @Test    public void testFindByLastName() {        Customer customer = new Customer("first", "last");        entityManager.persist(customer);        List<Customer> findByLastName = customers.findByLastName(customer.getLastName());        assertThat(findByLastName).extracting(Customer::getLastName).containsOnly(customer.getLastName());    }}

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