用Spring Data JPA 基于内存存储pojo的简单案例

poject结构如下：

Customer.java类是一个pojo类，代码如下：

package hello;

import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;

@Entity
public class Customer {

    @Id
    @GeneratedValue(strategy=GenerationType.AUTO)
    private long id;
    private String firstName;
    private String lastName;

    private 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);
    }

}

@Entity：说明该类是一个jpa entity,如果缺少@Table注解则默认匹配表Customer

@Id:jpa通过它定义识别对象Id，使用注解@GeneratedValue让id自动增长

toString：重写了对象打印出来的方法

CustomerRepository.java是jpa用来存储Customer对象的仓库。

package hello;

import java.util.List;

import org.springframework.data.jpa.repository.JpaRepository;

public interface CustomerRepository extends JpaRepository<Customer, Long> {

    List<Customer> findByLastName(String lastName);

}

CustomerRepository继承了JpaRepository，除了能使用基本的增删改查外，还可以自己定义查找方法，如：findByLastName，关键在于findBy后面的属性要与Customer中的属性对应，在eclipse中编写的时候如果没有对应的属性eclipse也会提示你属性找不到。

这里只定义了一个接口，并没有具体实现，包括findByLastName方法，为什么？这就是Spring Data Jpa的魅力之处，因为它致力于在运行程序的时候从Repository接口处自动为它创建相应实现类。通过@EnableJpaRepositories注解实现。

Application.java应用主类，Spring Data JPA测试类

package hello;

import static org.springframework.jdbc.datasource.embedded.EmbeddedDatabaseType.H2;

import java.util.List;

import javax.persistence.EntityManagerFactory;
import javax.sql.DataSource;

import org.springframework.context.annotation.AnnotationConfigApplicationContext;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import org.springframework.context.support.AbstractApplicationContext;
import org.springframework.data.jpa.repository.config.EnableJpaRepositories;
import org.springframework.jdbc.datasource.embedded.EmbeddedDatabaseBuilder;
import org.springframework.orm.jpa.JpaTransactionManager;
import org.springframework.orm.jpa.JpaVendorAdapter;
import org.springframework.orm.jpa.LocalContainerEntityManagerFactoryBean;
import org.springframework.orm.jpa.vendor.Database;
import org.springframework.orm.jpa.vendor.HibernateJpaVendorAdapter;
import org.springframework.transaction.PlatformTransactionManager;

@Configuration
@EnableJpaRepositories
public class Application {
    
    @Bean
    public DataSource dataSource() {
        return new EmbeddedDatabaseBuilder().setType(H2).build();
    }

    @Bean
    public LocalContainerEntityManagerFactoryBean entityManagerFactory(DataSource dataSource, JpaVendorAdapter jpaVendorAdapter) {
        LocalContainerEntityManagerFactoryBean lef = new LocalContainerEntityManagerFactoryBean();
        lef.setDataSource(dataSource);
        lef.setJpaVendorAdapter(jpaVendorAdapter);
        lef.setPackagesToScan("hello");
        return lef;
    }

    @Bean
    public JpaVendorAdapter jpaVendorAdapter() {
        HibernateJpaVendorAdapter hibernateJpaVendorAdapter = new HibernateJpaVendorAdapter();
        hibernateJpaVendorAdapter.setShowSql(false);
        hibernateJpaVendorAdapter.setGenerateDdl(true);
        hibernateJpaVendorAdapter.setDatabase(Database.H2);
        return hibernateJpaVendorAdapter;
    }

    @Bean
    public PlatformTransactionManager transactionManager(EntityManagerFactory entityManagerFactory) {
        return new JpaTransactionManager(entityManagerFactory);
    }


    public static void main(String[] args) {
        AbstractApplicationContext context = new AnnotationConfigApplicationContext(Application.class);
        CustomerRepository repository = context.getBean(CustomerRepository.class);

        // 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
        List<Customer> customers = repository.findAll();
        System.out.println("Customers found with findAll():");
        System.out.println("-------------------------------");
        for (Customer customer : customers) {
            System.out.println(customer);
        }
        System.out.println();

        // fetch an individual customer by ID
        Customer customer = repository.findOne(1L);
        System.out.println("Customer found with findOne(1L):");
        System.out.println("--------------------------------");
        System.out.println(customer);
        System.out.println();

        // fetch customers by last name
        List<Customer> bauers = repository.findByLastName("Bauer");
        System.out.println("Customer found with findByLastName('Bauer'):");
        System.out.println("--------------------------------------------");
        for (Customer bauer : bauers) {
            System.out.println(bauer);
        }

        context.close();
    }

}

@EnableJpaRepositories：告诉JPA找到继承了repository的接口，并为它创建相应的实现类，JpaRepository也继承了repository。

DataSource：声明一个存储对象的数据库

LocalContainerEntityManagerFactoryBean：对象的操作类，通过lef.setPackagesToScan("hello");避免了创建persistence.xml,它告诉JPA到hello包下面找bean类

JpaVendorAdapter：为LocalContainerEntityManagerFactoryBean定义了基于hibernate的JPA适配器

PlatformTransactionManager：用于处理事务持久化

最后运行程序，结果如下：

Customers found with findAll():
-------------------------------
Customer[id=1, firstName='Jack', lastName='Bauer']
Customer[id=2, firstName='Chloe', lastName='O'Brian']
Customer[id=3, firstName='Kim', lastName='Bauer']
Customer[id=4, firstName='David', lastName='Palmer']
Customer[id=5, firstName='Michelle', lastName='Dessler']

Customer found with findOne(1L):
--------------------------------
Customer[id=1, firstName='Jack', lastName='Bauer']

Customer found with findByLastName('Bauer'):
--------------------------------------------
Customer[id=1, firstName='Jack', lastName='Bauer']
Customer[id=3, firstName='Kim', lastName='Bauer']

该项目需要用到的jar包，通过maven的pom.xml获得

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>org.springframework</groupId>
    <artifactId>gs-accessing-data-jpa</artifactId>
    <version>0.1.0</version>

    <parent>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-parent</artifactId>
        <version>0.5.0.M4</version>
    </parent>

    <dependencies>
        <dependency>
            <groupId>org.springframework</groupId>
            <artifactId>spring-orm</artifactId>
        </dependency>
        <dependency>
            <groupId>org.springframework.data</groupId>
            <artifactId>spring-data-jpa</artifactId>
        </dependency>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-web</artifactId>
        </dependency>
        <dependency>
            <groupId>org.hibernate</groupId>
            <artifactId>hibernate-entitymanager</artifactId>
        </dependency>
        <dependency>
            <groupId>com.h2database</groupId>
            <artifactId>h2</artifactId>
        </dependency>
    </dependencies>

    <properties>
        <!-- use UTF-8 for everything -->
        <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
        <project.reporting.outputEncoding>UTF-8</project.reporting.outputEncoding>
        <start-class>hello.Application</start-class>
    </properties>

    <build>
        <plugins>
            <plugin> 
                <artifactId>maven-compiler-plugin</artifactId> 
                <version>2.3.2</version> 
            </plugin>
            <plugin>
                <groupId>org.springframework.boot</groupId>
                <artifactId>spring-boot-maven-plugin</artifactId>
            </plugin>
        </plugins>
    </build>

    <repositories>
        <repository>
            <id>spring-snapshots</id>
            <name>Spring Snapshots</name>
            <url>http://repo.spring.io/libs-snapshot</url>
            <snapshots>
                <enabled>true</enabled>
            </snapshots>
        </repository>
        <repository>
            <id>spring-milestones</id>
            <name>Spring Milestones</name>
            <url>http://repo.spring.io/libs-milestone</url>
            <snapshots>
                <enabled>false</enabled>
            </snapshots>
        </repository>
        <repository>
            <id>org.jboss.repository.releases</id>
            <name>JBoss Maven Release Repository</name>
            <url>https://repository.jboss.org/nexus/content/repositories/releases</url>
            <snapshots>
                <enabled>false</enabled>
            </snapshots>
        </repository>
    </repositories>

    <pluginRepositories>
        <pluginRepository>
            <id>spring-snapshots</id>
            <name>Spring Snapshots</name>
            <url>http://repo.spring.io/libs-snapshot</url>
            <snapshots>
                <enabled>true</enabled>
            </snapshots>
        </pluginRepository>
        <pluginRepository>
            <id>spring-milestones</id>
            <name>Spring Milestones</name>
            <url>http://repo.spring.io/libs-milestone</url>
            <snapshots>
                <enabled>false</enabled>
            </snapshots>
        </pluginRepository>
    </pluginRepositories>

</project>

这是从spring官网学习的，有不对的地方欢迎指导