JPA入门

1. JPA

1.1. JPA with H2

参考spring官网新建项目

entity：

 

src\main\java\hello\Customer.java

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;

    protected Customer(){}

    public Customer(String firstName,String lastName){
        this.firstName = firstName;
        this.lastName = lastName;
    }

    public String toString(){
        return String.format(
                "customer[id=%d,firstName='%s',lastName='%s']",
                id,firstName,lastName);
    }
}

 

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);
}

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.context.annotation.Bean;
import org.springframework.boot.autoconfigure.SpringBootApplication;

@SpringBootApplication
public 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) -> {
            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"));

            log.info("Customers found with finaAll():");
            log.info("------------------------------");
            for( Customer customer:repository.findAll()){
                log.info(customer.toString());
            }
            log.info("");
            repository.findById(1L)
                    .ifPresent(customer->{
                        log.info("Customer found with findById(1L):");
                        log.info("---------------------------------");
                        log.info(customer.toString());
                        log.info("");
                    });
            log.info("customer found with findBylastName('Bauer')");
            repository.findByLastName("Bauer").forEach(bauer->{
                log.info(bauer.toString());
            });
            log.info("");
        };
    }
}

 

1.2. JPA with mysql

在1.3.1的基础上进行如下修改：

1. 新增文件src\main\resources\application.properties

spring.jpa.hibername.ddl-auto=create
spring.datasource.url=jdbc:mysql://localhost:3306/mysql
spring.datasource.username=root
spring.datasource.password=123456

2. 修改pom.xml,作如下修改：

<!--<dependency>
    <groupId>com.h2database</groupId>
    <artifactId>h2</artifactId>
</dependency>-->
<dependency>
    <groupId>mysql</groupId>
    <artifactId>mysql-connector-java</artifactId>
</dependency>

3. 数据库新增表：

CREATE TABLE `NewTable` (

`ID`  int(10) NOT NULL AUTO_INCREMENT ,

`first_name`  varchar(255) CHARACTER SET utf8mb4 COLLATE utf8mb4_0900_ai_ci NULL DEFAULT NULL ,

`last_name`  varchar(255) CHARACTER SET utf8mb4 COLLATE utf8mb4_0900_ai_ci NULL DEFAULT NULL ,

PRIMARY KEY (`ID`)

)

ENGINE=InnoDB

DEFAULT CHARACTER SET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci

ROW_FORMAT=DYNAMIC

;

4. 构建

手工执行：mvnw spring-boot:run

5. 执行后效果：

问题：

[ERROR] Failed to execute goal org.springframework.boot:spring-boot-maven-plugin:2.0.5.RELEASE:run (default-cli) on project gs-accessing-data-jpa: An exception occurred while running. null: InvocationTar

getException: Failed to execute CommandLineRunner: error performing isolated work; SQL [n/a]; nested exception is org.hibernate.exception.SQLGrammarException: error performing isolated work: Table 'mysql

.hibernate_sequence' doesn't exist -> [Help 1]

解决：

对实体进行修改，GenerationType.AUTO -> GenerationType.IDENTITY

@Entity

public class Customer {

 

    @Id

    @GeneratedValue(strategy = GenerationType.IDENTITY)

private Long id;

 

问题：

The database returned no natively generated identity value

解决：

将对应字段，勾选自动递增，并设为主键

 

1.3. jpa with mysql for Web

来源：https://spring.io/guides/gs/accessing-data-mysql/

1. pom.xml

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>

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

    <parent>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-parent</artifactId>
        <version>2.0.5.RELEASE</version>
    </parent>

    <dependencies>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-web</artifactId>
        </dependency>
        
        <!-- JPA Data (We are going to use Repositories, Entities, Hibernate, etc...) -->
        
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-data-jpa</artifactId>
        </dependency>
        
        <!-- Use MySQL Connector-J -->
        
        <dependency>
            <groupId>mysql</groupId>
            <artifactId>mysql-connector-java</artifactId>
        </dependency>
        
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-test</artifactId>
            <scope>test</scope>
        </dependency>
    </dependencies>

    <properties>
        <java.version>1.8</java.version>
    </properties>

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

</project>

2. 目录结构

3. 相关文件

Users.java

package hello;

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

@Entity
public class Users {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Integer id;
    private String name;
    private String email;

    public Integer getId() {
        return id;
    }

    public String getName() {
        return name;
    }

    public String getEmail() {
        return email;
    }

    public void setId(Integer id) {
        this.id = id;
    }

    public void setName(String name) {
        this.name = name;
    }

    public void setEmail(String email) {
        this.email = email;
    }

}

UserRepository.java

package hello;

import org.springframework.data.repository.CrudRepository;

public interface  UserRepository extends CrudRepository<Users,Integer> {
}

MainController.java

package hello;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Controller;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RequestParam;
import org.springframework.web.bind.annotation.ResponseBody;

@Controller
@RequestMapping(path="/demo")
public class MainController {

    @Autowired
    private UserRepository userRepository;

    @GetMapping(path="/add")
    public @ResponseBody String addNewUser(
            @RequestParam String name, @RequestParam  String email){

        Users u = new Users();
        u.setName(name);
        u.setEmail(email);
        userRepository.save(u);
        return "saved";
    }

    @GetMapping(path="/all")
    public @ResponseBody Iterable<Users> getAllUsers(){
        return userRepository.findAll();
    }
}

Application.java

package hello;


import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;

@SpringBootApplication
public class Application {
    public static void main(String []args){
        SpringApplication.run(Application.class,args);
    }
}

数据库中表：

CREATE TABLE `NewTable` (

`id`  int(10) NOT NULL AUTO_INCREMENT ,

`name`  varchar(255) CHARACTER SET utf8mb4 COLLATE utf8mb4_0900_ai_ci NULL DEFAULT NULL ,

`email`  varchar(255) CHARACTER SET utf8mb4 COLLATE utf8mb4_0900_ai_ci NULL DEFAULT NULL ,

PRIMARY KEY (`id`)

)

ENGINE=InnoDB

DEFAULT CHARACTER SET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci

ROW_FORMAT=DYNAMIC

;

4. 测试

http://127.0.0.1:8080/demo/add?name=yangpb&email=123@123.com

返回：saved

http://127.0.0.1:8080/demo/all

返回：

[{"id":1,"name":"yangpb","email":"123@123.com"}]