SpringGuide：Accessing data with MySQL

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本文介绍了使用Spring Boot连接实际数据库MySQL的示例。从Spring Initializr开始，依次讲解了创建数据库、配置文件、实体模型、资料库、控制器等步骤，还说明了如何构建可执行JAR和测试应用，通过命令行添加用户并验证结果。

这个例子很不错，用实际的数据库MySQL，而不是用内存数据库，内存数据库估计大家平时一般也不会用，总感觉有点虚。

Starting with Spring Initializr

访问初始化程序的应用网站：https://start.spring.io/

依赖是三个分别是：1、 Spring Web Starter, 2、Spring Data JPA,3、 MySQL Driver

应用程序取名字：accessing data mysql

点击Gnerate 生成一个下载包。下载后用SpringToolSuite 在本地打开。

Create the Database（创建数据库）

创建数据库，你如果有数据库，就不用创建了，用就可以了。

创建用户，给一切权限。

mysql> create database db_example; -- Creates the new database
mysql> create user 'springuser'@'%' identified by 'ThePassword'; -- Creates the user
mysql> grant all on db_example.* to 'springuser'@'%'; -- Gives all privileges to the new user on the newly created database

这些你也可以在MySQL的图形界面去做，创建一个用户给它个DBA权限就可以了。

Create the `application.properties` File（创建配置文件）

这个就是配置文件，配置数据源，连接字符串什么的，具体如下：

spring.jpa.hibernate.ddl-auto=update
spring.datasource.url=jdbc:mysql://localhost:3306/test?serverTimezone=UTC
#jdbc:mysql://localhost:3306/library?characterEncoding=UTF-8&serverTimezone=Asia/Shanghai

spring.datasource.username=springlcg
spring.datasource.password=XXXXXXXXXXXX

密码根据实际情况自己改了。这一句中;spring.datasource.url=jdbc:mysql://localhost:3306/test?serverTimezone=UTC 问号后面是后加的，要不然会出错。

原文解释了一下几个参数的意义：MySQL默认就是none，这里用update，就是说根据给定的实体结构，Hibernate会更改库结构。

none: The default for MySQL. No change is made to the database structure.
update: Hibernate changes the database according to the given entity structures.
create: Creates the database every time but does not drop it on close.
create-drop: Creates the database and drops it when SessionFactory closes.

Create the `@Entity` Model（创建实体模型）

创建类：User.java

package com.example.accessingdatamysql;

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

@Entity // This tells Hibernate to make a table out of this class
public class User {
@Id
@GeneratedValue(strategy=GenerationType.AUTO)
private Integer id;

private String name;

private String email;

public Integer getId() {
return id;
}

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

public String getName() {
return name;
}

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

public String getEmail() {
return email;
}

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

Create the Repository（创建资料库）

创建类：UserRepository.java

package com.example.accessingdatamysql;

import org.springframework.data.repository.CrudRepository;

import com.example.accessingdatamysql.User;

// This will be AUTO IMPLEMENTED by Spring into a Bean called userRepository
// CRUD refers Create, Read, Update, Delete

public interface UserRepository extends CrudRepository<User, Integer> {

}

Spring automatically implements this repository interface in a bean that has the same name (with a change in the case — it is called userRepository).

Spring 自动实现这个repository接口在一个bean中,bean的名字就是小写后的:userRepository

Create a Controller（创建控制器）

创建类MainController.java

package com.example.accessingdatamysql;

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.PostMapping;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RequestParam;
import org.springframework.web.bind.annotation.ResponseBody;

@Controller // This means that this class is a Controller
@RequestMapping(path="/demo") // This means URL's start with /demo (after Application path)
public class MainController {
  @Autowired // This means to get the bean called userRepository
         // Which is auto-generated by Spring, we will use it to handle the data
  private UserRepository userRepository;

  @PostMapping(path="/add") // Map ONLY POST Requests
  public @ResponseBody String addNewUser (@RequestParam String name
      , @RequestParam String email) {
    // @ResponseBody means the returned String is the response, not a view name
    // @RequestParam means it is a parameter from the GET or POST request

    User n = new User();
    n.setName(name);
    n.setEmail(email);
    userRepository.save(n);
    return "Saved";
  }

  @GetMapping(path="/all")
  public @ResponseBody Iterable<User> getAllUsers() {
    // This returns a JSON or XML with the users
    return userRepository.findAll();
  }
}