前言

本文介绍如何使用spring boot jpa entity。

开发环境如下：

操作步骤

• 新建spring boot项目

• 使用navicat新建数据库db_example

• 修改配置文件./src/main/resources/application.properties

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

• 添加实体类book

package org.nick.bootstart.model;/*
 * @Author      : Nick
 * @Description :
 * @Date        : Create in 12:18 2018/4/30
 **/import lombok.AllArgsConstructor;import lombok.Data;import lombok.Generated;import lombok.NoArgsConstructor;import javax.persistence.*;import java.util.HashSet;import java.util.Objects;import java.util.Set;@Data@NoArgsConstructor@AllArgsConstructor@Entitypublic class Book {    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)    private long id;    private String title;    private String isbn;    public String publisher;    @ManyToMany
    @JoinTable(name = "author_book", joinColumns = @JoinColumn(name = "book_id"), inverseJoinColumns = @JoinColumn(name = "author_id"))    public Set<Author> authors = new HashSet<>();    @Override
    public boolean equals(Object o) {        if (this == o) return true;        if (o == null || getClass() != o.getClass()) return false;
        Book book = (Book) o;        return id == book.id &&
                Objects.equals(title, book.title) &&
                Objects.equals(isbn, book.isbn) &&
                Objects.equals(publisher, book.publisher);
    }    @Override
    public int hashCode() {        return Objects.hash(id);
    }    @Override
    public String toString() {        return "Book{" +                "id=" + id +                ", title='" + title + '\'' +                ", isbn='" + isbn + '\'' +                ", publisher='" + publisher + '\'' +                '}';
    }
}

• 添加Author

package org.nick.bootstart.model;/*
 * @Author      : Nick
 * @Description :
 * @Date        : Create in 12:36 2018/4/30
 **/import lombok.AllArgsConstructor;import lombok.Data;import lombok.NoArgsConstructor;import javax.persistence.*;import java.util.HashSet;import java.util.Set;@Data@NoArgsConstructor@AllArgsConstructor@Entitypublic class Author {    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)    private long id;    private String firstName;    private String lastName;    @ManyToMany(mappedBy = "authors")    private Set<Book> books = new HashSet<>();
}

• 执行程序

• 打开navicat 查看数据库，发现已经初始化了相关表；
  

  
  

总结

这里spring.jpa.hibernate.ddl-auto可以是none,update,create,create-drop。具体含义如下：

• none 不改变表结构

• update 通过entity结构改变数据库结构

• create 每次都打开的时候新建数据库，关闭的时候删除数据库

• create-drop 创建数据库，然后在SessionFactory关闭的时候删除数据库；