spring.jpa.properties.hibernate.hbm2ddl.auto=
有四种配置方式，分别如下：
是hibernate的配置属性，其主要作用是：自动创建、更新、验证数据库表结构。该参数的几种配置如下：
-
create：每次加载hibernate时都会删除上一次的生成的表，然后根据你的model类再重新来生成新表，哪怕两次没有任何改变也要这样执行，这就是导致数据库表数据丢失的一个重要原因。 -
create-drop：每次加载hibernate时根据model类生成表，但是sessionFactory一关闭,表就自动删除。 -
update：最常用的属性，第一次加载hibernate时根据model类会自动建立起表的结构（前提是先建立好数据库），以后加载hibernate时根据model类自动更新表结构，即使表结构改变了但表中的行仍然存在不会删除以前的行。要注意的是当部署到服务器后，表结构是不会被马上建立起来的，是要等应用第一次运行起来后才会。 -
validate：每次加载hibernate时，验证创建数据库表结构，只会和数据库中的表进行比较，不会创建新表，但是会插入新值。
在上面配置中要注意的是“spring.jpa.hibernate.ddl-auto”这个属性，它有五个value值选着分别是：
1. none 永远以数据表字段为准，不做任何修改
2. validate 加载hibernate时，验证创建数据库表结构，会和数据库中的表进行比较，不会创建新表，但是会插入新值
3. create 每次加载hibernate，重新创建数据库表结构，这就是导致数据库表数据丢失的原因
4. create-drop 加载hibernate时创建，退出是删除表结构
5. update 加载hibernate自动更新数据库结构
一个Sample，第一张图看下目录结构
JPA依赖
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-data-jpa</artifactId>
</dependency>
重点看下Dao包下的BookResposibility，自带的API，都不需要具体写实现
package com.example.demo.dao; import com.example.demo.entity.Book;
import org.springframework.data.jpa.repository.JpaRepository;
import org.springframework.data.jpa.repository.Modifying;
import org.springframework.data.jpa.repository.Query;
import org.springframework.transaction.annotation.Transactional;
import java.util.List; public interface BookRepository extends JpaRepository<Book, Long> { List<Book> findByAuthor(String author); List<Book> findAll(); List<Book> findByName(String name); Book findById(long id); @Transactional
@Modifying
@Query("update Book set name = ?1 where id = ?2")
int updateBookName(String name, long id); }
还有我的Controller文件，主要的新增，查询，查询列表，更新操作 CRUD
package com.example.demo.controller; import com.example.demo.entity.Book;
import com.example.demo.entity.BookPo;
import com.example.demo.dao.BookRepository;
import com.example.demo.response.ResVoid;
import com.example.demo.response.Response;
import io.swagger.annotations.ApiImplicitParam;
import io.swagger.annotations.ApiOperation;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.beans.factory.annotation.Value;
import org.springframework.web.bind.annotation.*; import java.text.SimpleDateFormat;
import java.util.Date;
import java.util.List; @RestController
public class BookController { @Value("${uid}")
String uid; @Autowired
private BookRepository bookRepository; @ApiOperation(value = "新增书籍信息", notes = "新增书籍信息")
@ApiImplicitParam(dataType = "BookPo", name = "bookpo", value = "新增书籍信息")
@RequestMapping(value = "/save", method = RequestMethod.POST)
public ResVoid save(@RequestBody BookPo bookpo) {
Date dt = new Date();
SimpleDateFormat sdf = new SimpleDateFormat("yyyyMMdd HH:mm:ss");
String date = sdf.format(dt);
String bookname = bookpo.getName();
String bookauthor = bookpo.getAuthor();
int booknumber = bookpo.getNumber();
float bookprice = bookpo.getPrice(); bookRepository.save(new Book(bookname, bookauthor, date, booknumber, bookprice, date, date, uid, uid));
ResVoid re = new ResVoid();
re.setCode(200);
re.setMessage("Success");
return re; //bookRepository.save(new Book("书名11", "作者", "上架时间", 100, 23, "创建时间", "更新时间", "创建uid", "更新uid"));
//bookRepository.save(new Book("书名12", "作者", "20190504", 100, 23, "20190504", date, "3", "4"));
//bookRepository.save(new Book("西游记10", "吴承恩", "20190504", 100, 20, "20190504", date, "2", "3"));
} @ApiOperation(value = "根据作者名字或者笔名查找书籍", notes = "根据作者名字或者笔名查找书籍")
@ApiImplicitParam(dataType = "String", name = "author", value = "根据作者名字或者笔名查找书籍", paramType = "path",required = true)
@RequestMapping(value = "/findByAuthor/{author}", method = RequestMethod.GET)
public Response findByAuthor(@PathVariable("author") String author) {
List<Book> result = bookRepository.findByAuthor(author);
System.out.println("一共有多少条呢----" + result.size());
for (Book book : result) {
System.out.println(book);
}
System.out.println("================================");
Response re = new Response();
re.setCode(200);
re.setMessage("Success");
re.setSize(result.size());
re.setObject(result);
return re;
} @ApiOperation(value = "根据书籍名称进行找书", notes = "根据书籍名称进行找书")
@ApiImplicitParam(dataType = "String", name = "name", value = "根据书籍名称进行找书", paramType = "path",required = true)
@RequestMapping(value = "/findByName/{name}", method = RequestMethod.GET)
public Response findByName(@PathVariable String name) {
List<Book> result = bookRepository.findByName(name); Response re = new Response();
re.setCode(200);
re.setMessage("Success");
re.setSize(result.size());
re.setObject(result);
return re;
} @ApiOperation(value = "更新书籍信息", notes = "更新书籍信息")
@ApiImplicitParam(dataType = "Book", name = "book", value = "更新书籍信息", required = true)
@RequestMapping(value = "/updateBookName", method = RequestMethod.POST)
public Response updateBookName(@RequestBody Book book) { int count = bookRepository.updateBookName("三国演义", 2);
System.out.println(count); List<Book> result = bookRepository.findAll(); Response re = new Response();
re.setCode(200);
re.setMessage("Success");
re.setSize(result.size());
re.setObject(result);
return re; } @ApiOperation(value = "更新书籍信息", notes = "更新书籍信息")
@ApiImplicitParam(dataType = "Book", name = "book", value = "更新书籍信息", required = true)
@RequestMapping(value = "/updateBook", method = RequestMethod.POST)
public Response updateBook(@RequestBody Book book) { Date dt = new Date();
SimpleDateFormat sdf = new SimpleDateFormat("yyyyMMdd HH:mm:ss");
String date = sdf.format(dt); Book book_orignial = bookRepository.findById(book.getId()); book.setOntime(book_orignial.getOntime());
book.setCreatuid(book_orignial.getCreatuid());
book.setCreatedt(book_orignial.getCreatedt());
book.setUpdatedt(date);
book.setUpdateuid(uid); bookRepository.save(book); List<Book> result = bookRepository.findAll(); Response re = new Response();
re.setCode(200);
re.setMessage("Success");
re.setSize(result.size());
re.setObject(result);
return re; }
}