spring.jpa.hibernate.ddl-auto 属性在 Spring 中是如何工作的? [英] How does spring.jpa.hibernate.ddl-auto property exactly work in Spring?

问题描述

我在处理我的 Spring Boot 应用程序项目时注意到，有时我的数据库在另一台服务器 (SQL Server) 上会出现连接超时错误.当我尝试使用 FlyWay 进行一些脚本迁移时，特别会发生这种情况，但经过多次尝试后它才起作用.

I was working on my Spring boot app project and noticed that, sometimes there is a connection time out error to my Database on another server(SQL Server). This happens specially when I try to do some script migration with FlyWay but it works after several tries.

然后我注意到我没有在我的属性文件中指定 spring.jpa.hibernate.ddl-auto.我做了一些研究，发现建议添加spring.jpa.hibernate.ddl-auto= create-drop 正在开发中.并将其更改为:spring.jpa.hibernate.ddl-auto= none in production.

Then I noticed that I didn't specify spring.jpa.hibernate.ddl-auto in my properties file. I did some research and found that it is recommended to add spring.jpa.hibernate.ddl-auto= create-drop in development. And change it to: spring.jpa.hibernate.ddl-auto= none in production.

但我实际上并不了解它是如何工作的，以及休眠如何使用 create-drop 或 none 值生成数据库模式.您能否从技术上解释它是如何工作的，以及在开发和生产服务器上使用此属性的建议是什么.谢谢

But I didn't actually understand how does it really work and how does hibernate generate database schema using create-drop or none value. Can you please explain technically how does it really work, and what are recommendations for using this property in development and on a production server. Thank you

推荐答案

为了记录，spring.jpa.hibernate.ddl-auto 属性是 Spring Data JPA 特定的，是他们指定的方式一个最终将在它知道的属性下传递给 Hibernate 的值，hibernate.hbm2ddl.auto.

For the record, the spring.jpa.hibernate.ddl-auto property is Spring Data JPA specific and is their way to specify a value that will eventually be passed to Hibernate under the property it knows, hibernate.hbm2ddl.auto.

值 create、create-drop、validate 和 update 基本上影响模式工具管理的方式将在启动时操作数据库架构.

The values create, create-drop, validate, and update basically influence how the schema tool management will manipulate the database schema at startup.

例如，update 操作将查询 JDBC 驱动程序的 API 以获取数据库元数据，然后 Hibernate 根据读取您的注释类或 HBM XML 映射来比较它创建的对象模型，并将尝试即时调整架构.

For example, the update operation will query the JDBC driver's API to get the database metadata and then Hibernate compares the object model it creates based on reading your annotated classes or HBM XML mappings and will attempt to adjust the schema on-the-fly.

例如，update 操作将尝试添加新列、约束等，但永远不会删除以前可能存在但不再作为对象模型一部分的列或约束.事先运行.

The update operation for example will attempt to add new columns, constraints, etc but will never remove a column or constraint that may have existed previously but no longer does as part of the object model from a prior run.

通常在测试用例场景中，您可能会使用 create-drop 以便创建架构、测试用例添加一些模拟数据、运行测试，然后在测试用例期间清理，模式对象被删除，留下一个空的数据库.

Typically in test case scenarios, you'll likely use create-drop so that you create your schema, your test case adds some mock data, you run your tests, and then during the test case cleanup, the schema objects are dropped, leaving an empty database.

在开发中，经常会看到开发人员使用 update 自动修改架构以在重新启动时添加新内容.但请再次理解，这不会删除先前执行中可能存在的不再需要的列或约束.

In development, it's often common to see developers use update to automatically modify the schema to add new additions upon restart. But again understand, this does not remove a column or constraint that may exist from previous executions that is no longer necessary.

在生产中，通常强烈建议您使用 none 或干脆不指定此属性.这是因为 DBA 检查数据库更改的迁移脚本是一种常见做法，尤其是当您的数据库在多个服务和应用程序之间共享时.

In production, it's often highly recommended you use none or simply don't specify this property. That is because it's common practice for DBAs to review migration scripts for database changes, particularly if your database is shared across multiple services and applications.

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