我构建了一个简单的SpringBoot应用程序(版本：2.4.3(。我有以下实体：
package com.ksteindl.logolo.domain;
import javax.persistence.Entity;
import javax.persistence.Id;
@Entity
public class Foo {
@Id
private String id;
private String foo;
public String getFoo() {
return foo;
}
public void setFoo(String foo) {
this.foo = foo;
}
public void setId(String id) {
this.id = id;
}
public String getId() {
return id;
}
}
和一个application.properties
spring.jpa.show-sql=true
spring.datasource.url = jdbc:mysql://localhost:3306/mysql
spring.datasource.username=admin
spring.datasource.password=admin
spring.jpa.database-platform=org.hibernate.dialect.MySQL5Dialect
spring.jpa.hibernate.ddl-auto=update
spring.jpa.properties.hibernate.dialect = org.hibernate.dialect.MySQL57Dialect
第一次跑步时，一切似乎都很好。这些表是根据Entites创建的。然而，当我重新启动我的应用程序时，我的每个实体/表都会出现异常。此外，如果我修改我的实体(例如在FooEntity中引入一个bar属性(，那么DB中对应的表就不会改变。在我看来，它试图重新创建表，但我不知道为什么。对于我拥有的每个表/实体，我都有以下例外：
Hibernate: create table foo (id varchar(255) not null, bar varchar(255), baz varchar(255), foo varchar(255), primary key (id)) engine=InnoDB
2021-04-02 16:34:17.000 WARN 16359 --- [ restartedMain] o.h.t.s.i.ExceptionHandlerLoggedImpl : GenerationTarget encountered exception accepting command : Error executing DDL "create table foo (id varchar(255) not null, bar varchar(255), baz varchar(255), foo varchar(255), primary key (id)) engine=InnoDB" via JDBC Statement
org.hibernate.tool.schema.spi.CommandAcceptanceException: Error executing DDL "create table foo (id varchar(255) not null, bar varchar(255), baz varchar(255), foo varchar(255), primary key (id)) engine=InnoDB" via JDBC Statement
at org.hibernate.tool.schema.internal.exec.GenerationTargetToDatabase.accept(GenerationTargetToDatabase.java:67) ~[hibernate-core-5.4.28.Final.jar:5.4.28.Final]
at org.hibernate.tool.schema.internal.AbstractSchemaMigrator.applySqlString(AbstractSchemaMigrator.java:559) [hibernate-core-5.4.28.Final.jar:5.4.28.Final]
at org.hibernate.tool.schema.internal.AbstractSchemaMigrator.applySqlStrings(AbstractSchemaMigrator.java:504) [hibernate-core-5.4.28.Final.jar:5.4.28.Final]
at org.hibernate.tool.schema.internal.AbstractSchemaMigrator.createTable(AbstractSchemaMigrator.java:277) [hibernate-core-5.4.28.Final.jar:5.4.28.Final]
at org.hibernate.tool.schema.internal.GroupedSchemaMigratorImpl.performTablesMigration(GroupedSchemaMigratorImpl.java:71) [hibernate-core-5.4.28.Final.jar:5.4.28.Final]
at org.hibernate.tool.schema.internal.AbstractSchemaMigrator.performMigration(AbstractSchemaMigrator.java:207) [hibernate-core-5.4.28.Final.jar:5.4.28.Final]
at org.hibernate.tool.schema.internal.AbstractSchemaMigrator.doMigration(AbstractSchemaMigrator.java:114) [hibernate-core-5.4.28.Final.jar:5.4.28.Final]
at org.hibernate.tool.schema.spi.SchemaManagementToolCoordinator.performDatabaseAction(SchemaManagementToolCoordinator.java:184) [hibernate-core-5.4.28.Final.jar:5.4.28.Final]
at org.hibernate.tool.schema.spi.SchemaManagementToolCoordinator.process(SchemaManagementToolCoordinator.java:73) [hibernate-core-5.4.28.Final.jar:5.4.28.Final]
at org.hibernate.internal.SessionFactoryImpl.<init>(SessionFactoryImpl.java:318) [hibernate-core-5.4.28.Final.jar:5.4.28.Final]
at org.hibernate.boot.internal.SessionFactoryBuilderImpl.build(SessionFactoryBuilderImpl.java:468) [hibernate-core-5.4.28.Final.jar:5.4.28.Final]
at org.hibernate.jpa.boot.internal.EntityManagerFactoryBuilderImpl.build(EntityManagerFactoryBuilderImpl.java:1259) [hibernate-core-5.4.28.Final.jar:5.4.28.Final]
at org.springframework.orm.jpa.vendor.SpringHibernateJpaPersistenceProvider.createContainerEntityManagerFactory(SpringHibernateJpaPersistenceProvider.java:58) [spring-orm-5.3.4.jar:5.3.4]
at org.springframework.orm.jpa.LocalContainerEntityManagerFactoryBean.createNativeEntityManagerFactory(LocalContainerEntityManagerFactoryBean.java:365) [spring-orm-5.3.4.jar:5.3.4]
at org.springframework.orm.jpa.AbstractEntityManagerFactoryBean.buildNativeEntityManagerFactory(AbstractEntityManagerFactoryBean.java:409) [spring-orm-5.3.4.jar:5.3.4]
at org.springframework.orm.jpa.AbstractEntityManagerFactoryBean.afterPropertiesSet(AbstractEntityManagerFactoryBean.java:396) [spring-orm-5.3.4.jar:5.3.4]
at org.springframework.orm.jpa.LocalContainerEntityManagerFactoryBean.afterPropertiesSet(LocalContainerEntityManagerFactoryBean.java:341) [spring-orm-5.3.4.jar:5.3.4]
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.invokeInitMethods(AbstractAutowireCapableBeanFactory.java:1845) [spring-beans-5.3.4.jar:5.3.4]
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.initializeBean(AbstractAutowireCapableBeanFactory.java:1782) [spring-beans-5.3.4.jar:5.3.4]
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:602) [spring-beans-5.3.4.jar:5.3.4]
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:524) [spring-beans-5.3.4.jar:5.3.4]
at org.springframework.beans.factory.support.AbstractBeanFactory.lambda$doGetBean$0(AbstractBeanFactory.java:335) [spring-beans-5.3.4.jar:5.3.4]
at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:234) ~[spring-beans-5.3.4.jar:5.3.4]
at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:333) [spring-beans-5.3.4.jar:5.3.4]
at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:208) [spring-beans-5.3.4.jar:5.3.4]
at org.springframework.context.support.AbstractApplicationContext.getBean(AbstractApplicationContext.java:1153) ~[spring-context-5.3.4.jar:5.3.4]
at org.springframework.context.support.AbstractApplicationContext.finishBeanFactoryInitialization(AbstractApplicationContext.java:907) ~[spring-context-5.3.4.jar:5.3.4]
at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:582) ~[spring-context-5.3.4.jar:5.3.4]
at org.springframework.boot.web.servlet.context.ServletWebServerApplicationContext.refresh(ServletWebServerApplicationContext.java:144) ~[spring-boot-2.4.3.jar:2.4.3]
at org.springframework.boot.SpringApplication.refresh(SpringApplication.java:767) ~[spring-boot-2.4.3.jar:2.4.3]
at org.springframework.boot.SpringApplication.refresh(SpringApplication.java:759) ~[spring-boot-2.4.3.jar:2.4.3]
at org.springframework.boot.SpringApplication.refreshContext(SpringApplication.java:426) ~[spring-boot-2.4.3.jar:2.4.3]
at org.springframework.boot.SpringApplication.run(SpringApplication.java:326) ~[spring-boot-2.4.3.jar:2.4.3]
at org.springframework.boot.SpringApplication.run(SpringApplication.java:1311) ~[spring-boot-2.4.3.jar:2.4.3]
at org.springframework.boot.SpringApplication.run(SpringApplication.java:1300) ~[spring-boot-2.4.3.jar:2.4.3]
at com.ksteindl.logolo.LogoloApplication.main(LogoloApplication.java:10) ~[classes/:na]
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[na:1.8.0_282]
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) ~[na:1.8.0_282]
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[na:1.8.0_282]
at java.lang.reflect.Method.invoke(Method.java:498) ~[na:1.8.0_282]
at org.springframework.boot.devtools.restart.RestartLauncher.run(RestartLauncher.java:49) ~[spring-boot-devtools-2.4.3.jar:2.4.3]
Caused by: java.sql.SQLSyntaxErrorException: Table 'foo' already exists
at com.mysql.cj.jdbc.exceptions.SQLError.createSQLException(SQLError.java:120) ~[mysql-connector-java-8.0.23.jar:8.0.23]
at com.mysql.cj.jdbc.exceptions.SQLExceptionsMapping.translateException(SQLExceptionsMapping.java:122) ~[mysql-connector-java-8.0.23.jar:8.0.23]
at com.mysql.cj.jdbc.StatementImpl.executeInternal(StatementImpl.java:762) ~[mysql-connector-java-8.0.23.jar:8.0.23]
at com.mysql.cj.jdbc.StatementImpl.execute(StatementImpl.java:646) ~[mysql-connector-java-8.0.23.jar:8.0.23]
at com.zaxxer.hikari.pool.ProxyStatement.execute(ProxyStatement.java:95) ~[HikariCP-3.4.5.jar:na]
at com.zaxxer.hikari.pool.HikariProxyStatement.execute(HikariProxyStatement.java) ~[HikariCP-3.4.5.jar:na]
at org.hibernate.tool.schema.internal.exec.GenerationTargetToDatabase.accept(GenerationTargetToDatabase.java:54) ~[hibernate-core-5.4.28.Final.jar:5.4.28.Final]
... 40 common frames omitted
有人能帮帮我吗？感谢
在某些情况下，Hibernate本身无法对数据库模式进行一些更改。
这就是为什么通常我们尽量不将ddl-auto设置为update，而更喜欢使用像liquibase或flyway这样的数据迁移工具。
在非常简单的情况下，比如添加一列，Hibernate会非常好地工作，因为它是一个不间断的更改。另一方面，例如，如果你想删除一列，它会变得更加微妙。在这种情况下，您的java实体将不再知道该列，但在数据库中它仍然存在。
这既是一个问题，也是一件好事，因为Hibernate可以确保您不会丢失数据，但这也是没有进行某些更改的原因。
正如我在上面告诉你的，我们更喜欢使用数据迁移工具，这些工具是为做这类事情而设计的。
例如，你可以看看Liquibase，这是一个非常好的工具。以下是一些有用的链接：
- https://www.liquibase.org/
- https://docs.liquibase.com/tools-integrations/springboot/springboot.html
- https://www.baeldung.com/liquibase-refactor-schema-of-java-app
但你也可以使用天桥(我不知道(：https://flywaydb.org/
最后，Liquibase和Flyway之间的比较非常有趣：https://dzone.com/articles/flyway-vs-liquibase
在2.4.3 SpringBoot中，JPA ddl auto=update行为似乎不适用于MySql 8。在PostgreSQL 13中，ddl auto=update行为运行良好，没有任何错误。我的application.yml
spring:
jpa:
hibernate:
ddl-auto: update
database-platform: org.hibernate.dialect.PostgreSQLDialect
datasource:
url: "jdbc:postgresql://localhost:5432/mydb"
username: myuser
password: mypassword
如果有人能反驳/谴责，那将是令人放心的。