尽管数据源可用，如何使 Spring Boot 应用程序运行

Question

在我的 Spring Boot 应用程序中，配置了一个数据源。 application.properties就像：

spring.datasource.url=jdbc:mysql://localhost/test_db
spring.datasource.username=test
spring.datasource.password=test

应用程序代码如下所示：

public class Foo {
    @Autowired
    private JdbcTemplate jdbcTemplate;

    private List<String> getDataFromDB(String user) {
        List<String> data = jdbcTemplate.query(
                    "SELECT ... FROM ...;", /* the detail ommited */);
        return data;
    }
}

该代码工作正常。 但是，如果数据库服务器不可用，Spring Boot 将无法启动：

2018-01-17 14:03:30.310 ERROR o.a.tomcat.jdbc.pool.ConnectionPool     [182][main][][][] Unable to create initial connections of pool.
com.mysql.jdbc.exceptions.jdbc4.CommunicationsException: Communications link failure

The last packet sent successfully to the server was 0 milliseconds ago. The driver has not received any packets from the server.
    ...
2018-01-17 14:03:30.314  WARN o.h.e.jdbc.internal.JdbcServicesImpl    [204][main][][][] HHH000342: Could not obtain connection to query metadata : Communications link failure

The last packet sent successfully to the server was 0 milliseconds ago. The driver has not received any packets from the server.
2018-01-17 14:03:30.318  WARN ationConfigEmbeddedWebApplicationContext[546][main][][][] Exception encountered during context initialization - cancelling refresh attempt: org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'entityManagerFactory' defined in class path resource [org/springframework/boot/autoconfigure/orm/jpa/HibernateJpaAutoConfiguration.class]: Invocation of init method failed; nested exception is org.hibernate.HibernateException: Access to DialectResolutionInfo cannot be null when 'hibernate.dialect' not set
2018-01-17 14:03:30.336 ERROR o.s.boot.SpringApplication              [827][main][][][] Application startup failed
org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'entityManagerFactory' defined in class path resource [org/springframework/boot/autoconfigure/orm/jpa/HibernateJpaAutoConfiguration.class]: Invocation of init method failed; nested exception is org.hibernate.HibernateException: Access to DialectResolutionInfo cannot be null when 'hibernate.dialect' not set
    ...

Process finished with exit code 1

我省略了多行错误消息。

在我的应用程序中，数据库不是必需的，它仅用于改进。 即使数据库不可用，我也想运行该应用程序，但会注意到此类失败。

如何实现这一目标？

Answer 1

尝试设置 spring.datasource.continue-on-error=true

Answer 2

如果无法通过设置以下参数连接到数据库，我可以让我的 Spring Boot 应用程序继续运行：

spring.jpa.database-platform=org.hibernate.dialect.MySQL5Dialect