为什么我无法在 Spring Boot 中使用 application.properties 文件中的属性？

Question

Why I fail to use data from application.properties in the PostDao class? Properties just not injected and DATABASE_URL and other variables are null .
PostDao.java is the class (src.main.java.site.model.PostDao) where I try to use properties.

@Component
public class PostDao
{
    @Value("${url}") public static String DATABASE_URL;
    @Value("${user}") public static String USER;
    @Value("${password}") public static String PASSWORD;
}

Here is my application.peoprerties file which stays in the resources directory

url=jdbc:postgresql://localhost:5432/news
user=postgres
password=123

Here is my config file (src.main.java.site.NewsSiteApplication)

@SpringBootApplication
public class NewsSiteApplication
{
    public static void main(String[] args)
    {
        SpringApplication.run(NewsSiteApplication.class, args);
    }
}

Answer 1

You can't inject in a static variable. You need a Spring-managed Bean for that. Try the following:

@Component
public class PostDao {
    @Value("${url}") public String databaseUrl;
    @Value("${user}") public String user;
    @Value("${password}") public String password;
}

As pointed out by @Turing85 (thanks for the hint ;) ), only constants should be written in UPPER_SNAKE_CASE , hence I changed the variables name.

Answer 2

Alternatively, you can do the following, the setter method will handle for the injection:

@Component
public class PostDao
{
    public static String DATABASE_URL;


    @Value("${url}")
    public void setUrlStatic(String name){
        PostDao.DATABASE_URL = name;
    }

    // And others..
}