/**
 * Callback interface for initializing a Spring {@link ConfigurableApplicationContext}
 * prior to being {@linkplain ConfigurableApplicationContext#refresh() refreshed}.
 *
 * <p>Typically used within web applications that require some programmatic initialization
 * of the application context. For example, registering property sources or activating
 * profiles against the {@linkplain ConfigurableApplicationContext#getEnvironment()
 * context's environment}. See {@code ContextLoader} and {@code FrameworkServlet} support
 * for declaring a "contextInitializerClasses" context-param and init-param, respectively.
 *
 * <p>{@code ApplicationContextInitializer} processors are encouraged to detect
 * whether Spring's {@link org.springframework.core.Ordered Ordered} interface has been
 * implemented or if the @{@link org.springframework.core.annotation.Order Order}
 * annotation is present and to sort instances accordingly if so prior to invocation.
 *
 * @author Chris Beams
 * @since 3.1
 * @param <C> the application context type
 * @see org.springframework.web.context.ContextLoader#customizeContext
 * @see org.springframework.web.context.ContextLoader#CONTEXT_INITIALIZER_CLASSES_PARAM
 * @see org.springframework.web.servlet.FrameworkServlet#setContextInitializerClasses
 * @see org.springframework.web.servlet.FrameworkServlet#applyInitializers
 */
public interface ApplicationContextInitializer<C extends ConfigurableApplicationContext> {

    /**
     * Initialize the given application context.
     * @param applicationContext the application to configure
     */
    void initialize(C applicationContext);

}

@Order(1)
public class FirstApplicationContextInitializer implements ApplicationContextInitializer<GenericApplicationContext> {

    @Override
    public void initialize(GenericApplicationContext context) {
        ConfigurableEnvironment environment = context.getEnvironment();
        Map<String,String> map = new HashMap<>();
        map.put("first","hello first");
        environment.getSystemProperties().putAll(map);
        System.out.println("firstApplicationContextInitializer is start");
    }
}

org.springframework.context.ApplicationContextInitializer=com.example.demo.initialize.FirstApplicationContextInitializer

@RestController
public class InitializeController {

    @Autowired
    ApplicationContext applicationContext;

    @GetMapping("/first")
    public String test(){
        String a = applicationContext.getEnvironment().getProperty("first");
        return a;
    }

    @GetMapping("/second")
    public String test1(){
        String a = applicationContext.getEnvironment().getProperty("second");
        return a;
    }

    @GetMapping("/third")
    public String test2(){
        String a = applicationContext.getEnvironment().getProperty("third");
        return a;
    }

}

 .   ____          _            __ _ _
 /\\ / ___'_ __ _ _(_)_ __  __ _ \ \ \ \
( ( )\___ | '_ | '_| | '_ \/ _` | \ \ \ \
 \\/  ___)| |_)| | | | | || (_| |  ) ) ) )
  '  |____| .__|_| |_|_| |_\__, | / / / /
 =========|_|==============|___/=/_/_/_/
 :: Spring Boot ::        (v2.2.6.RELEASE)
firstApplicationContextInitializer is start
2020-05-29 10:20:52.989  INFO 1576 --- [           main] com.example.demo.DemoApplication         : Starting DemoApplication on ganxinledeMacBook-Pro.local with PID 1576 (/Users/ganxinle/workspace/demo/target/classes started by ganxinle in /Users/ganxinle/workspace/demo)
2020-05-29 10:20:52.991  INFO 1576 --- [           main] com.example.demo.DemoApplication         : No active profile set, falling back to default profiles: default
2020-05-29 10:20:53.686  INFO 1576 --- [           main] o.s.b.w.embedded.tomcat.TomcatWebServer  : Tomcat initialized with port(s): 8080 (http)
2020-05-29 10:20:53.696  INFO 1576 --- [           main] o.apache.catalina.core.StandardService   : Starting service [Tomcat]
2020-05-29 10:20:53.696  INFO 1576 --- [           main] org.apache.catalina.core.StandardEngine  : Starting Servlet engine: [Apache Tomcat/9.0.33]
2020-05-29 10:20:53.749  INFO 1576 --- [           main] o.a.c.c.C.[Tomcat].[localhost].[/]       : Initializing Spring embedded WebApplicationContext
2020-05-29 10:20:53.749  INFO 1576 --- [           main] o.s.web.context.ContextLoader            : Root WebApplicationContext: initialization completed in 721 ms
2020-05-29 10:20:53.860  INFO 1576 --- [           main] o.s.s.concurrent.ThreadPoolTaskExecutor  : Initializing ExecutorService 'applicationTaskExecutor'
2020-05-29 10:20:53.979  INFO 1576 --- [           main] o.s.b.w.embedded.tomcat.TomcatWebServer  : Tomcat started on port(s): 8080 (http) with context path ''
2020-05-29 10:20:53.982  INFO 1576 --- [           main] com.example.demo.DemoApplication         : Started DemoApplication in 1.258 seconds (JVM running for 1.577)
2020-05-29 10:21:12.351  INFO 1576 --- [nio-8080-exec-1] o.a.c.c.C.[Tomcat].[localhost].[/]       : Initializing Spring DispatcherServlet 'dispatcherServlet'
2020-05-29 10:21:12.351  INFO 1576 --- [nio-8080-exec-1] o.s.web.servlet.DispatcherServlet        : Initializing Servlet 'dispatcherServlet'
2020-05-29 10:21:12.354  INFO 1576 --- [nio-8080-exec-1] o.s.web.servlet.DispatcherServlet        : Completed initialization in 3 ms