That's because Spring isn't managing your PagePopulationInterceptor instance. You are creating it yourself in the below （攔截器內使用@Autowired時出現了null，這是由於你的spring物件注入時機在你的攔截器之後了）

public @Override void addInterceptors(InterceptorRegistry registry) {
    registry.addInterceptor(new PagePopulationInterceptor());
}
 

change that to

@Bean
public PagePopulationInterceptor pagePopulationInterceptor() {
    return new PagePopulationInterceptor();
}

public @Override void addInterceptors(InterceptorRegistry registry) {
    registry.addInterceptor(pagePopulationInterceptor());
}

in this way, Spring will manage the lifecycle of the PagePopulationInterceptor instance since it's generated from a @Bean method. Spring will scan it for @Autowired targets and inject them.
This assumes that PagePopulationInterceptor is in a package to be @ComponentScaned.