UTF-8 encoding of application.properties attributes in Spring-Boot

In my application.properties I add some custom attributes.

custom.mail.property.subject-message=This is a äöüß problem

In this class I have the representation of the custom attributes.

@Component@ConfigurationProperties(prefix="custom.mail.property")public class MailProperties {    private String subjectMessage;    public String getSubjectMessage() {        return subjectMessage;    }    public void setSubjectMessage(String subjectMessage) {        this.subjectMessage = subjectMessage;    }

And here I use my MailProperties:

@Servicepublic class SimpleUnknownResponseMessage extends MailProperties implements UnknownResponseMessage{    private JavaMailSender javaMailSender;    @Autowired    public SimpleUnknownResponseMessage(JavaMailSender javaMailSender) {        this.javaMailSender = javaMailSender;    }    @Override    public void placeUnknownResponse(BookResponse bookResponse) {        MimeMessage message = javaMailSender.createMimeMessage();        try {            MimeMessageHelper helper = new MimeMessageHelper(message, "UTF-8");            helper.setSubject(this.getSubjectMessage());                        javaMailSender.send(message);        } catch (MessagingException e) {            e.printStackTrace();        }    }

While debugging I can see that my this.getSubjectMessage() variable has this value inside: This is a äöüà problem. So before sending my mail I already have an UTF-8 encoding problem.

I already checked the encoding of the application.properties file and its UTF-8.

My IDE(STS/Eclipse) and the project properties are also set on UTF-8.

How can I set the UTF-8 encoding for the text of my custom attributes in the application.properties file?