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 ä ö ü ß 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:

@Service
public 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 ä ö ü à 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?

As already mentioned in the comments .properties files are expected to be encoded in ISO 8859-1. One can use unicode escapes to specify other characters. There is also a tool available to do the conversion. This can for instance be used in the automatic build so that you still can use your favorite encoding in the source.

Be sure to read also the other answers for some alternatives. There are ways to specify the encoding for properties.