javaMailSender方法无法解析

Question

@PostMapping("/signup")
public ResponseEntity<?> registerUser(@Valid @RequestBody SignUpRequest signUpRequest) {
    if(userRepository.existsByUsername(signUpRequest.getUsername())) {
        return new ResponseEntity(new ApiResponse(false, "Username is already taken!"),
                HttpStatus.BAD_REQUEST);
    }

    if(userRepository.existsByEmail(signUpRequest.getEmail())) {
        return new ResponseEntity(new ApiResponse(false, "Email Address already in use!"),
                HttpStatus.BAD_REQUEST);
    }

    // Creating user's account
    User user = new User( signUpRequest.getUsername(),
            signUpRequest.getEmail(), signUpRequest.getPassword());

    user.setPassword(passwordEncoder.encode(user.getPassword()));

    //Role userRole = roleRepository.findByName(RoleName.ROLE_USER)
           // .orElseThrow(() -> new AppException("User Role not set."));
    //user.setRoles(Collections.singleton(userRole));

    User result = userRepository.save(user);

    URI location = ServletUriComponentsBuilder
            .fromCurrentContextPath().path("/api/users/{username}")
            .buildAndExpand(result.getUsername()).toUri();

    // Send mail
    SimpleMailMessage msg = new SimpleMailMessage();
    msg.setTo("to_1@gmail.com", "to_2@gmail.com", "to_3@yahoo.com");
    msg.setSubject("Testing from Spring Boot");
    msg.setText("Hello World \n Spring Boot Email");

    javaMailSender.send(msg);
    return ResponseEntity.created(location).body(new ApiResponse(true, "User registered successfully"));
}