HTTP 状态 415 - 请求实体的格式不受支持

我正在开发java restful Web服务。我让它为GET请求工作，但POST请求不起作用。我的控制器类是 。到目前为止，我已经做了这些：RestController

@RequestMapping(value = "/api/signup", method = RequestMethod.POST, consumes = MediaType.APPLICATION_JSON_VALUE, produces = MediaType.APPLICATION_JSON_VALUE)
public long signUp(@ModelAttribute ApiMemberModel apiMember) {
    memberService = new MemberDetailsService();
    Member m = memberService.createMember(apiMember.getUsername(), apiMember.getPassword(), apiMember.getEmail(), "");
    return m.getId();
}

还尝试了 RequestBody 而不是 ModelAttribute。

我使用Postman扩展来发送POST请求。例如：

{
    "username": "asd",
    "password": "sfsdg",
    "email": "sdfdsf@fdsfkg.com"
}

但是我得到错误：

description The server refused this request because the request entity is in a format not supported by the requested resource for the requested method.

我做错了什么？模型类为：

 public class ApiMemberModel {
    private String username;
    private String password;
    private String email;

    public ApiMemberModel(String username, String password, String email) {
        this.username = username;
        this.password = password;
        this.email = email;
    }

    public String getUsername() {
        return username;
    }

    public void setUsername(String username) {
        this.username = username;
    }

    public String getPassword() {
        return password;
    }

    public void setPassword(String password) {
        this.password = password;
    }

    public String getEmail() {
        return email;
    }

    public void setEmail(String email) {
        this.email = email;
    }
}