java - 如何将 Java 对象序列化为 JSON 并将其返回到 servlet 过滤器中？

Question

我有这个 javax.servlet.Filter 来检查是否允许客户端访问 API REST 资源。

@Component
public class AuthorizationRequestFilter implements Filter {

    public static final String AUTHORIZATION_TOKEN = "X-Access-Token";

    @Autowired
    @Qualifier("loginService")
    private ILoginService loginService;

    private void throwUnauthorized(ServletResponse res) throws IOException {

        HttpServletResponse response = (HttpServletResponse) res;

        response.reset();
        response.setHeader("Content-Type", "application/json;charset=UTF-8");
        response.sendError(HttpServletResponse.SC_UNAUTHORIZED);

    }

    private void throwForbidden(ServletResponse res) throws IOException {

        HttpServletResponse response = (HttpServletResponse) res;

        response.reset();
        response.setHeader("Content-Type", "application/json;charset=UTF-8");
        response.sendError(HttpServletResponse.SC_FORBIDDEN);

    }

    @Override
    public void doFilter(ServletRequest req, ServletResponse res, FilterChain chain) throws IOException, ServletException {

        HttpServletRequest request = (HttpServletRequest) req;

        String accessToken = request.getHeader(AUTHORIZATION_TOKEN);

        if (StringUtils.isEmpty(accessToken)) {
            throwUnauthorized(res);
        } else {
            AccountLoginData account = loginService.find(accessToken);
            if (account == null) {
                throwForbidden(res);
            }
        }

        chain.doFilter(req, res);

    }

    @Override
    public void destroy() {
    }

    @Override
    public void init(FilterConfig arg0) throws ServletException {
    }

}

它有效，但我想在这两个 throw*() 方法中使用适当的信息写入客户端 JSON。在这个应用程序的另一部分，我使用这些响应消息对象来通知客户端发生了什么。

比如没有找到记录时:

public class NotFoundResponseMessage extends ResponseMessage {

    public NotFoundResponseMessage(String message) {
        super(HttpStatus.NOT_FOUND, 1, message);
    }

}

和

public class ResponseMessage {

    private int status;
    private int code;
    private String message;
    private String reason;

    public ResponseMessage(int status, int code, String message, String reason) {

        Assert.notNull(reason, "Reason must not be null.");
        Assert.isTrue(status > 0, "Status must not be empty.");

        this.status = status;
        this.code = code;
        this.message = message;
        this.reason = reason;

    }

我的问题

我想在我的 javax.servlet.Filter 授权/身份验证过滤器中返回带有序列化对象(UnauthorizedResponseMessage 和 ForbiddenResponseMessage)的 JSON。我使用 Spring Boot 和 Jackson 库。

1. 如何手动将 ResponseMessage 序列化为它的 JSON 表示形式？
2. 如何在我的过滤器类中将此 JSON 写回给客户端？

编辑 1:

private void throwUnauthorized(ServletResponse res) throws IOException {

    HttpServletResponse response = (HttpServletResponse) res;

    response.reset();
    response.setHeader("Content-Type", "application/json;charset=UTF-8");
    response.setStatus(HttpServletResponse.SC_UNAUTHORIZED);
    response.getWriter().write("{\"foo\":\"boo\"}");

}

现在我可以写出 JSON 但返回 HTTP 500，因为:

java.lang.IllegalStateException: getWriter() has already been called for this response
    at org.apache.catalina.connector.Response.getOutputStream(Response.java:544)

Answer 1

使用Jackson将Object转换为JSON，下面是一个例子

ObjectMapper mapper = new ObjectMapper();
String Json =  mapper.writeValueAsString(object);