spring 26 MessageConverter

演示 - MessageConverter 的作用

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 private static void test4() throws IOException, HttpMediaTypeNotAcceptableException, NoSuchMethodException {
        MockHttpServletRequest request = new MockHttpServletRequest();
        MockHttpServletResponse response = new MockHttpServletResponse();
        ServletWebRequest webRequest = new ServletWebRequest(request, response);

        request.addHeader("Accept", "application/xml");
        response.setContentType("application/json");

        RequestResponseBodyMethodProcessor processor = new RequestResponseBodyMethodProcessor(
                List.of(
                        new MappingJackson2HttpMessageConverter(), new MappingJackson2XmlHttpMessageConverter()
                ));
        processor.handleReturnValue(
                new User("张三", 18),
                new MethodParameter(A28.class.getMethod("user"), -1),
                new ModelAndViewContainer(),
                webRequest
        );
        System.out.println(new String(response.getContentAsByteArray(), StandardCharsets.UTF_8));
    }

    @ResponseBody
    @RequestMapping(produces = "application/json")
    public User user() {
        return null;
    }

    private static void test3() throws IOException {
        MockHttpInputMessage message = new MockHttpInputMessage("""
                {
                    "name":"李四",
                    "age":20
                }
                """.getBytes(StandardCharsets.UTF_8));
        MappingJackson2HttpMessageConverter converter = new MappingJackson2HttpMessageConverter();
        if (converter.canRead(User.class, MediaType.APPLICATION_JSON)) {
            Object read = converter.read(User.class, message);
            System.out.println(read);
        }
    }

    private static void test2() throws IOException {
        MockHttpOutputMessage message = new MockHttpOutputMessage();
        MappingJackson2XmlHttpMessageConverter converter = new MappingJackson2XmlHttpMessageConverter();
        if (converter.canWrite(User.class, MediaType.APPLICATION_XML)) {
            converter.write(new User("李四", 20), MediaType.APPLICATION_XML, message);
            System.out.println(message.getBodyAsString());
        }
    }

    public static void test1() throws IOException {
        MockHttpOutputMessage message = new MockHttpOutputMessage();
        MappingJackson2HttpMessageConverter converter = new MappingJackson2HttpMessageConverter();
        if (converter.canWrite(User.class, MediaType.APPLICATION_JSON)) {
            converter.write(new User("张三", 18), MediaType.APPLICATION_JSON, message);
            System.out.println(message.getBodyAsString());
        }
    }

收获💡

1. MessageConverter 的作用
   • @ResponseBody 是返回值处理器解析的
   • 但具体转换工作是 MessageConverter 做的
2. 如何选择 MediaType 常见的 json xml
   • 首先看 @RequestMapping 上有没有指定 setContextType， @RequestMapping 中设置属性 produces
   • 其次看 request 的 Accept 头有没有指定 addHeader name：Accept 属性：MeidaType
   • 最后按 MessageConverter 的顺序, 谁能谁先转换