将HATEOAS对象广播为内容类型不是application/json的websocket消息

Question

我需要在RESTController上呈现下面的类(以及extends RepresentationModel的其他几个类(，并将其作为到websocket的广播消息。

@Getter
@JsonPropertyOrder(alphabetic=true)
@Setter
public class MessageWrapper extends RepresentationModel<MessageWrapper> {
private Object message;
private String simpleName;
public MessageWrapper(Object message) {
this.message = message;
this.simpleName = message.getClass().getSimpleName();
}
}

我使用以下代码将其呈现给响应主体：

@RequestMapping(
headers = {
"Content-Type=application/json",
"X-Requested-With=XMLHttpRequest"
},
method = RequestMethod.POST,
produces = "application/hal+json",
value = "/thread/{threadId}/message")
public HttpEntity<MessageWrapper> post(...) {
MessageWrapper wrapper = ...
HttpEntity<MessageWrapper> entity =
new ResponseEntity<HateoasWrapper>(wrapper, HttpStatus.OK);
return entity;
}

它会产生以下截断的响应：

HTTP/1.1 200 
...
Content-Type: application/hal+json
{
"_links": {
"previous": {
"href": "/thread/1044/message/86"
},
"self": {
"href": "/thread/1044/message/87"
}
},
"message": {
...
},
"simpleName":"..."
}

请注意响应的Content-Type和上面的_links对象属性。我为那个Content-Type和那个_links属性创建了一个javascript处理程序。现在，在稍后的另一个事件中，我需要将完全相同的JSON字符串广播到websocket。所以我做了这个：

@Autowired
private SimpMessagingTemplate messagingTemplate;
public void broadcastToUser(StompPrincipal principal, MessageWrapper wrapper) {
SimpMessageHeaderAccessor simpHeaderAccessor =
SimpMessageHeaderAccessor.create(SimpMessageType.MESSAGE);
simpHeaderAccessor.setUser(principal);
simpHeaderAccessor.setLeaveMutable(true);
messagingTemplate.convertAndSendToUser(
principal.getUsername(),
"/broadcast",
wrapper,
simpHeaderAccessor.getMessageHeaders());
}

它会产生以下信息：

a["MESSAGE
destination:/user/broadcast
content-type:application/json
subscription:sub-0
message-id:tluhntae-0
content-length:267
{
"links": [
{
"rel": "previous",
"href": "/thread/1044/message/86"
},
{
"rel": "self",
"href": "/thread/1044/message/87"
}
],
"message": {...},
"simpleName": "..."
}
"]

请注意，它具有links属性，而不是_links属性。还要注意的是，它没有像上面那样的对象值，而是有一个对象数组作为值。我想对上面的RESTController响应使用相同的javascript回调，但不能，因为links属性具有不同的结构。我可以在javascript端对其进行编组，但我宁愿不进行编组，而是尝试将其配置为按照我期望的方式进行渲染。我读到这是因为我将websocket消息呈现为application/json而不是application/hal+json，这就是它使用links而不是_links的原因。因此，在广播消息之前，我将消息的Content-Type设置为HAL：

MimeType applicationJson;

if(RepresentationModel.class.isAssignableFrom(body.getClass())) {
applicationJson = new MimeType("application", "hal+json");
} else {
applicationJson = new MimeType("application", "json");
}
simpHeaderAccessor.setContentType(applicationJson);

不过，它抛出了以下错误：

2020-10-26 23:40:40.799 ERROR 6200 --- [nio-8080-exec-4] o.a.c.c.C.[.[.[/].[dispatcherServlet]    : Servlet.service() for servlet [dispatcherServlet] in context with path [] threw exception [Request processing failed; nested exception is org.springframework.messaging.converter.MessageConversionException: Unable to convert payload with type='MessageWrapper', contentType='application/hal+json', converter=[CompositeMessageConverter[converters=[org.springframework.messaging.converter.StringMessageConverter@66389c40, org.springframework.messaging.converter.ByteArrayMessageConverter@724493b5, org.springframework.messaging.converter.MappingJackson2MessageConverter@1228bacc]]]] with root cause
org.springframework.messaging.converter.MessageConversionException: Unable to convert payload with type='MessageWrapper', contentType='application/hal+json', converter=[CompositeMessageConverter[converters=[org.springframework.messaging.converter.StringMessageConverter@66389c40, org.springframework.messaging.converter.ByteArrayMessageConverter@724493b5, org.springframework.messaging.converter.MappingJackson2MessageConverter@1228bacc]]]
at org.springframework.messaging.core.AbstractMessageSendingTemplate.doConvert(AbstractMessageSendingTemplate.java:187) ~[spring-messaging-5.3.0-M1.jar:5.3.0-M1]
at org.springframework.messaging.core.AbstractMessageSendingTemplate.convertAndSend(AbstractMessageSendingTemplate.java:150) ~[spring-messaging-5.3.0-M1.jar:5.3.0-M1]
at org.springframework.messaging.simp.SimpMessagingTemplate.convertAndSendToUser(SimpMessagingTemplate.java:230) ~[spring-messaging-5.3.0-M1.jar:5.3.0-M1]
at org.springframework.messaging.simp.SimpMessagingTemplate.convertAndSendToUser(SimpMessagingTemplate.java:211) ~[spring-messaging-5.3.0-M1.jar:5.3.0-M1]
at dev.gideon.chatapp.service.BroadcastService.broadcastToUser(BroadcastService.java:54) ~[classes/:na]
...

我真的需要将websocket消息的Content-Type设置为HAL，以便按照我预期的方式对其进行编组。有什么方法可以实现吗？我之所以提出这个问题，除了我不想做单独的javascript代码来处理具有CCD_ 17属性和具有对象值的CCD_，我还认为，用HALContent-Type广播它确实是合适的，这样除了我自己的客户端之外，其他客户端也会这样对待它，而不仅仅是普通的JSON广播消息。

Answer 1

Content-Typeapplication/hal+json的websocket广播消息没有默认的MessageConverter。我需要为我的自定义MIME类型实现我自己的MessageConverter。我创建了一个extends AbstractMessageConverter:转换器

public class HalMessageConverter extends AbstractMessageConverter {
public HalMessageConverter() {
MimeType hal = new MimeType("application", "hal+json");
this.addSupportedMimeTypes(hal);
}
@Override
protected boolean supports(Class<?> clazz) {
return RepresentationModel.class.isAssignableFrom(clazz);
}
@Override
protected Object convertToInternal(
Object payload,
MessageHeaders headers,
Object conversionHint) {
TypeConstrainedMappingJackson2HttpMessageConverter converter = 
new TypeConstrainedMappingJackson2HttpMessageConverter(RepresentationModel.class);
Module module = new Jackson2HalModule();
LinkRelationProvider provider = new DefaultLinkRelationProvider();
Jackson2HalModule.HalHandlerInstantiator instantiator =
new Jackson2HalModule.HalHandlerInstantiator(
provider,
CurieProvider.NONE,
MessageResolver.DEFAULTS_ONLY);
ObjectMapper mapper = converter.getObjectMapper();
mapper.disable(SerializationFeature.WRITE_DATES_AS_TIMESTAMPS);
mapper.registerModule(module);
mapper.setHandlerInstantiator(instantiator);
try {
String payloadJson = mapper.writeValueAsString(payload);
byte[] bytes = payloadJson.getBytes();
return bytes;
} catch(JsonProcessingException exception) {
return null;
}
}
}

我需要创建一个自定义类，因为方法addSupportedMimeTypes是protected。CCD_ 27保持转换逻辑。Jackson2HalModule类是我一直在寻找的。该类是当Response与application/hal+json的Content-Type一起发送时links变为_links的原因。

在创建了这个自定义MessageConverter之后，我将其添加到应用程序的默认转换器中：

@Configuration @EnableWebSocketMessageBroker
public class WebSocketConfig implements WebSocketMessageBrokerConfigurer {
@Override
public boolean configureMessageConverters(List<MessageConverter> messageConverters) {
messageConverters.add(new HalMessageConverter());
return false;
}
}