SpringBoot-底层是如何将对象转换JSON进行响应的

1.需求说明 : 演示返回 JSON 格式数据

2.应用实例

1. SpringBoot 支持返回 JSON 格式数据，在启用 WEB 开发场景时，已经引入了相关依赖

2.创建src\main\java\com\llp\springboot\controller\ResponseController.java

@RestController
public class ResponseController {
	//返回Monster数据-要求以json格式返回
    @GetMapping("/get/monster")
    public Monster getmonster(){
        //开发中, monster对象是从DB获取-这里模拟一个monster对象
        Monster monster = new Monster();
        monster.setId(100);
        monster.setName("奔波霸");
        monster.setAge(200);
        monster.setIsMarried(false);
        monster.setBirth(new Date());
        Car car = new Car();
        car.setName("奔驰");
        car.setPrice(222.2);
        monster.setCar(car);
        return monster;
    }
}

3.测试

4. Debug 一下 monster 对象以 Json 格式返回

AbstractJackson2HttpMessageConverter.java

@Override
protected void writeInternal(Object object, @Nullable Type type, HttpOutputMessage outputMessage)
      throws IOException, HttpMessageNotWritableException {
	//获取要转换的类型：application/json、application/xml 
   MediaType contentType = outputMessage.getHeaders().getContentType();
    //默认是UTF-8
   JsonEncoding encoding = getJsonEncoding(contentType);
	
   Class<?> clazz = (object instanceof MappingJacksonValue ?
         ((MappingJacksonValue) object).getValue().getClass() : object.getClass());
   ObjectMapper objectMapper = selectObjectMapper(clazz, contentType);
   Assert.state(objectMapper != null, "No ObjectMapper for " + clazz.getName());

   OutputStream outputStream = StreamUtils.nonClosing(outputMessage.getBody());
    //获取要转的generator对象
   try (JsonGenerator generator = objectMapper.getFactory().createGenerator(outputStream, encoding)) {
      writePrefix(generator, object);

      Object value = object;
      Class<?> serializationView = null;
      FilterProvider filters = null;
      JavaType javaType = null;

      if (object instanceof MappingJacksonValue) {
         MappingJacksonValue container = (MappingJacksonValue) object;
         value = container.getValue();
         serializationView = container.getSerializationView();
         filters = container.getFilters();
      }
      if (type != null && TypeUtils.isAssignable(type, value.getClass())) {
         javaType = getJavaType(type, null);
      }

      ObjectWriter objectWriter = (serializationView != null ?
            objectMapper.writerWithView(serializationView) : objectMapper.writer());
      if (filters != null) {
         objectWriter = objectWriter.with(filters);
      }
      if (javaType != null && javaType.isContainerType()) {
         objectWriter = objectWriter.forType(javaType);
      }
      SerializationConfig config = objectWriter.getConfig();
      if (contentType != null && contentType.isCompatibleWith(MediaType.TEXT_EVENT_STREAM) &&
            config.isEnabled(SerializationFeature.INDENT_OUTPUT)) {
         objectWriter = objectWriter.with(this.ssePrettyPrinter);
      }
       //将对象转换为指定的格式
      objectWriter.writeValue(generator, value);

      writeSuffix(generator, object);
      generator.flush();
   }
   catch (InvalidDefinitionException ex) {
      throw new HttpMessageConversionException("Type definition error: " + ex.getType(), ex);
   }
   catch (JsonProcessingException ex) {
      throw new HttpMessageNotWritableException("Could not write JSON: " + ex.getOriginalMessage(), ex);
   }
}