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首先调用AbstractMessageConverterMethodArgumentResolver类的readWithMessageConverters方法

	protected <T> Object readWithMessageConverters(HttpInputMessage inputMessage, MethodParameter parameter,
			Type targetType) throws IOException, HttpMediaTypeNotSupportedException, HttpMessageNotReadableException {

		MediaType contentType;
		boolean noContentType = false;
		try {
			contentType = inputMessage.getHeaders().getContentType();
		}
		catch (InvalidMediaTypeException ex) {
			throw new HttpMediaTypeNotSupportedException(ex.getMessage());
		}
		if (contentType == null) {
			noContentType = true;
			contentType = MediaType.APPLICATION_OCTET_STREAM;
		}

		Class<?> contextClass = parameter.getContainingClass();
		Class<T> targetClass = (targetType instanceof Class ? (Class<T>) targetType : null);
		if (targetClass == null) {
			ResolvableType resolvableType = ResolvableType.forMethodParameter(parameter);
			targetClass = (Class<T>) resolvableType.resolve();
		}

		HttpMethod httpMethod = (inputMessage instanceof HttpRequest ? ((HttpRequest) inputMessage).getMethod() : null);
		Object body = NO_VALUE;

		EmptyBodyCheckingHttpInputMessage message = null;
		try {
			message = new EmptyBodyCheckingHttpInputMessage(inputMessage);

			for (HttpMessageConverter<?> converter : this.messageConverters) {
				Class<HttpMessageConverter<?>> converterType = (Class<HttpMessageConverter<?>>) converter.getClass();
				GenericHttpMessageConverter<?> genericConverter =
						(converter instanceof GenericHttpMessageConverter ? (GenericHttpMessageConverter<?>) converter : null);
				if (genericConverter != null ? genericConverter.canRead(targetType, contextClass, contentType) :
						(targetClass != null && converter.canRead(targetClass, contentType))) {
					if (message.hasBody()) {
						HttpInputMessage msgToUse =
								getAdvice().beforeBodyRead(message, parameter, targetType, converterType);
						body = (genericConverter != null ? genericConverter.read(targetType, contextClass, msgToUse) :
								((HttpMessageConverter<T>) converter).read(targetClass, msgToUse));
						body = getAdvice().afterBodyRead(body, msgToUse, parameter, targetType, converterType);
					}
					else {
						body = getAdvice().handleEmptyBody(null, message, parameter, targetType, converterType);
					}
					break;
				}
			}
		}
		catch (IOException ex) {
			throw new HttpMessageNotReadableException("I/O error while reading input message", ex, inputMessage);
		}
		finally {
			if (message != null && message.hasBody()) {
				closeStreamIfNecessary(message.getBody());
			}
		}

		if (body == NO_VALUE) {
			if (httpMethod == null || !SUPPORTED_METHODS.contains(httpMethod) ||
					(noContentType && !message.hasBody())) {
				return null;
			}
			throw new HttpMediaTypeNotSupportedException(contentType,
					getSupportedMediaTypes(targetClass != null ? targetClass : Object.class));
		}

		MediaType selectedContentType = contentType;
		Object theBody = body;
		LogFormatUtils.traceDebug(logger, traceOn -> {
			String formatted = LogFormatUtils.formatValue(theBody, !traceOn);
			return "Read \"" + selectedContentType + "\" to [" + formatted + "]";
		});

		return body;
	}

该方法中

body = (genericConverter != null ?

genericConverter.read

(targetType, contextClass, msgToUse) :

((HttpMessageConverter) converter).read(targetClass, msgToUse));

会调用AbstractJackson2HttpMessageConverter的read方法：

	public Object read(Type type, @Nullable Class<?> contextClass, HttpInputMessage inputMessage)
			throws IOException, HttpMessageNotReadableException {

		JavaType javaType = getJavaType(type, contextClass);
		return readJavaType(javaType, inputMessage);
	}

该方法会调用getJavaType方法将HttpInputMessage转化为java对象

	private Object readJavaType(JavaType javaType, HttpInputMessage inputMessage) throws IOException {
		MediaType contentType = inputMessage.getHeaders().getContentType();
		Charset charset = getCharset(contentType);

		ObjectMapper objectMapper = selectObjectMapper(javaType.getRawClass(), contentType);
		Assert.state(objectMapper != null, "No ObjectMapper for " + javaType);

		boolean isUnicode = ENCODINGS.containsKey(charset.name()) ||
				"UTF-16".equals(charset.name()) ||
				"UTF-32".equals(charset.name());
		try {
			InputStream inputStream = StreamUtils.nonClosing(inputMessage.getBody());
			if (inputMessage instanceof MappingJacksonInputMessage) {
				Class<?> deserializationView = ((MappingJacksonInputMessage) inputMessage).getDeserializationView();
				if (deserializationView != null) {
					ObjectReader objectReader = objectMapper.readerWithView(deserializationView).forType(javaType);
					if (isUnicode) {
						return objectReader.readValue(inputStream);
					}
					else {
						Reader reader = new InputStreamReader(inputStream, charset);
						return objectReader.readValue(reader);
					}
				}
			}
			if (isUnicode) {
				return objectMapper.readValue(inputStream, javaType);
			}
			else {
				Reader reader = new InputStreamReader(inputStream, charset);
				return objectMapper.readValue(reader, javaType);
			}
		}
		catch (InvalidDefinitionException ex) {
			throw new HttpMessageConversionException("Type definition error: " + ex.getType(), ex);
		}
		catch (JsonProcessingException ex) {
			throw new HttpMessageNotReadableException("JSON parse error: " + ex.getOriginalMessage(), ex, inputMessage);
		}
	}