Spring MessageConverter

    	Object fromMessage(Message<?> message, Class<?> targetClass);

    	Message<?> toMessage(Object payload, @Nullable MessageHeaders headers);

	@Override
	@Nullable
	public final Object fromMessage(Message<?> message, Class<?> targetClass) {
		return fromMessage(message, targetClass, null);
	}

	@Override
	@Nullable
	public final Object fromMessage(Message<?> message, Class<?> targetClass, @Nullable Object conversionHint) {
		if (!canConvertFrom(message, targetClass)) {
			return null;
		}
		return convertFromInternal(message, targetClass, conversionHint);
	}


	// 子类实现
	@Nullable
	protected Object convertFromInternal(
			Message<?> message, Class<?> targetClass, @Nullable Object conversionHint) {

		return null;
	}

	@Override
	protected Object convertFromInternal(Message<?> message, Class<?> targetClass, @Nullable Object conversionHint) {
		Charset charset = getContentTypeCharset(getMimeType(message.getHeaders()));
		Object payload = message.getPayload();
		return (payload instanceof String ? payload : new String((byte[]) payload, charset));
	}

    @Override
    @Nullable
    public final Message<?> toMessage(Object payload, @Nullable MessageHeaders headers) {
        return toMessage(payload, headers, null);
    }

    @Override
    @Nullable
    public final Message<?> toMessage(Object payload, @Nullable MessageHeaders headers, @Nullable Object conversionHint) {
        if (!canConvertTo(payload, headers)) {
            return null;
        }
        // 子类实现
        Object payloadToUse = convertToInternal(payload, headers, conversionHint);
        if (payloadToUse == null) {
            return null;
        }

        MimeType mimeType = getDefaultContentType(payloadToUse);
        if (headers != null) {
            MessageHeaderAccessor accessor = MessageHeaderAccessor.getAccessor(headers, MessageHeaderAccessor.class);
            if (accessor != null && accessor.isMutable()) {
                if (mimeType != null) {
                    accessor.setHeaderIfAbsent(MessageHeaders.CONTENT_TYPE, mimeType);
                }
                // 创建信息对象
                return MessageBuilder.createMessage(payloadToUse, accessor.getMessageHeaders());
            }
        }

        MessageBuilder<?> builder = MessageBuilder.withPayload(payloadToUse);
        if (headers != null) {
            builder.copyHeaders(headers);
        }
        if (mimeType != null) {
            builder.setHeaderIfAbsent(MessageHeaders.CONTENT_TYPE, mimeType);
        }
        return builder.build();
    }