Analysis of the sending principle of OData metadata request of SAP UI5 application

When we use the OData model in our SAP UI5 application to read data from the remote server, please refer to my article for the specific implementation details: SAP UI5 Beginner Tutorial Part 24 - How to Use the OData Data Model , which can be found in Chrome In the developer tools, a request for metadata automatically made by the SAP UI5 framework was observed.

Requested url:

https://services.odata.org/V2/Northwind/Northwind.svc/$metadata?sap-language=EN

The request has no content-type field, only one accept field with the value: application/xml

There is also a MaxDataServiceVersion with a value of 3.0

In the header field of the response, there is no Accept field, only the Content-Type field, pay attention to the case:

The specific code location that initiates the HTTP request: the _loadMetadata method of ODataMetadata:

Trigger _loadMetadata in the constructor:

Create a request object based on the url:

Get supported languages for HTTP requests according to the following API: Accept-Language

sap.ui.getCore().getConfiguration().getLanguageTag()

Calculated to be: en-US

The bAsync flag is true, which means this is an asynchronous request:

The withCredentials flag is false:

Then use the traditional promise API to make the request:

If the metadata is loaded successfully, execute the initialization logic in its callback:

if (!this.oMetadata.isLoaded()) {
                this.oMetadata.attachFailed(this.onMetadataFailed);
            }

Next, call the OData API, pass in the request object just constructed, and send the request:

Enter datajs.js, first prepareRequest:

normalizeHeaders: Normalize the header fields, in fact, convert the header fields to lowercase:

Then call invokeRequest to send:

Hand over to httpClient:

The bottom layer of datajs.js is still based on XHR, namely XmlHttpRequest:

Use the browser's native XMLHttpRequest directly:

The fallback mechanism here uses the very old ActiveXObject, which modern browsers will not implement:

var createXmlHttpRequest = function () {
        /// <summary>Creates a XmlHttpRequest object.</summary>
        /// <returns type="XmlHttpRequest">XmlHttpRequest object.</returns>
        if (window.XMLHttpRequest) {
            return new window.XMLHttpRequest();
        }
        var exception;
        if (window.ActiveXObject) {
            try {
                return new window.ActiveXObject("Msxml2.XMLHTTP.6.0");
            } catch (_) {
                try {
                    return new window.ActiveXObject("Msxml2.XMLHTTP.3.0");
                } catch (e) {
                    exception = e;
                }
            }
        } else {
            exception = { message: "XMLHttpRequest not supported" };
        }
        throw exception;
    };

Finally call xhr.open: