Intercept and override HTTP requests from WebView

Question

I have a WebView in my application in which some site is opened (always the same, it is my own page). The site has some JS code which loads some images from the remote host.

I want to intercept requests to such images (by URL pattern) and give back my own content (i.e. another image), or leave request untouched depending on internal application logic.

Is it possible to do that?

EDIT: The current state of question...

WebView has an ability to set a WebViewClient (as noted by Axarydax). WebViewClient have two useful methods

• shouldOverrideUrlLoading
• onLoadResource

shouldOverrideUrlLoading is able to intercept any url loading, if loading is triggered by page interaction (i.e. link on a page is clicked, WebView.loadURL("") isn't triggering this method). It is also able to cancel URL loading by returning false. This approach isn't usable, cause' it is not able to intercept loading of page resources (and images, what i need to intercept, is such a page resource).

onLoadResource is triggering every time that page resource (and images! thx to jessyjones) are loading, but there is no way to cancel that. That makes this method not suitable for my task also.

Answer 1

It looks like API level 11 has support for what you need. See WebViewClient.shouldInterceptRequest().

Answer 2

Try this, I've used it in a personal wiki-like app:

webView.setWebViewClient(new WebViewClient() {
    @Override
    public boolean shouldOverrideUrlLoading(WebView view, String url) {
        if (url.startsWith("foo://")) {
            // magic
            return true;
        }
        return false;
    }
});

Answer 3

As far as I know, shouldOverrideUrlLoading is not called for images but rather for hyperlinks... I think the appropriate method is

@Override public void onLoadResource(WebView view, String url)

This method is called for every resource (image, styleesheet, script) that's loaded by the webview, but since it's a void, I haven't found a way to change that url and replace it so that it loads a local resource ...

Answer 4

Here is my solution I use for my app.

I have several asset folder with css / js / img anf font files.

The application gets all filenames and looks if a file with this name is requested. If yes, it loads it from asset folder.

//get list of files of specific asset folder
        private ArrayList listAssetFiles(String path) {

            List myArrayList = new ArrayList();
            String [] list;
            try {
                list = getAssets().list(path);
                for(String f1 : list){
                    myArrayList.add(f1);
                }
            } catch (IOException e) {
                e.printStackTrace();
            }
            return (ArrayList) myArrayList;
        }

        //get mime type by url
        public String getMimeType(String url) {
            String type = null;
            String extension = MimeTypeMap.getFileExtensionFromUrl(url);
            if (extension != null) {
                if (extension.equals("js")) {
                    return "text/javascript";
                }
                else if (extension.equals("woff")) {
                    return "application/font-woff";
                }
                else if (extension.equals("woff2")) {
                    return "application/font-woff2";
                }
                else if (extension.equals("ttf")) {
                    return "application/x-font-ttf";
                }
                else if (extension.equals("eot")) {
                    return "application/vnd.ms-fontobject";
                }
                else if (extension.equals("svg")) {
                    return "image/svg+xml";
                }
                type = MimeTypeMap.getSingleton().getMimeTypeFromExtension(extension);
            }
            return type;
        }

        //return webresourceresponse
        public WebResourceResponse loadFilesFromAssetFolder (String folder, String url) {
            List myArrayList = listAssetFiles(folder);
            for (Object str : myArrayList) {
                if (url.contains((CharSequence) str)) {
                    try {
                        Log.i(TAG2, "File:" + str);
                        Log.i(TAG2, "MIME:" + getMimeType(url));
                        return new WebResourceResponse(getMimeType(url), "UTF-8", getAssets().open(String.valueOf(folder+"/" + str)));
                    } catch (IOException e) {
                        e.printStackTrace();
                    }
                }
            }
            return null;
        }

        //@TargetApi(Build.VERSION_CODES.LOLLIPOP)
        @SuppressLint("NewApi")
        @Override
        public WebResourceResponse shouldInterceptRequest(final WebView view, String url) {
            //Log.i(TAG2, "SHOULD OVERRIDE INIT");
            //String url = webResourceRequest.getUrl().toString();
            String extension = MimeTypeMap.getFileExtensionFromUrl(url);
            //I have some folders for files with the same extension
            if (extension.equals("css") || extension.equals("js") || extension.equals("img")) {
                return loadFilesFromAssetFolder(extension, url);
            }
            //more possible extensions for font folder
            if (extension.equals("woff") || extension.equals("woff2") || extension.equals("ttf") || extension.equals("svg") || extension.equals("eot")) {
                return loadFilesFromAssetFolder("font", url);
            }

            return null;
        }

Answer 5

You don't mention the API version, but since API 11 there's the method WebViewClient.shouldInterceptRequest

Maybe this could help?

Answer 6

An ultimate solution would be to embed a simple http server listening on your 'secret' port on loopback. Then you can substitute the matched image src URL with something like http://localhost:123/.../mypic.jpg

Answer 7

This may helps:

@TargetApi(Build.VERSION_CODES.LOLLIPOP)
@Override
public WebResourceResponse shouldInterceptRequest(WebView view, WebResourceRequest request) {
    String url = request.getUrl().toString();
    WebResourceResponse response = super.shouldInterceptRequest(view, request);
    // load native js
    if (url != null && url.contains(INJECTION_TOKEN/* scheme define */)) {

        response = new WebResourceResponse(
                "text/javascript",
                "utf-8",
                loadJsInputStream(url, JsCache.getJsFilePath(path) /* InputStream */));
    }
    return response;
}