WebViewAssetLoader

public final class WebViewAssetLoader

Helper class to load local files including application's static assets and resources using http(s):// URLs inside a android.webkit.WebView class. Loading local files using web-like URLs instead of "file://" is desirable as it is compatible with the Same-Origin policy.

For more context about application's assets and resources and how to normally access them please refer to Android Developer Docs: App resources overview.

This class is expected to be used within shouldInterceptRequest, which is invoked on a different thread than application's main thread. Although instances are themselves thread-safe (and may be safely constructed on the application's main thread), exercise caution when accessing private data or the view system.

Using http(s):// URLs to access local resources may conflict with a real website. This means that local files should only be hosted on domains your organization owns (at paths reserved for this purpose) or the default domain reserved for this: appassets.androidplatform.net.

A typical usage would be like: 

final WebViewAssetLoader assetLoader = new WebViewAssetLoader.Builder()
         .addPathHandler("/assets/", new AssetsPathHandler(this))
         .build();

      

webView.setWebViewClient(new WebViewClientCompat() {
    @Override
    public WebResourceResponse shouldInterceptRequest(WebView view, WebResourceRequest request) {
        return assetLoader.shouldInterceptRequest(request.getUrl());
    }


      
@Override
    @SuppressWarnings("deprecation") // for API < 21
    public WebResourceResponse shouldInterceptRequest(WebView view, String url) {
        return assetLoader.shouldInterceptRequest(Uri.parse(url));
    }
});


      

WebSettings webViewSettings = webView.getSettings();
// Setting this off for security. Off by default for SDK versions >= 16.
webViewSettings.setAllowFileAccessFromFileURLs(false);
// Off by default, deprecated for SDK versions >= 30.
webViewSettings.setAllowUniversalAccessFromFileURLs(false);
// Keeping these off is less critical but still a good idea, especially if your app is not
// using file:// or content:// URLs.
webViewSettings.setAllowFileAccess(false);
webViewSettings.setAllowContentAccess(false);


      

// Assets are hosted under http(s)://appassets.androidplatform.net/assets/... .
// If the application's assets are in the "main/assets" folder this will read the file
// from "main/assets/www/index.html" and load it as if it were hosted on:
// https://appassets.androidplatform.net/assets/www/index.html
webview.loadUrl("https://appassets.androidplatform.net/assets/www/index.html");

Summary

Nested types

Handler class to open a file from assets directory in the application APK.
public final class WebViewAssetLoader.Builder

A builder class for constructing WebViewAssetLoader objects.

Handler class to open files from application internal storage.

A handler that produces responses for a registered path.

Handler class to open a file from resources directory in the application APK.

Constants
static final String
DEFAULT_DOMAIN = "appassets.androidplatform.net"

An unused domain reserved for Android applications to intercept requests for app assets.

Public methods
@Nullable WebResourceResponse

Attempt to resolve the url to an application resource or asset, and return a WebResourceResponse for the content.

Constants

DEFAULT_DOMAIN

Added in 1.1.0
public static final String DEFAULT_DOMAIN = "appassets.androidplatform.net"

An unused domain reserved for Android applications to intercept requests for app assets.

It is used by default unless the user specified a different domain.

Public methods

shouldInterceptRequest

Added in 1.1.0
@WorkerThread
public @Nullable WebResourceResponse shouldInterceptRequest(@NonNull Uri url)

Attempt to resolve the url to an application resource or asset, and return a WebResourceResponse for the content.

This method should be invoked from within shouldInterceptRequest.

Parameters
@NonNull Uri url

the URL to process.
Returns
@Nullable WebResourceResponse

WebResourceResponse if the request URL matches a registered URL, null otherwise.