Summary: Nested Classes | Constants | Methods | Inherited Methods

WebViewAssetLoader

Kotlin |Java

public final class WebViewAssetLoader
extends Object

java.lang.Object
↳	androidx.webkit.WebViewAssetLoader

---

Helper class to load local files including application's static assets and resources using http(s):// URLs inside a 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 WebViewClient.shouldInterceptRequest(WebView, WebResourceRequest), 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 WebViewClient() {
     @Override
     @RequiresApi(21)
     public WebResourceResponse shouldInterceptRequest(WebView view,
                                      WebResourceRequest request) {
         return assetLoader.shouldInterceptRequest(request.getUrl());
     }

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

 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 classes
class	WebViewAssetLoader.AssetsPathHandler Handler class to open a file from assets directory in the application APK.
class	WebViewAssetLoader.Builder A builder class for constructing WebViewAssetLoader objects.
class	WebViewAssetLoader.InternalStoragePathHandler Handler class to open files from application internal storage.
interface	WebViewAssetLoader.PathHandler A handler that produces responses for a registered path.
class	WebViewAssetLoader.ResourcesPathHandler Handler class to open a file from resources directory in the application APK.

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

Public methods
WebResourceResponse	shouldInterceptRequest(Uri url) Attempt to resolve the url to an application resource or asset, and return a WebResourceResponse for the content.

Inherited methods
From class java.lang.Object Object clone() boolean equals(Object arg0) void finalize() final Class<?> getClass() int hashCode() final void notify() final void notifyAll() String toString() final void wait(long arg0, int arg1) final void wait(long arg0) final void wait()

Constants

DEFAULT_DOMAIN

public static final String DEFAULT_DOMAIN

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.

Constant Value: "appassets.androidplatform.net"

Public methods

shouldInterceptRequest

public WebResourceResponse shouldInterceptRequest (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 WebViewClient.shouldInterceptRequest(android.webkit.WebView, String).

Parameters
url	Uri: the URL to process.

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