Best practice implementing JavascriptInterface using WebView in Android?

Question

I have a crash report of the same error like in this question: WebView methods on same thread error

There it is suggested to create a Runnable().

I don't understand why exactly this solves the problem. The error says "Webview methods on same Thread", but the answer suggests to create the method on the UI-Thread (Main Thread). But isn't the UI-Thread the one and only thread? Could someone explain this whole process in detail (considering I create a new Webview in every activity in the constructor)?

My code to implement Javascript functions/methods looks like this:

public class JS_Bind {
    private static final String TAG = "JS_Bind";
    private Context context;
    private AdvancedWebView mWebView;

    public JS_Bind(Context c, AdvancedWebView mWebView) {
        context = c;
        this.mWebView = mWebView;
    }
    @JavascriptInterface
    public void openActivity(String activityName) {
        try {
            Class activityClass = Class.forName(PACKAGE_NAME + "." + activityName);
            context.startActivity(new Intent(MainActivity.this, activityClass));
        } catch (ClassNotFoundException e) {
            Toast.makeText(context, "Invalid activity name: " + activityName, Toast.LENGTH_SHORT).show();
            e.printStackTrace();
        }
    }
    @JavascriptInterface
    public void makeToast(String toast) {
        Toast mToast = Toast.makeText(context, toast, Toast.LENGTH_SHORT);
        mToast.setGravity(Gravity.CENTER, 0, 0);
        mToast.show();
    }
    @JavascriptInterface
    public void external(String url) {
        mTracker.send(new HitBuilders.EventBuilder().setCategory("Action").setAction("External Link: " + url).build());
        Uri uri = Uri.parse(url);
        Intent intent = new Intent(Intent.ACTION_VIEW, uri);
        startActivity(intent);
    }
    @JavascriptInterface
    public String showToken() {
        return gcmToken;
    }
    @JavascriptInterface
    public int showUid() {
        SharedPreferences pref = getSharedPreferences("Pref", Activity.MODE_PRIVATE);
        int uid = pref.getInt("uid", 0);
        return uid;
    }
    @JavascriptInterface
    public void buyPremium() {
        bp.purchase(MainActivity.this, PRODUCT_ID);
    }
}

Do I have to change EVERY function to this code (first answer in the question I refered to):

@JavascriptInterface
    mWebView.post(new Runnable() {
        @Override
        public void makeToast() {
          // ...
        }
    });

?

By the way, this is how I create the webview in the constructor activies onCreate method:

mWebView = (AdvancedWebView) findViewById(R.id.webView);
mWebView.setListener(this, this);
mWebView.addJavascriptInterface(new JS_Bind(this, mWebView), "Android");
mWebView.setLayerType(View.LAYER_TYPE_SOFTWARE, null);

if (!DetectConnection.checkInternetConnection(this)) {
    mWebView.loadUrl("file:///android_asset/offline.html");
}
else {
        mWebView.loadUrl("http://example.com/tw3/index.php?s=home");
}

Answer 1

But isn't the UI-Thread the one and only thread?

No. WebView has its own pool of threads. There can be many other threads in an Android application.

Do I have to change EVERY function to this code

Not necessarily.

First, I do not see how you are getting that error (A WebView method was called on thread 'JavaBridge'. All WebView methods must be called on the same thread.) from your @JavascriptInterface methods shown above. That error is when you call a method on the WebView itself, and you are not doing that.

You will need to use runOnUiThread() (or equivalent techniques) if:

• Your @JavascriptInterface methods refer to the WebView itself, or

• Your @JavascriptInterface try to do something else that has to be done on the main application thread (which will yield a different error message, as it will not be tied specifically to WebView)

Your startActivity() call might need to be called on the main application thread — I forget if that can be called on a background thread or not. Similarly with your Toast work — while I think that can be done on a background thread, I am not certain of it. It has been ages since I tried doing either of those things from a background thread.

---

Also, please only use your code if you control every single byte of what is being displayed in the WebView. Exposing startActivity() to arbitrary Web content has significant security implications.