Uncaught TypeError when using a JavascriptInterface

Question

I'm currently displaying a bunch of data to the user as HTML in a webview. I have some links below each entry that should call a method in my app when clicked. The Android WebView's javascript interface seems to be the best (only?) way of handling these things. However, whenever I click the link, I get this error message: ERROR/Web Console(6112): Uncaught TypeError: Object [my namespace]@4075ff10 has no method 'edit' at [base URL]:55

I have the following interface declared:

public class JavaScriptInterface {
    Context context;

    JavaScriptInterface(Context c) {
        context = c;
    }

    public void edit(String postid) {
        Log.d("myApp", "EDIT!");
        //do stuff
    }
}

I then add it to my WebView:

final WebView threadView = (WebView) findViewById(R.id.webViewThread);
threadView.getSettings().setJavaScriptEnabled(true);
threadView.addJavascriptInterface(new JavaScriptInterface(this), "Android");

And, finally, I call this within my HTML as follows:

<div class="post-actions">
    <div class="right">
        <a onClick="Android.edit('4312244');">Edit</a>
    </div>
</div>

The real kicker is this all works when I'm debugging my app via the emulator or adb connection to my phone. When I build and publish the app, it breaks.

I'm at my wits end. Any help or advice would be greatly appreciated!

Answer 1

Same problem for my 2.3.3 mobile phone. But as I knew one app that worked and another not, I was not happy with this workaround. And I find out the differnce of my two apps. The one with the broken JavaScriptInterface uses Proguard. After a little search, I find a solution.

Short summary: interface JavascriptCallback, which is implemented by JavaScriptInterface and added rules for Proguard in proguard.conf:

public interface JavascriptCallback {

}

public class JavaScriptInterface implements JavascriptCallback {
    Context mContext;
    /** Instantiate the interface and set the context */
    JavaScriptInterface(Context c) {
        mContext = c;
    }
    /** Show a toast from the web page */
    public void showToast(String toast) {
        Toast.makeText(mContext, toast, Toast.LENGTH_SHORT).show();
    }
}

proguard.cfg:

-keep public class YOURPACKAGENAMEHERE.JavascriptCallback
-keep public class * implements YOURPACKAGENAMEHERE.JavascriptCallback
-keepclassmembers class * implements YOURPACKAGENAMEHERE.JavascriptCallback {
    <methods>;
}

Answer 2

So, I'm pleased to say that my problem has been solved. Basically, it's a known bug in Gingerbread, and is present on my 2.3.4 device. After some head scratching, I found this workaround concocted by Jason Shah at PhoneGap. The real kudos for this goes to him as my solution is a slightly modified version of the code in that post.

The WebView

In my onLoad method, I call the following function.

private void configureWebView() {
    try {
        if (Build.VERSION.RELEASE.startsWith("2.3")) {
            javascriptInterfaceBroken = true;
        }
    } catch (Exception e) {
        // Ignore, and assume user javascript interface is working correctly.
    }

    threadView = (WebView) findViewById(R.id.webViewThread);
    threadView.setWebViewClient(new ThreadViewClient());
    Log.d(APP_NAME, "Interface Broken? " + javascriptInterfaceBroken.toString());
    // Add javascript interface only if it's not broken
    iface = new JavaScriptInterface(this);
    if (!javascriptInterfaceBroken) {
        threadView.addJavascriptInterface(new JavaScriptInterface(this), "Android");
    }
}

There are several things going on here.

1. In contrast with the PhoneGap method, I'm using a startsWith comparison against the version string. This is because Build.VERSION.RELEASE is 2.3.4 on my reference device. Rather than test against all releases in the 2.3 series, I'm comfortable painting all devices with one brushstroke.

2. javascriptInterface is a bool initialized to false. JavaScriptInterface, instantiated as iface, is the class that normally handles JS events in my WebView.

3. ThreadViewClient is the meat and potatoes of my implementation. It's where all the logic for handling the workaround occurs.

The WebViewClient

In the class ThreadViewClient (which extends WebViewClient), I first account for the fact that the js handler that Android normally attaches isn't here. This means that, if I want to use the same javascript calls from within my WebView, I need to duplicate the interface. This is accomplished by inserting custom handlers into the content of your website once it has loaded...

@Override
public void onPageFinished(WebView view, String url) {
    super.onPageFinished(view, url);
    if (javascriptInterfaceBroken) {
        final String handleGingerbreadStupidity =
        "javascript:function shortSignature(id) { window.location='http://MyHandler:shortSignature:'+id; }; "
        + "javascript: function longSignature(text, username, forumnumber,threadnumber,pagenumber,postid) { var sep='[MyHandler]';"
            + "window.location='http://MyHandler:longSignature:' + encodeURIComponent(text + sep + username + sep + forumnumber + sep + threadnumber + sep + pagenumber + sep + postid);};"
      + "javascript: function handler() { this.shortSignature = shortSignature; this.longSignature = longSignature;}; "
      + "javascript: var Android = new handler();";
        view.loadUrl(handleGingerbreadStupidity);
    }
}

There's a lot to process there. In the javascript, I define an object handler that contains the functions that map to my js interface. An instance of it is then bound to "Android", which is the same interface name as that used by non-2.3 implementation. This allows for re-use of the code rendered within your webview content.

The functions take advantage of the fact that Android allows one to intercept all navigation that occurs within a WebView. In order to communicate with the outside program, they alter the window location to one with a special signature. I'll get into this in a bit.

Another thing I'm doing is concatenating the parameters of functions with more than one parameter. This allows me to reduce the code complexity within the location handler.

The location handler is also placed in ThreadViewClient...

@Override
public boolean shouldOverrideUrlLoading(WebView view, String url) {
    Method sMethod = null;
    Log.d(APP_NAME, "URL LOADING");
    if (javascriptInterfaceBroken) {
        if (url.contains("MyHandler")) {
            StringTokenizer st = new StringTokenizer(url, ":");
          st.nextToken(); // remove the 'http:' portion
          st.nextToken(); // remove the '//jshandler' portion
          String function = st.nextToken();
          String parameter = st.nextToken();
          Log.d(APP_NAME, "Handler: " + function + " " + parameter);
          try {
            if (function.equals("shortSignature")) {
                iface.shortSignature(parameter);
            } else if (function.equals("longSignature")) {
                iface.longSignature(parameter);
            } else {
                if (sMethod == null) {
                    sMethod = iface.getClass().getMethod(function, new Class[] { String.class });
                  }
                    sMethod.invoke(iface, parameter);
            }
        }
        //Catch & handle SecurityException, NoSuchMethodException, IllegalArgumentException, IllegalAccessException, InvocationTargetException
          return true;
        }
    }
    startActivity(new Intent(Intent.ACTION_VIEW, Uri.parse(url)));
    return true;
}

Here I am intercepting all URL load events that occur in the WebView. If the destination URL contains a magic string, the app attempts to parse it to extract out the method call. Rather than using the tokenizer to extract the individual parameters, I'm passing it to version of my longSignature method that can parse and handle it. This is detailed in the final part of this post.

If, by the time it has exited the "javascriptInterfaceBroken" block, execution has not be returned to the caller, this method treats the URL loading action as a normal link clicked event. In the case of my application I don't want to use the WebView for that, so I pass it off to the operating system via the ACTION_VIEW intent.

This is very similar to the implementation on Jason's blog. However I am bypassing reflection for the most part. I was attempting to use the method in the block with reflection to handle all of my bound functions, but due to my JavaScriptInterface being a nested class I was unable to look into it from another. However, since I defined the interface within the main Activity scope, its methods can be called directly.

Handling Concatenated Parameters

Finally, in my JavaScriptInterface, I created a handler to deal with the case of a concatenated parameter...

public void longSignature(String everything) {
    try {
            everything = URLDecoder.decode(everything, "UTF-8");
        } catch (UnsupportedEncodingException e) {
            Log.e(APP_NAME, e);
        }
    final String[] elements = everything.split("\\[MyHandler\\]");
    if (elements.length != 6) {
        Toast.makeText(getApplicationContext(), "[" + elements.length + "] wrong number of parameters!", Toast.LENGTH_SHORT).show();
    }
    else {
        longSignature(elements[0], elements[1], elements[2], elements[3], elements[4], elements[5]);
    }
}

Hooray polymorphism!

---

And that's my solution! There's a lot of room for improvement, but, for now, this is sufficient. Sorry if some of my conventions have raised your hackles - this is my first Android app and I am unfamiliar with some of the best practices and conventions. Good luck!

Answer 3

You have to annotate (@JavascriptInterface) methods in Java class that you want to make available to JavaScript.

    public class JavaScriptInterface {
Context context;

@JavascriptInterface
JavaScriptInterface(Context c) {
    context = c;
}

@JavascriptInterface
public void edit(String postid) {
    Log.d("myApp", "EDIT!");
    //do stuff
}    }

Its worked for me. Try out this.