Question

I have added a Javascript interface object to a WebView using the addJavascriptInterface() method and I am trying to invoke a method on that object using WebView.loadUrl().

The object is a simple class called JSInterface. It has a single method called echo() that takes no parameters and returns void. The URL I am passing to loadUrl() is "javascript:JSInterface.echo()" (JSInterface is also the interface name I used for addJavascriptInterface()).

When I run the attached app, the echo() method never executes. There are no errors in the logcat or anywhere else. What do I need to do to invoke echo() from Javascript?

This is the activity with all source.

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It is of course right after I post the question that I figure out what’s wrong. In the attached code I am calling addJavascriptInterface() from a thread other than the UI thread. Moving addJavascriptInterface() to onCreate() fixes the problem.

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package test.pg;

import android.app.Activity;
import android.os.Bundle;
import android.util.Log;
import android.webkit.WebView;

public class TestActivity2
  extends Activity
{
  WebView webview;

  public void onCreate(Bundle savedInstanceState)
  {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.main);
    webview = (WebView) findViewById(R.id.webview);
    webview.getSettings().setJavaScriptEnabled(true);
    webview.loadUrl("file:///android_asset/www/invoke.html");
    TimerThread t = new TimerThread(webview);
    t.start();
  }

  class TimerThread
    extends Thread
  {
    long    lastRun;
    WebView appView;

    public TimerThread(WebView appView)
    {
      lastRun = System.currentTimeMillis();
      this.appView = appView;
    }

    public void run()
    {
      try
      {
        while (true)
        {
          if (System.currentTimeMillis() - lastRun < 5000)
          {
            Thread.sleep(5000);
          }
          else
          {
            appView.addJavascriptInterface(new JSInterface(), "JSInterface");
            InvokerThread t = new InvokerThread(appView);
            t.start();
            lastRun = System.currentTimeMillis();
          }
        }
      }
      catch (Exception e)
      {
        Log.e("InvokerThread", "exception=" + e.toString(), e);
      }
    }
  }

  class InvokerThread
    extends Thread
  {
    WebView appView;

    public InvokerThread(WebView appView)
    {
      this.appView = appView;
    }

    public void run()
    {
      String function = "javascript:JSInterface.echo()";
      appView.loadUrl(function);
    }
  }

  class JSInterface
  {
    public void echo()
    {
      System.out.println("JSInterface.echo()");
      Log.i("JSInterface", "JSInterface.echo()");
    }
  }
}

Answer 1

It is of course right after I post the question that I figure out what’s wrong. In the attached code I am calling addJavascriptInterface() from a thread other than the UI thread. Moving addJavascriptInterface() to onCreate() fixes the problem.