android webview.loadUrl won't load another webpage

Question

In my simple android app I programmatically load a webpage in the WebView. It initially starts with a default webpage, and the next one is loaded based upon the user inputs for the first one. The JavaScript pass information back into the Android side of things via a message. Everything works fine, except when the second webpage doesn't load no matter what I do and which URL I give. When I load the second test, a warning appears "All WebView methods must be called on the same thread." But to my knowledge I'm not using multithreading, nor do I need to use multithreading.

Here's the relevant code:

public class MainActivity extends Activity {
    public WebView myWebView;
    public int state;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        requestWindowFeature(Window.FEATURE_NO_TITLE);
        getWindow().setFlags(WindowManager.LayoutParams.FLAG_FULLSCREEN, 
                WindowManager.LayoutParams.FLAG_FULLSCREEN);

        setContentView(R.layout.activity_main);

        myWebView = (WebView) findViewById(R.id.webview);

        WebSettings myWebViewSettings = myWebView.getSettings();
        myWebViewSettings.setJavaScriptEnabled(true);
        myWebViewSettings.setDomStorageEnabled(true);
        myWebViewSettings.setAllowFileAccessFromFileURLs(true);
        myWebViewSettings.setAllowUniversalAccessFromFileURLs(true);

        myWebView.addJavascriptInterface(new JavascriptHandler(), "cpjs");

        state = 0;
        loadNextTest();
    }

    public void webviewLoadURL(String url) {
        Log.d("app", "now loading " + url);
        myWebView.clearHistory();
        myWebView.clearFormData();
        myWebView.clearCache(true);
        myWebView.loadUrl(url);
    }

    final class JavascriptHandler {
           @JavascriptInterface
           public void sendToAndroid(String text) {
                if (text.equals("confirmed at target")) {
                    loadNextTest();
                }
           }
    }

    public void loadNextTest() {
        Log.d("app", "now loading test " + (state + 1));
        if (state == 0) {
            webviewLoadURL("file:///android_asset/test1.html");
            state = state + 1;

        } else if (state == 1) {
            webviewLoadURL("file:///android_asset/test2.html");
            // webviewLoadURL("http://www.google.com");
            state = state + 1;

        } else {
            webviewLoadURL("file:///android_asset/test3.html");
        }
    }
}

Here's the relevant log lines:

02-22 09:12:32.250: V/WebViewChromium(15117): Binding Chromium to the background looper Looper (main, tid 1) {41c7ec00}
02-22 09:12:32.250: I/chromium(15117): [INFO:library_loader_hooks.cc(112)] Chromium logging enabled: level = 0, default verbosity = 0
02-22 09:12:32.255: I/BrowserProcessMain(15117): Initializing chromium process, renderers=0
02-22 09:12:32.265: W/chromium(15117): [WARNING:proxy_service.cc(888)] PAC support disabled because there is no system implementation
02-22 09:12:32.315: D/dalvikvm(15117): GC_FOR_ALLOC freed 86K, 5% free 3217K/3364K, paused 8ms, total 8ms
02-22 09:12:32.315: I/dalvikvm-heap(15117): Grow heap (frag case) to 4.273MB for 1127536-byte allocation
02-22 09:12:32.325: D/dalvikvm(15117): GC_FOR_ALLOC freed <1K, 4% free 4318K/4468K, paused 11ms, total 11ms
02-22 09:12:32.345: D/dalvikvm(15117): GC_CONCURRENT freed <1K, 4% free 4317K/4468K, paused 1ms+6ms, total 17ms
02-22 09:12:32.365: D/app(15117): now loading test 1
02-22 09:12:32.365: D/app(15117): now loading file:///android_asset/test1.html
02-22 09:12:32.450: D/mali_winsys(15117): new_window_surface returns 0x3000
02-22 09:12:32.460: I/Icing(794): Indexing 5AA949AFB589F1D17D8668589402D01E615E228D from com.google.android.googlequicksearchbox
02-22 09:12:32.500: D/OpenGLRenderer(15117): Enabling debug mode 0
02-22 09:12:32.505: W/AwContents(15117): nativeOnDraw failed; clearing to background color.
02-22 09:12:32.540: I/ActivityManager(447): Displayed com.example.myApp/.MainActivity: +344ms
02-22 09:12:32.550: W/AwContents(15117): nativeOnDraw failed; clearing to background color.
02-22 09:12:32.635: I/Icing(794): Indexing done 5AA949AFB589F1D17D8668589402D01E615E228D
02-22 09:12:32.945: I/chromium(15117): [INFO:async_pixel_transfer_manager_android.cc(56)] Async pixel transfers not supported
02-22 09:12:32.960: I/chromium(15117): [INFO:async_pixel_transfer_manager_android.cc(56)] Async pixel transfers not supported
02-22 09:12:33.180: E/AndroidProtocolHandler(15117): Unable to open asset URL: file:///android_asset/backend/images/ajax-loader.gif
02-22 09:12:46.010: D/app(15117): now loading test 2
02-22 09:12:46.010: D/app(15117): now loading http://www.google.com
02-22 09:12:46.020: W/WebView(15117): java.lang.Throwable: A WebView method was called on thread 'JavaBridge'. All WebView methods must be called on the same thread. (Expected Looper Looper (main, tid 1) {41c7ec00} called on Looper (JavaBridge, tid 271) {41c6f5c8}, FYI main Looper is Looper (main, tid 1) {41c7ec00})
02-22 09:12:46.020: W/WebView(15117):   at android.webkit.WebView.checkThread(WebView.java:2063)
02-22 09:12:46.020: W/WebView(15117):   at android.webkit.WebView.clearHistory(WebView.java:1399)
02-22 09:12:46.020: W/WebView(15117):   at com.example.myApp.MainActivity.webviewLoadURL(MainActivity.java:68)
02-22 09:12:46.020: W/WebView(15117):   at com.example.myApp.MainActivity.loadNextTest(MainActivity.java:91)
02-22 09:12:46.020: W/WebView(15117):   at com.example.myApp.MainActivity$JavascriptHandler.sendToAndroid(MainActivity.java:78)
02-22 09:12:46.020: W/WebView(15117):   at com.android.org.chromium.base.SystemMessageHandler.nativeDoRunLoopOnce(Native Method)
02-22 09:12:46.020: W/WebView(15117):   at com.android.org.chromium.base.SystemMessageHandler.handleMessage(SystemMessageHandler.java:27)
02-22 09:12:46.020: W/WebView(15117):   at android.os.Handler.dispatchMessage(Handler.java:102)
02-22 09:12:46.020: W/WebView(15117):   at android.os.Looper.loop(Looper.java:136)
02-22 09:12:46.020: W/WebView(15117):   at android.os.HandlerThread.run(HandlerThread.java:61)
02-22 09:12:46.030: W/System.err(15117): java.lang.RuntimeException: java.lang.Throwable: A WebView method was called on thread 'JavaBridge'. All WebView methods must be called on the same thread. (Expected Looper Looper (main, tid 1) {41c7ec00} called on Looper (JavaBridge, tid 271) {41c6f5c8}, FYI main Looper is Looper (main, tid 1) {41c7ec00})
02-22 09:12:46.035: W/System.err(15117):    at android.webkit.WebView.checkThread(WebView.java:2073)
02-22 09:12:46.035: W/System.err(15117):    at android.webkit.WebView.clearHistory(WebView.java:1399)
02-22 09:12:46.035: W/System.err(15117):    at com.example.myApp.MainActivity.webviewLoadURL(MainActivity.java:68)
02-22 09:12:46.035: W/System.err(15117):    at com.example.myApp.MainActivity.loadNextTest(MainActivity.java:91)
02-22 09:12:46.035: W/System.err(15117):    at com.example.myApp.MainActivity$JavascriptHandler.sendToAndroid(MainActivity.java:78)
02-22 09:12:46.035: W/System.err(15117):    at com.android.org.chromium.base.SystemMessageHandler.nativeDoRunLoopOnce(Native Method)
02-22 09:12:46.040: W/System.err(15117):    at com.android.org.chromium.base.SystemMessageHandler.handleMessage(SystemMessageHandler.java:27)
02-22 09:12:46.040: W/System.err(15117):    at android.os.Handler.dispatchMessage(Handler.java:102)
02-22 09:12:46.040: W/System.err(15117):    at android.os.Looper.loop(Looper.java:136)
02-22 09:12:46.040: W/System.err(15117):    at android.os.HandlerThread.run(HandlerThread.java:61)
02-22 09:12:46.040: W/System.err(15117): Caused by: java.lang.Throwable: A WebView method was called on thread 'JavaBridge'. All WebView methods must be called on the same thread. (Expected Looper Looper (main, tid 1) {41c7ec00} called on Looper (JavaBridge, tid 271) {41c6f5c8}, FYI main Looper is Looper (main, tid 1) {41c7ec00})
02-22 09:12:46.045: W/System.err(15117):    at android.webkit.WebView.checkThread(WebView.java:2063)
02-22 09:12:46.045: W/System.err(15117):    ... 9 more

Any help is appreciated. Thanks.

Answer 1

As mentioned by ProfessorT, there are many threads running behind the scenes with the WebView, and callbacks from JavaScript via JavaScript interface objects are made on a background thread.

You can achieve what you like with some code like this inside loadNextTest:

myWebView.post(new Runnable() {
    @Override
    public void run() {
        webviewLoadURL("file:///android_asset/test1.html");
    }
});

Please also note that your state variable will also be written on the JavaScript background thread so you many need to synchronize your read/writes if they are coming from other threads too.