Calling Javascript method from within webview in Android

You should execute the javascript function when the page is loaded

webView.setWebViewClient(new WebViewClient() {
    public void onPageFinished(WebView view, String url) {
        webView.loadUrl("javascript:callJS()");
    }
});

When you put onload=callJS() means that the javascript function will be call when page is load. For debugging, you can put console.log("your text here") in your javascript function and you will get it in your android studio log. (Commonly with the tag I/chromium). Otherwise, you can use Remote Debugging on Android with Chrome. The documentation here https://developer.chrome.com/devtools/docs/remote-debugging.

Please try this at once:

Activity code:

public class MainActivity extends AppCompatActivity {
 private WebView mWebView = null;
    private ProgressBar mLoading;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        this.mWebView = (WebView)findViewById(R.id.wvPortal);
        mLoading = (ProgressBar) findViewById(R.id.pbLoading);
        WebSettings webSettings = mWebView.getSettings();
        webSettings.setJavaScriptEnabled(true);
        webSettings.setDomStorageEnabled(true);
        webSettings.setSupportZoom(true);
        webSettings.setLoadWithOverviewMode(true);
        webSettings.setUseWideViewPort(true);
        webSettings.setSupportMultipleWindows(true);
        mWebView.getSettings().setDomStorageEnabled(true);
        mWebView.setWebChromeClient(new WebChromeClient());
       /* To keep page navigation within the WebView and hence within the app,
        we need to create a subclass of WebViewClient,and override its shouldOverrideUrlLoading(WebView webView, String url) method.*/
        WebViewClientImpl webViewClient = new WebViewClientImpl(this);
        mWebView.setWebViewClient(webViewClient);
        mWebView.loadUrl("file:///android_asset/www/index.html");
        WebSettings mWebSettings = mWebView.getSettings();
        mWebSettings.setJavaScriptEnabled(true);
        Log.d("Test 0","Interface calls");
        //calling javascript interface:
        // 1st.parameter is the JavaScript interface object itself.
        // 2nd.parameter is the name of the global JavaScript variable which the JavaScript interface object is bound to.
        mWebView.addJavascriptInterface(new AppJavaScriptProxy(this,mWebView), "androidAppProxy");

    }
}

AppJavaScriptProxy class for creationg interface :

public class AppJavaScriptProxy {
    private Activity activity = null;
    private WebView webView  = null;
    public AppJavaScriptProxy(Activity activity,WebView webview) {
        this.activity = activity;
        this.webView  = webview;
        Log.d("Test 1","in Interface");
    }

    @JavascriptInterface
    public void showMessage(final String message) {
        Log.d("from javascript", "" + message);
        final Activity theActivity = this.activity;
        final WebView theWebView = this.webView;
        this.activity.runOnUiThread(new Runnable() {
            @Override
            public void run() {
                if (!theWebView.getUrl().startsWith("file:///android_asset/www/index.html")) {
                    return;
                }
                Toast toast = Toast.makeText(
                        theActivity.getApplicationContext(),
                        message,
                        Toast.LENGTH_SHORT);
                toast.show();
            }
        });
    }
}

Index.html call this script in html page.

<input type="button" value="call Methode" onClick="showMessage("Message from JavaScript")" />
<script>
    if(typeof androidAppProxy !== "undefined"){
    androidAppProxy.showMessage("Message from JavaScript");
} else {
    alert("Running outside Android app");
}
</script>

call this script on any button click on webview index page. Ref. Building webapps in webview

If above code snippet would helps you please vote my answer. Thanks!

write your code like this

WebView webView = new WebView(this);
webView.getSettings().setJavaScriptEnabled(true);
webView.loadUrl("file:///android_asset/temp.html");
webView.setWebViewClient(new WebViewClient() {
            @Override
            public void onPageFinished(WebView view, String url) {
                view.loadUrl("javascript:callJS()");
            }
        });