androidwebviewjsalert对话框不能弹出解决办法

    /** Called when the activity is first created. */
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.webviewmain);
        /** android内置浏览器对象 */
        webView = (WebView) this.findViewById(R.id.webView);

        /**
         * 加载放在assets目录下的android.html文件 注意url的地址前缀为： file:///android_asset/
         *
         * 其实可以把这个html布局文件放在公网中，这样方便随时更新维护 例如
         * webview.loadUrl("http://192.168.1.100:8080/Hello/index.html");
         */
        //webView.loadUrl("file:///android_asset/android.html");
        //webView.loadUrl("file:///android_asset/index.html");
        
        
        /** 允许javascript的运行 */
        webView.getSettings().setJavaScriptEnabled(true);
        final Context myApp = this;  
          
      
        /* WebChromeClient must be set BEFORE calling loadUrl! */  
        webView.setWebChromeClient(new WebChromeClient() {  
            @Override  
            public boolean onJsAlert(WebView view, String url, String message, final android.webkit.JsResult result)  
            {  
                new AlertDialog.Builder(MainActivity.this)  
                    .setTitle("javaScript dialog")  
                    .setMessage(message)  
                    .setPositiveButton(android.R.string.ok,  
                            new AlertDialog.OnClickListener()  
                            {  
                                public void onClick(DialogInterface dialog, int which)  
                                {  
                                    result.confirm();  
                                }  
                            })  
                    .setCancelable(false)  
                    .create()  
                    .show();  
          
                return true;  
            };  
        
            @Override
            public boolean onJsConfirm(WebView view, String url,
                            String message, final JsResult result) {
                    AlertDialog.Builder b = new AlertDialog.Builder(MainActivity.this);
                    b.setTitle("Confirm");
                    b.setMessage(message);
                    b.setPositiveButton(android.R.string.ok,
                                    new AlertDialog.OnClickListener() {
                                            public void onClick(DialogInterface dialog,
                                                            int which) {
                                                    result.confirm();
                                            }
                                    });
                    b.setNegativeButton(android.R.string.cancel,
                                    new DialogInterface.OnClickListener() {
                                            public void onClick(DialogInterface dialog,
                                                            int which) {
                                                    result.cancel();
                                            }
                                    });
                    b.setCancelable(false);
                    b.create();
                    b.show();
                    return true;
            };
/*
            @Override
            public boolean onJsPrompt(WebView view, String url, String message,
                            String defaultValue, final JsPromptResult result) {
                    final LayoutInflater factory = LayoutInflater
                                    .from(MainActivity.this);
                    final View v = factory.inflate(R.layout.prompt_dialog, null);
                    ((TextView) v.findViewById(R.id.prompt_message_text))
                                    .setText(message);
                    ((EditText) v.findViewById(R.id.prompt_input_field))
                                    .setText(defaultValue);

                    AlertDialog.Builder b = new AlertDialog.Builder(MainActivity.this);
                    b.setTitle("Prompt");
                    b.setView(v);
                    b.setPositiveButton(android.R.string.ok,
                                    new AlertDialog.OnClickListener() {
                                            public void onClick(DialogInterface dialog,
                                                            int which) {
                                                    String value = ((EditText) v
                                                                    .findViewById(R.id.prompt_input_field))
                                                                    .getText().toString();
                                                    result.confirm(value);
                                            }
                                    });
                    b.setNegativeButton(android.R.string.cancel,
                                    new DialogInterface.OnClickListener() {
                                            public void onClick(DialogInterface dialog,
                                                            int which) {
                                                    result.cancel();
                                            }
                                    });
                    b.setOnCancelListener(new DialogInterface.OnCancelListener() {
                            public void onCancel(DialogInterface dialog) {
                                    result.cancel();
                            }
                    });
                    b.show();
                    return true;
            };*/

           public void onProgressChanged(WebView view, int newProgress) {
                    MainActivity.this.getWindow().setFeatureInt(
                                    Window.FEATURE_PROGRESS, newProgress * 100);
                    super.onProgressChanged(view, newProgress);
            }

            public void onReceivedTitle(WebView view, String title) {
                    MainActivity.this.setTitle(title);
                    super.onReceivedTitle(view, title);
            }
   
        });