Android-WebView的onReceivedHttpAuthRequest()不再被调用

我正在使用基于webview的应用程序,我在webview中呈现URL.Url具有HTTP身份验证.

当我第一次启动url时,会调用onReceivedHttpAuthRequest()并显示一个对话框,供用户输入身份验证凭据,即auth用户名和密码.

@Override
public void onReceivedHttpAuthRequest(WebView view, HttpAuthHandler handler, String host, String realm) {
        final WebView mView = view;
        final HttpAuthHandler mHandler = handler;

        final EditText usernameInput = new EditText(mActivity);
        usernameInput.setHint("Username");

        final EditText passwordInput = new EditText(mActivity);
        passwordInput.setHint("Password");
        passwordInput.setInputType(InputType.TYPE_CLASS_TEXT | InputType.TYPE_TEXT_VARIATION_PASSWORD);

        LinearLayout ll = new LinearLayout(mActivity);
        ll.setOrientation(LinearLayout.VERTICAL);
        ll.addView(usernameInput);
        ll.addView(passwordInput);

        Builder authDialog = new AlertDialog
                .Builder(mActivity)
                .setTitle("Authentication")
                .setView(ll)
                .setCancelable(false)
                .setPositiveButton("OK", new DialogInterface.OnClickListener() {
                    public void onClick(DialogInterface dialog, int whichButton) {
                        mHandler.proceed(usernameInput.getText().toString(), passwordInput.getText().toString());
                        dialog.dismiss();
                    }
                })
                .setNegativeButton("Cancel", new DialogInterface.OnClickListener() {
                    public void onClick(DialogInterface dialog, int whichButton) {
                        dialog.dismiss();
                        mView.stopLoading();
                        onLoadListener.onAuthCancel((MyWebView)mView, mTitleTextView);
                    }
                });

        if(view!=null)
            authDialog.show();

    }

在提交请求时,请继续并加载URL.但是在我使用后退按钮(不在后台发送)退出应用程序后,如果我再次启动它并加载相同的URL,则直接加载URL而不要求提供onReceivedHttpAuthRequest()的凭据再也不会被调用.

我还使用以下代码清除app退出的凭据:

WebViewDatabase webDB = WebViewDatabase.getInstance(BrowserActivity.this);
    if(webDB!=null){
        if(webDB.hasFormData())
            webDB.clearFormData();
        if(webDB.hasUsernamePassword())
            webDB.clearUsernamePassword();
        if(webDB.hasHttpAuthUsernamePassword())
            webDB.clearHttpAuthUsernamePassword();
    }
webView.clearCache(true);

此外,我正在清除所有webview缓存,cookie和应用程序的缓存目录和webview数据库:

BrowserActivity.this.deleteDatabase("webview.db");
BrowserActivity.this.deleteDatabase("webviewCache.db");

我不知道为什么会这样.有没有人可以帮助我.至少在没有调用onReceivedHttpAuthRequest()的问题上？