android WebView stop running

Hello,

I have the following problem with a WebView Control. My webview is defined in a Activity as follows:

public class MyWebView extends Activity {
private WebView webview;
...

@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.mywebview);
...
webview = new WebView(getApplicationContext());
webview.setWebChromeClient(new WebChromeClient() { ... });
webview.setWebViewClient(new WebViewClient() { ... });
...
webview.loadUrl("http://www.google.de");
}

Everything works fine, the Webpage is opened. But if close the Activity (Back-Button pressed) and go back to my main-menue, the WebView seems not to be closed. The http-threads keeps running.

If I open about 10 several pages one after another, the Webpages are not nonger loadet, but "Webpage not available" is displayd in the WebView.

I tried to close the thread by adding the code:
webview.destroy();
webview = null;
or
myLinearLayout = null;

or
Method op = Class.forName("android.webkit.WebView").getMethod("onPause",(Class[]) null);
op.invoke(webview, (Object[]) null);

But there is no change - the http-threads are still running.

What is wrong? How to close the WebView? webview.destroy(); webview = null; e.g. does not work.

Thanx a lot for any suggestions!