In ActivtyMain:
	Database db = Database.newInstance();
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        db.setURL("http://192.168.30.1:28389/");
        try {

            getAllViews();
        } catch (Exception e) {
            Toast.makeText(this,"exception"+e.getMessage(),Toast.LENGTH_LONG).show();
        }

    }

My Database class:

package com.example.schueler.project.data;

import com.example.schueler.project.misc.ControllerSync;
import com.google.gson.Gson;
import com.google.gson.reflect.TypeToken;

import java.lang.reflect.Type;
import java.util.ArrayList;

public class Database {
    private static Database db=null;
    private static ControllerSync controller = null;
    private static String url = null;

    private Database() {
    }

    public static Database newInstance(){
        if(db==null)
            db= new Database();
        return db;
    }

    public static void setURL(String _url){
        url=_url;
    }

    public ArrayList<User> getUsers() throws Exception{
        Gson gson = new Gson();
        //each call needs an new instance of async !!
        controller = new ControllerSync(url);

        controller.execute("users");
        String strFromWebService = controller.get();
        System.out.println("=========" + strFromWebService);
        Type collectionType = new TypeToken<ArrayList<User>>() {}.getType();
        System.out.println("ct:"+collectionType);
        ArrayList<User> vec = gson.fromJson(strFromWebService, collectionType);

        return vec;
    }



}

And finally the controller class:

package com.example.schueler.project.misc;

import android.os.AsyncTask;
import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.net.URL;
import java.net.URLConnection;

import android.os.AsyncTask;
import java.io.BufferedOutputStream;
import java.io.BufferedWriter;
import java.io.OutputStreamWriter;
import java.net.HttpURLConnection;

public class ControllerSync extends AsyncTask<String, Void, String> {

    private static final String URI_2ND = "BummWebService/webresources/";
    private String url_1st;

    public ControllerSync(String url) throws Exception {
        this.url_1st = url;
    }

    @Override
    protected String doInBackground(String... command) {
        boolean isGet = true, isPut = false;

        BufferedReader reader = null;
        BufferedWriter writer = null;
        String content = null;
        URL url = null;

        try {
            if (command[0].equals("users")) {
                //call producerlist
                url = new URL(url_1st + URI_2ND + "users");
            }

            HttpURLConnection conn = (HttpURLConnection) url.openConnection();
            if (isPut) {
                conn.setRequestMethod("PUT");
                conn.setRequestProperty("Content-Type", "application/json");
                writer = new BufferedWriter(new OutputStreamWriter(conn.getOutputStream()));
                writer.write(command[1]); //product - object in json-format
                writer.flush();
                writer.close();
                conn.getResponseCode();
            }
            // get data from server
            reader = new BufferedReader(new InputStreamReader(
                    conn.getInputStream()));
            StringBuilder sb = new StringBuilder();
            String line = null;

            while ((line = reader.readLine()) != null) {
                sb.append(line);
            }

            content = sb.toString();
            reader.close();
            conn.disconnect();
        } catch (Exception ex) {
            content = "error in doInBackground: " + ex.getMessage();
        }
        return content;

    }
}