Android - 获取JSON时的java.lang.NullPointerException

Question

从服务器获取JSON我正在使用RestClient时遇到此错误。我真的无法弄清楚为什么。当我在doInBackground方法中使用相同的源代码时它可以工作，但是当我使用它时，否则我没有工作！我在浏览器中测试了网址并重新调整了JSON。我非常感谢您的帮助和感谢。

    inputSearch.addTextChangedListener(new TextWatcher() {

    @Override
    public void onTextChanged(CharSequence s, int arg1, int arg2,int arg3) {

    RestClient rc = new RestClient("url");

        try {
            rc.Execute(RequestMethod.GET);
        } catch (Exception e) {
            e.printStackTrace();
        }   
     Gson gson = new Gson();
     String reponse = rc.getResponse(); // this return null that's why I have java.lang.NullPointerException but I don't know why ? It don't get informations from the server     

     final Container container = gson.fromJson(reponse,Container.class);
    }
...

这是我的RestClient类：

public class RestClient {

    private static final int CONNECTION_TIMEOUT = 5000;
    private static final int SOCKET_TIMEOUT = 5000;
    private static final long MCC_TIMEOUT = 5000;

    private ArrayList <NameValuePair> params;
    private ArrayList <NameValuePair> headers;

    private String url;

    private int responseCode;
    private String message;

    private String response;

    public String getResponse() {
        return response;
    }

    public String getErrorMessage() {
        return message;
    }

    public int getResponseCode() {
        return responseCode;
    }

    public RestClient(String url)
    {
        this.url = url;
        params = new ArrayList<NameValuePair>();
        headers = new ArrayList<NameValuePair>();
    }

    public void AddParam(String name, String value)
    {
        params.add(new BasicNameValuePair(name, value));
    }

    public void AddHeader(String name, String value)
    {
        headers.add(new BasicNameValuePair(name, value));
    }

    public void Execute(RequestMethod method) throws Exception
    {
        switch(method) {
            case GET:
            {
                //add parameters
                String combinedParams = "";
                if(!params.isEmpty()){
                    combinedParams += "?";
                    for(NameValuePair p : params)
                    {
                        String paramString = p.getName() + "=" + URLEncoder.encode(p.getValue(),"UTF-8");
                        if(combinedParams.length() > 1)
                        {
                            combinedParams  +=  "&" + paramString;
                        }
                        else
                        {
                            combinedParams += paramString;
                        }
                    }
                }

                HttpGet request = new HttpGet(url + combinedParams);
                setTimeouts(request.getParams());

                //add headers
                for(NameValuePair h : headers)
                {
                    request.addHeader(h.getName(), h.getValue());
                }

                executeRequest(request, url);
                break;
            }
            case POST:
            {
                HttpPost request = new HttpPost(url);
                setTimeouts(request.getParams());

                //add headers
                for(NameValuePair h : headers)
                {
                    request.addHeader(h.getName(), h.getValue());
                }

                if(!params.isEmpty()){
                    request.setEntity(new UrlEncodedFormEntity(params, HTTP.UTF_8));
                }

                executeRequest(request, url);
                break;
            }
        }
    }

    private void executeRequest(HttpUriRequest request, String url)
    {
        HttpClient client = new DefaultHttpClient();

        HttpResponse httpResponse;

        try {
            httpResponse = client.execute(request);
            responseCode = httpResponse.getStatusLine().getStatusCode();
            message = httpResponse.getStatusLine().getReasonPhrase();

            HttpEntity entity = httpResponse.getEntity();

            if (entity != null) {

                InputStream instream = entity.getContent();
                response = convertStreamToString(instream);

                // Closing the input stream will trigger connection release
                instream.close();
            }

        } catch (ClientProtocolException e)  {
            client.getConnectionManager().shutdown();
            e.printStackTrace();
        } catch (IOException e) {
            client.getConnectionManager().shutdown();
            e.printStackTrace();
        }
    }

    private static String convertStreamToString(InputStream is) {

        BufferedReader reader = new BufferedReader(new InputStreamReader(is));
        StringBuilder sb = new StringBuilder();

        String line = null;
        try {
            while ((line = reader.readLine()) != null) {
                sb.append(line + "\n");
            }
        } catch (IOException e) {
            e.printStackTrace();
        } finally {
            try {
                is.close();
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
        return sb.toString();
    }

    private static void setTimeouts(HttpParams params) {
        params.setIntParameter(CoreConnectionPNames.CONNECTION_TIMEOUT, CONNECTION_TIMEOUT);
        params.setIntParameter(CoreConnectionPNames.SO_TIMEOUT, SOCKET_TIMEOUT);
        params.setLongParameter(ConnManagerPNames.TIMEOUT, MCC_TIMEOUT);
    }
}

这是我的RequestMethod

public enum RequestMethod {
    GET, POST
}

这是我的logcat：

FATAL EXCEPTION: main
java.lang.RuntimeException: Unable to start activity ComponentInfo{Activity}: java.lang.NullPointerException
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2180)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2230)
at android.app.ActivityThread.access$600(ActivityThread.java:141)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1234)
at android.os.Handler.dispatchMessage(Handler.java:99)
at android.os.Looper.loop(Looper.java:137)
at android.app.ActivityThread.main(ActivityThread.java:5039)
at java.lang.reflect.Method.invokeNative(Native Method)
at java.lang.reflect.Method.invoke(Method.java:511)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:793)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:560)
at dalvik.system.NativeStart.main(Native Method)
05-17 20:46:13.472: E/AndroidRuntime(2027): Caused by: java.lang.NullPointerException
at android.app.Activity.performCreate(Activity.java:5104)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1080)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2144)
... 11 more