Java - NullPointerException when getting JSONObject

Question

I'm trying to implement a login feature in my Android app using Lukencode's RestClient class. I have a class named UserFunctions to retrieve JSONObject via RestClient:

public class UserFunctions {

    private RestClient restClient;
    private String json;
    private Context mContext;

    private static String login_tag = "login";
    private static String register_tag = "register";

    // constructor
    public UserFunctions(Context context){
        restClient = new RestClient("http://10.0.2.2:81/HGourmet/user");
        mContext = context;
    }

    /**
     * function make Login Request
     * @param email
     * @param password
     * */
   public JSONObject loginUser(String email, String password){
        // Building Parameters      
        restClient.AddParam("tag", login_tag);
        restClient.AddParam("email", email);
        restClient.AddParam("password", password);

        // getting JSON Object
        try{
    restClient.Execute(RequestMethod.POST);
    }catch(Exception e){
        e.printStackTrace();
    }               
    json = restClient.getResponse();

        JSONObject jObj = null;
        try {
            jObj = new JSONObject(json);
        } catch (JSONException e) {
            Log.e("JSON Parser", "Error parsing data " + e.toString());
        }
        return jObj;
    }

And below is what I get in logcat:

01-06 09:01:05.805: E/Trace(1819): error opening trace file: No such file or directory (2)
01-06 09:02:05.485: E/AndroidRuntime(1819): FATAL EXCEPTION: main
01-06 09:02:05.485: E/AndroidRuntime(1819): java.lang.NullPointerException
01-06 09:02:05.485: E/AndroidRuntime(1819):     at org.json.JSONTokener.nextCleanInternal(JSONTokener.java:116)
01-06 09:02:05.485: E/AndroidRuntime(1819):     at org.json.JSONTokener.nextValue(JSONTokener.java:94)
01-06 09:02:05.485: E/AndroidRuntime(1819):     at org.json.JSONObject.<init>(JSONObject.java:154)
01-06 09:02:05.485: E/AndroidRuntime(1819):     at org.json.JSONObject.<init>(JSONObject.java:171)
01-06 09:02:05.485: E/AndroidRuntime(1819):     at com.hanu.hgourmet.library.UserFunctions.loginUser(UserFunctions.java:49)
01-06 09:02:05.485: E/AndroidRuntime(1819):     at com.hanu.hgourmet.LoginActivity$1.onClick(LoginActivity.java:56)
01-06 09:02:05.485: E/AndroidRuntime(1819):     at android.view.View.performClick(View.java:4202)
01-06 09:02:05.485: E/AndroidRuntime(1819):     at android.view.View$PerformClick.run(View.java:17340)
01-06 09:02:05.485: E/AndroidRuntime(1819):     at android.os.Handler.handleCallback(Handler.java:725)
01-06 09:02:05.485: E/AndroidRuntime(1819):     at android.os.Handler.dispatchMessage(Handler.java:92)
01-06 09:02:05.485: E/AndroidRuntime(1819):     at android.os.Looper.loop(Looper.java:137)
01-06 09:02:05.485: E/AndroidRuntime(1819):     at android.app.ActivityThread.main(ActivityThread.java:5039)
01-06 09:02:05.485: E/AndroidRuntime(1819):     at java.lang.reflect.Method.invokeNative(Native Method)
01-06 09:02:05.485: E/AndroidRuntime(1819):     at java.lang.reflect.Method.invoke(Method.java:511)
01-06 09:02:05.485: E/AndroidRuntime(1819):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:793)
01-06 09:02:05.485: E/AndroidRuntime(1819):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:560)
01-06 09:02:05.485: E/AndroidRuntime(1819):     at dalvik.system.NativeStart.main(Native Method)

I really hope that someone would help me pinpoint the reason of this problem. Thank you very much.

public JSONObject loginUser(String email, String password) returns jObj, but doesn't create jObj. Perhaps that is your issue? — Mike
– Mike, Commented Jan 6, 2013 at 8:53
jObj is already created when I call new GetUser().execute(); isn't it? — huong
– huong, Commented Jan 6, 2013 at 8:54
This is so frustrating. Don't you realise that the logcat stack trace will tell you exactly which line the error is on? Toasts are not a good way to debug. Step through your code with the debugger. In the meantime, please post the stack trace for the exception. — Simon
– Simon, Commented Jan 6, 2013 at 8:58
No jObj wont get initialised since GetUser is an AsyncTask which runs in different thread and after calling new GetUser().execute(); the control wont wait for AsyncTask to finish and control moves forward and return jObj which is not initialised. — Animesh Sinha
– Animesh Sinha, Commented Jan 6, 2013 at 9:03
@strawberryjam Post the stack trace... errr... Simon says post the stack trace. Without the stack trace odds are you're just going to get a complaint about the missing stack trace, or guesses at the problem. — Mike
– Mike, Commented Jan 6, 2013 at 9:05

JB Nizet · Accepted Answer · 2013-01-06 09:06:40Z

11

I won't expand on how you should do this, but only on why it happens. An AsyncTask, by definition, is asynchronous. This means that when you execute it, it does its job in parallel, in a different thread. So you can't expect it to be terminated right after execute() has been called (else it would be called SyncTask, and wouldn't have any reason to exist).

See AsyncTask as a toaster. When you start the toaster (execute the AsyncTask), it toasts your bread, but doesn't return the toasted bread immediately. It takes some time to toast it. While it's toasting your bread, you can do something else (and thus not freeze the UI thread completely). And when it has finished toasting your bread, it notifies you with a ding! that the bread is toasted. That's the same thing here. You should only start using json (the toasted bread), when the AsyncTask notifies you that it has finished executing.

answered Jan 6, 2013 at 9:06

JB Nizet

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4 Comments

huong Over a year ago

Thank you so much for enlightening me with this. I'll see what I can do to fix this.

Simon Over a year ago

Love the toaster analogy. +1

Tom Lubitz Over a year ago

To expand on this, it looks to me like getResponse() is null. it's getting passed to a JSONTokener, which tries to access the length of the input without a null check. Seems like it's actually a bug in JSONTokener.

sud007 Over a year ago

@TomLubitz Since I am facing this issue even without an Asynctask, I think the same.

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