Unable to convert a [text/plain,UTF-8] representation into an object of class java.lang.String

Question

I'm facing a strange problem while developing a restlet web service + Android app.

When I try to recover a String from the web service (deployed locally on Tomcat 7.0) using a simple get, if the string from the server is too big the Android app crashes with a NullPointerException.

Basically, the scheme is the following

Android app

String retrievedString = clientResource.get();

Web server

return serializedString;

When serializedString has a (not too) large number of characters (above 2k or something like that), retrievedString get a null value.

I don't know what is the problem here, but I think the most likely cause is a memory limit, due to Android heap, Java heap, system RAM or maybe even VirtualBox/Genymotion limitation.

What do you think of the problem and how can I solve it?

---

EDIT

As requested, here it is some more code; I tried to avoid unnecessary stuff and try/catch blocks.

Server

 ObjectMapper objectMapper = new ObjectMapper();
    FilterProvider excludeThreadFilter =
        new SimpleFilterProvider().addFilter("filter properties by name",
            SimpleBeanPropertyFilter.serializeAllExcept("email", "password",
                "threads", "messages", "subforumsSet", /*"parentSubforum",
                "registeredDate", "threadSubforum"*/));
    String st = objectMapper.writer(excludeThreadFilter).writeValueAsString(lt);
    return st;

Client

ClientResource cr = new ClientResource("...an ip address...");
    ThreadsInSubforumIdByPage tis = cr.wrap(ThreadsInSubforumIdByPage.class);

      // The line below is null
      String retrievedString = tis.getThreads();
      // The line below raise the NullPointerException
      List<Thread> retrievedList =
        DataHolder.getHolder().getObjectMapper().readValue(
          retrievedString, new TypeReference<List<Thread>>(){});

Exception

EDIT: Ok, I was misled by logcat color. It's only a few days that I'm using Android Studio and I haven't noticed at all that Log.e() messages are written in normal color.

So, this is what logcat reports

5 .../W/System.err﹕ Unable to convert a [text/plain,UTF-8] representation into an object of class java.lang.String
5 .../W/System.err﹕ [ 175  3749: 3749 W/System.err ]
    android.os.NetworkOnMainThreadException
5 .../W/System.err﹕ at android.os.StrictMode$AndroidBlockGuardPolicy.onNetwork(StrictMode.java:1117)
5 .../W/System.err﹕ at libcore.io.BlockGuardOs.recvfrom(BlockGuardOs.java:158)
5 .../W/System.err﹕ at libcore.io.IoBridge.recvfrom(IoBridge.java:525)
5 .../W/System.err﹕ at java.nio.SocketChannelImpl.readImpl(SocketChannelImpl.java:305)
5 .../W/System.err﹕ at java.nio.SocketChannelImpl.read(SocketChannelImpl.java:267)
5 .../W/System.err﹕ at org.restlet.engine.io.ReadableSocketChannel.read(ReadableSocketChannel.java:82)
5 .../W/System.err﹕ at org.restlet.engine.connector.Connection$1.read(Connection.java:232)
5 .../W/System.err﹕ at org.restlet.engine.io.Buffer.fill(Buffer.java:395)
5 .../W/System.err﹕ at org.restlet.engine.io.ReadableBufferedChannel.onFill(ReadableBufferedChannel.java:157)
5 .../W/System.err﹕ at org.restlet.engine.io.Buffer.process(Buffer.java:601)
5 .../W/System.err﹕ at org.restlet.engine.io.ReadableBufferedChannel.read(ReadableBufferedChannel.java:176)
5 .../W/System.err﹕ at org.restlet.engine.io.Buffer.fill(Buffer.java:395)
5 .../W/System.err﹕ at org.restlet.engine.io.NbChannelInputStream.onFill(NbChannelInputStream.java:189)
5 .../W/System.err﹕ at org.restlet.engine.io.Buffer.process(Buffer.java:601)
5 .../W/System.err﹕ at org.restlet.engine.io.NbChannelInputStream.read(NbChannelInputStream.java:307)
5 .../W/System.err﹕ at java.io.InputStream.read(InputStream.java:163)
5 .../W/System.err﹕ at org.restlet.engine.io.BioUtils.copy(BioUtils.java:81)
5 .../W/System.err﹕ at org.restlet.engine.io.NioUtils.copy(NioUtils.java:148)
5 .../W/System.err﹕ at org.restlet.representation.ReadableRepresentation.write(ReadableRepresentation.java:104)
5 .../W/System.err﹕ at org.restlet.representation.ChannelRepresentation.write(ChannelRepresentation.java:76)
5 .../W/System.err﹕ at org.restlet.representation.ChannelRepresentation.write(ChannelRepresentation.java:82)
5 .../W/System.err﹕ at org.restlet.representation.Representation.getText(Representation.java:397)
9 .../W/System.err﹕ at org.restlet.engine.converter.DefaultConverter.toObject(DefaultConverter.java:260)
9 .../W/System.err﹕ at org.restlet.service.ConverterService.toObject(ConverterService.java:170)
9 .../W/System.err﹕ at org.restlet.resource.Resource.toObject(Resource.java:828)
9 .../W/System.err﹕ at org.restlet.engine.resource.ClientInvocationHandler.invoke(ClientInvocationHandler.java:240)
9 .../W/System.err﹕ at $Proxy23.getThreads(Native Method)
9 .../W/System.err﹕ at com.gmail.tigerjack89.pervasive.forum.SubforumContentActivity$LoadThreadsInBackground.loadThreads(SubforumContentActivity.java:329)
9 .../W/System.err﹕ at com.gmail.tigerjack89.pervasive.forum.SubforumContentActivity$LoadThreadsInBackground.access$400(SubforumContentActivity.java:283)
9 .../W/System.err﹕ at com.gmail.tigerjack89.pervasive.forum.SubforumContentActivity$LoadThreadsInBackground$1.run(SubforumContentActivity.java:301)
9 .../W/System.err﹕ at android.os.Handler.handleCallback(Handler.java:615)
9 .../W/System.err﹕ at android.os.Handler.dispatchMessage(Handler.java:92)
9 .../W/System.err﹕ at android.os.Looper.loop(Looper.java:137)
9 .../W/System.err﹕ at android.app.ActivityThread.main(ActivityThread.java:4745)
9 .../W/System.err﹕ at java.lang.reflect.Method.invokeNative(Native Method)
9 .../W/System.err﹕ at java.lang.reflect.Method.invoke(Method.java:511)
9 .../W/System.err﹕ at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:786)
9 .../W/System.err﹕ at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553)
9 .../W/System.err﹕ at dalvik.system.NativeStart.main(Native Method)
    [ 187  3749: 3749 E/class com.gmail.tigerjack89.pervasive.forum.SubforumContentActivity$LoadThreadsInBackground NullPointerException
    java.lang.NullPointerException
            at com.fasterxml.jackson.core.JsonFactory.createParser(JsonFactory.java:822)
            at com.fasterxml.jackson.databind.ObjectMapper.readValue(ObjectMapper.java:2153)
            at com.gmail.tigerjack89.pervasive.forum.SubforumContentActivity$LoadThreadsInBackground.loadThreads(SubforumContentActivity.java:331)
            at com.gmail.tigerjack89.pervasive.forum.SubforumContentActivity$LoadThreadsInBackground.access$400(SubforumContentActivity.java:283)
            at com.gmail.tigerjack89.pervasive.forum.SubforumContentActivity$LoadThreadsInBackground$1.run(SubforumContentActivity.java:301)
            at android.os.Handler.handleCallback(Handler.java:615)
            at android.os.Handler.dispatchMessage(Handler.java:92)
            at android.os.Looper.loop(Looper.java:137)
            at android.app.ActivityThread.main(ActivityThread.java:4745)
            at java.lang.reflect.Method.invokeNative(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:511)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:786)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553)
            at dalvik.system.NativeStart.main(Native Method)

In the original question, I omitted that I'm using Jackson to serialize my entities because seems to be involved only indirectly in this problem. I mean, it raise an NullPointerException only when retrievedString is null.

---

AsyncTask code

As requested, here is a lot of code from my AsynkTask

  private class LoadThreadsInBackground extends LoadFromServerInBackground
  {
    public LoadThreadsInBackground()
    {
      super(SubforumContentActivity.this);
    }
    @Override
    protected Void doInBackground(final Boolean... params)
    {
      runOnUiThread(
        new Runnable()
        {
          @Override
          public void run()
          {
            currentThreadsPosition = threadsListView.getFirstVisiblePosition();

            // Appending new data to threadsList
            loadThreads(params[0]);

            // Setting new scroll position
            threadsListView.setSelectionFromTop(currentThreadsPosition + 1, 0);
          }
        });
      return (null);
    }

    private void loadThreads(boolean reload)
    {
      if (reload) threadList.clear();

      ClientResource cr = new ClientResource("IP address");
      cr.setChallengeResponse(getAuthentication());
      ThreadsInSubforumIdByPage tis = cr.wrap(ThreadsInSubforumIdByPage.class);

      List<Thread> retrievedList;
        String s = tis.getThreads();
        retrievedList =
          getObjectMapper().readValue(
            s, new TypeReference<List<Thread>>(){});
      threadList.addAll(retrievedList);

      if ((retrievedList == null) || (retrievedList.size() < Constants
        .RETRIEVED_PER_PAGE))
      {
        loadMoreThreadsButton.setEnabled(false);
        if (retrievedList != null && retrievedList.size() == 0)
        { currentThreadsPage--; }
      }
    }
  }

It extends this abstact class

public abstract class LoadFromServerInBackground extends AsyncTask<Boolean, Void, Void>
{
  private ProgressDialog mProgressDialog;
  private Activity mInvokerActivity;

  public LoadFromServerInBackground(Activity invokerActivity)
  {
    mInvokerActivity = invokerActivity;
  }

  @Override
  protected void onPreExecute()
  {
    // Showing progress dialog before sending http request
    mProgressDialog = new ProgressDialog(mInvokerActivity);
    mProgressDialog.setMessage("Please wait ...");
    mProgressDialog.setIndeterminate(true);
    mProgressDialog.setCancelable(false);
    mProgressDialog.show();
  }

  @Override
  protected void onPostExecute(Void unused)
  {
    // closing progress dialog
    mProgressDialog.dismiss();
  }
}

Answer 1

You're misusing your AsyncTask by calling runOnUiThread(). runOnUiThread() does exactly what it says it does: executes the code in the Runnable on the UI thread.

Network communication on Android must be performed on a background thread, runOnUiThread() breaks that requirement.

Try this version of your AsyncTask:

    private class LoadThreadsInBackground extends LoadFromServerInBackground<List<Thread>> {
    int currentThreadsPosition;
    boolean reload;

    public LoadThreadsInBackground() {
        super(SubforumContentActivity.this);
    }

    @Override
    protected void onPreExecute() {
        super.onPreExecute();
        currentThreadsPosition = threadsListView.getFirstVisiblePosition();
    }

    @Override
    protected List<Thread> doInBackground(final Boolean... params) {
        reload = params[0];
        // Load data from network
        return loadThreads();
    }

    @Override
    protected void onPostExecute(List<Thread> retrievedList) {
        super.onPostExecute(retrievedList);

        if (reload) {
            threadList.clear();
        }

        // Appending new data to threadsList
        threadList.addAll(retrievedList);

        if ((retrievedList == null) || (retrievedList.size() < Constants.RETRIEVED_PER_PAGE)) {
            loadMoreThreadsButton.setEnabled(false);
            if (retrievedList != null && retrievedList.size() == 0) {
                currentThreadsPage--; 
            }
        }

        threadsListView.setSelectionFromTop(currentThreadsPosition + 1, 0);
    }

    private List<Thread> loadThreads() {
        ClientResource cr = new ClientResource("IP address");
        cr.setChallengeResponse(getAuthentication());
        ThreadsInSubforumIdByPage tis = cr.wrap(ThreadsInSubforumIdByPage.class);

        String s = tis.getThreads();
        List<Thread> retrievedList = getObjectMapper().readValue(s, List<Thread>.class);
        return retrievedList;
    }
}

Then, have it extend this class:

public abstract class LoadFromServerInBackground<T> extends AsyncTask<Boolean, Void, T> {
    private ProgressDialog mProgressDialog;
    private Activity mInvokerActivity;

    public LoadFromServerInBackground(Activity invokerActivity) {
        mInvokerActivity = invokerActivity;
    }

    @Override
    protected void onPreExecute() {
        // Showing progress dialog before sending http request
        mProgressDialog = new ProgressDialog(mInvokerActivity);
        mProgressDialog.setMessage("Please wait ...");
        mProgressDialog.setIndeterminate(true);
        mProgressDialog.setCancelable(false);
        mProgressDialog.show();
    }

    @Override
    protected void onPostExecute(T unused) {
        // closing progress dialog
        mProgressDialog.dismiss();
    }
}