Android - Deserializing Large Json String from webservice (java.lang.OutOfMemoryError)

First of all, I must mention that I have seen other questions asked by others about this error, but I could not figure it out.

I am making a photo gallery for my application, the photos are inserted into the site database as bytes. I have prepared a web service that returns these pictures as json string. everything was fine until the number of photos from the website database increased (the returned json string got bigger and bigger)

when i try to read this json and put it in a string i get this error:

java.lang.RuntimeException: An error occured while executing doInBackground()
            at android.os.AsyncTask$3.done(AsyncTask.java:300)
            at java.util.concurrent.FutureTask.finishCompletion(FutureTask.java:355)
            at java.util.concurrent.FutureTask.setException(FutureTask.java:222)
            at java.util.concurrent.FutureTask.run(FutureTask.java:242)
            at android.os.AsyncTask$SerialExecutor$1.run(AsyncTask.java:231)
            at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1112)
            at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:587)
            at java.lang.Thread.run(Thread.java:841)
     Caused by: java.lang.OutOfMemoryError
            at java.lang.AbstractStringBuilder.enlargeBuffer(AbstractStringBuilder.java:94)
            at java.lang.AbstractStringBuilder.append0(AbstractStringBuilder.java:124)
            at java.lang.StringBuilder.append(StringBuilder.java:271)
            at org.kxml2.io.KXmlParser.readValue(KXmlParser.java:1345)
            at org.kxml2.io.KXmlParser.next(KXmlParser.java:390)
            at org.kxml2.io.KXmlParser.next(KXmlParser.java:310)
            at org.ksoap2.serialization.SoapSerializationEnvelope.readUnknown(SoapSerializationEnvelope.java:210)
            at org.ksoap2.serialization.SoapSerializationEnvelope.read(SoapSerializationEnvelope.java:366)
            at org.ksoap2.serialization.SoapSerializationEnvelope.readUnknown(SoapSerializationEnvelope.java:233)
            at org.ksoap2.serialization.SoapSerializationEnvelope.read(SoapSerializationEnvelope.java:366)
            at org.ksoap2.serialization.SoapSerializationEnvelope.parseBody(SoapSerializationEnvelope.java:121)
            at org.ksoap2.SoapEnvelope.parse(SoapEnvelope.java:137)
            at org.ksoap2.transport.Transport.parseResponse(Transport.java:63)
            at org.ksoap2.transport.HttpTransportSE.call(HttpTransportSE.java:100)
            at ir.pargansystem.orkidehtalaei.OrkidehWebService.Call(OrkidehWebService.java:42)
            at ir.pargansystem.orkidehtalaei.photogallery.PhotoGalleryViewPager$GetDataTask.doInBackground(PhotoGalleryViewPager.java:226)
            at ir.pargansystem.orkidehtalaei.photogallery.PhotoGalleryViewPager$GetDataTask.doInBackground(PhotoGalleryViewPager.java:218)
            at android.os.AsyncTask$2.call(AsyncTask.java:288)
            at java.util.concurrent.FutureTask.run(FutureTask.java:237)
            at android.os.AsyncTask$SerialExecutor$1.run(AsyncTask.java:231)
            at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1112)
            at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:587)
            at java.lang.Thread.run(Thread.java:841)

      

        
        
        
      

    

how can i read json and deserialize it without running out of memory? any help would be appreciated!

edit 1: web service code:

public String Call(String methodName) {
        String loginUser = "user";
        String loginPassword = "pass";

        SoapObject request = new SoapObject(WSDL_TARGET_NAMESPACE, methodName);
        PropertyInfo pi = new PropertyInfo();
        pi.setName("username");
        pi.setValue(loginUser);
        pi.setType(String.class);
        request.addProperty(pi);
        pi = new PropertyInfo();
        pi.setName("password");
        pi.setValue(loginPassword);
        pi.setType(String.class);
        request.addProperty(pi);

        SoapSerializationEnvelope envelope = new SoapSerializationEnvelope(
                SoapEnvelope.VER11);
        envelope.dotNet = true;

        envelope.setOutputSoapObject(request);

        HttpTransportSE httpTransport = new HttpTransportSE(SOAP_ADDRESS);
        Object response = null;
        try {
            httpTransport.call(SOAP_ACTION + methodName, envelope);
            response = envelope.getResponse();
        } catch (Exception exception) {
            response = exception.toString();
            exception.printStackTrace();
        }
        return response.toString();
    }

      

        
        
        
      

    

and calling the web service here:

private class GetDataTask extends AsyncTask<Void, Void, Void> {
        String jsonString;

        @Override
        protected Void doInBackground(Void... params) {
            // Simulates a background job.
            try {
            OrkidehWebService fws = new OrkidehWebService();
            jsonString = fws.Call("getGalleryData"); // I get the error here

            } catch (Exception e) {
                e.printStackTrace();
            }
            return null;
        }
@Override
    protected void onPostExecute(Void result) {
        try {
            Gson gson = new Gson();
            @SuppressWarnings("serial")
            Type collectionType = new TypeToken<List<PhotoGalleryEntity>>() {
            }.getType();

            lstPhotos = gson.fromJson(jsonString, collectionType);

            Collections.sort(lstPhotos, new Comparator<PhotoGalleryEntity>() {
                public int compare(PhotoGalleryEntity o1, PhotoGalleryEntity o2) {
                    return o1.getPriority() - o2.getPriority();
                }
            });
}