Fire Asynctask class exception

Question

The class below is responsible for obtaining data from a WCF Rest service:

public class MyAsyncTaskGeneric<T> extends AsyncTask<String, Void, T>{

    private final Class<T> typeGeneric;         

    public MyAsyncTaskGeneric(Class<T> typeGeneric) {
        this.typeGeneric = typeGeneric;     
    }

    @Override
    protected T doInBackground(String... params) {      
        T result = null;        
        try {               
            RestTemplate restTemplate = new RestTemplate();
            restTemplate.getMessageConverters().add(new GsonHttpMessageConverter());
            result = restTemplate.getForObject(params[0], typeGeneric);
        } catch (Exception e) {
            Log.i("Teste", e.getMessage());         
        } 
        return result;
    }
}

It is consumed by business class:

public class CidadeBll {    

    public CidadeBll() {

    }

    public Cidade[] GetCidades(String url) throws Exception {
        Cidade[] result = null;
        try {
            MyAsyncTaskGeneric<Cidade[]> myAsync = new MyAsyncTaskGeneric<Cidade[]>(Cidade[].class);
            result = myAsync.execute(url + "/cidades").get();
        } catch (Exception e) {
            throw e;
        }
        return result;
    }
}

Which in turn is consumed by an Activity:

btnGetCidades.setOnClickListener(new View.OnClickListener() {           
        @Override
        public void onClick(View v) {
            try {
                CidadeBll cidadeBll = new CidadeBll();
                Cidade[] cidades = cidadeBll.GetCidades(url2);

                if (cidades.length > 0){
                    Toast.makeText(getApplicationContext(), "Qtde de Cidades: " + cidades.length, Toast.LENGTH_LONG).show();
                }

            } catch (Exception e) {
                Toast.makeText(getApplicationContext(), "Erro:" + e.getMessage(), Toast.LENGTH_LONG).show();
            }
        }
    });

Considering possible exceptions triggered by WCF Rest, I wish the Asynctask class could trigger the exception for the business class, and the business class for Activity.

That’s possible?

Answer 1

It is possible yes. But before, I wanted to resurface that you are making a design mistake: a AsyncTask is intrinsically linked to the UI, so much so that the methods onPreExecute(), onProgressUpdate(Progress...) and onPostExecute(Result) have access to UI Thread (see The 4 Steps). So you don’t have to create Toasts in the Activity, but in the AsyncTask. Therefore, in the Activity you instance a AsyncTask and in the AsyncTask you instance an object of the business class to perform the work in background.

Returning to your question: you can add an attribute of the type Exception at its instance of AsyncTask and check the result in the method onPostExecute(Result). Here’s an example code:

Activity with Asynctask:

    btnGetCidades.setOnClickListener(new View.OnClickListener() {
        
        @Override
        public void onClick(View v) {
            
            new AsyncTask<String, Void, Cidade[]>() {
                
                // atributo que guarda excecao lancada
                private Exception e;
                
                @Override
                protected Cidade[] doInBackground(String... params) {
                    // em background
                    Cidade[] cidades = null;
                    
                    try {
                        CidadeBll cidadeBll = new CidadeBll();
                        cidades = cidadeBll.GetCidades(params + "/cidades");
                    } catch (Exception e) {
                        this.e = e;
                        return null;
                    }
                    
                    return cidades;
                }
                
                @Override
                protected void onPostExecute(Cidade[] cidades) {
                    // acesso a UI thread
                    if (cidades == null && e != null) {
                        // erro
                        Toast.makeText(
                                getApplicationContext(),
                                "Erro:" + e.getMessage(),
                                Toast.LENGTH_LONG).show();
                    } else {
                        // ok ..
                        if (cidades.length > 0){
                            Toast.makeText(
                                    getApplicationContext(),
                                    "Qtde de Cidades: " + cidades.length,
                                    Toast.LENGTH_LONG).show();
                        }
                    }
                }
            }.execute(url2);
        }
    });

Cidadebll.java (business class):

    public class CidadeBll {

        public CidadeBll() {
    
        }

        public Cidade[] GetCidades(String ... params) throws Exception {
            Cidade[] result = null;
    
            try {
                RestTemplate restTemplate = new RestTemplate();
                restTemplate.getMessageConverters().add(new GsonHttpMessageConverter());
                result = restTemplate.getForObject(params[0], Cidade[].class);
            } catch (Exception e) {
                Log.i("Teste", e.getMessage());
                
                throw e;
            }
            
            return result;
        }
    }

If the code of Activity has become large/confused can be found in another class, but keep the logic of modifying UI from the methods of AsyncTask who have access to the Thread UI.

I hope I’ve helped and for more information on how it works AsyncTask see the documentation.