Question

我在调用包含json数据的第二个活动（登录后）的意图时试图增加额外费用。在LoginActivity的BackgroundTask类中正确调用json数据（我知道，因为显示了我带有json字符串的Log），但是当我执行startChildActivityIntent.putExtra时，它表示json_string变量为null（这会导致TaskActivity出错。我希望有人能告诉我出了什么问题？我留下了一些评论，确切地说出了问题。这是代码：

LoginActivity：

package com.example.myApp;

some imports

public class LoginActivity extends AppCompatActivity {

    GoogleSignInOptions gso;
    GoogleSignInClient mGoogleSignInClient;
    SignInButton signInButton;
    private int RC_SIGN_IN = 6;
    String json_string;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_login);

        gso = new GoogleSignInOptions.Builder(GoogleSignInOptions.DEFAULT_SIGN_IN)
            .requestIdToken(getString(R.string.server_client_id))
            .requestEmail()
            .build();

        mGoogleSignInClient = GoogleSignIn.getClient(this, gso);
    }

    @Override
    protected void onStart() {
        super.onStart();

        GoogleSignInAccount account = GoogleSignIn.getLastSignedInAccount(this);
    updateUI(account);
    }

    private void updateUI(GoogleSignInAccount account) {
        if(account == null){
            signInButton = findViewById(R.id.sign_in_button);
            signInButton.setSize(SignInButton.SIZE_WIDE);

            signInButton.setOnClickListener(new View.OnClickListener() {
                @Override
                public void onClick(View view) {
                    signIn();
                }
            });

            signInButton.setVisibility(View.VISIBLE);
        } else {

            //here the json_string is correct
            new BackgroundTask().execute();

            Context context = LoginActivity.this;

            Class destinationActivity = TaskActivity.class;

            Intent startChildActivityIntent = new Intent(context, destinationActivity);

            //here the json_string is null again

            startChildActivityIntent.putExtra("json_data", json_string);

            startActivity(startChildActivityIntent);
        }
    }

    private void signIn() {
        Intent signInIntent = mGoogleSignInClient.getSignInIntent();
        startActivityForResult(signInIntent, RC_SIGN_IN);
    }

    @Override
    public void onActivityResult(int requestCode, int resultCode, Intent data) {
        super.onActivityResult(requestCode, resultCode, data);

        if (requestCode == RC_SIGN_IN) {
            Task<GoogleSignInAccount> task = GoogleSignIn.getSignedInAccountFromIntent(data);
            handleSignInResult(task);
        }
    }

    private void handleSignInResult(Task<GoogleSignInAccount> completedTask) {
        try {
            GoogleSignInAccount account = completedTask.getResult(ApiException.class);
            String idToken = account.getIdToken();

            updateUI(account);
        } catch (ApiException e) {
            updateUI(null);
        }
    }

    class BackgroundTask extends AsyncTask<Void, Void, String> {
        String json_url;
        String JSON_STRING;

        @Override
        protected void onPreExecute() {
            json_url = "https://my_url/json_get_data.php";
        }

        @Override
        protected void onProgressUpdate(Void... values) {
            super.onProgressUpdate(values);
        }

        @Override
        protected void onPostExecute(String result) {
            json_string = result;
            //this log is displayed with the json_string filled in correctly
            Log.i("onpostexecute","json: " + json_string);
        }

        @Override
        protected String doInBackground(Void... voids) {
            try {
                URL url = new URL(json_url);
                HttpURLConnection httpURLConnection = (HttpURLConnection) url.openConnection();
                InputStream inputStream = httpURLConnection.getInputStream();
                BufferedReader bufferedReader = new BufferedReader(new InputStreamReader(inputStream));
                StringBuilder stringBuilder = new StringBuilder();
                while ((JSON_STRING = bufferedReader.readLine()) != null){
                    stringBuilder.append(JSON_STRING + "\n");
                }

                bufferedReader.close();
                inputStream.close();
                httpURLConnection.disconnect();

                return stringBuilder.toString().trim();

            } catch (MalformedURLException e) {
                e.printStackTrace();
            } catch (IOException e) {
                e.printStackTrace();
            }

            return null;
        }
    }
}

TaskActivity：

package com.example.myApp;

some imports

public class TaskActivity extends AppCompatActivity {

    private static final String URL_DATA = "https://my_url/json_get_data.php";
    private RecyclerView recyclerView;
    private RecyclerView.Adapter scorpioAdapter;
    private List<ListItem> listItems;
    String json_string;
    JSONObject jsonObject;
    JSONArray jsonArray;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_task);

        recyclerView = findViewById(R.id.rv);
        recyclerView.setHasFixedSize(true);
        recyclerView.setLayoutManager(new LinearLayoutManager(this));

        listItems = new ArrayList<>();

        loadRecyclerViewData();
   }

    private void loadRecyclerViewData() {
        final ProgressDialog progressDialog = new ProgressDialog(this);
        progressDialog.setMessage("Loading data...");
        progressDialog.show();

        StringRequest stringRequest = new StringRequest(Request.Method.GET,
            URL_DATA,
            new Response.Listener<String>() {
                @Override
                public void onResponse(String s) {
                    progressDialog.dismiss();

                    json_string = getIntent().getExtras().getString("json_data");

                    try {
                        jsonObject = new JSONObject(json_string);
                        jsonArray = jsonObject.getJSONArray("server_response");
                        int count = 0;
                        int id;
                        String name, and, some, other, fields;

                        while (count < jsonObject.length()){
                            JSONObject o = jsonArray.getJSONObject(count);
                            id = o.getInt("id");
                            name = o.getString("name");
                            and= o.getString("and");
                            some= o.getInt("some");
                            other= o.getString("other");
                            fields= o.getString("fields");
                            ListItem item = new ListItem(id, name, and, some, other, fields);

                            listItems.add(item);
                        }

                        myAdapter = new myAdapter (listItems, getApplicationContext());
                        recyclerView.setAdapter(myAdapter);

                    } catch (JSONException e) {
                        e.printStackTrace();
                    }
                }
            },
            new Response.ErrorListener() {
                @Override
                public void onErrorResponse(VolleyError volleyError) {
                    Toast.makeText(getApplicationContext(), volleyError.getMessage(), Toast.LENGTH_LONG).show();
                }
            }
        );

        RequestQueue requestQueue = Volley.newRequestQueue(this);
        requestQueue.add(stringRequest);
    }
}

Answer 1

这是因为您正在调用后台任务，而不等待结果传递json_string作为额外内容。因此，json_string将为null，因为后台任务尚未完成执行。为避免这种情况，请尝试从TaskActivity

调用onPostExecute

@Override
    protected void onPostExecute(String result) {
        json_string = result;
        //this log is displayed with the json_string filled in correctly
        Log.i("onpostexecute","json: " + json_string);

         Intent startChildActivityIntent = new Intent(LoginActivity.this, TaskActivity.class);
         startChildActivityIntent.putExtra("json_data", json_string);
         startActivity(startChildActivityIntent);
    }

并删除已存在的意向调用

json extra在其他意图上为空

1 个答案: