java - Android Volley 同步请求不起作用

Question

我正在尝试使用 RequestFuture 向服务器发出一个同步请求，但它不起作用。使用异步完成时相同的请求工作正常。

这是我的代码:

 public void fetchModules(){
   JSONObject response = null;
    RequestQueue requestQueue = Volley.newRequestQueue(getContext());

    RequestFuture<JSONObject> future = RequestFuture.newFuture();
    JsonObjectRequest request = new JsonObjectRequest(Url.ALL_MODULES_URL,null,future,future);
    requestQueue.add(request);


    try {
         response = future.get(3, TimeUnit.SECONDS); // Blocks for at most 10 seconds.
    } catch (InterruptedException e) {
             Log.d(TAG,"interupted");
     } catch (ExecutionException e) {
        Log.d(TAG,"execution");
    } catch (TimeoutException e) {
        e.printStackTrace();
    }

    Log.d(TAG,response.toString());
}

我得到一个空指针异常:

java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String org.json.JSONObject.toString()' on a null object reference at com.maths.app.AllModules.fetchModules(AllModules.java:85) at com.maths.app.AllModules.onCreateView(AllModules.java:51) at android.support.v4.app.Fragment.performCreateView(Fragment.java:2080) at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1108) at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1290) at android.support.v4.app.BackStackRecord.run(BackStackRecord.java:801) at android.support.v4.app.FragmentManagerImpl.execPendingActions(FragmentManager.java:1677) at android.support.v4.app.FragmentManagerImpl$1.run(FragmentManager.java:536) at android.os.Handler.handleCallback(Handler.java:746) at android.os.Handler.dispatchMessage(Handler.java:95) at android.os.Looper.loop(Looper.java:148) at android.app.ActivityThread.main(ActivityThread.java:5443) at java.lang.reflect.Method.invoke(Native Method) at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:728) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:618)

它返回一个空响应。我该如何解决这个问题？

Answer 1

tl；博士；你被 try-catch 骗了

解释:因为 RequestFuture.get() 可能在 UI 线程上运行，您实际上在幕后得到了 java.util.concurrent.TimeoutException。 这是在主线程上执行调用时的默认行为。

try catch 阻止了应用程序崩溃，尽管如此，响应仍然是空引用，当您尝试记录结果时，它会导致应用程序崩溃。如果您评论以下行，您将看到该应用程序不再(在那里)崩溃。

Log.d(TAG,response.toString());

修复:在另一个线程上进行 RequestFuture 网络调用!

一种方法:

    public class TestVolley {

    private String TAG = "SO_TEST";
    private String url = "http://pokeapi.co/api/v2/pokemon-form/1/";


    public JSONObject fetchModules(Context ctx){
        JSONObject response = null;
        RequestQueue requestQueue = Volley.newRequestQueue(ctx);


        RequestFuture<JSONObject> future = RequestFuture.newFuture();
        JsonObjectRequest request = new JsonObjectRequest(url,null,future,future);
        requestQueue.add(request);


        try {
            response = future.get(3, TimeUnit.SECONDS); // Blocks for at most 10 seconds.
        } catch (InterruptedException e) {
            Log.d(TAG,"interrupted");
        } catch (ExecutionException e) {
            Log.d(TAG,"execution");
        } catch (TimeoutException e) {
            e.printStackTrace();
        }

        Log.d(TAG,response.toString());

        return response;
    }
}

将进行网络调用的AsyncTask:

public class MyVolleyAsyncTask extends AsyncTask<String,String, JSONObject> {

    private Context ctx;

    public MyVolleyAsyncTask(Context hostContext)
    {
        ctx = hostContext;
    }

    @Override
    protected JSONObject doInBackground(String... params) {

        // Method runs on a separate thread, make all the network calls you need
        TestVolley tester = new TestVolley();

        return tester.fetchModules(ctx);
    }


    @Override
    protected void onPostExecute(JSONObject result)
    {
       // runs on the UI thread
       // do something with the result
    }
}

主要 Activity :

public class MainActivity extends AppCompatActivity {

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

        // this is your old code which will crash the app
        //TestVolley tester = new TestVolley();
        //tester.fetchModules(this);

        // Works!
        new MyVolleyAsyncTask(this).execute();
    }
}

结果:

com.so.henriquems.testvolleyfuture D/SO_TEST: {"id":1,"pokemon":{"url":"http:\/\/pokeapi.co\/api\/v2\/pokemon\/1\/","name":"bulbasaur"},[...]

希望对你有帮助

干杯!