我总是得到这个java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.Context android.content.Context.getApplicationContext()' on a null object reference
我运行我的应用程序时出错。我看到了其他答案，但我不明白如何使用这些答案，我理解上下文存在一些问题，因为没有调用Activity，所以我也尝试传递Activity上下文成功...
这是我的代码：
修改 我已经做了你的答案并且它已经工作了，所以我删除了这个因为我认为它已经解决了。由于你的答案，我更新了代码（上下文作为参与者的距离类＆gt;到MySingleton类......）
public class MainActivity extends AppCompatActivity {
private Button b;
private TextView t;
private LocationManager locationManager;
private LocationListener listener;
@Override
protected void onCreate(@Nullable Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
t = (TextView) findViewById(R.id.textView);
b = (Button) findViewById(R.id.button);
locationManager = (LocationManager) getSystemService(LOCATION_SERVICE);
listener = new LocationListener() {
@Override
public void onLocationChanged(Location location) {
Log.d("Co", "onLocationChanged");
Distance distance=new Distance();
t.setText("\n " + location.getLongitude() + " " + location.getLatitude()+"\n "+distance.getDistance(location));
Location l2=new Location("");
//float distance=location.distanceTo(l2);
Log.d("Co", String.valueOf(distance));
}
@Override
public void onStatusChanged(String s, int i, Bundle bundle) {
}
@Override
public void onProviderEnabled(String s) {
}
@Override
public void onProviderDisabled(String s) {
Intent i = new Intent(Settings.ACTION_LOCATION_SOURCE_SETTINGS);
startActivity(i);
}
};
configure_button();
}
@Override
public void onRequestPermissionsResult(int requestCode, @NonNull String[] permissions, @NonNull int[] grantResults) {
switch (requestCode){
case 10: {
configure_button();
Log.d("Co", "premmsioenChecks");
}
break;
default:
break;
}
}
public void configure_button(){
// first check for permissions
if (ActivityCompat.checkSelfPermission(this, Manifest.permission.ACCESS_FINE_LOCATION) != PackageManager.PERMISSION_GRANTED && ActivityCompat.checkSelfPermission(this, Manifest.permission.ACCESS_COARSE_LOCATION) != PackageManager.PERMISSION_GRANTED) {
if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.M) {
requestPermissions(new String[]{Manifest.permission.ACCESS_COARSE_LOCATION,Manifest.permission.ACCESS_FINE_LOCATION,Manifest.permission.INTERNET}
,10);
Log.d("Co", "premmsioen");
}
return;
}
// this code won't execute IF permissions are not allowed, because in the line above there is return statement.
b.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View view) {
//noinspection MissingPermission
locationManager.requestLocationUpdates(LocationManager.GPS_PROVIDER, 0, 0, listener);
locationManager.requestLocationUpdates(LocationManager.NETWORK_PROVIDER, 0, 0, listener);
Log.d("Co", "onClick");
Location l2=new Location("");
}
});
}
}
公共班级距离{
static String url = "";
static String result="";
public String getDistance(Location location,Context context) {
Log.d("app","getDistance");
String origins=location.getLatitude()+","+location.getLongitude();
url="https://maps.googleapis.com/maps/api/distancematrix/json?&origins="+origins+"&destinations="+destinations;
Log.d("URL",url);
JsonObjectRequest jsObjRequest = new JsonObjectRequest
(Request.Method.GET, url, null, new Response.Listener<JSONObject>() {
@Override
public void onResponse(JSONObject response) {
Log.d("app","onRespone");
String distance=getText(response);
if (distance.contains("Error")) {
result=distance;
return;
}
else {
if (distance.contains("km")) {
int meters = Integer.parseInt(distance.replaceAll("[\\D]", "")) * 1000;
result= "The distance is " + meters + " meters";
return;
}
Log.d("7", String.valueOf(Integer.parseInt(distance.replaceAll("[\\D]", ""))));
result= "The distance is " + distance;
}
}
}, new Response.ErrorListener() {
@Override
public void onErrorResponse(VolleyError error) {
Log.d("app","Error Respone");
}
}
);
MySingleton.getInstance(context).addToRequestque(jsObjRequest);
return result;
}
public String getText(JSONObject response)
{
String text="";
try {
JSONArray rows = response.getJSONArray("rows");
Log.d("1", rows.toString());
if(rows.toString().contains("[]")) return text="Error";
JSONObject elements = rows.getJSONObject(0);
Log.d("2", elements.toString());
rows = elements.getJSONArray("elements");
Log.d("3", rows.toString());
elements = rows.getJSONObject(0);
Log.d("4", elements.toString());
Log.d("Error1","Error is @"+elements.getString("status"));
if( elements.getString("status").contains("OK"))
{
JSONObject d = elements.getJSONObject("distance");
Log.d("5", d.toString());
text = d.getString("text");
}
else {
Log.d("Error2", elements.getString("status"));
text = "Error-" + elements.getString("status");
}
}
catch (JSONException e)
{
text="Some error ocuerd";
}
return text;
}
}
public class MySingleton {
private static MySingleton mInstance;
private RequestQueue requestQueue;
public static Context mCtx;
private MySingleton(Context context)
{
mCtx=context;
requestQueue= getRequestQueue();
}
public RequestQueue getRequestQueue() {
if(requestQueue==null) {
requestQueue = Volley.newRequestQueue(mCtx.getApplicationContext());
}
return requestQueue;
}
public static synchronized MySingleton getInstance(Context context)
{
if(mInstance==null)
{
mInstance=new MySingleton(context);
}
return mInstance;
}
public <T> void addToRequestque(Request<T> request)
{
requestQueue.add(request);
}
}
这是logcat
06-24 16:19:26.771 25300-25300/com.example.elicahi.gateor D/Co: onClick
06-24 16:19:27.556 25300-25300/com.example.elicahi.gateor D/Co: onLocationChanged
06-24 16:19:27.579 25300-25300/com.example.elicahi.gateor D/AndroidRuntime: Shutting down VM
06-24 16:19:27.579 25300-25300/com.example.elicahi.gateor E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.elicahi.gateor, PID: 25300
java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.Context android.content.Context.getApplicationContext()' on a null object reference
at android.content.ContextWrapper.getApplicationContext(ContextWrapper.java:107)
at com.example.elicahi.gateor.MySingleton.getRequestQueue(MySingleton.java:24)
at com.example.elicahi.gateor.MySingleton.<init>(MySingleton.java:19)
at com.example.elicahi.gateor.MySingleton.getInstance(MySingleton.java:32)
at com.example.elicahi.gateor.Distance.getDistance(Distance.java:79)
at com.example.elicahi.gateor.MainActivity$1.onLocationChanged(MainActivity.java:54)
at android.location.LocationManager$ListenerTransport._handleMessage(LocationManager.java:285)
at android.location.LocationManager$ListenerTransport.-wrap0(LocationManager.java)
at android.location.LocationManager$ListenerTransport$1.handleMessage(LocationManager.java:230)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:148)
at android.app.ActivityThread.main(ActivityThread.java:5417)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)
答案 0 :(得分：11)
public class Distance extends Application
不要随意扩展类，只是因为您认为它会清除编译器错误。您在此处没有有效的Application子类，并且您没有正确使用它。
第1步：从extends Application移除Distance。
步骤2：让getDistance()将Context作为参数，并将其与MySingleton
第3步：将MainActivity getApplicationContext()传递给getDistance() Context
答案 1 :(得分：0)
这实际上不是答案，而只是一个想法。
我遇到了同样的问题（这就是我遇到的问题的方式），而我的问题实际上非常简单。在设置Singleton中的私有上下文之前，我试图设置RequestQueue。
private MySingleton(Context context) {
ctx = context; // queue = getRequestQueue(); <--- queue first but it needs context.
queue = getRequestQueue(); // ctx = context; <--- ctx being set here. queue can't find context.
}
public synchronized static MySingleton getInstance(Context context) {
if (instance == null) {
instance = new MySingleton(context);
}
return instance;
}
private RequestQueue getRequestQueue() {
if (queue == null) {
queue = Volley.newRequestQueue( ctx.getApplicationContext() ); // <--- Here is the crash;
}
return queue;
}
此外，在发现此问题之前，我确保上下文作为参数传递时不为null。这意味着问题必须出在MySingleton代码中。
// This code is run before MySingleton.getInstance() wherever it is in your app;
Context testParameter = getBaseContext();
Log.d("DEBUG_CONTEXT", testParameter.toString() );
因此，我只是在分享我的问题，希望它可以为您带来一些想法。祝你好运。
答案 2 :(得分：0)
对于使用flutter的Firebase Cloud Messaging（FCM）时遇到此错误的人，pubspec.yaml中可能有一个引起问题的软件包。
就我而言，keyboard_visibility是导致与FCM发生冲突的人。我不得不将其替换为flutter_keyboard_visibility。
答案 3 :(得分：0)
私有上下文mContext;
private Activity mActivity;
@Override
public void onViewCreated(@NonNull View view, @Nullable Bundle savedInstanceState) {
super.onViewCreated(view, savedInstanceState);
if (getActivity() == null) {
return;
}
else
{
mActivity = getActivity();
}
}
答案 4 :(得分：0)
对于 kotlin，你可以使用这个：
requireNotNull(this.activity).application
答案 5 :(得分：-2)
就像之前的回答（CommonsWare）一样，并把它放在：
Context context; // before onCreate in MainActivity
context = getApplicationContext(); // in onCreate in MainActivity