活动在通过intent第二次启动时崩溃

Question

我正在使用firebase设计一个简单的存储/检索数据的android应用程序。

Activity1是一个登录活动，当用户成功登录时，它会直接进入Activity2。第一次登录没有问题，一切都在正确的位置。我在Activity2中有一个导航抽屉，用于切换片段，在该导航抽屉中，用户可以注销并返回登录活动。以下是Activity2的摘录，它将用户带回登录活动

case R.id.nav_logout:
account_key = "";
Toast.makeText(getApplicationContext(), "Logged Out", Toast.LENGTH_LONG).show();
Intent intent = new Intent(getApplicationContext(), LoginActivity.class);
startActivity(intent);
finish();
break;

当我点击注销按钮时，它会将我带到Activity1。现在的问题是，当我再次尝试登录时，应用程序会崩溃，并出现以下错误：

E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.adzu.bfarsystem, PID: 2143
java.lang.NullPointerException: Attempt to invoke interface method 'android.view.MenuItem android.view.Menu.add(int, int, int, java.lang.CharSequence)' on a null object reference
at android.view.MenuInflater$MenuState.addItem(MenuInflater.java:540)
at android.view.MenuInflater.parseMenu(MenuInflater.java:193)
at android.view.MenuInflater.inflate(MenuInflater.java:114)
at androidx.appcompat.view.SupportMenuInflater.inflate(SupportMenuInflater.java:120)
at com.adzu.bfarmobile.activities.MainActivity.createSearchView(MainActivity.java:153)
at com.adzu.bfarmobile.activities.MainActivity$1.dataRetrieved(MainActivity.java:109)
at com.adzu.bfarmobile.entities.DatabaseReader$3.onChildAdded(DatabaseReader.java:64)
at com.google.firebase.database.core.ChildEventRegistration.fireEvent(ChildEventRegistration.java:79)
at com.google.firebase.database.core.view.DataEvent.fire(DataEvent.java:63)
at com.google.firebase.database.core.view.EventRaiser$1.run(EventRaiser.java:55)
at android.os.Handler.handleCallback(Handler.java:873)
at android.os.Handler.dispatchMessage(Handler.java:99)
at android.os.Looper.loop(Looper.java:193)
at android.app.ActivityThread.main(ActivityThread.java:6669)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:493)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:858)

以下是触发错误的行：

LoginActivity.java线路95-102

if (activated) {
Toast.makeText(getApplicationContext(), "Login Success!", Toast.LENGTH_LONG).show();
Intent newIntent = new Intent(getApplicationContext(), MainActivity.class);
newIntent.putExtra("account_key", dataSnapshot.getKey());
startActivity(newIntent);
finish();
}

MainActivity.java线路95-127

Intent intent = getIntent();
account_key = intent.getStringExtra("account_key");
DatabaseReference ref = FirebaseDatabase.getInstance().getReference("account");
DatabaseReader.readDataByKey(account_key, ref, new OnGetDataListener() {
@Override
public void dataRetrieved(DataSnapshot dataSnapshot) {
Account user = new Account((Map<String, Object>) dataSnapshot.getValue());
fragment1 = new ListFragment();
fragment1.setUser(user);
createSearchView();
showSearchView();
getSupportFragmentManager().beginTransaction().replace(R.id.fragment_container, fragment1).commit();
navigationView.setCheckedItem(R.id.nav_operatorlist);
TextView acct_name = findViewById(R.id.acct_name);
acct_name.setText("Logged in as: " + user.getUsername());
}
@Override
public void dataExists(DataSnapshot dataSnapshot) {
}
@Override
public void onStart() {
}
@Override
public void onFailure() {
}
});

MainActivity.java线路141-153

@Override
public boolean onCreateOptionsMenu(Menu menu) {
this.menu = menu;
return super.onCreateOptionsMenu(menu);
}
public void createSearchView(){
MenuInflater inflater = getMenuInflater();
inflater.inflate(R.menu.search_menu, menu);
MenuItem searchItem = menu.findItem(R.id.action_search);
searchView = findViewById(R.id.search_view);
searchView.setMenuItem(searchItem);
expandableLayout = findViewById(R.id.expandable_layout);

DatabaseReader.java第64-89行

public static void readDataByKey(String account_key, DatabaseReference ref, final OnGetDataListener listener) {
listener.onStart();
Query query = ref.orderByKey().equalTo(account_key);
query.addChildEventListener(new ChildEventListener() {
@Override
public void onChildAdded(@NonNull DataSnapshot dataSnapshot, @Nullable String s) {
listener.dataRetrieved(dataSnapshot);
}
@Override
public void onChildChanged(@NonNull DataSnapshot dataSnapshot, @Nullable String s) {
}
@Override
public void onChildRemoved(@NonNull DataSnapshot dataSnapshot) {
}
@Override
public void onChildMoved(@NonNull DataSnapshot dataSnapshot, @Nullable String s) {
}
@Override
public void onCancelled(@NonNull DatabaseError databaseError) {
listener.onFailure();
}
});
}

如有任何帮助，我们将不胜感激。谢谢

Answer 1

您可能会遇到竞争条件。在系统调用onCreateOptionsMenu之前，您可能已经调用了createSearchView，因此菜单为null。

尝试让onCreateOptionsMenu在第行后调用createSearchView

this.menu = menu;