我为我的应用制作了启动画面。在启动屏幕的加载时，我想检查用户是否登录。
如果用户登录，那么我想显示他的主要活动，如果不登录，那么我想将他重定向到LoginActivitiy。
我使用firebase作为我的后端，我正在检查Asynctask上的用户。 This thing i have seen at here.
我在onStart（）中添加了 FirebaseAuth侦听器。
我已经理解为什么它会给我错误但我不明白在哪里添加它以便我的应用运行正常。
如果有人可以帮助我，或者给出一些更好的建议来做同样的事情会有所帮助 谢谢你...
public class MainActivity extends AppCompatActivity {
private FirebaseDatabase mDataBase;
private DatabaseReference mUserDatabase;
protected FirebaseAuth mAuth;
private FirebaseAuth.AuthStateListener mAuthStateListener;
private String mCurrentUser;
private static int SPLASH_TIME_OUT = 2000;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.welcome_screen);
mDataBase = FirebaseDatabase.getInstance();
mUserDatabase = mDataBase.getReference("users");
mUserDatabase.keepSynced(true);
startProcessing();
}
private void startProcessing() {
new userCheckOperation().execute();
}
protected void userIsLogIn() {
mAuth = FirebaseAuth.getInstance();
mAuthStateListener = new FirebaseAuth.AuthStateListener() {
@Override
public void onAuthStateChanged(@NonNull FirebaseAuth firebaseAuth) {
if (firebaseAuth.getCurrentUser() == null) {
Intent loginIntent = new Intent(MainActivity.this, LoginActivity.class);
//user won't go back
loginIntent.setFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP);
startActivity(loginIntent);
finish();
}
}
};
}
@Override
protected void onStart() {
super.onStart();
mAuth.addAuthStateListener(mAuthStateListener);
}
private void checkUserExists() {
if (mAuth.getCurrentUser() != null) {
mCurrentUser = mAuth.getCurrentUser().getUid();
mUserDatabase.addValueEventListener(new ValueEventListener() {
@Override
public void onDataChange(DataSnapshot dataSnapshot) {
if (!dataSnapshot.hasChild(mCurrentUser)) {
Intent setupIntent = new Intent(MainActivity.this, SetupActivity.class);
//user won't go back
setupIntent.setFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP);
startActivity(setupIntent);
finish();
} else {
if (mCurrentUser.equals("nKnlkU2fWGeLlP2QDc8CLz21Fet1")) {
Intent admin_mainIntent = new Intent(MainActivity.this, AdminMainActivity.class);
//user won't go back
admin_mainIntent.setFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP);
startActivity(admin_mainIntent);
finish();
} else {
Intent user_mainIntent = new Intent(MainActivity.this, UserMainActivity.class);
//user won't go back
user_mainIntent.setFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP);
startActivity(user_mainIntent);
finish();
}
}
}
@Override
public void onCancelled(DatabaseError databaseError) {
}
});
}
}
@Override
public void onStop() {
super.onStop();
if (mAuthStateListener != null) {
mAuth.removeAuthStateListener(mAuthStateListener);
}
}
private class userCheckOperation extends AsyncTask<Void,Void,Void>{
@Override
protected Void doInBackground(Void... voids) {
for (int i = 0; i < 5; i++) {
try {
Thread.sleep(1000);
} catch (InterruptedException e) {
Thread.interrupted();
}
}
return null;
}
@Override
protected void onPostExecute(Void aVoid) {
super.onPostExecute(aVoid);
userIsLogIn();
checkUserExists();
}
}
}
E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.lenovo.jdstudio, PID: 17986
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.lenovo.jdstudio/com.example.lenovo.jdstudio.MainActivity}: java.lang.NullPointerException
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2377)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2429)
at android.app.ActivityThread.access$800(ActivityThread.java:151)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1342)
at android.os.Handler.dispatchMessage(Handler.java:110)
at android.os.Looper.loop(Looper.java:193)
at android.app.ActivityThread.main(ActivityThread.java:5333)
at java.lang.reflect.Method.invokeNative(Native Method)
at java.lang.reflect.Method.invoke(Method.java:515)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:828)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:644)
at dalvik.system.NativeStart.main(Native Method)
Caused by: java.lang.NullPointerException
at com.example.lenovo.jdstudio.MainActivity.onStart(MainActivity.java:94)
at android.app.Instrumentation.callActivityOnStart(Instrumentation.java:1174)
at android.app.Activity.performStart(Activity.java:5353)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2340)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2429)
at android.app.ActivityThread.access$800(ActivityThread.java:151)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1342)
at android.os.Handler.dispatchMessage(Handler.java:110)
at android.os.Looper.loop(Looper.java:193)
at android.app.ActivityThread.main(ActivityThread.java:5333)
at java.lang.reflect.Method.invokeNative(Native Method)
at java.lang.reflect.Method.invoke(Method.java:515)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:828)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:644)
at dalvik.system.NativeStart.main(Native Method)
答案 0 :(得分：0)
简单使用这个，我个人推荐这个方法：
//Get Firebase auth instance
FirebaseAuth auth = FirebaseAuth.getInstance();
if (auth.getCurrentUser() != null) {
// User is logged in - send it to home screen \\
} else {
//User is not logged in - send it to login screen \\
}
您可以将此代码放在onCreate（）方法中，并且可以在用户登录时将用户发送到主活动，否则将用户发送到登录屏幕
对于你的启动画面：
public class SplashScreenActivity extends AppCompatActivity {
private static final int SPLASH_TIME_OUT = 2000;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_splash_screen);
new Handler().postDelayed(new Runnable() {
/*
* Showing splash screen with a timer. This will be useful when you
* want to show case your app logo / company
*/
@Override
public void run() {
//Get Firebase auth instance
FirebaseAuth auth = FirebaseAuth.getInstance();
if (auth.getCurrentUser() != null) {
// User is logged in - send it to home screen \\
} else {
//User is not logged in - send it to login screen \\
}
}
}, SPLASH_TIME_OUT);
} // End On-Create \\
} // End Class \\