Question

我正在尝试使用简单的登录构建一个Android应用程序并从本教程中注销应用程序 http://www.androidhive.info/2012/08/android-session-management-using-shared-preferences/

BUt log out会导致应用程序崩溃 Logcat报告

09-26 04:53:03.204: E/AndroidRuntime(1926): FATAL EXCEPTION: main
09-26 04:53:03.204: E/AndroidRuntime(1926): Process: com.learn2crack.tab, PID: 1926
09-26 04:53:03.204: E/AndroidRuntime(1926): java.lang.NullPointerException
09-26 04:53:03.204: E/AndroidRuntime(1926):     at com.learn2crack.tab.Attendance$1.onClick(Attendance.java:53)
09-26 04:53:03.204: E/AndroidRuntime(1926):     at android.view.View.performClick(View.java:4438)
09-26 04:53:03.204: E/AndroidRuntime(1926):     at android.view.View$PerformClick.run(View.java:18422)
09-26 04:53:03.204: E/AndroidRuntime(1926):     at android.os.Handler.handleCallback(Handler.java:733)
09-26 04:53:03.204: E/AndroidRuntime(1926):     at android.os.Handler.dispatchMessage(Handler.java:95)
09-26 04:53:03.204: E/AndroidRuntime(1926):     at android.os.Looper.loop(Looper.java:136)
09-26 04:53:03.204: E/AndroidRuntime(1926):     at android.app.ActivityThread.main(ActivityThread.java:5017)
09-26 04:53:03.204: E/AndroidRuntime(1926):     at java.lang.reflect.Method.invokeNative(Native Method)
09-26 04:53:03.204: E/AndroidRuntime(1926):     at java.lang.reflect.Method.invoke(Method.java:515)
09-26 04:53:03.204: E/AndroidRuntime(1926):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779)
09-26 04:53:03.204: E/AndroidRuntime(1926):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595)
09-26 04:53:03.204: E/AndroidRuntime(1926):     at dalvik.system.NativeStart.main(Native Method)

注销功能

SessionManager session;
    AlertDialogManager alert = new AlertDialogManager();
public void logoutUser(){



   // Clearing all data from Shared Preferences
    editor.clear();
    editor.commit();

    // After logout redirect user to Loing Activity
    Intent i = new Intent(_context, Login.class);
    // Closing all the Activities
    i.addFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP);

    // Add new Flag to start new Activity
    i.setFlags(Intent.FLAG_ACTIVITY_NEW_TASK);

    // Staring Login Activity
    _context.startActivity(i);
}

调用函数的类

public class Attendance extends Fragment{
    SessionManager session;
    AlertDialogManager alert = new AlertDialogManager();
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
            Bundle savedInstanceState) 
{


        View attendance = inflater.inflate(R.layout.attedance, container, false);





        final ImageButton logoutbtn = (ImageButton)attendance.findViewById(R.id.logoutbtn);




        logoutbtn.setOnClickListener(new OnClickListener(){
            @Override
            public void onClick(View v) {



                    // Clear the session data
                    // This will clear all session data and 
                    // redirect user to LoginActivity
                    session.logoutUser();
                }
            });
        return attendance;}


}

Answer 1

您还没有初步化您的SessionManager类，这就是您获得NPE的原因：

public class Attendance extends Fragment{
    SessionManager session;
    AlertDialogManager alert = new AlertDialogManager();
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
            Bundle savedInstanceState) 
    {
        View attendance = inflater.inflate(R.layout.attedance, container, false);

        session = new SessionManager(); //<-- Add this

        final ImageButton logoutbtn = (ImageButton)attendance.findViewById(R.id.logoutbtn);
        logoutbtn.setOnClickListener(new OnClickListener(){
            @Override
            public void onClick(View v) {
                    // Clear the session data
                    // This will clear all session data and 
                    // redirect user to LoginActivity
                    session.logoutUser();
                }
            });
        return attendance;
    }
}

Answer 2

问题是因为您尚未初始化SessionManager课程。所以在onCreateView()方法中初始化它。像

session = new SessionManager();

如果它有一个Context那么这个

session = new SessionManager(getActivity());

注销按钮导致崩溃？

2 个答案: