Question

The class contain this parameter like thie

public class Global {
    public static String USER_ID = "";
    public static String USER_NAME = "";
    public static String USER_PASSWORD = "";
}

the USER_ID,I set once when do login.(for example Global.USER_ID = “12345”).
And a thread running on the background do that every 5 minute post some data(contains this USER_ID) to webservice.

the question is
when the thread running long long time (about one day).
I found the parameter USER_ID return back to “”, is not the “12345”.

Why???

anyone know it?

Answer 1

I think your class is removed from the memory by class loader (during GC) and it is loaded next time you use Global class.

My Suggestion will be to use Singelton class for this you can have something like this

public class Global {
    public String USER_ID = "";
    public String USER_NAME = "";
    public String USER_PASSWORD = "";

    //constructor 
   private Global(){
    // just to prevent object creation from outside
   }
    //static reference
    public static Global global=null;
    public Global getInstance(){    
     return ((global!=null)? global : (global=new Global()) ;
    }
}

Now wherever you want to use this class you can get this object, the singelton logic ensures that their is only one object which is shared among all.

You can get the reference in your threads using

global = Global.getInstance();

Now you can use variables i.e. USER_ID using global.USER_ID

Since you have a reference of global object GC will not remove that from memory till your thread is alive.

NOTE
I have used public variables just for the illustration however they are not recommended in most cases.

EDIT

For saving user data I recommend SharedPreferences and you can use them as follows:-

private final String YourAppName_PREFS_NAME = "chooseAMeaningFulName";
private final String YourAppName_USERNAME = "username";
private final String YourAppName_PASSWORD = "password";

 /*This function will save the user name and password for providing Automatically login in future*/

       public void saveUserData() {
          if ( rememberMeChckbox.isChecked() ) {
            //      Log.i("loginFrag", "Saving userName: Pass " + userName + " : " + password);
            loginActivity.getSharedPreferences(YourAppName_PREFS_NAME, Context.MODE_PRIVATE).edit().putString(YourAppName_USERNAME, userName)
                  .putString(YourAppName_PASSWORD, password).commit();
          } else {
            loginActivity.getSharedPreferences(YourAppName_PREFS_NAME, Context.MODE_PRIVATE).edit().remove(RBR_USERNAME).remove(RBR_PASSWORD).commit();
          }
        }

        /*This function returns the previous data from the SharedPreferences and fills the UserName and Password Text Box*/
        private void getPreviousUserData() {
          SharedPreferences preferences = loginActivity.getSharedPreferences(YourAppName_NAME, Context.MODE_PRIVATE);
          userName = preferences.getString(YourAppName_USERNAME, null);
          password = preferences.getString(YourAppName_PASSWORD, null);
          //      Log.i("loginFrag", "Got userName: Pass " + userName + " : " + password);
          if ( userName != null ) {
            uText.setText(userName);
          } else {
            uText.setText("android");
          }
          if ( password != null ) {
            passText.setText(password);
          } else {
            passText.setText("android");
          }
        }