Android - save Object to SharedPreferences and get it anywhere in the app

问题

In my app I have a custom User class which holds some regular data (name etc...). I need to save that object and get it anywhere and anytime in other pages of the app. I made a helper class public final class GeneralMethods with many methods which I use a lot (static, of course).
In order to save the data Im using Gson library. I made this method:

public static void saveData(Context con, String variable, String data)
{
    SharedPreferences prefs = PreferenceManager.getDefaultSharedPreferences(con);
    prefs.edit().putString(variable, data).apply();
}

To save an object, I use this method as follows:

Gson gson = new Gson();
String stringUser = gson.toJson(newUser);    
GeneralMethods.saveData(VerificationActivity.this,"userObject",stringUser);

To load the data back, I'm using this static method:

public static String getData(Context con, String variable, String defaultValue)
{
    SharedPreferences prefs = PreferenceManager.getDefaultSharedPreferences(con);
    String data = prefs.getString(variable, defaultValue);
    return data;
}

I dont really know how to get the data back, this is what I've done so far:

Gson gson = new Gson();
String user="";
String value="";
user = GeneralMethods.getData(SplashScreenActivity.this,value,"userObject");

Im struggling with the getData method, how do I parse the data from String back to the User type?

EDIT
I tried the suggestions bellow and I always get NULL. Maybe I dont save the object in the right way?

EDIT2
It seems Im not generating the object correctly and therefore nothing is being saved. This is the user "Singleton" class:

public class User implements Serializable {

    private static User userInstance=null; //the only instance of the class
    private static String userName; //userName = the short phone number
    private User(){}

    public static User getInstance(){
        if(userInstance ==null){
            userInstance = new User();
        }
        return userInstance;
    }

    public static User getUserInstance() {
        return userInstance;
    }

    public String getUserName(){
        return this.userName;
    }

    public static void setUserName(String userName) {
        User.userName = userName;
    }

    public static void init(String _userName) {
        User.setUserName(_userName);
    }
}

This is how i setup the object with the relevant data (user name as the constructor parameter):

   User.init(name);

This is how i convert the object to a String:

   Gson gson = new Gson();
    String stringUser = gson.toJson(User.getInstance());
GeneralMethods.saveData(VerificationActivity.this,"userObject",stringUser);

回答1:

Replace your existing User class with below

public class User implements Serializable
{

    private static User userInstance = null; // the only instance of the class
    private String userName; // userName = the short phone number
    private User(){}
    public static User getInstance()
    {
        if (userInstance == null)
        {
            userInstance = new User();
        }
        return userInstance;
    }

    public String getUserName()
    {
        return userName;
    }

    public void setUserName(String p_userName)
    {
        userName = p_userName;
    }

    @Override
    public String toString()
    {
        return "User [userName=" + getUserName() + "]";
    }
}

Initialize User Name

User m_user = User.getInstance();
m_user.setUserName(name);

Convert the object to a String

Gson gson = new Gson();
String stringUser = gson.toJson(m_user);
GeneralMethods.saveData(VerificationActivity.this,"userObject",stringUser);

回答2:

I'd suggest you to use Dependency Injection pattern.

I usually create Preferences class which exposes getters and setters to quickly read and save values to SharedPreferences. I inject the same instance of Preferences anywhere in the code using Dagger as Dependency Injector.

Why use Singleton object over public static helpers?

If you have a helper class of utility functions that you're using directly, it creates a hidden dependency; you have no control over who can use it, or where. Injecting that same helper class via a stateless singleton instance lets you control where and how it's being used, and replace it / mock it / etc. when you need to.

Read more here.

Example of Preferences class:

@Singleton
public class Preferences {

    private SharedPreferences sharedPreferences;
    private final String NOTIFICATIONS_ENABLED = "NOTIFICATIONS_ENABLED";

    @Inject
    public Preferences(Context context) {
        sharedPreferences = context.getSharedPreferences("Preferences", 0);
    }

    public void setNotificationsEnabled(boolean enabled){
        SharedPreferences.Editor edit = sharedPreferences.edit();
        edit.putBoolean(NOTIFICATIONS_ENABLED, enabled);
        edit.commit();           
    }

    public boolean isNotificationsEnabled(){
        return sharedPreferences.getBoolean(NOTIFICATIONS_ENABLED, true);
    }
}

and how to use it in Activity:

public class MainActivity extends Activity {

    @Inject
    NavigationController navigationController;

    @Inject
    Preferences preferences;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        SharedObjectGraph.getObjectGraph().inject(this);

        if(preferences.isNotificationsEnabled()){
            // do something
        }
    }

回答3:

We can do this by using Gson library. And shared preference is accessible through out the application. I have created my own class to access shared pref with getter and setter of all type like String, Int, Object etc.

To use Gson Library, Below is the code. If you want the class that I have created then let me know

You can use gson.jar to store class objects into SharedPreferences. You can downlaod this jar from here https://code.google.com/p/google-gson/downloads/list

Or add GSON dependency in Gradle file

compile 'com.google.code.gson:gson:2.5'

To Save

 Editor prefsEditor = mPrefs.edit();
 Gson gson = new Gson();
 String json = gson.toJson(MyObject);
 prefsEditor.putString("MyObject", json);
 prefsEditor.commit();

To Retreive

Gson gson = new Gson();
String json = mPrefs.getString("MyObject", "");
MyObject obj = gson.fromJson(json, MyObject.class);

Hope this helps you Summved

回答4:

You need to just replace one line of code

user = GeneralMethods.getData(SplashScreenActivity.this,value,"userObject");

To

user = GeneralMethods.getData(SplashScreenActivity.this,"userObject",value);

回答5:

For getting the data back-

//Creating a shared preference
SharedPreferences  mPrefs = getPreferences(MODE_PRIVATE);

Gson gson = new Gson();
    String json = mPrefs.getString("MyObject", "");
    MyObject obj = gson.fromJson(json, MyObject.class);

Hope this helps :)

回答6:

You are doing it correctly, but i think you have switched the two fields in the getData method.

You have this method:

public static String getData(Context con, String variable, String defaultValue)
{....}

But you are sending in this:

user = GeneralMethods.getData(SplashScreenActivity.this,value,"userObject");

That means that your variable is this.value and your defaultValue is "userObject". I believe you want to have it the other way around

回答7:

public static String getData(Context con, String variable)
{
SharedPreferences prefs = PreferenceManager.getDefaultSharedPreferences(con);
String data = prefs.getString(variable, null);
return data;
}

Gson gson = new Gson();
String json = GeneralMethods.getData(SplashScreenActivity.this,"userObject");
if(json!=null){
  MyObject obj = gson.fromJson(json, MyObject.class);
}

回答8:

Try this

This will help you to create a class of shared preference and use it any where in any class.

1. Create a class named as MySharedPreferences

   public class MySharedPreferences
   {
    public static final String mySharedPrefrences = "MyPrefs" ;
    public static SharedPreferences sharedPreferences;
    public static SharedPreferences.Editor editor;
     private static MySharedPreferences instance;
   
   public static MySharedPreferences with(Context context) {
   if (instance == null) {
       instance = new MySharedPreferences(context);
   }
   return instance;
   }
   
   public MySharedPreferences(Context context)
   {
   sharedPreferences=context.getSharedPreferences(mySharedPrefrences, Context.MODE_PRIVATE);
   }
   
   public static String  getUserId(String str_userid)
   {
   if (sharedPreferences!= null) {
       return sharedPreferences.getString(str_userid, "");
   }
   return "";
   }
   
   public void saveUserId(String str_userId_key,String str_userId_value)
   {
   editor = sharedPreferences.edit();
   editor.putString(str_userId_key,str_userId_value);
   editor.commit();
   }
   
   public void clearAllData(Context context)
   {
   sharedPreferences = context.getSharedPreferences(mySharedPrefrences, Context.MODE_PRIVATE);
   SharedPreferences.Editor editor = sharedPreferences.edit();
   editor.clear().apply();
   }
   }
   

2. And use this like

   MySharedPreferences yourPrefrence =MySharedPreferences.with(getActivity());yourPrefrence.saveUserId("str_userId_key",et_title.getText().toString().trim());
   String value = yourPrefrence.getUserId("str_userId_key");
   

来源：https://stackoverflow.com/questions/29406945/android-save-object-to-sharedpreferences-and-get-it-anywhere-in-the-app