Android SharedPreferences 最佳实践

Question

In an application I have been building we rely on SharedPreferences quite a bit, this got me thinking about what is best practice when it comes to accessing SharedPreferences. For instance many people say the appropriate way to access it is via this call:

PreferenceManager.getDefaultSharedPreferences(Context context)

However it seems like this could be dangerous. If you have a large application that is relying on SharedPreferences you could have key duplication, especially in the case of using some third party library that relies on SharedPreferences as well. It seems to me that the better call to use would be:

Context.getSharedPreferences(String name, int mode)

This way if you have a class that heavily relies on SharedPreferences you can create a preference file that is used only by your class. You could use the fully qualified name of the class to ensure that the file will most likely not be duplicated by someone else.

Also based on this SO question: Should accessing SharedPreferences be done off the UI Thread? , it seems that accesses SharedPreferences should be done off the UI thread which makes sense.

Are there any other best practices Android developers should be aware of when using SharedPreferences in their applications?

Answer 1

I've wrote a little article that can also be found here . It describes what SharedPreferences is :

Best Practice: SharedPreferences

Android provides many ways of storing application data. One of those ways leads us to the SharedPreferences object which is used to store private primitive data in key-value pairs.

All logic are based only on three simple classes:

• SharedPreferences
• SharedPreferences.Editor
• SharedPreferences.OnSharedPreferenceChangeListener

SharedPreferences

SharedPreferences is main of them. It's responsible for getting (parsing) stored data, provides interface for getting Editor object and interfaces for adding and removing OnSharedPreferenceChangeListener

• To create SharedPreferences you will need Context object (can be an application Context )
• getSharedPreferences method parses Preference file and creates Map object for it

• You can create it in few modes provided by Context. You should always use MODE_PRIVATE, as all the other modes are deprecated since API level 17.

   // parse Preference file SharedPreferences preferences = context.getSharedPreferences("com.example.app", Context.MODE_PRIVATE); // get values from Map preferences.getBoolean("key", defaultValue) preferences.get..("key", defaultValue) // you can get all Map but be careful you must not modify the collection returned by this // method, or alter any of its contents. Map<String, ?> all = preferences.getAll(); // get Editor object SharedPreferences.Editor editor = preferences.edit(); //add on Change Listener preferences.registerOnSharedPreferenceChangeListener(mListener); //remove on Change Listener preferences.unregisterOnSharedPreferenceChangeListener(mListener); // listener example SharedPreferences.OnSharedPreferenceChangeListener mOnSharedPreferenceChangeListener = new SharedPreferences.OnSharedPreferenceChangeListener() { @Override public void onSharedPreferenceChanged(SharedPreferences sharedPreferences, String key) { } };

Editor

SharedPreferences.Editor is an Interface used for modifying values in a SharedPreferences object. All changes you make in an editor are batched, and not copied back to the original SharedPreferences until you call commit() or apply()

• Use simple interface to put values in Editor
• Save values synchronous with commit() or asynchronous with apply which is faster. In fact of using different threads using commit() is safer. Thats why I prefer to use commit() .

• Remove single value with remove() or clear all values with clear()

  // get Editor object SharedPreferences.Editor editor = preferences.edit(); // put values in editor editor.putBoolean("key", value); editor.put..("key", value); // remove single value by key editor.remove("key"); // remove all values editor.clear(); // commit your putted values to the SharedPreferences object synchronously // returns true if success boolean result = editor.commit(); // do the same as commit() but asynchronously (faster but not safely) // returns nothing editor.apply();

Performance & Tips

• SharedPreferences is a Singleton object so you can easily get as many references as you want, it opens file only when you call getSharedPreferences first time, or create only one reference for it.

   // There are 1000 String values in preferences SharedPreferences first = context.getSharedPreferences("com.example.app", Context.MODE_PRIVATE); // call time = 4 milliseconds SharedPreferences second = context.getSharedPreferences("com.example.app", Context.MODE_PRIVATE); // call time = 0 milliseconds SharedPreferences third = context.getSharedPreferences("com.example.app", Context.MODE_PRIVATE); // call time = 0 milliseconds

• As SharedPreferences is a Singleton object you can change any of It's instances and not be scared that their data will be different

  first.edit().putInt("key",15).commit(); int firstValue = first.getInt("key",0)); // firstValue is 15 int secondValue = second.getInt("key",0)); // secondValue is also 15

• Remember the larger the Preference object is the longer get , commit , apply , remove and clear operations will be. So it's highly recommended to separate your data in different small objects.

• Your Preferences will not be removed after Application update. So there are cases when you need to create some migration scheme. For example you have Application that parse local JSON in start of application, to do this only after first start you decided to save boolean flag wasLocalDataLoaded . After some time you updated that JSON and released new application version. Users will update their applications but they will not load new JSON because they already done it in first application version.

   public class MigrationManager { private final static String KEY_PREFERENCES_VERSION = "key_preferences_version"; private final static int PREFERENCES_VERSION = 2; public static void migrate(Context context) { SharedPreferences preferences = context.getSharedPreferences("pref", Context.MODE_PRIVATE); checkPreferences(preferences); } private static void checkPreferences(SharedPreferences thePreferences) { final double oldVersion = thePreferences.getInt(KEY_PREFERENCES_VERSION, 1); if (oldVersion < PREFERENCES_VERSION) { final SharedPreferences.Editor edit = thePreferences.edit(); edit.clear(); edit.putInt(KEY_PREFERENCES_VERSION, currentVersion); edit.commit(); } } }

• SharedPreferences are stored in an xml file in the app data folder

  // yours preferences /data/data/YOUR_PACKAGE_NAME/shared_prefs/YOUR_PREFS_NAME.xml // default preferences /data/data/YOUR_PACKAGE_NAME/shared_prefs/YOUR_PACKAGE_NAME_preferences.xml

Android guide.

Sample Code

public class PreferencesManager {

    private static final String PREF_NAME = "com.example.app.PREF_NAME";
    private static final String KEY_VALUE = "com.example.app.KEY_VALUE";

    private static PreferencesManager sInstance;
    private final SharedPreferences mPref;

    private PreferencesManager(Context context) {
        mPref = context.getSharedPreferences(PREF_NAME, Context.MODE_PRIVATE);
    }

    public static synchronized void initializeInstance(Context context) {
        if (sInstance == null) {
            sInstance = new PreferencesManager(context);
        }
    }

    public static synchronized PreferencesManager getInstance() {
        if (sInstance == null) {
            throw new IllegalStateException(PreferencesManager.class.getSimpleName() +
                    " is not initialized, call initializeInstance(..) method first.");
        }
        return sInstance;
    }

    public void setValue(long value) {
        mPref.edit()
                .putLong(KEY_VALUE, value)
                .commit();
    }

    public long getValue() {
        return mPref.getLong(KEY_VALUE, 0);
    }

    public void remove(String key) {
        mPref.edit()
                .remove(key)
                .commit();
    }

    public boolean clear() {
        return mPref.edit()
                .clear()
                .commit();
    }
}

Answer 2

If you have a large application that is relying on SharedPreferences you could have key duplication, especially in the case of using some third party library that relies on SharedPreferences as well.

Libraries should not use that particular SharedPreferences . The default SharedPreferences should only be used by the application.

This way if you have a class that heavily relies on SharedPreferences you can create a preference file that is used only by your class.

You are certainly welcome to do this. I wouldn't, at the application level, as the primary reason for SharedPreferences is to have them be shared among the components in the application. A development team should have no problem managing this namespace, just as they should have no problem managing names of classes, packages, resources, or other project-level stuff. Moreover, the default SharedPreferences are what your PreferenceActivity will use.

However, going back to your libraries point, reusable libraries should use a separate SharedPreferences for their library only. I would not base it on a class name, because then you are one refactoring away from breaking your app. Instead, pick a name that is unique (eg, based on the library name, such as "com.commonsware.cwac.wakeful.WakefulIntentService" ) but stable.

it seems that accesses SharedPreferences should be done off the UI thread which makes sense.

Ideally, yes. I recently released a SharedPreferencesLoader that helps with this.

Are there any other best practices Android developers should be aware of when using SharedPreferences in their applications?

Don't over-rely upon them. They are stored in XML files and are not transactional. A database should be your primary data store, particularly for data you really don't want to lose.

Answer 3

In kotlin, use of SharedPreferences can be simplified in the following way.

class Prefs(context: Context) {

    companion object {
        private const val PREFS_FILENAME = "app_prefs"

        private const val KEY_MY_STRING = "my_string"
        private const val KEY_MY_BOOLEAN = "my_boolean"
        private const val KEY_MY_ARRAY = "string_array"
    }

    private val sharedPrefs: SharedPreferences =
        context.getSharedPreferences(PREFS_FILENAME, Context.MODE_PRIVATE)

    var myString: String
        get() = sharedPrefs.getString(KEY_MY_STRING, "") ?: ""
        set(value) = sharedPrefs.edit { putString(KEY_MY_STRING, value) }

    var myBoolean: Boolean
        get() = sharedPrefs.getBoolean(KEY_MY_BOOLEAN, false)
        set(value) = sharedPrefs.edit { putBoolean(KEY_MY_BOOLEAN, value) }

    var myStringArray: Array<String>
        get() = sharedPrefs.getStringSet(KEY_MY_ARRAY, emptySet())?.toTypedArray()
            ?: emptyArray()
        set(value) = sharedPrefs.edit { putStringSet(KEY_MY_ARRAY, value.toSet()) }

Here, sharedPrefs.edit{...} is provided by the android core ktx library and should be implemented by adding dependency implementation "androidx.core:core-ktx:1.0.2" in appliation level build.gradle .

You can get the instance of SharedPreferences by using code:

val prefs = Prefs(context)

Furthermore, you can create the Singleton object of Prefs and use from anywhere within the app.

val prefs: Prefs by lazy {
    Prefs(App.instance)
}

where, App extends Application and should be included in AndroidManifest.xml

App.kt

class App:Application() {
    companion object {
        lateinit var instance: App
    }

    override fun onCreate() {
        super.onCreate()
        instance = this
    }
}

AndroidManifest.xml

<?xml version="1.0" encoding="utf-8"?>
<manifest .....

   <application
        android:name=".App"
        ....

Example Usage:

// get stored value
val myString = prefs.myString

// store value
prefs.myString = "My String Value"

// get stored array
val myStringArray = prefs.myStringArray

// store array
prefs.myStringArray = arrayOf("String 1","String 2","String 3")

Answer 4

This is my way

for write

SharedPreferences settings = context.getSharedPreferences("prefs", 0);
SharedPreferences.Editor editore = settings.edit();
editore.putString("key", "some value");
editore.apply();

to read

SharedPreferences settings = getSharedPreferences("prefs", 0);
Strings value = settings.getString("key", "");

Answer 5

Let's assume in a project, with multiple developers working on it, they are defining SharedPreference within an Activity like this:

SharedPreferences sharedPref = context.getSharedPreferences("prefName", 0);

At one point or another two developers can define the SharedPreference with the same name or insert equivalent Key - Value pairs, which will lead to problems in using the keys.

The solution relies on two options, whether to use;

1. SharedPreferences Singleton that uses String keys.

2. SharedPreferences Singleton that uses Enum keys.

Personally and According to this Sharepreference Documentation , I prefer to use Enum keys as it enforces stricter control when there are multiple programmers working on a project. A programmer has no choice but to declare a new key in the appropriate enum class and so all the keys are in the same place.

And to avoid boilerplate code writing create the SharedPreference singleton. This SharedPreferences singleton Class help to centralize and simplify reading and writing of SharedPreferences in your Android app.

The source code for the two provided solutions can be found in GitHub