android中getSharedPreferences方法实现原理

如题，我们在调用getSharedPreferences方法时，是直接在我们的Activity类中这样调用的，如下：
sp = this.getSharedPreferences("userInfo", Context.MODE_PRIVATE);
但是这个方法本身是一个抽象方法，而我们的Activity类中是继承了这个方法，也没有方法的具体实现代码，那么这个方法究竟是如何工作的呢，它的实现原理又是什么呢

看Context的实现类android.app.ContextImpl，在activity的父类中只是以Context接口方式声明的变量而已。

在android.jar中没有找到android.app.ContextImpl类呀

在Context的子类ContextImpl中有这个方法：

@Override
public SharedPreferences getSharedPreferences(String name, int mode) {
SharedPreferencesImpl sp;
synchronized (ContextImpl.class) {
if (sSharedPrefs == null) {
sSharedPrefs = new ArrayMap>();
}

        final String packageName = getPackageName();
        ArrayMap<String, SharedPreferencesImpl> packagePrefs = sSharedPrefs.get(packageName);
        if (packagePrefs == null) {
            packagePrefs = new ArrayMap<String, SharedPreferencesImpl>();
            sSharedPrefs.put(packageName, packagePrefs);
        }

        // At least one application in the world actually passes in a null
        // name.  This happened to work because when we generated the file name
        // we would stringify it to "null.xml".  Nice.
        if (mPackageInfo.getApplicationInfo().targetSdkVersion <
                Build.VERSION_CODES.KITKAT) {
            if (name == null) {
                name = "null";
            }
        }

        sp = packagePrefs.get(name);
        if (sp == null) {
            File prefsFile = getSharedPrefsFile(name);
            sp = new SharedPreferencesImpl(prefsFile, mode);
            packagePrefs.put(name, sp);
            return sp;
        }
    }
    if ((mode & Context.MODE_MULTI_PROCESS) != 0 ||
        getApplicationInfo().targetSdkVersion < android.os.Build.VERSION_CODES.HONEYCOMB) {
        // If somebody else (some other process) changed the prefs
        // file behind our back, we reload it.  This has been the
        // historical (if undocumented) behavior.
        sp.startReloadIfChangedUnexpectedly();
    }
    return sp;
}