base/android/java/src/org/chromium/base/ContextUtils.java

// Copyright 2015 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

package org.chromium.base;

import android.content.Context;
import android.content.SharedPreferences;
import android.preference.PreferenceManager;

import org.chromium.base.annotations.JNINamespace;
import org.chromium.base.annotations.MainDex;

/**
 * This class provides Android application context related utility methods.
 */
@JNINamespace("base::android")
@MainDex
public class ContextUtils {
    private static final String TAG = "ContextUtils";
    private static Context sApplicationContext;

    /**
     * Initialization-on-demand holder. This exists for thread-safe lazy initialization.
     */
    private static class Holder {
        // Not final for tests.
        private static SharedPreferences sSharedPreferences = fetchAppSharedPreferences();
    }

    /**
     * Get the Android application context.
     *
     * Under normal circumstances there is only one application context in a process, so it's safe
     * to treat this as a global. In WebView it's possible for more than one app using WebView to be
     * running in a single process, but this mechanism is rarely used and this is not the only
     * problem in that scenario, so we don't currently forbid using it as a global.
     *
     * Do not downcast the context returned by this method to Application (or any subclass). It may
     * not be an Application object; it may be wrapped in a ContextWrapper. The only assumption you
     * may make is that it is a Context whose lifetime is the same as the lifetime of the process.
     */
    public static Context getApplicationContext() {
        return sApplicationContext;
    }

    /**
     * Initializes the java application context.
     *
     * This should be called exactly once early on during startup, before native is loaded and
     * before any other clients make use of the application context through this class.
     *
     * @param appContext The application context.
     */
    public static void initApplicationContext(Context appContext) {
        // Conceding that occasionally in tests, native is loaded before the browser process is
        // started, in which case the browser process re-sets the application context.
        if (sApplicationContext != null && sApplicationContext != appContext) {
            throw new RuntimeException("Attempting to set multiple global application contexts.");
        }
        initJavaSideApplicationContext(appContext);
    }

    /**
     * Initialize the native Android application context to be the same as the java counter-part.
     */
    public static void initApplicationContextForNative() {
        if (sApplicationContext == null) {
            throw new RuntimeException("Cannot have native global application context be null.");
        }
        nativeInitNativeSideApplicationContext(sApplicationContext);
    }

    /**
     * Only called by the static holder class and tests.
     *
     * @return The application-wide shared preferences.
     */
    private static SharedPreferences fetchAppSharedPreferences() {
        return PreferenceManager.getDefaultSharedPreferences(sApplicationContext);
    }

    /**
     * This is used to ensure that we always use the application context to fetch the default shared
     * preferences. This avoids needless I/O for android N and above. It also makes it clear that
     * the app-wide shared preference is desired, rather than the potentially context-specific one.
     *
     * @return application-wide shared preferences.
     */
    public static SharedPreferences getAppSharedPreferences() {
        return Holder.sSharedPreferences;
    }

    /**
     * Occasionally tests cannot ensure the application context doesn't change between tests (junit)
     * and sometimes specific tests has its own special needs, initApplicationContext should be used
     * as much as possible, but this method can be used to override it.
     *
     * @param appContext The new application context.
     */
    @VisibleForTesting
    public static void initApplicationContextForTests(Context appContext) {
        initJavaSideApplicationContext(appContext);
        Holder.sSharedPreferences = fetchAppSharedPreferences();
    }

    private static void initJavaSideApplicationContext(Context appContext) {
        if (appContext == null) {
            throw new RuntimeException("Global application context cannot be set to null.");
        }
        sApplicationContext = appContext;
    }

    private static native void nativeInitNativeSideApplicationContext(Context appContext);
}