Activity以及Application對象中Context的創(chuàng)建時機

在Android中，Activity和Application實例都是通過反射創(chuàng)建的，具體可參考源碼ActivityThread類中的performLaunchActivity方法，在這里我們也能看到Activity以及Applictaion對象中Context的創(chuàng)建時機。

private Activity performLaunchActivity(ActivityClientRecord r, Intent customIntent) {
    ActivityInfo aInfo = r.activityInfo;
    if (r.loadedApk == null) {
        r.loadedApk = getLoadedApk(aInfo.applicationInfo, r.compatInfo,
                Context.CONTEXT_INCLUDE_CODE);
    }

    ComponentName component = r.intent.getComponent();
    if (component == null) {
        component = r.intent.resolveActivity(
                mInitialApplication.getPackageManager());
        r.intent.setComponent(component);
    }

    if (r.activityInfo.targetActivity != null) {
        component = new ComponentName(r.activityInfo.packageName,
                r.activityInfo.targetActivity);
    }

    ContextImpl appContext = createBaseContextForActivity(r);
    Activity activity = null;
    try {
        java.lang.ClassLoader cl = appContext.getClassLoader();
        activity = mInstrumentation.newActivity(
                cl, component.getClassName(), r.intent);
        StrictMode.incrementExpectedActivityCount(activity.getClass());
        r.intent.setExtrasClassLoader(cl);
        r.intent.prepareToEnterProcess();
        if (r.state != null) {
            r.state.setClassLoader(cl);
        }
    } catch (Exception e) {
        if (!mInstrumentation.onException(activity, e)) {
            throw new RuntimeException(
                    "Unable to instantiate activity " + component
                            + ": " + e.toString(), e);
        }
    }

    try {
        Application app = r.loadedApk.makeApplication(false, mInstrumentation);

        if (localLOGV) Slog.v(TAG, "Performing launch of " + r);
        if (localLOGV) Slog.v(
                TAG, r + ": app=" + app
                        + ", appName=" + app.getPackageName()
                        + ", pkg=" + r.loadedApk.getPackageName()
                        + ", comp=" + r.intent.getComponent().toShortString()
                        + ", dir=" + r.loadedApk.getAppDir());

        if (activity != null) {
            CharSequence title = r.activityInfo.loadLabel(appContext.getPackageManager());
            Configuration config = new Configuration(mCompatConfiguration);
            if (r.overrideConfig != null) {
                config.updateFrom(r.overrideConfig);
            }
            if (DEBUG_CONFIGURATION) Slog.v(TAG, "Launching activity "
                    + r.activityInfo.name + " with config " + config);
            Window window = null;
            if (r.mPendingRemoveWindow != null && r.mPreserveWindow) {
                window = r.mPendingRemoveWindow;
                r.mPendingRemoveWindow = null;
                r.mPendingRemoveWindowManager = null;
            }
            appContext.setOuterContext(activity);
            activity.attach(appContext, this, getInstrumentation(), r.token,
                    r.ident, app, r.intent, r.activityInfo, title, r.parent,
                    r.embeddedID, r.lastNonConfigurationInstances, config,
                    r.referrer, r.voiceInteractor, window, r.configCallback);

            if (customIntent != null) {
                activity.mIntent = customIntent;
            }
            r.lastNonConfigurationInstances = null;
            checkAndBlockForNetworkAccess();
            activity.mStartedActivity = false;
            int theme = r.activityInfo.getThemeResource();
            if (theme != 0) {
                activity.setTheme(theme);
            }

            activity.mCalled = false;
            if (r.isPersistable()) {
                mInstrumentation.callActivityOnCreate(activity, r.state, r.persistentState);
            } else {
                mInstrumentation.callActivityOnCreate(activity, r.state);
            }
            if (!activity.mCalled) {
                throw new SuperNotCalledException(
                        "Activity " + r.intent.getComponent().toShortString() +
                                " did not call through to super.onCreate()");
            }
            r.activity = activity;
            r.stopped = true;
            if (!r.activity.mFinished) {
                activity.performStart();
                r.stopped = false;
            }
            if (!r.activity.mFinished) {
                if (r.isPersistable()) {
                    if (r.state != null || r.persistentState != null) {
                        mInstrumentation.callActivityOnRestoreInstanceState(activity, r.state,
                                r.persistentState);
                    }
                } else if (r.state != null) {
                    mInstrumentation.callActivityOnRestoreInstanceState(activity, r.state);
                }
            }
            if (!r.activity.mFinished) {
                activity.mCalled = false;
                if (r.isPersistable()) {
                    mInstrumentation.callActivityOnPostCreate(activity, r.state,
                            r.persistentState);
                } else {
                    mInstrumentation.callActivityOnPostCreate(activity, r.state);
                }
                if (!activity.mCalled) {
                    throw new SuperNotCalledException(
                            "Activity " + r.intent.getComponent().toShortString() +
                                    " did not call through to super.onPostCreate()");
                }
            }
        }
        r.paused = true;

        mActivities.put(r.token, r);

    } catch (SuperNotCalledException e) {
        throw e;

    } catch (Exception e) {
        if (!mInstrumentation.onException(activity, e)) {
            throw new RuntimeException(
                    "Unable to start activity " + component
                            + ": " + e.toString(), e);
        }
    }

    return activity;
}

Activity

在構建Activity實例之前，通過createBaseContextForActivity方法創(chuàng)建了Context實現類的實例。

ContextImpl appContext = ContextImpl.createActivityContext(
                this, r.loadedApk, r.activityInfo, r.token, displayId, r.overrideConfig);

然后，通過attach方法依附在Activity中。

activity.attach(appContext, this, getInstrumentation(), r.token,
                        r.ident, app, r.intent, r.activityInfo, title, r.parent,
                        r.embeddedID, r.lastNonConfigurationInstances, config,
                        r.referrer, r.voiceInteractor, window, r.configCallback);

在這個方法里，又調用了attachBaseContext方法，傳遞給ContextWrapper類（activity的祖父類）中的mBase 字段

protected void attachBaseContext(Context base) {
    if (mBase != null) {
        throw new IllegalStateException("Base context already set");
    }
    mBase = base;
}

所以，當我們在Activity中調用getResource方法時，層層調用后，實際上調用的是ContextWrapper類中的getResources()方法:

public Resources getResources() {
    return mBase.getResources();
}

交給mBase來處理，也就是ContextImpl中的getResources()方法:

public Resources getResources() {
    return mResources;
}

而這個mResources是通過ContextImpl中的setResources方法創(chuàng)建的：

static ContextImpl createActivityContext(ActivityThread mainThread,
                                         LoadedApk loadedApk, ActivityInfo activityInfo, IBinder activityToken, int displayId,
                                         Configuration overrideConfiguration) {
    if (loadedApk == null) throw new IllegalArgumentException("loadedApk");

    String[] splitDirs = loadedApk.getSplitResDirs();
    ClassLoader classLoader = loadedApk.getClassLoader();

    if (loadedApk.getApplicationInfo().requestsIsolatedSplitLoading()) {
        Trace.traceBegin(Trace.TRACE_TAG_RESOURCES, "SplitDependencies");
        try {
            classLoader = loadedApk.getSplitClassLoader(activityInfo.splitName);
            splitDirs = loadedApk.getSplitPaths(activityInfo.splitName);
        } catch (NameNotFoundException e) {
            // Nothing above us can handle a NameNotFoundException, better crash.
            throw new RuntimeException(e);
        } finally {
            Trace.traceEnd(Trace.TRACE_TAG_RESOURCES);
        }
    }

    ContextImpl context = new ContextImpl(null, mainThread, loadedApk, activityInfo.splitName,
            activityToken, null, 0, classLoader);

    // Clamp display ID to DEFAULT_DISPLAY if it is INVALID_DISPLAY.
    displayId = (displayId != Display.INVALID_DISPLAY) ? displayId : Display.DEFAULT_DISPLAY;

    final CompatibilityInfo compatInfo = (displayId == Display.DEFAULT_DISPLAY)
            ? loadedApk.getCompatibilityInfo()
            : CompatibilityInfo.DEFAULT_COMPATIBILITY_INFO;

    final ResourcesManager resourcesManager = ResourcesManager.getInstance();

    // Create the base resources for which all configuration contexts for this Activity
    // will be rebased upon.
    context.setResources(resourcesManager.createBaseActivityResources(activityToken,
            loadedApk.getResDir(),
            splitDirs,
            loadedApk.getOverlayDirs(),
            loadedApk.getApplicationInfo().sharedLibraryFiles,
            displayId,
            overrideConfiguration,
            compatInfo,
            classLoader));
    context.mDisplay = resourcesManager.getAdjustedDisplay(displayId,
            context.getResources());
    return context;
}

Application

在performLaunchActivity方法中，我們可以看到通過makeApplication得到Application實例，

Application app = r.loadedApk.makeApplication(false, mInstrumentation);

而makeApplication方法中，

ContextImpl appContext = ContextImpl.createAppContext(mActivityThread, this);
app = mActivityThread.mInstrumentation.newApplication(cl, appClass, appContext);

最終調用attach方法，依附在Application對象上

static public Application newApplication(Class<?> clazz, Context context)
        throws InstantiationException, IllegalAccessException,
        ClassNotFoundException {
    Application app = (Application)clazz.newInstance();
    app.attach(context);
    return app;
}

同Activity一樣，attach方法又調用了attachBaseContext方法，傳遞給ContextWrapper類（activity的祖父類）中的mBase 字段。
順便，我們也來看一下Application中getResource方法得到的Resource對象。和Activity一樣，它最終也是通過setResource方法得到的，不同的是構造參數不一樣：

static ContextImpl createAppContext(ActivityThread mainThread, LoadedApk loadedApk) {
    if (loadedApk == null) throw new IllegalArgumentException("loadedApk");
    ContextImpl context = new ContextImpl(null, mainThread, loadedApk, null, null, null, 0,
            null);
    context.setResources(loadedApk.getResources());
    return context;
}

public Resources getResources() {
    if (mResources == null) {
        final String[] splitPaths;
        try {
            splitPaths = getSplitPaths(null);
        } catch (NameNotFoundException e) {
            // This should never fail.
            throw new AssertionError("null split not found");
        }

        mResources = ResourcesManager.getInstance().getResources(
                null,
                mResDir,
                splitPaths,
                mOverlayDirs,
                mApplicationInfo.sharedLibraryFiles,
                Display.DEFAULT_DISPLAY,
                null,
                getCompatibilityInfo(),
                getClassLoader());
    }
    return mResources;
}
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