在Android中，Activity和Application实例都是通过反射创建的，具体可参考源码ActivityThread类中的performLaunchActivity方法，在这里我们也能看到Activity以及Applictaion对象中Context的创建时机。
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
在构建Activity实例之前，通过createBaseContextForActivity方法创建了Context实现类的实例。
ContextImpl appContext = ContextImpl.createActivityContext(
this, r.loadedApk, r.activityInfo, r.token, displayId, r.overrideConfig);
然后，通过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);
在这个方法里，又调用了attachBaseContext方法，传递给ContextWrapper类（activity的祖父类）中的mBase 字段
protected void attachBaseContext(Context base) {
if (mBase != null) {
throw new IllegalStateException("Base context already set");
}
mBase = base;
}
所以，当我们在Activity中调用getResource方法时，层层调用后，实际上调用的是ContextWrapper类中的getResources()方法:
public Resources getResources() {
return mBase.getResources();
}
交给mBase来处理，也就是ContextImpl中的getResources()方法:
public Resources getResources() {
return mResources;
}
而这个mResources是通过ContextImpl中的setResources方法创建的：
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，我们可以看到通过makeApplication得到Application实例，
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);
最终调用attach方法，依附在Application对象上
static public Application newApplication(Class<?> clazz, Context context)
throws InstantiationException, IllegalAccessException,
ClassNotFoundException {
Application app = (Application)clazz.newInstance();
app.attach(context);
return app;
}
同Activity一样，attach方法又调用了attachBaseContext方法，传递给ContextWrapper类（activity的祖父类）中的mBase 字段。
顺便，我们也来看一下Application中getResource方法得到的Resource对象。和Activity一样，它最终也是通过setResource方法得到的，不同的是构造参数不一样：
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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