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本文以Android内核剖析为基准，结合android4.0源代码，进行理解总结。主要阐述了如下认识：
1、何为Context，如何认识Context。
2、Application、Activity、Service等实例化流程。
3、Context的实例化对象ContextImpl对象是如何和Application、Activity、Service发生联系的
4、初步认识下，Application、Activity等的生命周期onCreate、onStart方法的调用时机。
1、Context认知。
Context译为场景，一个应用程序可以认为是一个工作环境，在这个工作环境中可以存在许多场景，coding代码的场景 ，打电话的场景，开会的场景。这些场景可以类比不同的Activity，service。
2、从两个角度认识Context。
第一：Activity继承自Context，同时Activity还实现了其他的interface，我们可以这样看，activity在语法上extends了Context，其本质上是一个Context，但同时其实现了许多interface，扩充了Context的功能，扩充之后的类成为Activity或者Service。
第二：Context本质上包含了场景的所有元素，故而设定其为abstract，Activity和Service继承自Context，它们本质上可以认为就是Context。
3、Context继承关系图
4、Application对象的ContextImpl对象创建过程。
step 1、Ams通过远程Binder调用ActivityThread的内部类ApplicationThread的bingApplication方法，参数包括ApplicationInfo，这个对象由Ams创建，通过IPC传递到ActivityThread的内部类ApplicationThread中。
public final void bindApplication(String processName,step 2、构建AppBindData对象，如上代码所示。
ApplicationInfo appInfo, List<ProviderInfo> providers,
ComponentName instrumentationName, String profileFile,
ParcelFileDescriptor profileFd, boolean autoStopProfiler,
Bundle instrumentationArgs, IInstrumentationWatcher instrumentationWatcher,
int debugMode, boolean isRestrictedBackupMode, boolean persistent,
Configuration config, CompatibilityInfo compatInfo,
Map<String, IBinder> services, Bundle coreSettings) {
if (services != null) {
// Setup the service cache in the ServiceManager
ServiceManager.initServiceCache(services);
}
setCoreSettings(coreSettings);
AppBindData data = new AppBindData();
data.processName = processName;
data.appInfo = appInfo;
data.providers = providers;
data.instrumentationName = instrumentationName;
data.instrumentationArgs = instrumentationArgs;
data.instrumentationWatcher = instrumentationWatcher;
data.debugMode = debugMode;
data.restrictedBackupMode = isRestrictedBackupMode;
data.persistent = persistent;
data.config = config;
data.compatInfo = compatInfo;
data.initProfileFile = profileFile;
data.initProfileFd = profileFd;
data.initAutoStopProfiler = false;
queueOrSendMessage(H.BIND_APPLICATION, data);
}
step 3、调用H Handler，执行handleBindApplication（）方法。
static final class AppBindData {
LoadedApk info;
String processName;
ApplicationInfo appInfo;
List<ProviderInfo> providers;
ComponentName instrumentationName;
Bundle instrumentationArgs;
IInstrumentationWatcher instrumentationWatcher;
int debugMode;
boolean restrictedBackupMode;
boolean persistent;
Configuration config;
CompatibilityInfo compatInfo;
/** Initial values for {@link Profiler}. */
String initProfileFile;
ParcelFileDescriptor initProfileFd;
boolean initAutoStopProfiler;
public String toString() {
return "AppBindData{appInfo=" + appInfo + "}";
}
}
private void handleBindApplication(AppBindData data) {
mBoundApplication = data;
mConfiguration = new Configuration(data.config);
mCompatConfiguration = new Configuration(data.config);
//..........
TimeZone.setDefault(null);
/*
* Initialize the default locale in this process for the reasons we set the time zone.
*/
Locale.setDefault(data.config.locale);
data.info = getPackageInfoNoCheck(data.appInfo, data.compatInfo);//data.info对象为LoadApk，此时data.info为null，使用getPackageINfoNoCheck创建此对象。
if (data.instrumentationName != null) {//该条件尽在Android Unit Test工程时会执行到，此处直接看else语句
ContextImpl appContext = new ContextImpl();
appContext.init(data.info, null, this);
InstrumentationInfo ii = null;
try {
ii = appContext.getPackageManager().
getInstrumentationInfo(data.instrumentationName, 0);
} catch (PackageManager.NameNotFoundException e) {
}
if (ii == null) {
throw new RuntimeException(
"Unable to find instrumentation info for: "
+ data.instrumentationName);
}
mInstrumentationAppDir = ii.sourceDir;
mInstrumentationAppPackage = ii.packageName;
mInstrumentedAppDir = data.info.getAppDir();
ApplicationInfo instrApp = new ApplicationInfo();
instrApp.packageName = ii.packageName;
instrApp.sourceDir = ii.sourceDir;
instrApp.publicSourceDir = ii.publicSourceDir;
instrApp.dataDir = ii.dataDir;
instrApp.nativeLibraryDir = ii.nativeLibraryDir;
LoadedApk pi = getPackageInfo(instrApp, data.compatInfo,
appContext.getClassLoader(), false, true);
ContextImpl instrContext = new ContextImpl();
instrContext.init(pi, null, this);
try {
java.lang.ClassLoader cl = instrContext.getClassLoader();
mInstrumentation = (Instrumentation)
cl.loadClass(data.instrumentationName.getClassName()).newInstance();
} catch (Exception e) {
throw new RuntimeException(
"Unable to instantiate instrumentation "
+ data.instrumentationName + ": " + e.toString(), e);
}
mInstrumentation.init(this, instrContext, appContext,
new ComponentName(ii.packageName, ii.name), data.instrumentationWatcher);
if (mProfiler.profileFile != null && !ii.handleProfiling
&& mProfiler.profileFd == null) {
mProfiler.handlingProfiling = true;
File file = new File(mProfiler.profileFile);
file.getParentFile().mkdirs();
Debug.startMethodTracing(file.toString(), 8 * 1024 * 1024);
}
try {
mInstrumentation.onCreate(data.instrumentationArgs);
}
catch (Exception e) {
throw new RuntimeException(
"Exception thrown in onCreate() of "
+ data.instrumentationName + ": " + e.toString(), e);
}
} else {
mInstrumentation = new Instrumentation();//初始化Instrumentation对象，一个应用程序对应一个Instrumentation对象
}
Application app = data.info.makeApplication(data.restrictedBackupMode, null);
mInitialApplication = app;
try {
mInstrumentation.callApplicationOnCreate(app);//调用Application程序都应的onCreate方法。
} catch (Exception e) {
if (!mInstrumentation.onException(app, e)) {
throw new RuntimeException(
"Unable to create application " + app.getClass().getName()
+ ": " + e.toString(), e);
}
}
}
第三步可以又可以分为三小步。
step 3.1、给AppBindData的info变量赋值。
data.info = getPackageInfoNoCheck(data.appInfo, data.compatInfo);//data.info对象为LoadApk，此时data.info为null，使用getPackageINfoNoCheck创建此对象。step 3.2、初始化Instrumentation对象。
mInstrumentation = new Instrumentation();//初始化Instrumentation对象，一个应用程序对应一个Instrumentation对象step 3.3、创建Application对象。
Application app = data.info.makeApplication(data.restrictedBackupMode, null);
我们着重看一下step 3.1和step3.3.
step 3.1：mPackages和mResourcePackages集合，以packageName为key值，我们知道一个应用程序中的packageName是相同的，也就是说，此处一旦创建，其他地方再次调用此函数，就不需要创建了。总结：也就是说一个应用程序中的LoadedApk对象是唯一的。此处的LoadedApk，也被称为packageInfo。
public final LoadedApk getPackageInfoNoCheck(ApplicationInfo ai,
CompatibilityInfo compatInfo) {
return getPackageInfo(ai, compatInfo, null, false, true);
}
private LoadedApk getPackageInfo(ApplicationInfo aInfo, CompatibilityInfo compatInfo,
ClassLoader baseLoader, boolean securityViolation, boolean includeCode) {/*includeCode 默认为true*/
synchronized (mPackages) {
WeakReference<LoadedApk> ref;
if (includeCode) {//1、首先从mPackages或者mResourcePackages 集合中以packageName为Key值，获取LoadApk对象。
ref = mPackages.get(aInfo.packageName);
} else {
ref = mResourcePackages.get(aInfo.packageName);
}
LoadedApk packageInfo = ref != null ? ref.get() : null;
if (packageInfo == null || (packageInfo.mResources != null
&& !packageInfo.mResources.getAssets().isUpToDate())) {
if (localLOGV) Slog.v(TAG, (includeCode ? "Loading code package "
: "Loading resource-only package ") + aInfo.packageName
+ " (in " + (mBoundApplication != null
? mBoundApplication.processName : null)
+ ")");
packageInfo =
new LoadedApk(this, aInfo, compatInfo, this, baseLoader,
securityViolation, includeCode &&
(aInfo.flags&ApplicationInfo.FLAG_HAS_CODE) != 0);//2、如果packageInfo对象为null，则new初始化此对象
if (includeCode) {//3、最后将创建的此packageInfo对象，加入到mPackages或者mResourcePackages集合中。
mPackages.put(aInfo.packageName,
new WeakReference<LoadedApk>(packageInfo));
} else {
mResourcePackages.put(aInfo.packageName,
new WeakReference<LoadedApk>(packageInfo));
}
}
return packageInfo;
}
}
step 3.3、 总结：每个应用程序都存在一个Application，用户可以在AndroidManifest中重写它，如果不重写也存在一个默认的Application对象。
framework/base/core/java/android/app/LoadedApk.java
public Application makeApplication(boolean forceDefaultAppClass,
Instrumentation instrumentation) {
if (mApplication != null) {
return mApplication;
}
Application app = null;
String appClass = mApplicationInfo.className;
if (forceDefaultAppClass || (appClass == null)) {
appClass = "android.app.Application";//1、每个工程都存在一个Application对象，默认的Application对象为android.app.Application，客户端可以重写
}
try {
java.lang.ClassLoader cl = getClassLoader();
ContextImpl appContext = new ContextImpl();//2、创建ContextImpl对象，这才是Context的实际实现类
appContext.init(this, null, mActivityThread);//3、执行ContextImpl对象的init方法，initResource等对象
app = mActivityThread.mInstrumentation.newApplication(//4、以appContext为参数得到Application对象。
cl, appClass, appContext);
appContext.setOuterContext(app);
} catch (Exception e) {
if (!mActivityThread.mInstrumentation.onException(app, e)) {
throw new RuntimeException(
"Unable to instantiate application " + appClass
+ ": " + e.toString(), e);
}
}
mActivityThread.mAllApplications.add(app);//5、将创建的Application对象，加入到A来了Application中。
mApplication = app;
if (instrumentation != null) {//6、此时的instrumentation为null。
try {
instrumentation.callApplicationOnCreate(app);
} catch (Exception e) {
if (!instrumentation.onException(app, e)) {
throw new RuntimeException(
"Unable to create application " + app.getClass().getName()
+ ": " + e.toString(), e);
}
}
}
return app;
}
5、Activity中Context的创建过程
step 1、Ams通过远程Binder调用ActivityThread的Application的scheduleLaunchActivity方法，参数包括ActivityInfo，这个对象由Ams创建，通过IPC传递到ActivityThread的内部类ApplicationThread中。
public final void scheduleLaunchActivity(Intent intent, IBinder token, int ident,step 2、构建ActivityClientRecord对象，如上代码所示。
ActivityInfo info, Configuration curConfig, CompatibilityInfo compatInfo,
Bundle state, List<ResultInfo> pendingResults,
List<Intent> pendingNewIntents, boolean notResumed, boolean isForward,
String profileName, ParcelFileDescriptor profileFd, boolean autoStopProfiler) {
ActivityClientRecord r = new ActivityClientRecord();
r.token = token;
r.ident = ident;
r.intent = intent;
r.activityInfo = info;
r.compatInfo = compatInfo;
r.state = state;
r.pendingResults = pendingResults;
r.pendingIntents = pendingNewIntents;
r.startsNotResumed = notResumed;
r.isForward = isForward;
r.profileFile = profileName;
r.profileFd = profileFd;
r.autoStopProfiler = autoStopProfiler;
updatePendingConfiguration(curConfig);
queueOrSendMessage(H.LAUNCH_ACTIVITY, r);
}
step 3、调用H Handler，执行handleLaunchActivity（）方法。
其中step 3，又可分为10小步。
private Activity performLaunchActivity(ActivityClientRecord r, Intent customIntent) {
// System.out.println("##### [" + System.currentTimeMillis() + "] ActivityThread.performLaunchActivity(" + r + ")");
ActivityInfo aInfo = r.activityInfo;
if (r.packageInfo == null) {//1、如果packageInfo为null,则调用getPackageInfo的得到LoadedApk
r.packageInfo = getPackageInfo(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);
}
Activity activity = null;
try {//2、调用mInstrumentation的newActivity方法，得到Activity对象
java.lang.ClassLoader cl = r.packageInfo.getClassLoader();
activity = mInstrumentation.newActivity(
cl, component.getClassName(), r.intent);
StrictMode.incrementExpectedActivityCount(activity.getClass());
r.intent.setExtrasClassLoader(cl);
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.packageInfo.makeApplication(false, mInstrumentation);//3、获取Application对象
if (localLOGV) Slog.v(TAG, "Performing launch of " + r);
if (localLOGV) Slog.v(
TAG, r + ": app=" + app
+ ", appName=" + app.getPackageName()
+ ", pkg=" + r.packageInfo.getPackageName()
+ ", comp=" + r.intent.getComponent().toShortString()
+ ", dir=" + r.packageInfo.getAppDir());
if (activity != null) {//4、创建ContextImpl对象
ContextImpl appContext = new ContextImpl();
appContext.init(r.packageInfo, r.token, this);
appContext.setOuterContext(activity);
CharSequence title = r.activityInfo.loadLabel(appContext.getPackageManager());
Configuration config = new Configuration(mCompatConfiguration);
if (DEBUG_CONFIGURATION) Slog.v(TAG, "Launching activity "
+ r.activityInfo.name + " with config " + config);
activity.attach(appContext, this, getInstrumentation(), r.token,
r.ident, app, r.intent, r.activityInfo, title, r.parent,
r.embeddedID, r.lastNonConfigurationInstances, config);//5、执行Activity的attach方法，将此ContextImpl对象，设置给Activity，activity会调用attachBaseContext
if (customIntent != null) {
activity.mIntent = customIntent;
}
r.lastNonConfigurationInstances = null;
activity.mStartedActivity = false;
int theme = r.activityInfo.getThemeResource();//6、设置主题
if (theme != 0) {
activity.setTheme(theme);
}
activity.mCalled = false;
mInstrumentation.callActivityOnCreate(activity, r.state);//7、执行Activity的onCreate方法
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();//8、执行Activity的onStart方法
r.stopped = false;
}
if (!r.activity.mFinished) {
if (r.state != null) {
mInstrumentation.callActivityOnRestoreInstanceState(activity, r.state);//9、质细腻感onRestoresInstanceState方法
}
}
if (!r.activity.mFinished) {
activity.mCalled = false;
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);//10、将包含activity信息集的r对象，也就是ActivityClientRecord，加入到mActivities中，r.token为key值。
} 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;
}
总结：
activity的packageInfo对象和application的packageInfo是同一个对象。
6、Service中Context的创建过程
step 1、Ams通过远程Binder调用ActivityThread的内部类ApplicationThread的scheduleCreateService方法，参数包括serviceInfo，这个对象由Ams创建，通过IPC传递到ActivityThread的内部类ApplicationThread中。public final void scheduleCreateService(IBinder token,step 2、构建CreateServiceData对象，如上代码所示。
ServiceInfo info, CompatibilityInfo compatInfo) {
CreateServiceData s = new CreateServiceData();
s.token = token;
s.info = info;
s.compatInfo = compatInfo;
queueOrSendMessage(H.CREATE_SERVICE, s);
}
step 3、调用H Handler，执行handleCreateService（）方法。
其中step 3又可分为一下5步。
private void handleCreateService(CreateServiceData data) {
// If we are getting ready to gc after going to the background, well
// we are back active so skip it.
unscheduleGcIdler();
LoadedApk packageInfo = getPackageInfoNoCheck(
data.info.applicationInfo, data.compatInfo);//1、得到packageInfo，调用getPackageInfoNoCheck
Service service = null;
try {
java.lang.ClassLoader cl = packageInfo.getClassLoader();
service = (Service) cl.loadClass(data.info.name).newInstance();
} catch (Exception e) {
if (!mInstrumentation.onException(service, e)) {
throw new RuntimeException(
"Unable to instantiate service " + data.info.name
+ ": " + e.toString(), e);
}
}
try {
if (localLOGV) Slog.v(TAG, "Creating service " + data.info.name);
ContextImpl context = new ContextImpl();//2、创建ContextImpl对象
context.init(packageInfo, null, this);
Application app = packageInfo.makeApplication(false, mInstrumentation);//3、得到Application对象
context.setOuterContext(service);
service.attach(context, this, data.info.name, data.token, app,
ActivityManagerNative.getDefault());//4、调用service的attach方法，将实例化的ContextImpl设置给Service
service.onCreate();
mServices.put(data.token, service);//5、将service对象加入到mService集合中，key值为data.token。
try {
ActivityManagerNative.getDefault().serviceDoneExecuting(
data.token, 0, 0, 0);
} catch (RemoteException e) {
// nothing to do.
}
} catch (Exception e) {
if (!mInstrumentation.onException(service, e)) {
throw new RuntimeException(
"Unable to create service " + data.info.name
+ ": " + e.toString(), e);
}
}
}
综上所述：
1、无论是Application还是Activity、Service，他们的LoadedApk对象都是同一个，或者说packageInfo为同一个对象。
2、在创建ContextImpl对象时，Application和SErvice通过getPackageInfoNoCheck方法，Activity通过getPackageInfo方法得到。
3、一个应用程序中Context的个数 = Activity的数量+Service的数量 +1。这里的1代表Application。
4、应用程序中包含着多个ContextImpl对象，其内部的PackageInfo却是同一个。这样设计意味着ContextImpl是一个轻量级类，PackageInfo是一个重量级类，所有和包相关的操作封装到PackageInfo中，有利于代码的封装与隐藏。
class ContextImpl extends Context {
private final static String TAG = "ApplicationContext";
private final static boolean DEBUG = false;
private static final HashMap<String, SharedPreferencesImpl> sSharedPrefs =
new HashMap<String, SharedPreferencesImpl>();
/*package*/ LoadedApk mPackageInfo;