以下分别通过Context认知角度，继承关系，对象创建等方面android中Context做了深入的解释，一起学习下。
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, 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 2、构建AppBindData对象，如上代码所示。
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, 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 2、构建ActivityClientRecord对象，如上代码所示。
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 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, ServiceInfo info, CompatibilityInfo compatInfo) { CreateServiceData s = new CreateServiceData(); s.token = token; s.info = info; s.compatInfo = compatInfo; queueOrSendMessage(H.CREATE_SERVICE, s); }step 2、构建CreateServiceData对象，如上代码所示。
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;以上就是本篇文章的全部内容，希望大家通过学习能够对Context有更深入的理解。
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