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,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,true); } private LoadedApk getPackageInfo(ApplicationInfo aInfo,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,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,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); } } } 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,Bundle state,List<ResultInfo> pendingResults,List<Intent> pendingNewIntents,boolean notResumed,boolean isForward,String profilename,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 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,getInstrumentation(),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,this); Application app = packageInfo.makeApplication(false,mInstrumentation);//3、得到Application对象 context.setouterContext(service); service.attach(context,data.info.name,data.token,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); } 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 *** 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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