android中Context深入详解

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 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, 
        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。