Context家族及详解
Context大家太熟悉了，大家都会用。比如启动组件、获取系统服务、获取权限相关、包管理、类加载、文件管理和资源相关等功能。
这里先讲讲Context的类关系：
家族关系
开局一张图、内容全靠编：
其实Activity、Service、Application其实是亲戚关系。
其中Activity被包裹了一层ContextThemeWrapper，相比service多处理了关于Theme的。比如setTheme() initializeTheme()等方法。
ContextWrapper是一个包装类， ContextImpl是具体的实现类，比如startActivity,startService等方法的具体实现。所以这是一个典型的装饰者模式。
何时创建
根据前面所说，Context中所有实现均由ContextImpl来实现的，那么ContextImpl又何时创建的了？所以ContextWrapper中的Context引用指向的是一个ContextImpl实例。
知道android启动流程的，最后会走ActivityThread的main()方法，然后会初始化Looper巴拉巴拉之类的，然后会走到performLaunchActivity方法这里：
private Activity performLaunchActivity(ActivityClientRecord r, Intent customIntent) {
// System.out.println("##### [" + System.currentTimeMillis() + "] ActivityThread.performLaunchActivity(" + r + ")");
ActivityInfo aInfo = r.activityInfo;
if (r.packageInfo == null) {
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);
}
//获取上下文 关于activity的
ContextImpl appContext = createBaseContextForActivity(r);
Activity activity = null;
try {
//反射创建activity
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 {
//创建Application，里面会创建Application的上下文
Application app = r.packageInfo.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.packageInfo.getPackageName()
+ ", comp=" + r.intent.getComponent().toShortString()
+ ", dir=" + r.packageInfo.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对应
截取上面Activity相关
//创建ContextImpl
ContextImpl appContext = createBaseContextForActivity(r);
Activity activity = null;
try {
//反射创建activity，
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);
}
}
...
appContext.setOuterContext(activity);
//这里也要看绑定attach（）方法
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);
...
主要看createBaseContextForActivity方法，会创建ContextImpl
private ContextImpl createBaseContextForActivity(ActivityClientRecord r) {
final int displayId;
try {
displayId = ActivityManager.getService().getActivityDisplayId(r.token);
} catch (RemoteException e) {
throw e.rethrowFromSystemServer();
}
//ContextImpl 内部创建
ContextImpl appContext = ContextImpl.createActivityContext(
this, r.packageInfo, r.activityInfo, r.token, displayId, r.overrideConfig);
final DisplayManagerGlobal dm = DisplayManagerGlobal.getInstance();
// For debugging purposes, if the activity's package name contains the value of
// the "debug.use-second-display" system property as a substring, then show
// its content on a secondary display if there is one.
String pkgName = SystemProperties.get("debug.second-display.pkg");
if (pkgName != null && !pkgName.isEmpty()
&& r.packageInfo.mPackageName.contains(pkgName)) {
for (int id : dm.getDisplayIds()) {
if (id != Display.DEFAULT_DISPLAY) {
Display display =
dm.getCompatibleDisplay(id, appContext.getResources());
appContext = (ContextImpl) appContext.createDisplayContext(display);
break;
}
}
}
return appContext;
}
通过构造器创建ContextImpl
static ContextImpl createActivityContext(ActivityThread mainThread,
LoadedApk packageInfo, ActivityInfo activityInfo, IBinder activityToken, int displayId,
Configuration overrideConfiguration) {
if (packageInfo == null) throw new IllegalArgumentException("packageInfo");
...
ContextImpl context = new ContextImpl(null, mainThread, packageInfo, activityInfo.splitName,
activityToken, null, 0, classLoader);
...
return context;
}
创建好ContextImpl了，再回到上面的，当中会执行Application的创建和绑定（后面会讲），看这行activity.attach() 方法：
final void attach(Context context, ActivityThread aThread,
Instrumentation instr, IBinder token, int ident,
Application application, Intent intent, ActivityInfo info,
CharSequence title, Activity parent, String id,
NonConfigurationInstances lastNonConfigurationInstances,
Configuration config, String referrer, IVoiceInteractor voiceInteractor,
Window window, ActivityConfigCallback activityConfigCallback) {
...
//这里的context 就是 ContextImpl
attachBaseContext(context);
...
}
先走Activity自身的attachBaseContext()方法
@Override
protected void attachBaseContext(Context newBase) {
//这里的父类 是 ContextThemeWrapper
super.attachBaseContext(newBase);
newBase.setAutofillClient(this);
}
跟进ContextThemeWrapper中的attachBaseContext()方法：
@Override
protected void attachBaseContext(Context newBase) {
//这里的父类是 ContextWrapper
super.attachBaseContext(newBase);
}
跟进ContextWrapper中的attachBaseContext()方法：
protected void attachBaseContext(Context base) {
if (mBase != null) {
throw new IllegalStateException("Base context already set");
}
mBase = base;
}
到这了，已经将ContextImpl的引用传递到了ContextWrapper中，而mBase就是这个引用，我们使用的类似startActivity()等方法最终是由ContextImpl中执行的，比如：
@Override
public void startActivity(Intent intent) {
mBase.startActivity(intent);
}
Application对应
在回到上面performLaunchActivity方法的代码，这里把Application相关抽取下来：
//其实就一行哈哈
...
Application app = r.packageInfo.makeApplication(false, mInstrumentation);
...
看Application的创建, 在LoadedApk的makeApplication()方法：
public Application makeApplication(boolean forceDefaultAppClass,
Instrumentation instrumentation) {
...
Application app = null
//创建ContextImpl
ContextImpl appContext = ContextImpl.createAppContext(mActivityThread, this);
//这里由仪表盘创建Application
app = mActivityThread.mInstrumentation.newApplication(
cl, appClass, appContext);
appContext.setOuterContext(app);
...
return app;
}
先看ContextImpl的创建：
static ContextImpl createAppContext(ActivityThread mainThread, LoadedApk packageInfo) {
if (packageInfo == null) throw new IllegalArgumentException("packageInfo");
ContextImpl context = new ContextImpl(null, mainThread, packageInfo, null, null, null, 0,
null);
context.setResources(packageInfo.getResources());
return context;
}
还要接着看Instrumentation的newApplication()方法：
//Instrumentation中newApplication
public Application newApplication(ClassLoader cl, String className, Context context)
throws InstantiationException, IllegalAccessException,
ClassNotFoundException {
//反射创建Application
Application app = getFactory(context.getPackageName())
.instantiateApplication(cl, className);
//这里走了Application的attach（）方法了
app.attach(context);
return app;
}
//AppComponentFactory中instantiateApplication
public @NonNull Application instantiateApplication(@NonNull ClassLoader cl,
@NonNull String className)
throws InstantiationException, IllegalAccessException, ClassNotFoundException {
return (Application) cl.loadClass(className).newInstance();
}
看到上面attach()方法，传入的是ContextImpl的引用,看Application的attach
/* package */ final void attach(Context context) {
//自己没实现这个方法，由父类实现了 ，就是ContextWrapper
attachBaseContext(context);
mLoadedApk = ContextImpl.getImpl(context).mPackageInfo;
}
到这里又跟上面Activity的绑定一样了：
protected void attachBaseContext(Context base) {
if (mBase != null) {
throw new IllegalStateException("Base context already set");
}
mBase = base;
}
到这了，已经将ContextImpl的引用传递到了ContextWrapper中，而mBase就是这个引用，完成了与Application的相关了。
Service对应
具体的Service启动流程不说了，最后还是会走到ActivityThread 的 handleCreateService() 方法中:
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);
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);
//创建ContextImpl
ContextImpl context = ContextImpl.createAppContext(this, packageInfo);
context.setOuterContext(service);
Application app = packageInfo.makeApplication(false, mInstrumentation);
//绑定方法
service.attach(context, this, data.info.name, data.token, app,
ActivityManager.getService());
service.onCreate();
mServices.put(data.token, service);
try {
ActivityManager.getService().serviceDoneExecuting(
data.token, SERVICE_DONE_EXECUTING_ANON, 0, 0);
} catch (RemoteException e) {
throw e.rethrowFromSystemServer();
}
} catch (Exception e) {
if (!mInstrumentation.onException(service, e)) {
throw new RuntimeException(
"Unable to create service " + data.info.name
+ ": " + e.toString(), e);
}
}
}
这里创建ContextImpl就不看了，主要看service.attach()方法：
public final void attach(
Context context,
ActivityThread thread, String className, IBinder token,
Application application, Object activityManager) {
//自己没有实现，调用父类方法 ContextWrapper
attachBaseContext(context);
mThread = thread; // NOTE: unused - remove?
mClassName = className;
mToken = token;
mApplication = application;
mActivityManager = (IActivityManager)activityManager;
mStartCompatibility = getApplicationInfo().targetSdkVersion
< Build.VERSION_CODES.ECLAIR;
}
到这了，已经将ContextImpl的引用传递到了ContextWrapper中，而mBase就是这个引用，完成了与Service的相关了。
面试点
一个应用包含多少个Context个数？
看了前面的分析得到：
Context个数 = Activity个数 + Application（只有一个） + Service个数
Application能启动一个Activity嘛？
要区分来看，对于不同android版本也不同。
在android 9.0 执行如下代码：
Intent intent=new Intent();
intent.setClass(context, LoginActivity.class);
//intent.setFlags(Intent.FLAG_ACTIVITY_NEW_TASK);
context.startActivity(intent);
这里会报错的：
（"Calling startActivity() from outside of an Activity "
+ " context requires the FLAG_ACTIVITY_NEW_TASK flag."
+ " Is this really what you want?")
但是对于android 6，android 7，android 8 却不会报错，而对比源码如下
@Override
public void startActivity(Intent intent, Bundle options) {
warnIfCallingFromSystemProcess();
// Calling start activity from outside an activity without FLAG_ACTIVITY_NEW_TASK is
// generally not allowed, except if the caller specifies the task id the activity should
// be launched in.
if ((intent.getFlags()&Intent.FLAG_ACTIVITY_NEW_TASK) == 0
&& options != null && ActivityOptions.fromBundle(options).getLaunchTaskId() == -1) {
throw new AndroidRuntimeException(
"Calling startActivity() from outside of an Activity "
+ " context requires the FLAG_ACTIVITY_NEW_TASK flag."
+ " Is this really what you want?");
}
mMainThread.getInstrumentation().execStartActivity(
getOuterContext(), mMainThread.getApplicationThread(), null,
(Activity) null, intent, -1, options);
}
由于options为空，直接跳过if判断，所以不会报错。
对于9.0代码如下：
@Override
public void startActivity(Intent intent, Bundle options) {
warnIfCallingFromSystemProcess();
// Calling start activity from outside an activity without FLAG_ACTIVITY_NEW_TASK is
// generally not allowed, except if the caller specifies the task id the activity should
// be launched in. A bug was existed between N and O-MR1 which allowed this to work. We
// maintain this for backwards compatibility.
final int targetSdkVersion = getApplicationInfo().targetSdkVersion;
if ((intent.getFlags() & Intent.FLAG_ACTIVITY_NEW_TASK) == 0
&& (targetSdkVersion < Build.VERSION_CODES.N
|| targetSdkVersion >= Build.VERSION_CODES.P)
&& (options == null
|| ActivityOptions.fromBundle(options).getLaunchTaskId() == -1)) {
throw new AndroidRuntimeException(
"Calling startActivity() from outside of an Activity "
+ " context requires the FLAG_ACTIVITY_NEW_TASK flag."
+ " Is this really what you want?");
}
mMainThread.getInstrumentation().execStartActivity(
getOuterContext(), mMainThread.getApplicationThread(), null,
(Activity) null, intent, -1, options);
}
多判断了ActivityOptions.fromBundle(options).getLaunchTaskId() == -1这个，条件还是会成立的，就会抛出报错信息。
综上所述：对于版本不同，还要不同处理，针对版本判断，或者统一加上FLAG_ACTIVITY_NEW_TASK标签，但是这会新增一个task。所以真正解决问题的是：万不得已，不要用Application去启动一个Activity。
附图一张，抄来的：
数字1：启动Activity在这些类中是可以的，但是可能需要创建一个新的task。一般情况不推荐。
数字2：在这些类中去layout inflate是合法的，但是会使用系统默认的主题样式，如果你自定义了某些样式可能不会被使用。
数字3：在receiver为null时允许，在4.2或以上的版本中，用于获取黏性广播的当前值。（可以无视）
Context为何会发生泄漏，怎么解决？
单例里引用一个Context，而且这个Context刚好是Activity所对应的。当Activity销毁，但是单例中的Context还持有Activity的引用，就会造成内存泄漏。
解决：换成Application所对于的Context就行了，自身生命周期同引用一样，不存在泄漏。
ContentProvider的中的Context那来的？
这里由事关android的启动流程了，下次再讲这个流程，还是挺复杂的。只需知道ActivityThread的attach()方法会一步步调用到handleBindApplication方法：
private void handleBindApplication(AppBindData data) {
app = data.info.makeApplication(data.restrictedBackupMode, null);
// Propagate autofill compat state
app.setAutofillCompatibilityEnabled(data.autofillCompatibilityEnabled);
//这里就是Application
mInitialApplication = app;
// don't bring up providers in restricted mode; they may depend on the
// app's custom Application class
if (!data.restrictedBackupMode) {
if (!ArrayUtils.isEmpty(data.providers)) {
//传入的是Application
installContentProviders(app, data.providers);
// For process that contains content providers, we want to
// ensure that the JIT is enabled "at some point".
mH.sendEmptyMessageDelayed(H.ENABLE_JIT, 10*1000);
}
}
}
所以ContentProvider 的 Context 是在 Applicaiton 创建之后，被ContentProvider再利用了。
总结
其实所有的这些，只需弄懂android的启动流程，再看这些就不说很复杂了。源码的设计模式运用也非常优雅，值得多看。
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