源码分析 --- Context

1.1 Context是什么?

1) Context是一个抽象类，其通用实现在ContextImpl类中。
2) Context：是一个访问application环境全局信息的接口，通过它可以访问application的资源和相关的类，其主要功能如下：
• 启动Activity

• 启动和停止Service

• 发送广播消息(Intent)

• 注册广播消息(Intent)接收者

• 可以访问APK中各种资源(如Resources和AssetManager等)

• 可以访问Package的相关信息

• APK的各种权限管理
从以上分析可以看出，Context就是一个对APK包无所不知的大管家，大家需要什么，直接问它就可以了。

1.1.1 Context与View的关系

View与Context(或Activity)的关系类似于明星与经纪人的关系，所以创建View时，必须明确指定其Context(即经纪人或大管家)，否则View就成不了明星。

1.2 Context家族关系

Context类中基本都是抽象的方法

ContextImpl类是Context的实现类，实现了Context中定义的抽象的方法

ContextWrapper只是简单的一个代理类，包含一个ContextImpl的对象，也实现了Context中定义的抽象的方法，但实际上调用的是ContextImpl中的方法实现

1.3 Context关键函数

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public abstract class Context {

// 获取应用程序包的AssetManager实例

public abstract AssetManager getAssets();

// 获取应用程序包的Resources实例

public abstract Resources getResources();

// 获取PackageManager实例，以查看全局package信息

public abstract PackageManager getPackageManager();

// 获取应用程序包的ContentResolver实例

public abstract ContentResolver getContentResolver();

// 它返回当前进程的主线程的Looper，此线程分发调用给应用组件(activities, services等)

public abstract Looper getMainLooper();

// 返回当前进程的单实例全局Application对象的Context

public abstract Context getApplicationContext();

// 从string表中获取本地化的、格式化的字符序列

public final CharSequence getText(int resId) {

return getResources().getText(resId);

}

// 从string表中获取本地化的字符串

public final String getString(int resId) {

return getResources().getString(resId);

}

public final String getString(int resId, Object... formatArgs) {

return getResources().getString(resId, formatArgs);

}

// 返回一个可用于获取包中类信息的class loader

public abstract ClassLoader getClassLoader();

// 返回应用程序包名

public abstract String getPackageName();

// 返回应用程序信息

public abstract ApplicationInfo getApplicationInfo();

// 根据文件名获取SharedPreferences

public abstract SharedPreferences getSharedPreferences(String name,

int mode);

// 其根目录为: Environment.getExternalStorageDirectory()

public abstract File getExternalFilesDir(String type);

// 返回应用程序obb文件路径

public abstract File getObbDir();

// 启动一个新的activity

public abstract void startActivity(Intent intent);

// 启动一个新的activity

public void startActivityAsUser(Intent intent, UserHandle user) {

throw new RuntimeException("Not implemented. Must override in a subclass.");

}

// 启动一个新的activity

// intent: 将被启动的activity的描述信息

// options: 描述activity将如何被启动

public abstract void startActivity(Intent intent, Bundle options);

// 启动多个新的activity

public abstract void startActivities(Intent[] intents);

// 启动多个新的activity

public abstract void startActivities(Intent[] intents, Bundle options);

// 广播一个intent给所有感兴趣的接收者，异步机制

public abstract void sendBroadcast(Intent intent);

// 广播一个intent给所有感兴趣的接收者，异步机制

public abstract void sendBroadcast(Intent intent,String receiverPermission);

public abstract void sendOrderedBroadcast(Intent intent,String receiverPermission);

public abstract void sendOrderedBroadcast(Intent intent,

String receiverPermission, BroadcastReceiver resultReceiver,

Handler scheduler, int initialCode, String initialData,

Bundle initialExtras);

public abstract void sendBroadcastAsUser(Intent intent, UserHandle user);

public abstract void sendBroadcastAsUser(Intent intent, UserHandle user,

String receiverPermission);

// 注册一个BroadcastReceiver，且它将在主activity线程中运行

public abstract Intent registerReceiver(BroadcastReceiver receiver,

IntentFilter filter);

public abstract Intent registerReceiver(BroadcastReceiver receiver,

IntentFilter filter, String broadcastPermission, Handler scheduler);

public abstract void unregisterReceiver(BroadcastReceiver receiver);

// 请求启动一个application service

public abstract ComponentName startService(Intent service);

// 请求停止一个application service

public abstract boolean stopService(Intent service);

// 连接一个应用服务，它定义了application和service间的依赖关系

public abstract boolean bindService(Intent service, ServiceConnection conn,

int flags);

// 断开一个应用服务，当服务重新开始时，将不再接收到调用，

// 且服务允许随时停止

public abstract void unbindService(ServiceConnection conn);

public abstract Object getSystemService(String name);

public abstract int checkPermission(String permission, int pid, int uid);

// 返回一个新的与application name对应的Context对象

public abstract Context createPackageContext(String packageName,

int flags) throws PackageManager.NameNotFoundException;

// 返回基于当前Context对象的新对象，其资源与display相匹配

public abstract Context createDisplayContext(Display display);

}

1.5 ContextWrapper

它只是对Context类的一种封装，它的构造函数包含了一个真正的Context引用，即ContextImpl对象。

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/**

* Proxying implementation of Context that simply delegates all of its calls to

* another Context. Can be subclassed to modify behavior without changing

* the original Context.

*/

public class ContextWrapper extends Context {

Context mBase;

//该属性指向一个ContextIml实例

public ContextWrapper(Context base) {

mBase = base;

}

/**

* Set the base context for this ContextWrapper. All calls will then be

* delegated to the base context. Throws

* IllegalStateException if a base context has already been set.

*

* @param base The new base context for this wrapper.

* 创建Application、Service、Activity，会调用该方法给mBase属性赋值

*/

protected void attachBaseContext(Context base) {

if (mBase != null) {

throw new IllegalStateException("Base context already set");

}

mBase = base;

}

@Override

public Looper getMainLooper() {

return mBase.getMainLooper();

}

@Override

public Object getSystemService(String name) {

return mBase.getSystemService(name);

}

@Override

public void startActivity(Intent intent) {

mBase.startActivity(intent);

}

// 也实现了Context的抽象方法getResources，但是只是一层代理，实际上调用的是ContextIml中实现的方法

@Override

public Resources getResources()

{

return mBase.getResources();

}

}

1.6 ContextThemeWrapper

该类内部包含了主题(Theme)相关的接口，即Android:theme属性指定的。只有Activity需要主题，Service不需要主题，所以Service直接继承于ContextWrapper类。

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/**

* A ContextWrapper that allows you to modify the theme from what is in the

* wrapped context.

*/

public class ContextThemeWrapper extends ContextWrapper {

private Context mBase;

private int mThemeResource;

private Resources.Theme mTheme;

private LayoutInflater mInflater;

private Configuration mOverrideConfiguration;

private Resources mResources;

public ContextThemeWrapper() {

super(null);

}

public ContextThemeWrapper(Context base, int themeres) {

super(base);

mBase = base;

mThemeResource = themeres;

}

@Override protected void attachBaseContext(Context newBase) {

super.attachBaseContext(newBase);

mBase = newBase;

}

@Override public void setTheme(int resid) {

mThemeResource = resid;

initializeTheme();

}

@Override public Resources.Theme getTheme() {

if (mTheme != null) {

return mTheme;

}

mThemeResource = Resources.selectDefaultTheme(mThemeResource,

getApplicationInfo().targetSdkVersion);

initializeTheme();

return mTheme;

}

}

2. 何时创建Context

应用程序在以下几种情况下创建Context实例：

1) 创建Application对象时， 而且整个App共一个Application对象

2) 创建Service对象时

3) 创建Activity对象时

因此应用程序App共有的Context数目公式为：
总Context实例个数 = Service个数 + Activity个数 + 1（Application对应的Context实例）

ActivityThread类中（就是UI线程也是主线程）消息处理函数与本节相关的内容如下：

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public void handleMessage(Message msg) {

if (DEBUG_MESSAGES) Slog.v(TAG, ">>> handling: " + codeToString(msg.what));

switch (msg.what) {

case LAUNCH_ACTIVITY: { // 创建Activity对象

Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "activityStart");

ActivityClientRecord r = (ActivityClientRecord)msg.obj;

r.packageInfo = getPackageInfoNoCheck(

r.activityInfo.applicationInfo, r.compatInfo);

handleLaunchActivity(r, null);

Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);

} break;

case BIND_APPLICATION: // 创建Application对象

Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "bindApplication");

AppBindData data = (AppBindData)msg.obj;

handleBindApplication(data);

Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);

break;

case CREATE_SERVICE: // 创建Service对象

Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "serviceCreate");

handleCreateService((CreateServiceData)msg.obj);

Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);

break;

case BIND_SERVICE: // Bind Service对象

Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "serviceBind");

handleBindService((BindServiceData)msg.obj);

Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);

break;

}

}

2.1 创建Application对象时创建Context实例

每个应用程序在第一次启动时，都会首先创建一个Application对象。从startActivity流程可知，创建Application的时机在handleBindApplication()方法中，该函数位于 ActivityThread.Java类中
，相关代码如下：

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// ActivityThread.java

private void handleBindApplication(AppBindData data) {

try {

// If the app is being launched for full backup or restore, bring it up in

// a restricted environment with the base application class.

Application app = data.info.makeApplication(data.restrictedBackupMode, null);

mInitialApplication = app;

...

} finally {

StrictMode.setThreadPolicy(savedPolicy);

}

}

// 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";

}

try {

java.lang.ClassLoader cl = getClassLoader();

ContextImpl appContext = new ContextImpl(); // 创建ContextImpl实例

appContext.init(this, null, mActivityThread);

app = mActivityThread.mInstrumentation.newApplication(

cl, appClass, appContext);

appContext.setOuterContext(app); // 将Application实例传递给Context实例

} catch (Exception e) {

...

}

mActivityThread.mAllApplications.add(app);

mApplication = app;

return app;

}

把创建的ContextImpl实例通过mActivityThread.mInstrumentation.newApplication传递进去
看下这个函数的实现：

public Application newApplication(ClassLoader cl, String className, Context context)

throws InstantiationException, IllegalAccessException,

ClassNotFoundException {

return newApplication(cl.loadClass(className), context);

}

// 也是通过反射创建Application的实例

static public Application newApplication(Class<?> clazz, Context context)

throws InstantiationException, IllegalAccessException,

ClassNotFoundException {

Application app = (Application)clazz.newInstance();

app.attach(context);

return app;

}

看下Application中attach的实现

/* package */ final void attach(Context context) {

attachBaseContext(context); // 因为Application也是继承的ContextWrapper这个类。这个方法的实现在ContextWrapper类里

mLoadedApk = ContextImpl.getImpl(context).mPackageInfo;

}

//ContextWrapper类的attachBaseContext的实现

protected void
attachBaseContext(Context base) {

if (mBase != null) {

throw new IllegalStateException("Base context already set");

}

mBase = base; //传进来的ContextImpl实例保存到了ContextWrappr中的mBase变量中。

}

2.2 创建Activity对象时创建Context实例

通过startActivity()或startActivityForResult()请求启动一个Activity时，如果系统检测需要新建一个Activity对象时，就会回调handleLaunchActivity()方法，（UI线程的handleMessage中处理的）该方法继而调用performLaunchActivity()方法，去创建一个Activity实例，并且回调onCreate()，onStart()方法等，函数都位于
ActivityThread.java类 ，相关代码如下：

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private void handleLaunchActivity(ActivityClientRecord r, Intent customIntent) {

...

Activity a = performLaunchActivity(r, customIntent); // 到下一步

if (a != null) {

r.createdConfig = new Configuration(mConfiguration);

Bundle oldState = r.state;

handleResumeActivity(r.token, false, r.isForward,

!r.activity.mFinished && !r.startsNotResumed);

...

}

...

}

private Activity performLaunchActivity(ActivityClientRecord r, Intent customIntent) {

...

Activity activity = null;

try {

java.lang.ClassLoader cl = r.packageInfo.getClassLoader();

// 这个方法中反射调用了onCreate方法创建一个activity的实例，具体可以看startActivity的源码分析

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) {

...

}

try {

Application app = r.packageInfo.makeApplication(false, mInstrumentation);

if (activity != null) {

Context appContext = createBaseContextForActivity(r, activity);

// 创建Activity的Context

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);

// 和application的实现一样，attach函数里也是调用了ContextWrapper类的attachBaseContext的实现，把创建的Context实例保存到了Activity对象的mBace变量里

activity.attach(appContext, this, getInstrumentation(), r.token,

r.ident, app, r.intent, r.activityInfo, title, r.parent,

r.embeddedID, r.lastNonConfigurationInstances, config);

if (customIntent != null) {

activity.mIntent = customIntent;

}

r.lastNonConfigurationInstances = null;

activity.mStartedActivity = false;

int theme = r.activityInfo.getThemeResource();

if (theme != 0) {

activity.setTheme(theme);

}

mActivities.put(r.token, r);

} catch (SuperNotCalledException e) {

...

} catch (Exception e) {

...

}

return activity;

}

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private Context createBaseContextForActivity(ActivityClientRecord r,

final Activity activity) {

ContextImpl appContext = new ContextImpl(); // 创建ContextImpl实例

appContext.init(r.packageInfo, r.token, this);

appContext.setOuterContext(activity);

// 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.

Context baseContext = appContext;

String pkgName = SystemProperties.get("debug.second-display.pkg");

if (pkgName != null && !pkgName.isEmpty()

&& r.packageInfo.mPackageName.contains(pkgName)) {

DisplayManagerGlobal dm = DisplayManagerGlobal.getInstance();

for (int displayId : dm.getDisplayIds()) {

if (displayId != Display.DEFAULT_DISPLAY) {

Display display = dm.getRealDisplay(displayId);

baseContext = appContext.createDisplayContext(display);

break;

}

}

}

return baseContext;

}

所以在动态加载activity的时候，只是反射调用了onCreate方法创建了要启动的activity类的对象，但是没有创建ContextImpl这个对象，也就是没有对activity中的mBase做初始化的流程，所以创建的activity不能直接使用自己的context.因为为null.也就是没有上下文环境。

2.3 创建Service对象时创建Context实例

通过startService或者bindService时，如果系统检测到需要新创建一个Service实例，就会回调handleCreateService()方法，(也是通过UI线程的handlemessage方法实现的)完成相关数据操作。handleCreateService()函数位于 ActivityThread.java类，如下：

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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();

// 反射创建server类的对象

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);

// 也是先创建对象再创建Context实例，并且通过attach方法设置到对象的mBase变量中

ContextImpl context = new ContextImpl(); // 创建ContextImpl实例

context.init(packageInfo, null, this);

Application app = packageInfo.makeApplication(false, mInstrumentation);

context.setOuterContext(service);

service.attach(context, this, data.info.name, data.token, app,

ActivityManagerNative.getDefault());

service.onCreate();

mServices.put(data.token, service);

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);

}

}

}

3. 小结

通过对ContextImp的分析可知，其方法的大多数操作都是直接调用其属性mPackageInfo(该属性类型为PackageInfo)的相关方法而来。这说明ContextImp是一种轻量级类，而PackageInfo才是真正重量级的类。一个App里的不同的activity和service类对应不同的ContextImpl实例，但是这些不同的ContextImpl实例都对应同一个packageInfo对象。