文章參考《Android進(jìn)階解密》一書
一、概述
Context也就是上下文對象，是Android的常用類，Android四大組件都會涉及到Context，比如我們啟動Service會調(diào)用ContextWrapper以及ContextImpl的startService方法，ContextWrapper以及ContextImpl就是Context的關(guān)聯(lián)類。
二、Context的關(guān)聯(lián)類
Context意為上下文，是一個應(yīng)用程序環(huán)境信息的接口。在我們使用時一般分為兩種：
- 使用Context調(diào)用方法，比如啟動Activity、訪問資源、調(diào)用系統(tǒng)級服務(wù)等
- 調(diào)用方法時傳入Context，比如彈出Toast、Dialog等
Activity、Service、Application都間接地繼承自Context，因此我們可以計算出一個應(yīng)用程序的Context數(shù)量，等于Activity和Service的數(shù)量加1,1指的是Application數(shù)量。
Context是一個抽象類，它的內(nèi)部定義了很多方法以及靜態(tài)常量，它的具體實(shí)現(xiàn)類為ContextImpl。和Context相關(guān)聯(lián)的類，除了ContextImpl,還有ContextWrapper、ContextThemeWrapper和Activity等，如下圖：
從上圖可以看出ContextWrapper、Context繼承自Context，ContextWrapper內(nèi)部包含Context類型的mBase對象，mBase對象只想ContextImpl.ContextImpl提供了很多功能，ContextWrapper是一個裝飾類，對ContextImpl進(jìn)行包裝，ContextWrapper主要是起到了方法傳遞的功能，ContextWrapper幾乎所有的方法都是調(diào)用ContextImpl的相應(yīng)方法實(shí)現(xiàn)的。ContextThemeWrapper、Activity、Service都繼承自ContextWrapper，這樣它們就可以通過mBase使用Context的功能了。Activity有主題的相關(guān)設(shè)置，所以繼承了ContextThemeWrapper里面有設(shè)置或者gettheme的方法。<br />Context的關(guān)聯(lián)類采用了裝飾模式，主要有以下優(yōu)點(diǎn)：
- 使用者可以更方便的使用Context
- 如果ContextImpl發(fā)生改變，不需要修改其它地方的代碼
- ContextImpl的實(shí)現(xiàn)不會暴露給使用者，使用者也無需要關(guān)心ContextImpl的實(shí)現(xiàn)。
- 通過組合而非繼承的方式，拓展ContextImpl的功能，在運(yùn)行時選擇不同的裝飾類，實(shí)現(xiàn)不同的功能。
下面我們來分別講解一下個Context的創(chuàng)建過程。
三、Application Context的創(chuàng)建過程
調(diào)用getApplicationContext來獲取全局的Application Context。應(yīng)用程序啟動完成后，應(yīng)用程序就會有一個全局的Application Context。我們從應(yīng)用程序的啟動過程開始。<br />ActivityThread作為應(yīng)用程序的主線程管理類，它會調(diào)用它的內(nèi)部類ApplicationThread色scheduleLaunchActivity方法來啟動Activity
// we use token to identify this activity without having to send the
// activity itself back to the activity manager. (matters more with ipc)
@Override
public final void scheduleLaunchActivity(Intent intent, IBinder token, int ident,
ActivityInfo info, Configuration curConfig, Configuration overrideConfig,
CompatibilityInfo compatInfo, String referrer, IVoiceInteractor voiceInteractor,
int procState, Bundle state, PersistableBundle persistentState,
List<ResultInfo> pendingResults, List<ReferrerIntent> pendingNewIntents,
boolean notResumed, boolean isForward, ProfilerInfo profilerInfo) {
updateProcessState(procState, false);
ActivityClientRecord r = new ActivityClientRecord();
r.token = token;
r.ident = ident;
r.intent = intent;
r.referrer = referrer;
r.voiceInteractor = voiceInteractor;
r.activityInfo = info;
r.compatInfo = compatInfo;
r.state = state;
r.persistentState = persistentState;
r.pendingResults = pendingResults;
r.pendingIntents = pendingNewIntents;
r.startsNotResumed = notResumed;
r.isForward = isForward;
r.profilerInfo = profilerInfo;
r.overrideConfig = overrideConfig;
updatePendingConfiguration(curConfig);
sendMessage(H.LAUNCH_ACTIVITY, r);
}
應(yīng)用啟動以后啟動Activity會通過Binder跨進(jìn)程調(diào)用ActivityManagerService（AMS）里面，AMS里面持有ApplicationThread的IBinder引用，跨進(jìn)程調(diào)用scheduleLaunchActivity方法，scheduleLaunchActivity最后通過Handler發(fā)送H.LAUNCH_ACTIVITY消息，最后調(diào)用ActivityThread的handleLaunchActivity方法。
private void handleLaunchActivity(ActivityClientRecord r, Intent customIntent, String reason) {
// If we are getting ready to gc after going to the background, well
// we are back active so skip it.
unscheduleGcIdler();
mSomeActivitiesChanged = true;
if (r.profilerInfo != null) {
mProfiler.setProfiler(r.profilerInfo);
mProfiler.startProfiling();
}
// Make sure we are running with the most recent config.
handleConfigurationChanged(null, null);
if (localLOGV) Slog.v(
TAG, "Handling launch of " + r);
// Initialize before creating the activity
WindowManagerGlobal.initialize();
Activity a = performLaunchActivity(r, customIntent);
if (a != null) {
r.createdConfig = new Configuration(mConfiguration);
reportSizeConfigurations(r);
Bundle oldState = r.state;
handleResumeActivity(r.token, false, r.isForward,
!r.activity.mFinished && !r.startsNotResumed, r.lastProcessedSeq, reason);
............
}
} else {
// If there was an error, for any reason, tell the activity manager to stop us.
try {
ActivityManager.getService()
.finishActivity(r.token, Activity.RESULT_CANCELED, null,
Activity.DONT_FINISH_TASK_WITH_ACTIVITY);
} catch (RemoteException ex) {
throw ex.rethrowFromSystemServer();
}
}
}
然后調(diào)用performLaunchActivity方法
private Activity performLaunchActivity(ActivityClientRecord r, Intent customIntent) {
// System.out.println("##### [" + System.currentTimeMillis() + "] ActivityThread.performLaunchActivity(" + r + ")");
。。。。。。。
try {
Application app = r.packageInfo.makeApplication(false, mInstrumentation);
。。。。。。。
return activity;
}
這里簡化了一下代碼，最后會調(diào)用ActivityClientRecord.packageInfo.makeApplication()
public Application makeApplication(boolean forceDefaultAppClass,
Instrumentation instrumentation) {
if (mApplication != null) {
return mApplication;
}
Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "makeApplication");
Application app = null;
String appClass = mApplicationInfo.className;
if (forceDefaultAppClass || (appClass == null)) {
appClass = "android.app.Application";
}
try {
。。。。。。。
ContextImpl appContext = ContextImpl.createAppContext(mActivityThread, this);
app = mActivityThread.mInstrumentation.newApplication(
cl, appClass, appContext);
appContext.setOuterContext(app);
} catch (Exception e) {
。。。。。。。。
}
mActivityThread.mAllApplications.add(app);
mApplication = app;
if (instrumentation != null) {
try {
instrumentation.callApplicationOnCreate(app);
} catch (Exception e) {
。。。。。。。。。。。
}
}
return app;
}
當(dāng)mApplication不為null時，直接返回mApplication，如果第一次啟動程序mApplication為null，通過ContextImpl.createAppContext來創(chuàng)建ContextImpl。mActivityThread.mInstrumentation.newApplication中傳入?yún)?shù)ClassLoader和上面的ContextImpl對象創(chuàng)建Application，然后又將app賦值給ContextImpl的mOuterContext，這樣COntext也持有Application的引用了，最后將Application賦值給LoadedApk的mApplication，mApplication就是Application類型的對象。下面的代碼就是Application的創(chuàng)建方法：
static public Application newApplication(Class<?> clazz, Context context)
throws InstantiationException, IllegalAccessException,
ClassNotFoundException {
Application app = (Application)clazz.newInstance();
app.attach(context);
return app;
}
這里是通過反射來創(chuàng)建Application，并調(diào)用了attach方法，將ContextImpl傳進(jìn)去，最后返回該Application。
/**
* @hide
*/
/* package */ final void attach(Context context) {
attachBaseContext(context);
mLoadedApk = ContextImpl.getImpl(context).mPackageInfo;
}
這里調(diào)用了attachBaseContext(context)，在Application的父類ContextWrapper里面調(diào)用
protected void attachBaseContext(Context base) {
if (mBase != null) {
throw new IllegalStateException("Base context already set");
}
mBase = base;
}
將ContextImpl賦值給mBase,ContextImpl也是Context的實(shí)現(xiàn)類，將ContextImpl賦值給ContextWrapper的mBase,這樣ContextWrapper就可以用mBase調(diào)用Context里面的方法了，而Application繼承自ContextWrapper，所以Application也可以調(diào)用Context里面的方法了。
四、Activity Context的創(chuàng)建過程
Activity的Context也是在performLaunchActivity方法創(chuàng)建的。
private Activity performLaunchActivity(ActivityClientRecord r, Intent customIntent) {
// System.out.println("##### [" + System.currentTimeMillis() + "] ActivityThread.performLaunchActivity(" + r + ")");
。。。。。。
ContextImpl appContext = createBaseContextForActivity(r);
Activity activity = null;
try {
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 {
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);
}
}
return activity;
}
首先通過 createBaseContextForActivity(r)創(chuàng)建ContextImpl實(shí)例appContext，將創(chuàng)建的Activity設(shè)置到ContextImpl的outContext，這樣Context里面就可以訪問Activity的變量和方法，最后Activity的attach方法，將ContextImpl對象傳進(jìn)去。這后面跟Application差不多，會在Activity的attach方法調(diào)用attachBaseContext(context)方法，將ContextImpl復(fù)制給ContextWrapper的mBase引用，Activity繼承自ContextWrapper，這樣Activity就可以調(diào)用Context里面的方法了。
五、Service Context的創(chuàng)建過程
Service的Context創(chuàng)建其實(shí)跟上面兩個差不多，只不過是在Service類里面執(zhí)行，此處就不過多敘述了。大家自己去看源碼就可以了
