Android深入四大组件(一)应用程序启动过程(前篇)

activity_cover-1024x576_副本.png

前言

在此前的文章中,我讲过了Android系统启动流程和Android应用进程启动过程,这一篇顺理成章来学习Android 7.0的应用程序的启动过程。分析应用程序的启动过程其实就是分析根Activity的启动过程。

1.Launcher请求ActivityManageService

Android系统启动流程(四)Launcher启动过程与系统启动流程这篇文章我讲过Launcher启动后会将已安装应用程序的快捷图标显示到界面上,当我们点击应用程序的快捷图标时就会调用Launcher的startActivitySafely方法,如下所示。
packages/apps/Launcher3/src/com/android/launcher3/Launcher.java

public boolean startActivitySafely(View v, Intent intent, Object tag) {
...
try {
success = startActivity(v, intent, tag);
} catch (ActivityNotFoundException e) {
Toast.makeText(this, R.string.activity_not_found, Toast.LENGTH_SHORT).show();
Log.e(TAG, "Unable to launch. tag=" + tag + " intent=" + intent, e);
}
return success;
}

startActivitySafely函数中主要就是调用了startActivity函数:
packages/apps/Launcher3/src/com/android/launcher3/Launcher.java

private boolean startActivity(View v, Intent intent, Object tag) {
intent.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK);//1
try {
...
if (user == null || user.equals(UserHandleCompat.myUserHandle())) {
StrictMode.VmPolicy oldPolicy = StrictMode.getVmPolicy();
try {
StrictMode.setVmPolicy(new StrictMode.VmPolicy.Builder().detectAll()
.penaltyLog().build());
startActivity(intent, optsBundle);//2
} finally {
StrictMode.setVmPolicy(oldPolicy);
}
} else {
launcherApps.startActivityForProfile(intent.getComponent(), user,
intent.getSourceBounds(), optsBundle);
}
return true;
} catch (SecurityException e) {
...
}
return false;
}

在注释1处设置Flag为Intent.FLAG_ACTIVITY_NEW_TASK,这样根Activity会在新的任务栈中启动。在注释2处调用了startActivity函数:
frameworks/base/core/java/android/app/Activity.java

Override
public void startActivity(Intent intent, @Nullable Bundle options) {
if (options != null) {
startActivityForResult(intent, -1, options);
} else {
startActivityForResult(intent, -1);
}
}

startActivity会调用startActivityForResult函数,其中第二个参数为-1,表示Launcher不需要知道Activity启动的结果,startActivityForResult函数的代码如下所示。
frameworks/base/core/java/android/app/Activity.java

public void startActivityForResult(@RequiresPermission Intent intent, int requestCode,
@Nullable Bundle options) {
if (mParent == null) {
Instrumentation.ActivityResult ar =
mInstrumentation.execStartActivity(
this, mMainThread.getApplicationThread(), mToken, this,
intent, requestCode, options);
...
} else {
...
}
}

mParent是Activity类型的,表示当前Activity的父类。因为目前根Activity还没有创建出来,因此,mParent == null成立。接着调用Instrumentation的execStartActivity方法,Instrumentation主要用来监控应用程序和系统的交互,execStartActivity方法的代码如下所示。
frameworks/base/core/java/android/app/Instrumentation.java

public ActivityResult execStartActivity(
Context who, IBinder contextThread, IBinder token, Activity target,
Intent intent, int requestCode, Bundle options) {
...
try {
intent.migrateExtraStreamToClipData();
intent.prepareToLeaveProcess(who);
int result = ActivityManagerNative.getDefault()
.startActivity(whoThread, who.getBasePackageName(), intent,
intent.resolveTypeIfNeeded(who.getContentResolver()),
token, target != null ? target.mEmbeddedID : null,
requestCode, 0, null, options);
checkStartActivityResult(result, intent);
} catch (RemoteException e) {
throw new RuntimeException("Failure from system", e);
}
return null;
}

首先会调用ActivityManagerNative的getDefault来获取ActivityManageService(以后简称为AMS)的代理对象,接着调用它的startActivity方法。首先我们先来查看ActivityManagerNative的getDefault方法做了什么:
frameworks/base/core/java/android/app/ActivityManagerNative.java

static public IActivityManager getDefault() {
return gDefault.get();
}
private static final Singleton<IActivityManager> gDefault = new Singleton<IActivityManager>() {
protected IActivityManager create() {
IBinder b = ServiceManager.getService("activity");//1
if (false) {
Log.v("ActivityManager", "default service binder = " + b);
}
IActivityManager am = asInterface(b);//2
if (false) {
Log.v("ActivityManager", "default service = " + am);
}
return am;
}
};
}

getDefault方法调用了gDefault的get方法,我们接着往下看,gDefault 是一个Singleton类。注释1处得到名为”activity”的Service引用,也就是IBinder类型的ActivityManagerService的引用。接着在注释2处将它封装成ActivityManagerProxy(以后简称为AMP)类型对象,并将它保存到gDefault中,此后调用ActivityManagerNative的getDefault方法就会直接获得AMS的代理AMP对象。
回到Instrumentation类的execStartActivity方法中,从上面得知就是调用AMP的startActivity,其中AMP是ActivityManagerNative的内部类,代码如下所示。
frameworks/base/core/java/android/app/ActivityManagerNative.java

public int startActivity(IApplicationThread caller, String callingPackage, Intent intent,
String resolvedType, IBinder resultTo, String resultWho, int requestCode,
int startFlags, ProfilerInfo profilerInfo, Bundle options) throws RemoteException {
Parcel data = Parcel.obtain();
Parcel reply = Parcel.obtain();
data.writeInterfaceToken(IActivityManager.descriptor);
data.writeStrongBinder(caller != null ? caller.asBinder() : null);
data.writeString(callingPackage);
intent.writeToParcel(data, 0);
data.writeString(resolvedType);
data.writeStrongBinder(resultTo);
data.writeString(resultWho);
data.writeInt(requestCode);
data.writeInt(startFlags);
if (profilerInfo != null) {
data.writeInt(1);
profilerInfo.writeToParcel(data, Parcelable.PARCELABLE_WRITE_RETURN_VALUE);
} else {
data.writeInt(0);
}
if (options != null) {
data.writeInt(1);
options.writeToParcel(data, 0);
} else {
data.writeInt(0);
}
mRemote.transact(START_ACTIVITY_TRANSACTION, data, reply, 0);//1
reply.readException();
int result = reply.readInt();
reply.recycle();
data.recycle();
return result;
}

首先会将传入的参数写入到Parcel类型的data中。在注释1处通过IBinder类型对象mRemote向AMS发送一个START_ACTIVITY_TRANSACTION类型的进程间通信请求。那么服务端AMS就会从Binder线程池中读取我们客户端发来的数据,最终会调用ActivityManagerNative的onTransact方法中执行,如下所示。
frameworks/base/core/java/android/app/ActivityManagerNative.java

@Override
public boolean onTransact(int code, Parcel data, Parcel reply, int flags)
throws RemoteException {
switch (code) {
case START_ACTIVITY_TRANSACTION:
{
...
int result = startActivity(app, callingPackage, intent, resolvedType,
resultTo, resultWho, requestCode, startFlags, profilerInfo, options);
reply.writeNoException();
reply.writeInt(result);
return true;
}
}

onTransact中会调用AMS的startActivity方法,如下所示。
frameworks/base/services/core/java/com/android/server/am/ActivityManagerService.java

@Override
public final int startActivity(IApplicationThread caller, String callingPackage,
Intent intent, String resolvedType, IBinder resultTo, String resultWho, int requestCode,
int startFlags, ProfilerInfo profilerInfo, Bundle bOptions) {
return startActivityAsUser(caller, callingPackage, intent, resolvedType, resultTo,
resultWho, requestCode, startFlags, profilerInfo, bOptions,
UserHandle.getCallingUserId());
}

为了便于理解,给出Launcher到AMS调用过程的时序图。

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