主要内容
基本分析过程是一个Activity,a调用startActivity()启动b Activity（），Android系统内部进行的一些工作过程，主要分为三个过程，a Activity中的操作， AMS中的操作，B Activity中的操作，当中会有WMS相关的操作，基本目的是比较全面的分析一个Activity启动的全过程。
第一个A Activity中的工作
/framework/base/core/java/android/app/Activity.java
这个类中有许多的以startActivity开头的方法：
public void startActivityForResult(Intent intent, int requestCode)，
public void startActivityForResult(Intent intent, int requestCode, Bundle options)，
public void startActivity(Intent intent)，
public void startActivity(Intent intent, Bundle options)，
public void startActivities(Intent[] intents)，
public void startActivities(Intent[] intents, Bundle options)，
public boolean startActivityIfNeeded(Intent intent, int requestCode)，
public boolean startActivityIfNeeded(Intent intent, int requestCode, Bundle options)，
public void startActivityFromChild(Activity child, Intent intent,int requestCode) ，
public void startActivityFromChild(Activity child, Intent intent, int requestCode, Bundle options)
参数少的方法都是直接调用参数多的方法，其中参数多的方法中的参数options为null.
2.1 startActivity(Intent intent, Bundle options)
这个方法应该是最常用的方法，他的第一个参数是启动目标Activity的意图对象，Intent对象实质是一个Parcelable对象，因为这个对象需要进行跨进程传递给AMS，以使AMS能够根据这个对象找到正确的目标Activity信息。第二个参数用于传递数据，因为也是跨进程的，所以Bundle本身是一个Parcelable对象。
这个方法的实现就是直接调用startActivityForResule（）方法。
public void startActivity(Intent intent, Bundle options) {
if (options != null) {
startActivityForResult(intent, -1, options);
} else {
// Note we want to go through this call for compatibility with
// applications that may have overridden the method.
startActivityForResult(intent, -1);
}
}
2.2 startActivityForResult(Intent intent, int requestCode, Bundle options)
这个方法多了一个int型的参数，这个参数其实是一个返回值，如果这个值大于0，则这个值会在此Activity退出时返回给调用他的Activity。当直接startActivity()时，这个值是-1，所以默认是不会返回的。
public void startActivityForResult(Intent intent, int requestCode, Bundle options) {
if (mParent == null) {
Instrumentation.ActivityResult ar =
mInstrumentation.execStartActivity(
this, mMainThread.getApplicationThread(), mToken, this,
intent, requestCode, options);
if (ar != null) {
mMainThread.sendActivityResult(
mToken, mEmbeddedID, requestCode, ar.getResultCode(),
ar.getResultData());
}
if (requestCode >= 0) {
// If this start is requesting a result, we can avoid making
// the activity visible until the result is received. Setting
// this code during onCreate(Bundle savedInstanceState) or onResume() will keep the
// activity hidden during this time, to avoid flickering.
// This can only be done when a result is requested because
// that guarantees we will get information back when the
// activity is finished, no matter what happens to it.
mStartedActivity = true;
}
} else {
if (options != null) {
mParent.startActivityFromChild(this, intent, requestCode, options);
} else {
// Note we want to go through this method for compatibility with
// existing applications that may have overridden it.
mParent.startActivityFromChild(this, intent, requestCode);
}
}
}
这个方法根据mParent是否为空来进行分支处理。这个变量声明为Activity mParent，他也是一个Activity,这个变量在Activity本地创建时会被赋值，具体是在Activity在attach的时候会被赋值。
final void attach(Context context, ActivityThread aThread,这个方法attach()是在ActivityThread-->handleLaunchActivity-->performLaunchActivity中调用的，调用activity.attach(..., r.parent,...);这个r为一个ActivityClientRecord对象，是一个ActivityRecord的本地对象。mParent赋值具体原则是如果新建的Activity是ActivityTask的根Activity,则mParent为null,否则为ActivityTask的根Activity
Instrumentation instr, IBinder token, int ident,
Application application, Intent intent, ActivityInfo info,
CharSequence title, Activity parent, String id,
NonConfigurationInstances lastNonConfigurationInstances,
Configuration config) {
attachBaseContext(context);
mFragments.attachActivity(this);
mWindow = PolicyManager.makeNewWindow(this);<span style="white-space:pre"> </span>//这里申请一个surface
mWindow.setCallback(this);
mWindow.getLayoutInflater().setPrivateFactory(this);
...
mParent = parent;
...
}
如果mParent不为空，则调用startActivityFromChild()方法，否则调用mInstrumentation.execStartActivity(...);方法；而startActivityFromChild(..)方法的实现也是调用了mInstrumentation.execStartActivity(...)方法
public void startActivityFromChild(Activity child, Intent intent,即startActivity方法最终都会调用mInstrumentation.execStartActivity(...)方法，最终调用AMS启动新的Activity的操作也是在这个方法中执行的。
int requestCode, Bundle options) {
Instrumentation.ActivityResult ar =
mInstrumentation.execStartActivity(
this, mMainThread.getApplicationThread(), mToken, child,
intent, requestCode, options);
if (ar != null) {
mMainThread.sendActivityResult(
mToken, child.mEmbeddedID, requestCode,
ar.getResultCode(), ar.getResultData());
}
}
2.3 Instrumentation.java
/framework/base/core/java/android/app/Activity.java
/**这个类的对象会在Application之前创建对象，用于监督系统和Application直接的交互。
* Base class for implementing application instrumentation code. When running
* with instrumentation turned on, this class will be instantiated for you
* before any of the application code, allowing you to monitor all of the
* interaction the system has with the application. An Instrumentation
* implementation is described to the system through an AndroidManifest.xml's
* <instrumentation> tag.
*/
public class Instrumentation
public ActivityResult execStartActivity(可以知道他调用的是ActivityManagerNative里的startActivity方法。ActivityManagerNative是ActivityManagerService的本地代理类，由他代理和AMS之间的跨进程交互。这个类里面有很多的callActivityOnXx的方法，对应Activity的OnCreate等方法，即Activity的生命周期方法是在这个类里面调用的。ActivityManagerNative.startActivity方法会将调用者的进程信息，目标Activity的信息传递给AMS，一遍AMS去真正启动一个Activity组件。
Context who, IBinder contextThread, IBinder token, Activity target,
Intent intent, int requestCode, Bundle options) {
IApplicationThread whoThread = (IApplicationThread) contextThread;
if (mActivityMonitors != null) {
synchronized (mSync) {
final int N = mActivityMonitors.size();
for (int i=0; i<N; i++) {
final ActivityMonitor am = mActivityMonitors.get(i);
if (am.match(who, null, intent)) {
am.mHits++;
if (am.isBlocking()) {
return requestCode >= 0 ? am.getResult() : null;
}
break;
}
}
}
}
try {
intent.migrateExtraStreamToClipData();
intent.prepareToLeaveProcess();
int result = ActivityManagerNative.getDefault()
.startActivity(whoThread, who.getBasePackageName(), intent,
intent.resolveTypeIfNeeded(who.getContentResolver()),
token, target != null ? target.mEmbeddedID : null,
requestCode, 0, null, null, options);
checkStartActivityResult(result, intent);
} catch (RemoteException e) {
}
return null;
}
2.4 ActivityManagerNative
public int startActivity(IApplicationThread caller, Intent intent,方法一开始获取两个Parcel对象，data和reply,分别用于往AMS写数据，和从AMS获取结果。
String resolvedType, IBinder resultTo, String resultWho, int requestCode,
int startFlags, String profileFile,
ParcelFileDescriptor profileFd, Bundle options) throws RemoteException {
Parcel data = Parcel.obtain();
Parcel reply = Parcel.obtain();
data.writeInterfaceToken(IActivityManager.descriptor);
data.writeStrongBinder(caller != null ? caller.asBinder() : null);
intent.writeToParcel(data, 0);
data.writeString(resolvedType);
data.writeStrongBinder(resultTo);
data.writeString(resultWho);
data.writeInt(requestCode);
data.writeInt(startFlags);
data.writeString(profileFile);
if (profileFd != null) {
data.writeInt(1);
profileFd.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);
reply.readException();
int result = reply.readInt();
reply.recycle();
data.recycle();
return result;
}
然后开始往data中写入相关数据：
data.writeStrongBinder(caller != null ? caller.asBinder() : null);写入调用者进程信息。
intent.writeToParcel(data, 0);写入Intent信息
最重要的语句：
mRemote.transact(START_ACTIVITY_TRANSACTION, data, reply, 0);这段代码将开启AMS远程服务,注意这里传递的flag是
START_ACTIVITY_TRANSACTION<span style="font-family: Arial, Helvetica, sans-serif;">。mRemote对象是一个Binder的客户端，由ServiceManager返回，具体代码为：</span>
protected IActivityManager create() {
IBinder b = ServiceManager.getService("activity");
if (false) {
Log.v("ActivityManager", "default service binder = " + b);
}
IActivityManager am = asInterface(b);
if (false) {
Log.v("ActivityManager", "default service = " + am);
}
return am;
}
可见mRemote是通过ServiceManager获取的系统服务。transact方法里面会进行跨进程调用，具体使用Binder机制如何通信这里先不说，这一步的最后会调用ActivityManagerService里面的onTransact方法，而这个方法的具体实现也是在ActivityManagerNative里面，就是说在客户端和服务器端都有一个此类的对象。
/framework/base/core/java/android/app/ActivityManagerNative.java
这个方法有1500多行，会处理所有四大组件的相关事务，
public boolean onTransact(int code, Parcel data, Parcel reply, int flags)
throws RemoteException {
switch (code) {
case START_ACTIVITY_TRANSACTION:
{
data.enforceInterface(IActivityManager.descriptor);
IBinder b = data.readStrongBinder();
IApplicationThread app = ApplicationThreadNative.asInterface(b);
Intent intent = Intent.CREATOR.createFromParcel(data);
String resolvedType = data.readString();
IBinder resultTo = data.readStrongBinder();
String resultWho = data.readString();
int requestCode = data.readInt();
int startFlags = data.readInt();
String profileFile = data.readString();
ParcelFileDescriptor profileFd = data.readInt() != 0
? data.readFileDescriptor() : null;
Bundle options = data.readInt() != 0
? Bundle.CREATOR.createFromParcel(data) : null;
int result = startActivity(app, intent, resolvedType,
resultTo, resultWho, requestCode, startFlags,
profileFile, profileFd, options);
reply.writeNoException();
reply.writeInt(result);
return true;
}
<span style="white-space:pre"> </span>...
<span style="white-space:pre"> </span>}这个方法首先从data中取出数据，然后执行Activity启动过程中最重要的方法startActivity()；