Android Activity启动源码详解

我们都知道启动一个Activity最普通的方法就是Activity的startActivity(Intent intent) 方法

Activity#startActivity

@Override
public void startActivity(Intent intent) {
this.startActivity(intent, null);
}

接下来看startActivity（intent，null）

@Override
public void startActivity(Intent intent, @Nullable 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);
}
}

毫无疑问，走的是startActivityForResult（intent, -1）这个分支

public void startActivityForResult(Intent intent, int requestCode) {
startActivityForResult(intent, requestCode, null);
}

Activity#startActivityForResult

public void startActivityForResult(Intent intent, int requestCode, @Nullable 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;
}

final View decor = mWindow != null ? mWindow.peekDecorView() : null;
if (decor != null) {
decor.cancelPendingInputEvents();
}
// TODO Consider clearing/flushing other event sources and events for child windows.
} 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);
}
}
if (options != null && !isTopOfTask()) {
mActivityTransitionState.startExitOutTransition(this, options);
}
}

在这里看android源码了，因为sdk里面Instrumentation只保留了部分代码

如果本地没有下载好android源码，可以移步到http://grepcode.com/file/repository.grepcode.com/java/ext/com.google.android/android/5.1.1_r1/android/app/Instrumentation.java#Instrumentation

Instrumentation#execStartActivity

1474    public ActivityResult More ...execStartActivity(
1475            Context who, IBinder contextThread, IBinder token, Activity target,
1476            Intent intent, int requestCode, Bundle options) {
1477        IApplicationThread whoThread = (IApplicationThread) contextThread;
1478        if (mActivityMonitors != null) {
1479            synchronized (mSync) {
1480                final int N = mActivityMonitors.size();
1481                for (int i=0; i<N; i++) {
1482                    final ActivityMonitor am = mActivityMonitors.get(i);
1483                    if (am.match(who, null, intent)) {
1484                        am.mHits++;
1485                        if (am.isBlocking()) {
1486                            return requestCode >= 0 ? am.getResult() : null;
1487                        }
1488                        break;
1489                    }
1490                }
1491            }
1492        }
1493        try {
1494            intent.migrateExtraStreamToClipData();
1495            intent.prepareToLeaveProcess();
1496            int result = ActivityManagerNative.getDefault()
1497                .startActivity(whoThread, who.getBasePackageName(), intent,
1498                        intent.resolveTypeIfNeeded(who.getContentResolver()),
1499                        token, target != null ? target.mEmbeddedID : null,
1500                        requestCode, 0, null, options);
1501            checkStartActivityResult(result, intent);
1502        } catch (RemoteException e) {
1503        }
1504        return null;
1505    }