一、系统环境启动
image
Android手机开机启动Linux内核，它会去加载system/core/init/init.rc文件，启动init进程。
init进程的主要作用：
-
负责开关机界面
-
文件系统的创建和挂载
-
启动Zygote(孵化器)进程
-
启动ServiceManager，它是Binder的“DNS”，管理着所有Android系统服务
在init进程启动后会去fork一个叫Zygote的进程，顾名思义，Zygote相当于一个孵化器，它的作用主要是：
-
创建一个名叫zygote的socket，用来和客户端进程通信
-
启动SystemServer进程
-
当发来socket请求，它可以fork自身创建出一个新的进程
SystemServer是Zygote创建的第一个进程，它的作用如下：
-
启动binder线程池，保证与其他进程通信
-
启动各个系统服务并将其注册到ServiceManager中
- BootstrapServices（引导服务）：ActivityManagerService、PowerManagerService、UserManagerService等
-
CoreServices（核心服务）：BatteryService、UsageService等
-
OtherServices（其他服务）：AccountManagerService、WindowManagerService等
// SystemServer.java
// Start services.
try {
traceBeginAndSlog("StartServices");
startBootstrapServices();
startCoreServices();
startOtherServices();
SystemServerInitThreadPool.shutdown();
} catch (Throwable ex) {
Slog.e("System", "******************************************");
Slog.e("System", "************ Failure starting system services", ex);
throw ex;
} finally {
traceEnd();
}
可以看到SystemServer创建了应用所需要的各项系统服务，它们运行在system_server(系统进程)，应用进程通过binder通信调用相应的服务，在AMS被创建后，将首先启动Lancher进程，Lancher也是一个应用，它被启动之后，会从PMS获取已安装的应用列表展示给用户，用户点击图标会打开对应的应用。
二、Activity相关角色介绍
image
-
ActivityStackSupervisor
- ActivityStack的管理者，内部管理了mHomeStack、mFocusedStack和mLastFocusedStack三个ActivityStack
-
ActivityStack
- 保存TaskRecord
-
TaskRecord
- 以栈的方式保存ActivityRecord，也就是常说的任务栈
-
ActivityRecord
- 保存了Activity相关信息，对应一个Activity
image
通过adb来dump出相关信息：
//执行 adb shell dumpsys activity activities
...
TaskRecord{3f6aa9e #24 A=com.google.android.apps.messaging U=0 StackId=10 sz=2}
userId=0 effectiveUid=u0a115 mCallingUid=u0a103 mUserSetupComplete=true mCallingPackage=com.google.android.apps.nexuslauncher
affinity=com.google.android.apps.messaging
intent={act=android.intent.action.MAIN cat=[android.intent.category.LAUNCHER] flg=0x10200000 cmp=com.google.android.apps.messaging/.ui.ConversationListActivity}
origActivity=com.google.android.apps.messaging/.ui.ConversationListActivity
mActivityComponent=com.google.android.apps.messaging/.ui.conversationlist.ConversationListActivity
autoRemoveRecents=false isPersistable=true numFullscreen=2 activityType=1
rootWasReset=true mNeverRelinquishIdentity=true mReuseTask=false mLockTaskAuth=LOCK_TASK_AUTH_PINNABLE
Activities=[ActivityRecord{25f7fa9 u0 com.google.android.apps.messaging/.ui.ConversationListActivity t24}, ActivityRecord{f796dbd u0 com.google.android.apps.messaging/.ui.conversation.ConversationActivity t24}]
askedCompatMode=false inRecents=true isAvailable=true
mRootProcess=ProcessRecord{774c038 4253:com.google.android.apps.messaging/u0a115}
stackId=10
hasBeenVisible=true mResizeMode=RESIZE_MODE_RESIZEABLE_VIA_SDK_VERSION mSupportsPictureInPicture=false isResizeable=true lastActiveTime=454547 (inactive for 633s)
...
三、应用启动入口
image
从上面时序图可以看到，启动Activity前置操作主要如下几步：
- 桌面Activity调用startActivityForResult
- Instrumentation调用execStartActivity通过binder通信将intent发送给AMS(位于系统进程)
- AMS收到消息之后会先校验intent请求的合法性
- 检验成功后，ActivityStack调用resumeTopActivityInnerLocked，其中会调用startPausingLocked去将当前栈顶Activity的状态改为Pause，这里也是通过binder通信
- 然后ActivityStackSupervisor会判断当前请求启动的Activity所属进程是否启动（若manifest未设置process属性默认应用主进程），未启动则调用Process.strat()通过Zygote进程去fork出一个新的应用进程（系统进程和Zygote通过socket通信），在该进程中通过反射去调用ActivityThread.main()，即为应用启动的主入口。
// ActivityThread.java
public static void main(String[] args) {
Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "ActivityThreadMain");
// CloseGuard defaults to true and can be quite spammy. We
// disable it here, but selectively enable it later (via
// StrictMode) on debug builds, but using DropBox, not logs.
CloseGuard.setEnabled(false);
Environment.initForCurrentUser();
// Set the reporter for event logging in libcore
EventLogger.setReporter(new EventLoggingReporter());
// Make sure TrustedCertificateStore looks in the right place for CA certificates
final File configDir = Environment.getUserConfigDirectory(UserHandle.myUserId());
TrustedCertificateStore.setDefaultUserDirectory(configDir);
Process.setArgV0("<pre-initialized>");
Looper.prepareMainLooper();
// Find the value for {@link #PROC_START_SEQ_IDENT} if provided on the command line.
// It will be in the format "seq=114"
long startSeq = 0;
if (args != null) {
for (int i = args.length - 1; i >= 0; --i) {
if (args[i] != null && args[i].startsWith(PROC_START_SEQ_IDENT)) {
startSeq = Long.parseLong(
args[i].substring(PROC_START_SEQ_IDENT.length()));
}
}
}
ActivityThread thread = new ActivityThread();
thread.attach(false, startSeq);
if (sMainThreadHandler == null) {
sMainThreadHandler = thread.getHandler();
}
if (false) {
Looper.myLooper().setMessageLogging(new
LogPrinter(Log.DEBUG, "ActivityThread"));
}
// End of event ActivityThreadMain.
Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
Looper.loop();
throw new RuntimeException("Main thread loop unexpectedly exited");
}
ActivityThread的main方法主要作用如下：
- 初始化了主线程Looper对象
- 初始化ActivityThread对象同时调用attach方法，通过AMS为应用绑定一个Application，执行Application.onCreate()
- 指定主线程的Handler为H
- 调用Looper.loop()开启消息轮询，之后的操作将通过Android消息机制来实现，H中定义了各种类型的消息
class H extends Handler {
public static final int BIND_APPLICATION = 110;
public static final int EXIT_APPLICATION = 111;
public static final int RECEIVER = 113;
public static final int CREATE_SERVICE = 114;
public static final int SERVICE_ARGS = 115;
public static final int STOP_SERVICE = 116;
public static final int CONFIGURATION_CHANGED = 118;
public static final int CLEAN_UP_CONTEXT = 119;
public static final int GC_WHEN_IDLE = 120;
public static final int BIND_SERVICE = 121;
public static final int UNBIND_SERVICE = 122;
public static final int DUMP_SERVICE = 123;
public static final int LOW_MEMORY = 124;
public static final int PROFILER_CONTROL = 127;
public static final int CREATE_BACKUP_AGENT = 128;
public static final int DESTROY_BACKUP_AGENT = 129;
public static final int SUICIDE = 130;
public static final int REMOVE_PROVIDER = 131;
public static final int ENABLE_JIT = 132;
public static final int DISPATCH_PACKAGE_BROADCAST = 133;
public static final int SCHEDULE_CRASH = 134;
public static final int DUMP_HEAP = 135;
public static final int DUMP_ACTIVITY = 136;
public static final int SLEEPING = 137;
public static final int SET_CORE_SETTINGS = 138;
public static final int UPDATE_PACKAGE_COMPATIBILITY_INFO = 139;
public static final int DUMP_PROVIDER = 141;
public static final int UNSTABLE_PROVIDER_DIED = 142;
public static final int REQUEST_ASSIST_CONTEXT_EXTRAS = 143;
public static final int TRANSLUCENT_CONVERSION_COMPLETE = 144;
public static final int INSTALL_PROVIDER = 145;
public static final int ON_NEW_ACTIVITY_OPTIONS = 146;
public static final int ENTER_ANIMATION_COMPLETE = 149;
public static final int START_BINDER_TRACKING = 150;
public static final int STOP_BINDER_TRACKING_AND_DUMP = 151;
public static final int LOCAL_VOICE_INTERACTION_STARTED = 154;
public static final int ATTACH_AGENT = 155;
public static final int APPLICATION_INFO_CHANGED = 156;
public static final int RUN_ISOLATED_ENTRY_POINT = 158;
public static final int EXECUTE_TRANSACTION = 159;
public static final int RELAUNCH_ACTIVITY = 160;
...
}
四、Activity启动流程
当AMS的attachApplication被应用侧调用后，会执行到realStartActivityLocked方法，如下：
// ActivityStackSupervisor.java
final boolean realStartActivityLocked(ActivityRecord r, ProcessRecord app,
boolean andResume, boolean checkConfig) throws RemoteException {
...
// Create activity launch transaction.
final ClientTransaction clientTransaction = ClientTransaction.obtain(app.thread,
r.appToken);
clientTransaction.addCallback(LaunchActivityItem.obtain(new Intent(r.intent),
System.identityHashCode(r), r.info,
// TODO: Have this take the merged configuration instead of separate global
// and override configs.
mergedConfiguration.getGlobalConfiguration(),
mergedConfiguration.getOverrideConfiguration(), r.compat,
r.launchedFromPackage, task.voiceInteractor, app.repProcState, r.icicle,
r.persistentState, results, newIntents, mService.isNextTransitionForward(),
profilerInfo));
// Set desired final state.
final ActivityLifecycleItem lifecycleItem;
if (andResume) {
lifecycleItem = ResumeActivityItem.obtain(mService.isNextTransitionForward());
} else {
lifecycleItem = PauseActivityItem.obtain();
}
clientTransaction.setLifecycleStateRequest(lifecycleItem);
// Schedule transaction.
mService.getLifecycleManager().scheduleTransaction(clientTransaction);
...
return true;
}
在ActivityStackSupervisor.realStartActivityLocked方法中为ClientTransaction对象添加LaunchActivityItem的callback，然后设置当前的生命周期状态，最后调用ClientLifecycleManager.scheduleTransaction方法执行。
// TransactionExecutor.java
public void execute(ClientTransaction transaction) {
final IBinder token = transaction.getActivityToken();
log("Start resolving transaction for client: " + mTransactionHandler + ", token: " + token);
executeCallbacks(transaction);
executeLifecycleState(transaction);
mPendingActions.clear();
log("End resolving transaction");
}
ClientLifecycleManager会执行LaunchActivityItem.excute()，最终会调到ActivityThread.performLaunchActivity()
// ActivityThread.java
private Activity performLaunchActivity(ActivityClientRecord r, Intent customIntent) {
ActivityInfo aInfo = r.activityInfo;
if (r.packageInfo == null) {
r.packageInfo = getPackageInfo(aInfo.applicationInfo, r.compatInfo,
Context.CONTEXT_INCLUDE_CODE);
}
ComponentName component = r.intent.getComponent();
if (component == null) {
component = r.intent.resolveActivity(
mInitialApplication.getPackageManager());
r.intent.setComponent(component);
}
if (r.activityInfo.targetActivity != null) {
component = new ComponentName(r.activityInfo.packageName,
r.activityInfo.targetActivity);
}
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);
if (customIntent != null) {
activity.mIntent = customIntent;
}
r.lastNonConfigurationInstances = null;
checkAndBlockForNetworkAccess();
activity.mStartedActivity = false;
int theme = r.activityInfo.getThemeResource();
if (theme != 0) {
activity.setTheme(theme);
}
activity.mCalled = false;
if (r.isPersistable()) {
mInstrumentation.callActivityOnCreate(activity, r.state, r.persistentState);
} else {
mInstrumentation.callActivityOnCreate(activity, r.state);
}
if (!activity.mCalled) {
throw new SuperNotCalledException(
"Activity " + r.intent.getComponent().toShortString() +
" did not call through to super.onCreate()");
}
r.activity = activity;
}
r.setState(ON_CREATE);
mActivities.put(r.token, r);
} catch (SuperNotCalledException e) {
throw e;
} catch (Exception e) {
if (!mInstrumentation.onException(activity, e)) {
throw new RuntimeException(
"Unable to start activity " + component
+ ": " + e.toString(), e);
}
}
return activity;
}
这里对Activity的ComponentName、ContextImpl、Activity以及Application对象进行了初始化并相互关联，然后会设置Activity的主题，最后调用Instrumentation.callActivityOnCreate方法
//Instrumentation.java
public void callActivityOnCreate(Activity activity, Bundle icicle,
PersistableBundle persistentState) {
prePerformCreate(activity);
activity.performCreate(icicle, persistentState);
postPerformCreate(activity);
}
//Activity.java
final void performCreate(Bundle icicle, PersistableBundle persistentState) {
mCanEnterPictureInPicture = true;
restoreHasCurrentPermissionRequest(icicle);
if (persistentState != null) {
onCreate(icicle, persistentState);
} else {
onCreate(icicle);
}
writeEventLog(LOG_AM_ON_CREATE_CALLED, "performCreate");
mActivityTransitionState.readState(icicle);
mVisibleFromClient = !mWindow.getWindowStyle().getBoolean(
com.android.internal.R.styleable.Window_windowNoDisplay, false);
mFragments.dispatchActivityCreated();
mActivityTransitionState.setEnterActivityOptions(this, getActivityOptions());
}
callActivityOnCreate被调用后就会执行Activity的performCrate方法，最后会执行Activity的onCreate方法，这样ClientTransaction的callback就被执行完了，接下来执行TransactionExecutor .executeLifecycleState方法
// TransactionExecutor.java
/** Transition to the final state if requested by the transaction. */
private void executeLifecycleState(ClientTransaction transaction) {
final ActivityLifecycleItem lifecycleItem = transaction.getLifecycleStateRequest();
if (lifecycleItem == null) {
// No lifecycle request, return early.
return;
}
log("Resolving lifecycle state: " + lifecycleItem);
final IBinder token = transaction.getActivityToken();
final ActivityClientRecord r = mTransactionHandler.getActivityClient(token);
if (r == null) {
// Ignore requests for non-existent client records for now.
return;
}
// Cycle to the state right before the final requested state.
cycleToPath(r, lifecycleItem.getTargetState(), true /* excludeLastState */);
// Execute the final transition with proper parameters.
lifecycleItem.execute(mTransactionHandler, token, mPendingActions);
lifecycleItem.postExecute(mTransactionHandler, token, mPendingActions);
}
/** Transition the client between states. */
@VisibleForTesting
public void cycleToPath(ActivityClientRecord r, int finish) {
cycleToPath(r, finish, false /* excludeLastState */);
}
/**
* Transition the client between states with an option not to perform the last hop in the
* sequence. This is used when resolving lifecycle state request, when the last transition must
* be performed with some specific parameters.
*/
private void cycleToPath(ActivityClientRecord r, int finish,
boolean excludeLastState) {
final int start = r.getLifecycleState();
log("Cycle from: " + start + " to: " + finish + " excludeLastState:" + excludeLastState);
final IntArray path = mHelper.getLifecyclePath(start, finish, excludeLastState);
performLifecycleSequence(r, path);
}
/** Transition the client through previously initialized state sequence. */
private void performLifecycleSequence(ActivityClientRecord r, IntArray path) {
final int size = path.size();
for (int i = 0, state; i < size; i++) {
state = path.get(i);
log("Transitioning to state: " + state);
switch (state) {
case ON_CREATE:
mTransactionHandler.handleLaunchActivity(r, mPendingActions,
null /* customIntent */);
break;
case ON_START:
mTransactionHandler.handleStartActivity(r, mPendingActions);
break;
case ON_RESUME:
mTransactionHandler.handleResumeActivity(r.token, false /* finalStateRequest */,
r.isForward, "LIFECYCLER_RESUME_ACTIVITY");
break;
case ON_PAUSE:
mTransactionHandler.handlePauseActivity(r.token, false /* finished */,
false /* userLeaving */, 0 /* configChanges */, mPendingActions,
"LIFECYCLER_PAUSE_ACTIVITY");
break;
case ON_STOP:
mTransactionHandler.handleStopActivity(r.token, false /* show */,
0 /* configChanges */, mPendingActions, false /* finalStateRequest */,
"LIFECYCLER_STOP_ACTIVITY");
break;
case ON_DESTROY:
mTransactionHandler.handleDestroyActivity(r.token, false /* finishing */,
0 /* configChanges */, false /* getNonConfigInstance */,
"performLifecycleSequence. cycling to:" + path.get(size - 1));
break;
case ON_RESTART:
mTransactionHandler.performRestartActivity(r.token, false /* start */);
break;
default:
throw new IllegalArgumentException("Unexpected lifecycle state: " + state);
}
}
}
在executeLifecycleState方法里面，会先去调用TransactionExecutor.cycleToPath执行当前生命周期状态之前的状态，然后执行ActivityLifecycleItem.execute方法。执行cycleToPath时，因为下一个状态是
ON_START，所以会执行mTransactionHandler.handleStartActivity()，最终会执行Activity.onStart()。
同样的方式，ResumeActivityItem.execute方法会被执行，最终执行Activity.onResume方法，这样Activity就启动了。
五、前一个Activity调用onStop
在要打开的Activity执行完onResume之后，ActivityThread.handleResumeActivity()方法被执行，它会创建一个Idler，并且添加到主线程的消息队列中去。在这之后的流程会创建StopActivityItem最终去执行前一个Activity的onStop(如果Activity还是可见状态则不会执行)
六、应用启动流程总结
- 系统开机后先启动init进程，init进程fork出一个Zygote进程作为其他应用进程的孵化器
- Zygote进程会先创建一个SystemServer进程
- SystemServer负责初始化各项系统服务，同时启动桌面应用
- 桌面应用通过PMS获取已安装应用列表展示给用户
- 用户点击桌面图标通过startActivity向AMS发起打开应用的请求
- AMS识别有效请求后，会先将当前栈顶的Activity改为pause状态，为应用请求创建进程，最终通过ActivityThread.main()进入应用的主入口
- Activity所在应用的进程和主线程完成初始化之后绑定Application，然后开始启动Activity，首先对Activity的ComponentName、ContextImpl、Activity以及Application对象进行了初始化并相互关联，然后设置Activity主题，最后执行onCreate->onStart->onResume方法完成Activity的启动
- 上述流程都执行完毕后，会去执行之前栈顶Activity的onStop
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