android9 服务加载方法,Activity启动过程（以Android9.0为例）

一、Android Application启动流程分析

1. App基础理论

每个Android App都在一个独立空间里, 意味着其运行在一个单独的进程中, 拥有自己的VM, 被系统分配一个唯一的user ID.

与众多基于Linux内核的系统类似, 启动系统时, bootloader启动内核和init进程. init进程分裂出更多名为"daemons(守护进程)"的底层的Linux进程, 诸如android debug deamon, USB deamon等. 这些守护进程处理底层硬件相关的接口。

随后, init进程会启动一个非常有意思的进程---"Zygote". 顾名思义, 这是一个Android平台的非常基础的进程(起到了预加载资源和类到虚拟机提高应用程序提高的作用)。 这个进程初始化了第一个VM, 并且预加载了framework和众多App所需要的通用资源。 然后它开启一个Socket接口来监听请求, 根据请求孵化出新的VM来管理新的App进程。 一旦收到新的请求, Zygote会基于自身预先加载的VM来孵化出一个新的VM创建一个新的进程。

Zygote会孵化出一个超级管理进程---System Server。 SystemServer会启动所有系统核心服务, 例如Activity Manager Service, 硬件相关的Service等。接下来Zygote孵化出第一个App进程Launcher，也就是桌面App。

2. 启动App流程

我们点击了一个桌面图标启动一个App的Activity，Launcher里会执行：

Intent intent = new Intent("xxx");

startActivity(intent);

此时会通过Binder IPC机制, 最终调用到ActivityManagerService。 该Service会执行如下操作:

分析intent对象的指向信息，验证用户是否有足够的权限。

如果有权限, ActivityManagerService需要检查并在新的task中启动目标activity。

检查这个进程的ProcessRecord是否存在了，即是否存在该进程，如果ProcessRecord是null，就需要创建新的进程。

3. 创建新的进程

ActivityManagerService调用startProcessLocked()方法来创建新的进程, 该方法会通过前面讲到的socket通道传递参数给Zygote进程。 Zygote孵化自身fork出一个新的进程，在子进程中会接着关闭socket，调用ZygoteInit.invokeStaticMain(cloader, className, mainArgs)，即调用ActivityThread.main()。新的应用进程会从ActivityThread的main()函数处开始执行。

//ActivityThread.main()方法

public static void main(String[] args) {

...

//创建Looper

Looper.prepareMainLooper();

ActivityThread thread = new ActivityThread();

//主要做一些新进程起来后的工作，比如设置ProcessRecord信息、attach application，启动该进程的Activity

thread.attach(false);

...

Looper.loop();

}

4. 启动Activity

经过前两个步骤之后, 系统已经拥有了该application的进程. 后面的调用顺序就是普通的从一个已经存在的进程中启动一个新进程的activity了。

实际调用方法是realStartActivity()，是下面要重点阐述的内容。

二、Activity的启动流程

这里各个API版本的启动稍微有一点点差别，但是主要流程还是不变的。这里以Android 9.0为例。

1. 相关类简介

Instrumentation：

用于实现应用程序测试代码的基类。这个类将在任何应用程序代码之前为您实例化(估计是在Android系统的启动的时候就已经实例化了)，允许您监视系统与应用程序的所有交互。可以通过AndroidManifest.xml的标签描述该类的实现。

ActivityManager：

该类提供与Activity、Service和Process相关的信息以及交互方法， 可以被看作是ActivityManagerService的辅助类。

ActivityManagerService：

Android中最核心的服务，主要负责系统中四大组件的启动、切换、调度及应用程序的管理和调度等工作。

ActivityThread：

管理应用程序进程中主线程的执行，根据Activity管理者的请求调度和执行activities、broadcasts及其相关的操作。

ActivityStack：

负责单个Activity栈的状态和管理。

ActivityStackSupervisor：

负责所有Activity栈的管理。内部管理了mHomeStack、mFocusedStack和mLastFocusedStack三个Activity栈。其中，mHomeStack管理的是Launcher相关的Activity栈；mFocusedStack管理的是当前显示在前台Activity的Activity栈；mLastFocusedStack管理的是上一次显示在前台Activity的Activity栈。

ClientLifecycleManager：

用来管理多个客户端生命周期执行请求的管理类。

2. 发出启动请求

我们调用startActivity(intent)，会调用的是startActivityForResult方法来传递启动请求

// 位于android.app.Activity.java中

@Override

public void startActivity(Intent intent) {

this.startActivity(intent, null);

}

public void startActivityForResult(@RequiresPermission Intent intent, int requestCode,

@Nullable Bundle options) {

if (mParent == null) {

options = transferSpringboardActivityOptions(options);

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) {

mStartedActivity = true;

}

cancelInputsAndStartExitTransition(options);

} else { //不是第一次启动

...

}

}

startActivityForResult()我们只需要关注mParent == null的部分就好，在API13以后嵌入多个子Activity已经过时了(取而代之应用Fragment)。mParent为空会去调用Instrumentation.execStartActivity()方法:

//android.app.Instrumentation.java中

public ActivityResult execStartActivity(

Context who, IBinder contextThread, IBinder token, Activity target,

Intent intent, int requestCode, Bundle options) {

IApplicationThread whoThread = (IApplicationThread) contextThread;

Uri referrer = target != null ? target.onProvideReferrer() : null;

if (referrer != null) {

intent.putExtra(Intent.EXTRA_REFERRER, referrer);

}

......

try {

intent.migrateExtraStreamToClipData();

intent.prepareToLeaveProcess(who);

int result = ActivityManager.getService()

.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;

}

可以看到实际上调用了ActivityManager.getService().startActivity，这里实际上是个IPC远程调用，去请求ActivityManagerService(简称AMS)的服务，我们转到AMS去看看。

3. ActivityManagerService接收并处理启动请求

转到AMS的startActivity()康康：

//frameworks/base/services/core/java/com/android/server/am/ActivityManagerService.java中

@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());

}

public final int startActivityAsUser(IApplicationThread caller, String callingPackage,

Intent intent, String resolvedType, IBinder resultTo, String resultWho, int requestCode,

int startFlags, ProfilerInfo profilerInfo, Bundle bOptions, int userId,

boolean validateIncomingUser) {

enforceNotIsolatedCaller("startActivity");

userId = mActivityStartController.checkTargetUser(userId, validateIncomingUser,

Binder.getCallingPid(), Binder.getCallingUid(), "startActivityAsUser");

// TODO: Switch to user app stacks here.

return mActivityStartController.obtainStarter(intent, "startActivityAsUser")

.setCaller(caller)

.setCallingPackage(callingPackage)

.setResolvedType(resolvedType)

.setResultTo(resultTo)

.setResultWho(resultWho)

.setRequestCode(requestCode)

.setStartFlags(startFlags)

.setProfilerInfo(profilerInfo)

.setActivityOptions(bOptions)

.setMayWait(userId)

.execute();

}

经过多个方法调用会去执行startActivityAsUser方法，在startActivityAsUser方法最后会通过ActivityStartController.obtainStarter方法获得一个包含所有启动信息的ActivityStarter对象并调用execute方法执行。

//frameworks/base/services/core/java/com/android/server/am/ActivityStarter.java中

int execute() {

try {

// for transactional diffs and preprocessing.

if (mRequest.mayWait) {

return startActivityMayWait(mRequest.caller, mRequest.callingUid,

mRequest.callingPackage, mRequest.intent, mRequest.resolvedType,

........

);

} else {

return startActivity(mRequest.caller, mRequest.intent, mRequest.ephemeralIntent,

mRequest.resolvedType, mRequest.activityInfo, mRequest.resolveInfo,

.........

);

}

} finally {

onExecutionComplete();

}

}

因为在ActivityManagerService.startActivityAsUser中调用了.setMayWait(userId)方法，所以这里 mRequest.mayWait值为true，会去调用startActivityMayWait方法。

//frameworks/base/services/core/java/com/android/server/am/ActivityStarter.java

private int startActivityMayWait(IApplicationThread caller, int callingUid,

String callingPackage, Intent intent, String resolvedType,

IVoiceInteractionSession voiceSession, IVoiceInteractor voiceInteractor,

IBinder resultTo, String resultWho, int requestCode, int startFlags,

ProfilerInfo profilerInfo, WaitResult outResult,

Configuration globalConfig, SafeActivityOptions options, boolean ignoreTargetSecurity,

int userId, TaskRecord inTask, String reason,

boolean allowPendingRemoteAnimationRegistryLookup) {

...

int res = startActivity(caller, intent, ephemeralIntent, resolvedType, aInfo, rInfo,

voiceSession, voiceInteractor, resultTo, resultWho, requestCode, callingPid,

callingUid, callingPackage, realCallingPid, realCallingUid, startFlags, options,

ignoreTargetSecurity, componentSpecified, outRecord, inTask, reason,

allowPendingRemoteAnimationRegistryLookup);

...

return res;

}

}

private int startActivity(final ActivityRecord r, ActivityRecord sourceRecord,

IVoiceInteractionSession voiceSession, IVoiceInteractor voiceInteractor,

int startFlags, boolean doResume, ActivityOptions options, TaskRecord inTask,

ActivityRecord[] outActivity) {

...

result = startActivityUnchecked(r, sourceRecord, voiceSession, voiceInteractor,

startFlags, doResume, options, inTask, outActivity);

...

postStartActivityProcessing(r, result, mTargetStack);

return result;

}

经过多次辗转调用ActivityStarter的startActivity()方法，然后调用startActivityUnchecked()方法

//frameworks/base/services/core/java/com/android/server/am/ActivityStarter.java

private int startActivityUnchecked(final ActivityRecord r, ActivityRecord sourceRecord,

IVoiceInteractionSession voiceSession, IVoiceInteractor voiceInteractor,

int startFlags, boolean doResume, ActivityOptions options, TaskRecord inTask,

ActivityRecord[] outActivity) {

.....//省略n行代码

if (mDoResume) {

final ActivityRecord topTaskActivity =

mStartActivity.getTask().topRunningActivityLocked();

if (!mTargetStack.isFocusable()

|| (topTaskActivity != null && topTaskActivity.mTaskOverlay

&& mStartActivity != topTaskActivity)) {

// If the activity is not focusable, we can't resume it, but still would like to

// make sure it becomes visible as it starts (this will also trigger entry

// animation). An example of this are PIP activities.

// Also, we don't want to resume activities in a task that currently has an overlay

// as the starting activity just needs to be in the visible paused state until the

// over is removed.

mTargetStack.ensureActivitiesVisibleLocked(null, 0, !PRESERVE_WINDOWS);

// Go ahead and tell window manager to execute app transition for this activity

// since the app transition will not be triggered through the resume channel.

mService.mWindowManager.executeAppTransition();

} else {

// If the target stack was not previously focusable (previous top running activity

// on that stack was not visible) then any prior calls to move the stack to the

// will not update the focused stack. If starting the new activity now allows the

// task stack to be focusable, then ensure that we now update the focused stack

// accordingly.

if (mTargetStack.isFocusable() && !mSupervisor.isFocusedStack(mTargetStack)) {

mTargetStack.moveToFront("startActivityUnchecked");

}

mSupervisor.resumeFocusedStackTopActivityLocked(mTargetStack, mStartActivity,

mOptions);

}

} else if (mStartActivity != null) {

mSupervisor.mRecentTasks.add(mStartActivity.getTask());

}

//调用到这里

mSupervisor.updateUserStackLocked(mStartActivity.userId, mTargetStack);

mSupervisor.handleNonResizableTaskIfNeeded(mStartActivity.getTask(), preferredWindowingMode,

preferredLaunchDisplayId, mTargetStack);

return START_SUCCESS;

}

这个方法里会根据启动标志位和Activity启动模式来决定如何启动一个Activity以及是否要调用deliverNewIntent方法通知Activity有一个Intent试图重新启动它。无论以何种模式启动最终都会调用ActivityStackSupervisor.resumeFocusedStackTopActivityLocked方法。

//frameworks/base/services/core/java/com/android/server/am/ActivityStackSupervisor.java

boolean resumeFocusedStackTopActivityLocked(

ActivityStack targetStack, ActivityRecord target, ActivityOptions targetOptions) {

...

if (targetStack != null && isFocusedStack(targetStack)) {

return targetStack.resumeTopActivityUncheckedLocked(target, targetOptions);

}

final ActivityRecord r = mFocusedStack.topRunningActivityLocked();

if (r == null || !r.isState(RESUMED)) {

mFocusedStack.resumeTopActivityUncheckedLocked(null, null);

} else if (r.isState(RESUMED)) {

// Kick off any lingering app transitions form the MoveTaskToFront operation.

mFocusedStack.executeAppTransition(targetOptions);

}

...

return false;

}

//frameworks/base/services/core/java/com/android/server/am/ActivityStack.java

boolean resumeTopActivityUncheckedLocked(ActivityRecord prev, ActivityOptions options) {

...

result = resumeTopActivityInnerLocked(prev, options);

...

return result;

}

private boolean resumeTopActivityInnerLocked(ActivityRecord prev, ActivityOptions options) {

...

boolean pausing = mStackSupervisor.pauseBackStacks(userLeaving, next, false);

if (mResumedActivity != null) {

pausing |= startPausingLocked(userLeaving, false, next, false);

}

...

mStackSupervisor.startSpecificActivityLocked(next, true, true);

...

return true;

}

ActivityStackSupervisor.resumeFocusedStackTopActivityLocked方法会最终调用startSpecificActivityLocked。

4.根据需要创建应用进程

上面分析到ActivityStackSupervisor.startSpecificActivityLocked方法：

//frameworks/base/services/core/java/com/android/server/am/ActivityStackSupervisor.java

void startSpecificActivityLocked(ActivityRecord r,

boolean andResume, boolean checkConfig) {

// Is this activity's application already running?

ProcessRecord app = mService.getProcessRecordLocked(r.processName,

r.info.applicationInfo.uid, true);

...

if (app != null && app.thread != null) {

...

realStartActivityLocked(r, app, andResume, checkConfig);

return;

} catch (RemoteException e) {

Slog.w(TAG, "Exception when starting activity "

+ r.intent.getComponent().flattenToShortString(), e);

}

}

mService.startProcessLocked(r.processName, r.info.applicationInfo, true, 0,

"activity", r.intent.getComponent(), false, false, true);

}

可以看出，在这个方法中会去根据进程和线程是否存在判断App是否已经启动，如果已经启动，就会调用realStartActivityLocked方法继续处理。如果没有启动则调用ActivityManagerService.startProcessLocked方法创建新的进程处理。对应了第一部分的进程创建的部分。

ActivityManagerService.startProcessLocked方法经过多次跳转最终会通过Process.start方法来为应用创建进程。

//frameworks/base/services/core/java/com/android/server/am/ActivityManagerService.java

private ProcessStartResult startProcess(String hostingType, String entryPoint,

ProcessRecord app, int uid, int[] gids, int runtimeFlags, int mountExternal,

String seInfo, String requiredAbi, String instructionSet, String invokeWith,

long startTime) {

...

startResult = Process.start(entryPoint,

app.processName, uid, uid, gids, runtimeFlags, mountExternal,

app.info.targetSdkVersion, seInfo, requiredAbi, instructionSet,

app.info.dataDir, invokeWith,

new String[] {PROC_START_SEQ_IDENT + app.startSeq});

...

}

//frameworks/base/core/java/android/os/Process.java

public static final ProcessStartResult start(final String processClass,

final String niceName,

int uid, int gid, int[] gids,

int runtimeFlags, int mountExternal,

int targetSdkVersion,

String seInfo,

String abi,

String instructionSet,

String appDataDir,

String invokeWith,

String[] zygoteArgs) {

return zygoteProcess.start(processClass, niceName, uid, gid, gids,

runtimeFlags, mountExternal, targetSdkVersion, seInfo,

abi, instructionSet, appDataDir, invokeWith, zygoteArgs);

}

//frameworks/base/core/java/android/os/ZygoteProcess.java

public final Process.ProcessStartResult start(final String processClass,

final String niceName,..........) {

try {

return startViaZygote(processClass, niceName, uid, gid, gids,

runtimeFlags, mountExternal, targetSdkVersion, seInfo,

abi, instructionSet, appDataDir, invokeWith, false /* startChildZygote */,

zygoteArgs);

} catch (ZygoteStartFailedEx ex) {

Log.e(LOG_TAG,

"Starting VM process through Zygote failed");

throw new RuntimeException(

"Starting VM process through Zygote failed", ex);

}

}

private Process.ProcessStartResult startViaZygote(final String processClass,

final String niceName,

final int uid, final int gid,

.........)

throws ZygoteStartFailedEx {

...

synchronized(mLock) {

return zygoteSendArgsAndGetResult(openZygoteSocketIfNeeded(abi), argsForZygote);

}

}

private ZygoteState openZygoteSocketIfNeeded(String abi) throws ZygoteStartFailedEx {

Preconditions.checkState(Thread.holdsLock(mLock), "ZygoteProcess lock not held");

if (primaryZygoteState == null || primaryZygoteState.isClosed()) {

try {

primaryZygoteState = ZygoteState.connect(mSocket);

} catch (IOException ioe) {

throw new ZygoteStartFailedEx("Error connecting to primary zygote", ioe);

}

maybeSetApiBlacklistExemptions(primaryZygoteState, false);

maybeSetHiddenApiAccessLogSampleRate(primaryZygoteState);

}

if (primaryZygoteState.matches(abi)) {

return primaryZygoteState;

}

// The primary zygote didn't match. Try the secondary.

if (secondaryZygoteState == null || secondaryZygoteState.isClosed()) {

try {

secondaryZygoteState = ZygoteState.connect(mSecondarySocket);

} catch (IOException ioe) {

throw new ZygoteStartFailedEx("Error connecting to secondary zygote", ioe);

}

maybeSetApiBlacklistExemptions(secondaryZygoteState, false);

maybeSetHiddenApiAccessLogSampleRate(secondaryZygoteState);

}

if (secondaryZygoteState.matches(abi)) {

return secondaryZygoteState;

}

throw new ZygoteStartFailedEx("Unsupported zygote ABI: " + abi);

}

/**

* 可以看一下这里注释写的很清楚

* Sends an argument list to the zygote process, which starts a new child

* and returns the child's pid. Please note: the present implementation

* replaces newlines in the argument list with spaces.

*

* @throws ZygoteStartFailedEx if process start failed for any reason

*/

@GuardedBy("mLock")

private static Process.ProcessStartResult zygoteSendArgsAndGetResult(

ZygoteState zygoteState, ArrayList args)

throws ZygoteStartFailedEx {

......

//fork一个新进程

Process.ProcessStartResult result = new Process.ProcessStartResult();

// Always read the entire result from the input stream to avoid leaving

// bytes in the stream for future process starts to accidentally stumble

// upon.

result.pid = inputStream.readInt();

result.usingWrapper = inputStream.readBoolean();

if (result.pid < 0) {

throw new ZygoteStartFailedEx("fork() failed");

}

return result;

}

可以发现其最终调用了Zygote并通过socket通信的方式让Zygote进程fork出一个新的进程，并根据传递的”android.app.ActivityThread”字符串，反射出该对象并执行ActivityThread的main方法对其进行初始化。

5.启动Activity

回到ActivityStackSupervisor.startSpecificActivityLocked方法，如果已经启动，就会调用realStartActivityLocked方法继续处理：

//frameworks/base/services/core/java/com/android/server/am/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。

//frameworks/base/services/core/java/com/android/server/am/ClientLifecycleManager.java

void scheduleTransaction(ClientTransaction transaction) throws RemoteException {

final IApplicationThread client = transaction.getClient();

transaction.schedule();

...

}

frameworks/base/core/java/android/app/servertransaction/ClientTransaction.java

public void schedule() throws RemoteException {

mClient.scheduleTransaction(this);

}

可以看到执行了mClient的scheduleTransaction方法，这里mClient实际上是一个IApplicationThread类型，ActivityThread的内部类ApplicationThread派生这个接口类并实现了对应的方法，所以是一个IPC远程调用。因此又回到客户端了。

//android.app.ActivityThread中的内部类ApplicationThread

@Override

public void scheduleTransaction(ClientTransaction transaction) throws RemoteException {

ActivityThread.this.scheduleTransaction(transaction);

}

//android.app.ActivityThread

void scheduleTransaction(ClientTransaction transaction) {

transaction.preExecute(this);

sendMessage(ActivityThread.H.EXECUTE_TRANSACTION, transaction);

}

这里向ActivityThread的Handler发送了一个消息，Handler有一个很简洁的名字H,我们看一下他处理消息EXECUTE_TRANSACTION的代码：

public void handleMessage(Message msg) {

...

case EXECUTE_TRANSACTION:

final ClientTransaction transaction = (ClientTransaction) msg.obj;

mTransactionExecutor.execute(transaction);

if (isSystem()) {

// Client transactions inside system process are recycled on the client side

// instead of ClientLifecycleManager to avoid being cleared before this

// message is handled.

transaction.recycle();

}

// TODO(lifecycler): Recycle locally scheduled transactions.

break;

...

}

//android.app.servertransaction.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");

}

executeLifecycleState(transaction)我们在后面再说(在7. Activity的onStart()和onResume()里讲)。先看TransactionExecutor.execute()被执行,然后执行了executeCallbacks()方法。

public void executeCallbacks(ClientTransaction transaction) {

final List callbacks = transaction.getCallbacks();

...

for (int i = 0; i < size; ++i) {

final ClientTransactionItem item = callbacks.get(i);

log("Resolving callback: " + item);

final int postExecutionState = item.getPostExecutionState();

final int closestPreExecutionState = mHelper.getClosestPreExecutionState(r,

item.getPostExecutionState());

if (closestPreExecutionState != UNDEFINED) {

cycleToPath(r, closestPreExecutionState);

}

//callback得到执行

item.execute(mTransactionHandler, token, mPendingActions);

item.postExecute(mTransactionHandler, token, mPendingActions);

if (r == null) {

// Launch activity request will create an activity record.

r = mTransactionHandler.getActivityClient(token);

}

if (postExecutionState != UNDEFINED && r != null) {

// Skip the very last transition and perform it by explicit state request instead.

final boolean shouldExcludeLastTransition =

i == lastCallbackRequestingState && finalState == postExecutionState;

cycleToPath(r, postExecutionState, shouldExcludeLastTransition);

}

}

...

}

也就是说每一项ClientTransactionItem都会得到执行，

之前在realStartActivityLocked中添加了CallbackclientTransaction.addCallback(LaunchActivityItem.obtain(...))，此时在客户端这个LaunchActivityItem就会被执行了，会执行它的execute()方法：

//frameworks/base/core/java/android/app/servertransaction/LaunchActivityItem.java

@Override

public void execute(ClientTransactionHandler client, IBinder token,

PendingTransactionActions pendingActions) {

Trace.traceBegin(TRACE_TAG_ACTIVITY_MANAGER, "activityStart");

ActivityClientRecord r = new ActivityClientRecord(token, mIntent, mIdent, mInfo,

mOverrideConfig, mCompatInfo, mReferrer, mVoiceInteractor, mState, mPersistentState,

mPendingResults, mPendingNewIntents, mIsForward,

mProfilerInfo, client);

client.handleLaunchActivity(r, pendingActions, null /* customIntent */);

Trace.traceEnd(TRACE_TAG_ACTIVITY_MANAGER);

}

//frameworks/base/core/java/android/app/ActivityThread.java

public Activity handleLaunchActivity(ActivityClientRecord r,

PendingTransactionActions pendingActions, Intent customIntent) {

...

final Activity a = performLaunchActivity(r, customIntent);

...

return a;

}

又回到了了我们的ActivityThread，接下来康康performLaunchActivity()

6. 创建Activity

performLaunchActivity()主要完成了以下几件事：

从ActivityClientRecord获取待启动的Activity信息

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);

}

...

通过Instrumentation的newActivity()创建Activity，这里是用反射的方式创建的。

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);

}

}

...

通过LoadApk的makeApplication()方法尝试创建Application对象。

Application app = r.packageInfo.makeApplication(false, mInstrumentation);

//android.app.LoadApk.java

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 {

java.lang.ClassLoader cl = getClassLoader();

if (!mPackageName.equals("android")) {

Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER,

"initializeJavaContextClassLoader");

initializeJavaContextClassLoader();

Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);

}

ContextImpl appContext = ContextImpl.createAppContext(mActivityThread, this);

app = mActivityThread.mInstrumentation.newApplication(

cl, appClass, appContext);

appContext.setOuterContext(app);

} catch (Exception e) {

if (!mActivityThread.mInstrumentation.onException(app, e)) {

Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);

throw new RuntimeException(

"Unable to instantiate application " + appClass

+ ": " + e.toString(), e);

}

}

mActivityThread.mAllApplications.add(app);

mApplication = app;

if (instrumentation != null) {

try {

instrumentation.callApplicationOnCreate(app);

} catch (Exception e) {

...

}

}

// Rewrite the R 'constants' for all library apks.

SparseArray packageIdentifiers = getAssets().getAssignedPackageIdentifiers();

final int N = packageIdentifiers.size();

for (int i = 0; i < N; i++) {

final int id = packageIdentifiers.keyAt(i);

if (id == 0x01 || id == 0x7f) {

continue;

}

rewriteRValues(getClassLoader(), packageIdentifiers.valueAt(i), id);

}

Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);

return app;

}

可以看到在makeApplication()方法中，如果Application已经创建了就不用再创建了直接返回。创建Application同样用的是反射，然后instrumentation.callApplicationOnCreate(app)被调用，再这里面Application的onCreate()方法得到了执行。

创建的ContextImp对象通过Activity的attach()方法，使得二者获得关联，并做一些重要的数据初始化工作，

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);

//android.app.Activity.java

final void attach(Context context, ActivityThread aThread,

Instrumentation instr, IBinder token, int ident,

Application application, Intent intent, ActivityInfo info,

CharSequence title, Activity parent, String id,

NonConfigurationInstances lastNonConfigurationInstances,

Configuration config, String referrer, IVoiceInteractor voiceInteractor,

Window window, ActivityConfigCallback activityConfigCallback) {

attachBaseContext(context);

mFragments.attachHost(null /*parent*/);

mWindow = new PhoneWindow(this, window, activityConfigCallback);

mWindow.setWindowControllerCallback(this);

mWindow.setCallback(this);

mWindow.setOnWindowDismissedCallback(this);

mWindow.getLayoutInflater().setPrivateFactory(this);

...

...

mWindow.setWindowManager(

(WindowManager)context.getSystemService(Context.WINDOW_SERVICE),

mToken, mComponent.flattenToString(),

(info.flags & ActivityInfo.FLAG_HARDWARE_ACCELERATED) != 0);

if (mParent != null) {

mWindow.setContainer(mParent.getWindow());

}

mWindowManager = mWindow.getWindowManager();

mCurrentConfig = config;

mWindow.setColorMode(info.colorMode);

setAutofillCompatibilityEnabled(application.isAutofillCompatibilityEnabled());

enableAutofillCompatibilityIfNeeded();

}

比如在attach()方法中完成了PhoneWindow的创建并与Activity获得关联。

调用Activity的onCreate()方法

if (r.isPersistable()) {

mInstrumentation.callActivityOnCreate(activity, r.state, r.persistentState);

} else {

mInstrumentation.callActivityOnCreate(activity, r.state);

}

...

//更新Activity状态

r.setState(ON_CREATE);

mActivities.put(r.token, r);

到这里Activity的创建已经完成了。

7. Activity的onStart()和onResume()

回到TransactionExecutor的execute()方法，前面说完了executeCallbacks()执行完。

//android.app.servertransaction.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");

}

接下来该说executeLifecycleState(transaction)了。

//android.app.servertransaction.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.

//执行这里

cycleToPath(r, lifecycleItem.getTargetState(), true /* excludeLastState */);

// Execute the final transition with proper parameters.

lifecycleItem.execute(mTransactionHandler, token, mPendingActions);

lifecycleItem.postExecute(mTransactionHandler, token, mPendingActions);

}

private void cycleToPath(ActivityClientRecord r, int finish,

boolean excludeLastState) {

final int start = r.getLifecycleState();

log("Cycle from: " + start + " to: " + finish + " excludeLastState:" + excludeLastState);

//这里计算了路径，path中包含了ON_START ON_RESUME

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);

}

}

}

先执行cycleToPath方法，生命周期状态是从ON_CREATE状态到ON_RESUME状态，中间有一个ON_START状态，执行到performLifecycleSequence()，path中包含ON_START和ON_RESUME，所以会先后执行handleStartActivity方法和handleResumeActivity方法，这里生命周期已经看得很清楚了。接下来熟悉的onStart和onResume方法就会得到执行了。

三、总结

Activity启动过程包括APP的启动和创建其实是很复杂的，这里用到了IPC和ActivityManagerService通信，Android9.0和8.0以及之前不同的地方就是使用了一个LifeCycleManager来统一管理Activity的生命周期，然后在TransactionExecutor中处理和执行各种回调。

有纰漏之处，欢迎指正。