本篇内容介绍了“Android应用程序的启动流程是什么”的有关知识，在实际案例的操作过程中，不少人都会遇到这样的困境，接下来就让小编带领大家学习一下如何处理这些情况吧！希望大家仔细阅读，能够学有所成！
应用进程的启动流程
本文基于Android 11，主要分析应用程序的启动流程，会直接定位到ActivityStackSupervisor.startSpecificActivity函数开始，因为该函数前面的内容主要在Activity的启动流程中，可以通过这部分的文章来阅读。
看源码流程，需要戒骄戒躁，心态好。配合源码使用，建议先收藏，夜深人静，心血来潮再看。
通过分析应用进程的启动流程，可以得到：
在Framework层，现在不止有AMS负责请求Zygote进程创建新进程，还有ATMS、ActivityStarter、ActivityTaskManger、ActivityTaskS在协助分担一些参数和逻辑的检查。
每个进程都是通过fork Zygote进程而来，且获得Java虚拟机。也就是说每一个应用进程都有自己的虚拟机。
应用进程是通过Soket去请求Zygote进程fork自己的。
每个进程都有自己的Binder线程池用于IPC。
每个应用进程的主线程在ActivityThread,其main函数会创建消息循环机制。
1、ActivityStackSupervisor.startSpecificActivity
ATMS有一个ProcessMap<WindowProcessController>类型的mProcessNames ,用于存储封装了已启动进程信息ProcessRecord和窗口信息Windows的WindowProcessController实例。WindowProcessController用于协调ActivityManger管理ProcessReocrd和WindwManger管理WIndow和Activity的关系。
void startSpecificActivity(ActivityRecord r, boolean andResume, boolean checkConfig) {
// Is this activity's application already running?
final WindowProcessController wpc =
mService.getProcessController(r.processName, r.info.applicationInfo.uid);
boolean knownToBeDead = false;
if (wpc != null && wpc.hasThread()) {
realStartActivityLocked(r, wpc, andResume, checkConfig);
return;
...
knownToBeDead = true;
}
r.notifyUnknownVisibilityLaunchedForKeyguardTransition();
final boolean isTop = andResume && r.isTopRunningActivity();
mService.startProcessAsync(r, knownToBeDead, isTop, isTop ? "top-activity" : "activity");
}这里的mService是ActivityTaskManagerService的实例，通过getProcessController函数获得当前wpc对象，判断当前启动应用进程是否启动wpc != null && wpc.hasThread(),如果条件成立，则开始真正启动一个未启动过的Activity,通过realStartActivityLocked；条件不成立，则调用mService的startProcessAsync启动当前Activity的所在的进程。即startSpecificActivity函数是启动进程和启动Activity的一个分界点。
2、ATMS.startProcessAsync
PooledLambda.obtainMessage函数是Lambda的调用方式，表示调用ActivityManagerInternal的startProcess函数，后续则是其参数。并返回一个Message对象，发给Handler类型的mH。
void startProcessAsync(ActivityRecord activity, boolean knownToBeDead, boolean isTop,
String hostingType) {
final Message m = PooledLambda.obtainMessage(ActivityManagerInternal::startProcess,
mAmInternal, activity.processName, activity.info.applicationInfo, knownToBeDead,
isTop, hostingType, activity.intent.getComponent());
mH.sendMessage(m);
}抽象类ActivityManagerInternal的继承类定义在ActivityManagerService的内部类LocalService。
public final class LocalService extends ActivityManagerInternal
3、LocalService.startProcess
@Override
public void startProcess(String processName, ApplicationInfo info, boolean knownToBeDead,
boolean isTop, String hostingType, ComponentName hostingName) {
startProcessLocked(processName, info, knownToBeDead, 0 /* intentFlags */,
new HostingRecord(hostingType, hostingName, isTop),
ZYGOTE_POLICY_FLAG_LATENCY_SENSITIVE, false /* allowWhileBooting */,
false /* isolated */, true /* keepIfLarge */);
}4、startProcessLocked函数
final ProcessRecord startProcessLocked(String processName,
ApplicationInfo info, boolean knownToBeDead, int intentFlags,
HostingRecord hostingRecord, int zygotePolicyFlags, boolean allowWhileBooting,
boolean isolated, boolean keepIfLarge) {
return mProcessList.startProcessLocked(processName, info, knownToBeDead, intentFlags,
hostingRecord, zygotePolicyFlags, allowWhileBooting, isolated, 0 /* isolatedUid */,
keepIfLarge, null /* ABI override */, null /* entryPoint */,
null /* entryPointArgs */, null /* crashHandler */);
}5、ProcessList.startProcessLocked
ProcessList类的startProcessLocked函数，有几个重载函数，第一个调用。
在 ！isolated，判断了启动IntentFlag是否后台运行，是的话，直接拒绝。否则清理AMS中发生过Crash的进程（当前应用）。
分析一：创立当前应用进程的描述ProcessRecord。
判断当前系统是否启动完毕，未启动完毕，将进程信息缓存到AMS的mProcessesOnHold中。
分析二：调用了另外一个重载函数。
final ProcessRecord startProcessLocked(String processName, ApplicationInfo info,
boolean knownToBeDead, int intentFlags, HostingRecord hostingRecord,
int zygotePolicyFlags, boolean allowWhileBooting, boolean isolated, int isolatedUid,
boolean keepIfLarge, String abiOverride, String entryPoint, String[] entryPointArgs,
Runnable crashHandler) {
long startTime = SystemClock.uptimeMillis();
ProcessRecord app;
//isolated传递进来是false，
if (!isolated) {
//从mProcessNames缓存获取，由于是首次创建，null
app = getProcessRecordLocked(processName, info.uid, keepIfLarge);
checkSlow(startTime, "startProcess: after getProcessRecord");
//判断要启动进程是否后台运行，直接return null
if ((intentFlags & Intent.FLAG_FROM_BACKGROUND) != 0) {
if (mService.mAppErrors.isBadProcessLocked(info)) {
return null;
}
} else {
//重置进程的crash状态,使其处于正常状态
mService.mAppErrors.resetProcessCrashTimeLocked(info);
if (mService.mAppErrors.isBadProcessLocked(info)) {
mService.mAppErrors.clearBadProcessLocked(info);
if (app != null) {
app.bad = false;
}
}
}
} else {
app = null;
}
ProcessRecord precedence = null;
if (app != null && app.pid > 0) {
if ((!knownToBeDead && !app.killed) || app.thread == null) {
app.addPackage(info.packageName, info.longVersionCode, mService.mProcessStats);
return app;
}
ProcessList.killProcessGroup(app.uid, app.pid);
precedence = app;
app = null;
}
if (app == null) {
// 分析一、创建新的应用进程描述ProcessRocrd
//内部会将自己添加到mProcessNames中
app = newProcessRecordLocked(info, processName, isolated, isolatedUid, hostingRecord);
if (app == null) {
return null;
}
//此时三者都是null
app.crashHandler = crashHandler;
app.isolatedEntryPoint = entryPoint;
app.isolatedEntryPointArgs = entryPointArgs;
if (precedence != null) {
app.mPrecedence = precedence;
precedence.mSuccessor = app;
}
} else {
app.addPackage(info.packageName, info.longVersionCode, mService.mProcessStats);
}
// If the system is not ready yet, then hold off on starting this
// process until it is.
if (!mService.mProcessesReady
&& !mService.isAllowedWhileBooting(info)
&& !allowWhileBooting) {
if (!mService.mProcessesOnHold.contains(app)) {
mService.mProcessesOnHold.add(app);
}
if (DEBUG_PROCESSES) Slog.v(TAG_PROCESSES,
"System not ready, putting on hold: " + app);
checkSlow(startTime, "startProcess: returning with proc on hold");
return app;
}
分析二：
final boolean success =
startProcessLocked(app, hostingRecord, zygotePolicyFlags, abiOverride);
checkSlow(startTime, "startProcess: done starting proc!");
return success ? app : null;
}6、ProcessList.startProcessLocked重载
再次调用另外一个重载函数。
final boolean startProcessLocked(ProcessRecord app, HostingRecord hostingRecord,
int zygotePolicyFlags, String abiOverride) {
return startProcessLocked(app, hostingRecord, zygotePolicyFlags,
false /* disableHiddenApiChecks */, false /* disableTestApiChecks */,
false /* mountExtStorageFull */, abiOverride);
}重载函数,这个重载函数处理逻辑很长，主要给前面创建的ProcessRecord类型的app设置各种属性。例如外部存储挂载模式，应用进程运行模式，abi架构等等，其中包括最重要一点就是分析一，确定要启动进程的的类名：android.app.ActivityThread。分析二，继续调用重载函数。
boolean startProcessLocked(ProcessRecord app, HostingRecord hostingRecord,
int zygotePolicyFlags, boolean disableHiddenApiChecks, boolean disableTestApiChecks,
boolean mountExtStorageFull, String abiOverride) {
...
app.gids = gids;
app.setRequiredAbi(requiredAbi);
app.instructionSet = instructionSet;
final String seInfo = app.info.seInfo
+ (TextUtils.isEmpty(app.info.seInfoUser) ? "" : app.info.seInfoUser);
//分析一：确定要启动应用程序的类名
final String entryPoint = "android.app.ActivityThread";
//分析二：调用另外一个重载函数
return startProcessLocked(hostingRecord, entryPoint, app, uid, gids,
runtimeFlags, zygotePolicyFlags, mountExternal, seInfo, requiredAbi,
instructionSet, invokeWith, startTime);
} catch (RuntimeException e) {
...
}
}重载函数：也是设置一些属性，然后调用startProcess函数。
boolean startProcessLocked(HostingRecord hostingRecord, String entryPoint, ProcessRecord app,
int uid, int[] gids, int runtimeFlags, int zygotePolicyFlags, int mountExternal,
String seInfo, String requiredAbi, String instructionSet, String invokeWith,
long startTime) {
...
final Process.ProcessStartResult startResult = startProcess(hostingRecord,
entryPoint, app,
uid, gids, runtimeFlags, zygotePolicyFlags, mountExternal, seInfo,
requiredAbi, instructionSet, invokeWith, startTime);
handleProcessStartedLocked(app, startResult.pid, startResult.usingWrapper,
startSeq, false);
...
}
}7、ProcessList.startProcess
ProcessList类的startProcess函数会根据hostingRecord属性mHostingZygote判断走不同的创建分支，前面创建使用默认值，所以走了else分支。通过 Process.start函数创建新的应用进程。
Process.start的一路调用:
Process.start=>ZygoteProcess.start=>ZygoteState.start=>ZygoteState.startViaZygote
8、ZygoteState.startViaZygote
startViaZygote函数，主要是将传递进来的参数拼接成成字符串和收集起来。其中processClass是
private Process.ProcessStartResult startViaZygote(...)
throws ZygoteStartFailedEx {
//根据传递进来的参数，拼接成字符串并收集到ArrayList<String>类型argsForZygote
//将作为新应用程序的主函数的参数
return zygoteSendArgsAndGetResult(openZygoteSocketIfNeeded(abi),
zygotePolicyFlags,
argsForZygote);
}9、ZygoteState.openZygoteSocketIfNeeded
zygoteSendArgsAndGetResult的第一个参数，调用了openZygoteSocketIfNeeded函数。尝试建立与Socket的连接（如果之前未建立的话）。我们知道Zygote进程在创建的过程，会调用runSelectLoop函数，创建Server端的Socket,一直等待来自AMS的Client端的Socket创建进程请求。
private ZygoteState openZygoteSocketIfNeeded(String abi) throws ZygoteStartFailedEx {
try {
//建立和Zygote的Socket连接
attemptConnectionToPrimaryZygote();
//匹配abi的架构。在Zygote的创建对应四种模式:32,32_64和64，64_32
//32,64
if (primaryZygoteState.matches(abi)) {
return primaryZygoteState;
}
//主要架构模式不配，匹配第二种 32_64,64_32
if (mZygoteSecondarySocketAddress != null) {
// The primary zygote didn't match. Try the secondary.
attemptConnectionToSecondaryZygote();
if (secondaryZygoteState.matches(abi)) {
return secondaryZygoteState;
}
}
} catch (IOException ioe) {
throw new ZygoteStartFailedEx("Error connecting to zygote", ioe);
}
throw new ZygoteStartFailedEx("Unsupported zygote ABI: " + abi);
}attemptConnectionToPrimaryZygote函数主要通过底层的LocalSocket创建与Zygote进程的Socket连接，并获得输入流zygoteInputStream和输出流zygoteOutputWriter。
private void attemptConnectionToPrimaryZygote() throws IOException {
if (primaryZygoteState == null || primaryZygoteState.isClosed()) {
primaryZygoteState =
ZygoteState.connect(mZygoteSocketAddress, mUsapPoolSocketAddress);
maybeSetApiBlacklistExemptions(primaryZygoteState, false);
maybeSetHiddenApiAccessLogSampleRate(primaryZygoteState);
}
}和Zygote进程的Server端Socket建立连接后，就是开始往Socket写数据了。
10、attemptZygoteSendArgsAndGetResult
回到第8步调用了zygoteSendArgsAndGetResult函数，又调用了attemptZygoteSendArgsAndGetResult函数。
zygoteSendArgsAndGetResult=>attemptZygoteSendArgsAndGetResult
11、attemptZygoteSendArgsAndGetResult
到这里，通过Socket的方式向Zygote进程写进前面拼接好的参数，Zygote在Server端的Socket接收到数据之后，会执行创建动作。在返回的result.pid>=0表示创建成功，并运行在新的进程。
private Process.ProcessStartResult attemptZygoteSendArgsAndGetResult(
ZygoteState zygoteState, String msgStr) throws ZygoteStartFailedEx {
try {
final BufferedWriter zygoteWriter = zygoteState.mZygoteOutputWriter;
final DataInputStream zygoteInputStream = zygoteState.mZygoteInputStream;
zygoteWriter.write(msgStr);
zygoteWriter.flush();
Process.ProcessStartResult result = new Process.ProcessStartResult();
result.pid = zygoteInputStream.readInt();
result.usingWrapper = zygoteInputStream.readBoolean();
if (result.pid < 0) {
throw new ZygoteStartFailedEx("fork() failed");
}
return result;
} catch (IOException ex) {
zygoteState.close();
Log.e(LOG_TAG, "IO Exception while communicating with Zygote - "
+ ex.toString());
throw new ZygoteStartFailedEx(ex);
}
}12、Zygote.main
在Zygote的启动流程过程，调用了ZygoteInit的main函数，因为Zygote是通过fork自身来创建其他进程，所以需要根据传递进来的参数，进行判断是启动什么类型的进程，例如自身isPrimaryZygote=true，或者SystemServer进程。然后通过ZygoteServer.runSelectLoop函数，等待其他进程请求创建新的进程。
public static void main(String argv[]) {
ZygoteServer zygoteServer = null;
Runnable caller;
try {
...
boolean startSystemServer = false;
String zygoteSocketName = "zygote";
String abiList = null;
boolean enableLazyPreload = false;
for (int i = 1; i < argv.length; i++) {
if ("start-system-server".equals(argv[i])) {
startSystemServer = true; //判断是否SystemServer进程
} else if ("--enable-lazy-preload".equals(argv[i])) {
enableLazyPreload = true;
} else if (argv[i].startsWith(ABI_LIST_ARG)) {
abiList = argv[i].substring(ABI_LIST_ARG.length());
} else if (argv[i].startsWith(SOCKET_NAME_ARG)) {
//SCOKET_NAME_ARG="--socket-name=",根据参数得到SocketName
zygoteSocketName = argv[i].substring(SOCKET_NAME_ARG.length());
} else {
throw new RuntimeException("Unknown command line argument: " + argv[i]);
}
}
//PRIMARY_SOCKET_NAME=zygote
final boolean isPrimaryZygote = zygoteSocketName.equals(Zygote.PRIMARY_SOCKET_NAME);
gcAndFinalize();
Zygote.initNativeState(isPrimaryZygote);
ZygoteHooks.stopZygoteNoThreadCreation();
zygoteServer = new ZygoteServer(isPrimaryZygote);
if (startSystemServer) {
//启动SystemServer进程
Runnable r = forkSystemServer(abiList, zygoteSocketName, zygoteServer);
if (r != null) {
r.run();
return;
}
}
//循环等待AMS来请求创建新的进程
caller = zygoteServer.runSelectLoop(abiList);
} catch (Throwable ex) {
Log.e(TAG, "System zygote died with exception", ex);
throw ex;
} finally {
if (zygoteServer != null) {
zygoteServer.closeServerSocket();
}
}
//调用新的进程主函数
if (caller != null) {
caller.run();
}
}13、ZygoteServer.runSelectLoo
这里只关注ZygoteServer.runSelectLoop函数，接受Socket客户端数据。
/**
* Runs the zygote process's select loop. Accepts new connections as
* they happen, and reads commands from connections one spawn-request's
* worth at a time.
*/
Runnable runSelectLoop(String abiList) {
while (true) {
...
ZygoteConnection connection = peers.get(pollIndex);
final Runnable command = connection.processOneCommand(this);
...
if (mIsForkChild) {
return command;
}
....
}
}14、ZygoteConnection.processOneCommand
runSelctLoop主要是从循环中检测是否有连接建立，建立之后执行ZygoteConnection的processOneCommand函数，并返回一个Runable类型的command对象。
Runnable processOneCommand(ZygoteServer zygoteServer) {
...
args = Zygote.readArgumentList(mSocketReader);
//根据参数内容，作其他类型的处理
...
//创建进程,调用底层nativeForkAndSpecialize方法，通过fork当前进程来创建一个子线程。
pid = Zygote.forkAndSpecialize(parsedArgs.mUid, parsedArgs.mGid, parsedArgs.mGids,
parsedArgs.mRuntimeFlags, rlimits, parsedArgs.mMountExternal, parsedArgs.mSeInfo,
parsedArgs.mNiceName, fdsToClose, fdsToIgnore, parsedArgs.mStartChildZygote,
parsedArgs.mInstructionSet, parsedArgs.mAppDataDir, parsedArgs.mIsTopApp,
parsedArgs.mPkgDataInfoList, parsedArgs.mWhitelistedDataInfoList,
parsedArgs.mBindMountAppDataDirs, parsedArgs.mBindMountAppStorageDirs);
...
if (pid == 0) {
//设置mIsForkChild=true
zygoteServer.setForkChild();
//关闭Socket连接
zygoteServer.closeServerSocket();
IoUtils.closeQuietly(serverPipeFd);
serverPipeFd = null;
//执行子进程内容
return handleChildProc(parsedArgs, childPipeFd, parsedArgs.mStartChildZygote);
}
...
}15、handleChildProc
handleChildProc函数。
private Runnable handleChildProc(ZygoteArguments parsedArgs,
FileDescriptor pipeFd, boolean isZygote) {
...
if (!isZygote) {
return ZygoteInit.zygoteInit(parsedArgs.mTargetSdkVersion,
parsedArgs.mDisabledCompatChanges,
parsedArgs.mRemainingArgs, null /* classLoader */);
} else {
return ZygoteInit.childZygoteInit(parsedArgs.mTargetSdkVersion,
parsedArgs.mRemainingArgs, null /* classLoader */);
}
}16、 ZygoteInit.zygoteInit
public static final Runnable zygoteInit(int targetSdkVersion, long[] disabledCompatChanges,
String[] argv, ClassLoader classLoader) {
RuntimeInit.commonInit();
ZygoteInit.nativeZygoteInit();//为新进程创建Binder线程池
return RuntimeInit.applicationInit(targetSdkVersion, disabledCompatChanges, argv,
classLoader);
}以前还以为每个进程共用一个Binder线程池，现在知道每个进程都有自己的Binder线程池进行IPC。
17、RuntimeInit.applicationInit
protected static Runnable applicationInit(int targetSdkVersion, long[] disabledCompatChanges,
String[] argv, ClassLoader classLoader) {
final Arguments args = new Arguments(argv);
return findStaticMain(args.startClass, args.startArgs, classLoader);
}这里的args.startClass就是Socket客户端传递下来的android.app.ActivityThread。
18、RuntimeInit.findStaticMain
RuntimeInit.findStaticMain函数主要通过反射创建ActivityThread类的实例，并反射主函数main，然后封装到MethodAndArgsCaller实例中返回。
protected static Runnable findStaticMain(String className, String[] argv,
ClassLoader classLoader) {
...
Class<?> cl = Class.forName(className, true, classLoader);
Method m = cl.getMethod("main", new Class[] {