Android Zygote启动构造流程及进程创建详解
更新时间：2023年07月17日 10:13:17 作者：Yink_Liu
这篇文章主要为大家介绍了Android Zygote启动构造流程及进程创建详解，有需要的朋友可以借鉴参考下，希望能够有所帮助，祝大家多多进步，早日升职加薪
一、前言
本文主要讲解内容
1、系统启动zygote、zygote的构造流程、主要做了什么
2、如何创建一个新的进程
3、systemserver的ams创建应用如何建立socket联系，以及如何收发消息
先简单提一下开机流程
init.rc -> zygote.rc -> app_main.cpp -> AndroidRuntime.cpp （启动虚拟机，注册jni，启动zygote） -> ZygoteInit.java -> SystemServer进程
其中init是我们系统启动的第一个进程，正是它通过linux的forck方法，创建我们系统的最重要的进程之一zygote.
zygote的启动过程以及作用：启动dalvik虚拟机，加载系统必须的一些资源，启动framework的systemserver进程。最后等待app请求创建应用进程
zygote在fork一个新的进程时会克隆出和之前zygote几乎一样的进程包含zygote的资源，新进程不需要进行初始化操作，只会修改一些必要参数
由于源码过多，本文的代码都会精简要点，最好结合源码阅读,源码基于androidR，各安卓版本代码可能有小区别但整体不会变化很大
二、启动流程
2.1、init.rc启动的地方
关于zygote的rc文件有几个地方：
./system/core/rootdir/init.zygote32_64.rc ./system/core/rootdir/init.zygote32.rc ./system/core/rootdir/init.zygote64_32.rc ./system/core/rootdir/init.zygote64.rc
他们的32和64分别对应两个启动的地方app_process32和app_process64
例如：zygote64_32.rc，那么它以app_process64为主，app_process32为辅，它两都会启动，也就是有两个zygote进程
./system/core/rootdir/init.zygote64.rc
service zygote /system/bin/app_process64 -Xzygote /system/bin --zygote --start-system-server
class main
priority -20
user root
group root readproc reserved_disk
socket zygote stream 660 root system
socket usap_pool_primary stream 660 root system
onrestart exec_background - system system -- /system/bin/vdc volume abort_fuse
onrestart write /sys/power/state on
onrestart restart audioserver
onrestart restart cameraserver
onrestart restart media
onrestart restart netd
onrestart restart wificond
writepid /dev/cpuset/foreground/tasks上面就是开始启动zygote的地方，不管是启动32/64,都比较类似，执行手机里的/system/bin/app_process它对应的地方
2.2、app_main init主入口
app_main.cpp frameworks\base\cmds\app_process 11671 2022/2/23 241
app_main.cpp的main函数就成了入口，它主要做了如下几个事情：
//1、构建runtime
AppRuntime runtime(argv[0], computeArgBlockSize(argc, argv));
//2、解析运行参数
while (i < argc) {
const char* arg = argv[i++];
if (strcmp(arg, "--zygote") == 0) {
zygote = true;
niceName = ZYGOTE_NICE_NAME;
} else if (strcmp(arg, "--start-system-server") == 0) {
startSystemServer = true;
} else if (strcmp(arg, "--application") == 0) {
application = true;
} else if (strncmp(arg, "--nice-name=", 12) == 0) {
niceName.setTo(arg + 12);
} else if (strncmp(arg, "--", 2) != 0) {
className.setTo(arg);
break;
} else {
--i;
break;
}
}
//3、runtime start zygoteinit
if (zygote) {
runtime.start("com.android.internal.os.ZygoteInit", args, zygote);
} else if (className) {
runtime.start("com.android.internal.os.RuntimeInit", args, zygote);
} else {
fprintf(stderr, "Error: no class name or --zygote supplied.\n");
app_usage();
LOG_ALWAYS_FATAL("app_process: no class name or --zygote supplied.");
}这里的runtime只是一个调用，实现在
AndroidRuntime.cpp frameworks\base\core\jni
void AndroidRuntime::start(const char* className, const Vector<String8>& options, bool zygote)
{
//这句日志实际输出02-21 11:29:59.487 496 496 D AndroidRuntime: >>>>>> START com.android.internal.os.ZygoteInit uid 0 <<<<<<
//达标className = com.android.internal.os.ZygoteInit
ALOGD(">>>>>> START %s uid %d <<<<<<\n",
className != NULL ? className : "(unknown)", getuid());
//打印关键日志boot_progress_start 表示开机过程上层开始
for (size_t i = 0; i < options.size(); ++i) {
if (options[i] == startSystemServer) {
primary_zygote = true;
/* track our progress through the boot sequence */
const int LOG_BOOT_PROGRESS_START = 3000;
LOG_EVENT_LONG(LOG_BOOT_PROGRESS_START, ns2ms(systemTime(SYSTEM_TIME_MONOTONIC)));
}
}
1、启动虚拟机
/* start the virtual machine */
JniInvocation jni_invocation;
jni_invocation.Init(NULL);
JNIEnv* env;
if (startVm(&mJavaVM, &env, zygote, primary_zygote) != 0) {
return;
}
onVmCreated(env);
2、注册JNI
/*
* Register android functions.
*/
if (startReg(env) < 0) {
ALOGE("Unable to register all android natives\n");
return;
}
3、调用ZygoteInit的main函数
//这就是上面备注的className = ZygoteIni
char* slashClassName = toSlashClassName(className != NULL ? className : "");
jclass startClass = env->FindClass(slashClassName);
if (startClass == NULL) {
ALOGE("JavaVM unable to locate class '%s'\n", slashClassName);
/* keep going */
} else {
jmethodID startMeth = env->GetStaticMethodID(startClass, "main",
"([Ljava/lang/String;)V");2.3、ZygoteInit.java#main zygote入口
public static void main(String argv[]) {
1、加载进程的资源和类（这里可以多线程加载class文件优化开机速度）
preload(bootTimingsTraceLog);
2、创建zygoteServer
zygoteServer = new ZygoteServer(isPrimaryZygote);
if (startSystemServer) {
3、把sokeckname传进去systemserver中，并开始运行systemserver
Runnable r = forkSystemServer(abiList, zygoteSocketName, zygoteServer);
if (r != null) {
r.run();
return;
}
4、最后开启loop等待消息
caller = zygoteServer.runSelectLoop(abiList);
}看一下forkSystemServer中关键的两个地方
private static Runnable forkSystemServer(String abiList, String socketName,
ZygoteServer zygoteServer) {
1、zygote调用forkSystemServer这个native方法创建出系统进程
pid = Zygote.forkSystemServer(
parsedArgs.mUid, parsedArgs.mGid,
parsedArgs.mGids,
parsedArgs.mRuntimeFlags,
null,
parsedArgs.mPermittedCapabilities,
parsedArgs.mEffectiveCapabilities);
2、给应用创建新的进程，走handleSystemServerProcess逻辑
/* For child process */
if (pid == 0) {
if (hasSecondZygote(abiList)) {
waitForSecondaryZygote(socketName);
}
zygoteServer.closeServerSocket();
return handleSystemServerProcess(parsedArgs);
}
}这里下一步就开始启动我们熟悉的systemserver服务。关于Systemserver就不过多阐述了，它主要启动了我们整个android系统的各种service
三、zygote创建子进程流程
3.1、调用流程
创建一个应用流程这里就不详述了，都大同小异，这里直接跟踪到关键点，当我们跟踪启动流程跟踪到
ActivityStack.java frameworks\base\services\core\java\com\android\server\wm
ActivityStack#resumeTopActivityInnerLocked
mStackSupervisor.startSpecificActivity(next, true, false);
startSpecificActivity主要判断当前应用是否进程已经在运行,会跟踪改到ActivityTaskManager#startProcessAsync方法
final Message m = PooledLambda.obtainMessage(ActivityManagerInternal::startProcess, mAmInternal, activity.processName, activity.info.applicationInfo, knownToBeDead, isTop, hostingType, activity.intent.getComponent());即来到ActivityManagerServie#startProcess
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 */);
即来到ActivityManagerServie#startProcessLocked - ProcessList#startProcessLocked -
ProcessList.java frameworks\base\services\core\java\com\android\server\am
private Process.ProcessStartResult startProcess(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) {
Process.ProcessStartResult startResult;
if (hostingRecord.usesWebviewZygote()) {
startResult = startWebView(entryPoint,
app.processName, uid, uid, gids, runtimeFlags, mountExternal,
app.info.targetSdkVersion, seInfo, requiredAbi, instructionSet,
app.info.dataDir, null, app.info.packageName, app.mDisabledCompatChanges,
new String[]{PROC_START_SEQ_IDENT + app.startSeq});
} else if (hostingRecord.usesAppZygote()) {
final AppZygote appZygote = createAppZygoteForProcessIfNeeded(app);
// We can't isolate app data and storage data as parent zygote already did that.
startResult = appZygote.getProcess().start(entryPoint,
app.processName, uid, uid, gids, runtimeFlags, mountExternal,
app.info.targetSdkVersion, seInfo, requiredAbi, instructionSet,
app.info.dataDir, null, app.info.packageName,
/*zygotePolicyFlags=*/ ZYGOTE_POLICY_FLAG_EMPTY, isTopApp,
app.mDisabledCompatChanges, pkgDataInfoMap, whitelistedAppDataInfoMap,
false, false,
new String[]{PROC_START_SEQ_IDENT + app.startSeq});
} else {
//modify for prefork blank process begin
PreForkArgs preforkArgs = new PreForkArgs(entryPoint,
app.processName, uid, uid, gids, runtimeFlags, mountExternal,
app.info.targetSdkVersion, seInfo, requiredAbi, instructionSet,
app.info.dataDir, invokeWith, app.info.packageName, zygotePolicyFlags,
isTopApp, app.mDisabledCompatChanges, pkgDataInfoMap,
whitelistedAppDataInfoMap, bindMountAppsData, bindMountAppStorageDirs,
new String[] {PROC_START_SEQ_IDENT + app.startSeq});
startResult = mService.handlePreForkStartProcess(preforkArgs);
if (startResult == null) {
startResult = Process.start(entryPoint,
app.processName, uid, uid, gids, runtimeFlags, mountExternal,
app.info.targetSdkVersion, seInfo, requiredAbi, instructionSet,
app.info.dataDir, invokeWith, app.info.packageName, zygotePolicyFlags,
isTopApp, app.mDisabledCompatChanges, pkgDataInfoMap,
whitelistedAppDataInfoMap, bindMountAppsData, bindMountAppStorageDirs,
new String[]{PROC_START_SEQ_IDENT + app.startSeq});
}
//modify for prefork blank process end
}
}其中主要是appZygote.getProcess().start这一句，调用栈
ZygoteProcess中start - startViaZygote -
ygoteProces - zygoteSendArgsAndGetResult - attemptZygoteSendArgsAndGetResult
3.2、连接socket发送数据
1、sokeck建立的地方
startViaZygote - openZygoteSocketIfNeeded - attemptConnectionToPrimaryZygote
@GuardedBy("mLock")
private void attemptConnectionToPrimaryZygote() throws IOException {
primaryZygoteState =
ZygoteState.connect(mZygoteSocketAddress, mUsapPoolSocketAddress);
//构建好消息这里发送
maybeSetApiBlacklistExemptions(primaryZygoteState, false);
}
static ZygoteState connect(@NonNull LocalSocketAddress zygoteSocketAddress,
@Nullable LocalSocketAddress usapSocketAddress) {
DataInputStream zygoteInputStream;
BufferedWriter zygoteOutputWriter;
final LocalSocket zygoteSessionSocket = new LocalSocket();
if (zygoteSocketAddress == null) {
throw new IllegalArgumentException("zygoteSocketAddress can't be null");
}
try {
//关键点，这里连接上了zygote的socket
zygoteSessionSocket.connect(zygoteSocketAddress);
zygoteInputStream = new DataInputStream(zygoteSessionSocket.getInputStream());
zygoteOutputWriter =
new BufferedWriter(
new OutputStreamWriter(zygoteSessionSocket.getOutputStream()),
Zygote.SOCKET_BUFFER_SIZE);
}2、通过zygoteState的BufferedWriter，用socket发消息给zygote
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();
// Always read the entire result from the input stream to avoid leaving
// bytes in the stream for future process starts to accidentally stumble
// upon.
Process.ProcessStartResult result = new Process.ProcessStartResult();
result.pid = zygoteInputStream.readInt();
result.usingWrapper = zygoteInputStream.readBoolean();
if (result.pid < 0) {
throw new ZygoteStartFailedEx("fork() failed");
}
上面这段代码，通过zygoteWriter把消息发给zygote，即发送流程
3.3、接收流程
接着上面的第二部分的ZygoteInit#main函数中，new了一个ZygoteServer
3.3.1 创建socket
ZygoteServer(boolean isPrimaryZygote) {
mUsapPoolEventFD = Zygote.getUsapPoolEventFD();
if (isPrimaryZygote) {
mZygoteSocket = Zygote.createManagedSocketFromInitSocket(Zygote.PRIMARY_SOCKET_NAME);
static LocalServerSocket createManagedSocketFromInitSocket(String socketName) {
int fileDesc;
// fullSocketName = “ANDROID_SOCKET_” + “zygote";
final String fullSocketName = ANDROID_SOCKET_PREFIX + socketName;
try {
String env = System.getenv(fullSocketName);
fileDesc = Integer.parseInt(env);
} catch (RuntimeException ex) {
throw new RuntimeException("Socket unset or invalid: " + fullSocketName, ex);
}
try {
FileDescriptor fd = new FileDescriptor();
fd.setInt$(fileDesc);
return new LocalServerSocket(fd);
} catch (IOException ex) {
throw new RuntimeException(
"Error building socket from file descriptor: " + fileDesc, ex);
}
}
3.3.2 sokeck创建后消息发送到了哪里
这里又接着上面的第二部分的ZygoteInit#main函数中第四步zygoteServer.runSelectLoop(abiList);
ZygoteServer创建了socket，runSelectLoop会执行
Runnable runSelectLoop(String abiList) {
while(true) {
try {
ZygoteConnection connection = peers.get(pollIndex);
// while循环到这句话会执行forck操作，进行新的进程的创建
final Runnable command = connection.processOneCommand(this);
// TODO (chriswailes): Is this extra check necessary?
if (mIsForkChild) {
// We're in the child. We should always have a command to run at
// this stage if processOneCommand hasn't called "exec".
if (command == null) {
throw new IllegalStateException("command == null");
}
return command;
}
}
}
3.4 创建子进程
ZygoteConnection#processOneCommand
Runnable processOneCommand(ZygoteServer zygoteServer) {
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);
//穿件完后给ams回复
return handleChildProc(parsedArgs, childPipeFd, parsedArgs.mStartChildZygote);
}
//Zygote#forkAndSpecialize
static int forkAndSpecialize(int uid, int gid, int[] gids, int runtimeFlags,
int[][] rlimits, int mountExternal, String seInfo, String niceName, int[] fdsToClose,
int[] fdsToIgnore, boolean startChildZygote, String instructionSet, String appDataDir,
boolean isTopApp, String[] pkgDataInfoList, String[] whitelistedDataInfoList,
boolean bindMountAppDataDirs, boolean bindMountAppStorageDirs) {
ZygoteHooks.preFork();
int pid = nativeForkAndSpecialize(
uid, gid, gids, runtimeFlags, rlimits, mountExternal, seInfo, niceName, fdsToClose,
fdsToIgnore, startChildZygote, instructionSet, appDataDir, isTopApp,
pkgDataInfoList, whitelistedDataInfoList, bindMountAppDataDirs,
bindMountAppStorageDirs);
子进程的初始化操作就不详述了大概说下调用栈
handleChildProc - ZygoteInit.zygoteInit - RuntimeInit.applicationInit - RuntimeInit.findStaticMain
四、写在最后
本文意在分析zygote的构建，以及如何通过socke构建新的进程。下面说两个比较常规的问题
为啥系统其它进程都用binder而这里采用socket呢?
首先binder是多线程的，zygote的fork函数是不允许多线程的，不然容易造成死锁（copy on write）
为啥运行app不新建一个进程而采用zygote fork？
因为应用是独立运行在dalvik中，他们的进程空间是分开的。如果每个应用都是新建进程，那么zygote加载的系统资源就会重复创建添加浪费系统资源。且zygote孵化也可节约创建的时间。
以上就是Android Zygote启动构造流程及进程创建详解的详细内容，更多关于Android Zygote启动的资料请关注脚本之家其它相关文章！
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