壮心未与年俱老，死去犹能作鬼雄。这篇文章主要讲述Android系统启动流程分析相关的知识，希望能为你提供帮助。
随着android版本的升级,aosp项目中的代码也有了些变化,本文基于Android 7.0分析Android系统启动流程.当我们按下电源键后,整个Android设备大体经过了一下过程:

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今天我们只想来分析init进程及其后的过程,也就是下图所示部分:

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init进程 init进程会解析init.rc文件(关于init.rc中的语法,可以参见之前写的深入分析AIL语言及init.rc文件),加载相关分区,并启动相关服务.

init进程在/system/core/init/init.cpp
init.rc文件在/system/core/rootdir下
init.rc文件由parser.cpp解析,在/system/core/init/init_parser.cpp

在init.rc中,Zygote进程被启动.Zygote进程是其他所有进程的孵化器.init.rc通过include引入init.zygote.rc,这里以init.zygote64.rc为例:

service zygote /system/bin/app_process64 -Xzygote /system/bin --zygote --start-system-server class main priority -20 user root group root readproc socket zygote stream 660 root system onrestart write /sys/android_power/request_state wake onrestart write /sys/power/state on onrestart restart audioserver onrestart restart cameraserver onrestart restart media onrestart restart netd writepid /dev/cpuset/foreground/tasks

对个脚本简单分析:

1. service zygote /system/bin/app_process64 :service命令告诉init进程要创建一个名字为zygote的进程,这个zygote进程执行的程序是/system/bin/app_process64,后面是传给app_process64程序的参数.
2. socket zygote stream 660 root system:表示zygote进程需要一个名为”zygote”的socket,该socket用来实现进程间的通信.当新启动一个应用时,ActivityManagerService想向该Socket发起请求,请求zygote进程fork出一个新的进程.
3. 后面的onrestart表示zygote重启时需要执行的动作.

Zygote进程启动 上面说到init进程会根据init.rc执行相关的操作,其中有一项就是创建Zygote进程.Zygote进程所对应的程序是/system/bin/app_process,
位于/frameworks/base/cmds/app_process/app_main.cpp,其入口函数是main():

int main(int argc, char* const argv[]) { if (prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0) < 0) { LOG_ALWAYS_FATAL(" PR_SET_NO_NEW_PRIVS failed: %s" , strerror(errno)); }if (!LOG_NDEBUG) { String8 argv_String; for (int i = 0; i < argc; + + i) { argv_String.append(" \\" " ); argv_String.append(argv[i]); argv_String.append(" \\" " ); } ALOGV(" app_process main with argv: %s" , argv_String.string()); }AppRuntime runtime(argv[0], computeArgBlockSize(argc, argv)); // Process command line arguments // ignore argv[0] argc--; argv+ + ; const char* spaced_commands[] = { " -cp" , " -classpath" }; bool known_command = false; int i; for (i = 0; i < argc; i+ + ) { if (known_command = = true) { runtime.addOption(strdup(argv[i])); ALOGV(" app_process main add known option ' %s' " , argv[i]); known_command = false; continue; }for (int j = 0; j < static_cast< int> (sizeof(spaced_commands) / sizeof(spaced_commands[0])); + + j) { if (strcmp(argv[i], spaced_commands[j]) = = 0) { known_command = true; ALOGV(" app_process main found known command ' %s' " , argv[i]); } }if (argv[i][0] != ' -' ) { break; } if (argv[i][1] = = ' -' & & argv[i][2] = = 0) { + + i; // Skip --. break; }runtime.addOption(strdup(argv[i])); ALOGV(" app_process main add option ' %s' " , argv[i]); }// Parse runtime arguments.Stop at first unrecognized option. bool zygote = false; bool startSystemServer = false; bool application = false; String8 niceName; String8 className; + + i; // Skip unused " parent dir" argument. 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) { //init.zygote64.rc中接受的参数,表示启动SystemServer组件 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; } }Vector< String8> args; if (!className.isEmpty()) {args.add(application ? String8(" application" ) : String8(" tool" )); runtime.setClassNameAndArgs(className, argc - i, argv + i); if (!LOG_NDEBUG) { String8 restOfArgs; char* const* argv_new = argv + i; int argc_new = argc - i; for (int k = 0; k < argc_new; + + k) { restOfArgs.append(" \\" " ); restOfArgs.append(argv_new[k]); restOfArgs.append(" \\" " ); } ALOGV(" Class name = %s, args = %s" , className.string(), restOfArgs.string()); } } else { // We' re in zygote mode. maybeCreateDalvikCache(); if (startSystemServer) { args.add(String8(" start-system-server" )); }char prop[PROP_VALUE_MAX]; if (property_get(ABI_LIST_PROPERTY, prop, NULL) = = 0) { LOG_ALWAYS_FATAL(" app_process: Unable to determine ABI list from property %s." , ABI_LIST_PROPERTY); return 11; }String8 abiFlag(" --abi-list= " ); abiFlag.append(prop); args.add(abiFlag); // In zygote mode, pass all remaining arguments to the zygote // main() method. for (; i < argc; + + i) { args.add(String8(argv[i])); } }if (!niceName.isEmpty()) { runtime.setArgv0(niceName.string(), true /* setProcName */); }if (zygote) { //此处见到了我们熟悉的ZygoteInit,但该方法的具体实现在//AndroidRuntime.start() 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." ); } }

上述代码总体比较简单,主要是处理相关参数,并创建AppRuntime,由于在init.rc文件中,app_process启动参数被设置为--zygote --start-system-server,因此会执行runtime.start(" com.android.internal.os.ZygoteInit" , args, zygote),现在我们来看看AppRuntime的具体实现,它同样在
在/frameworks/base/cmds/app_process/app_main.cpp:

class AppRuntime : public AndroidRuntime { public: AppRuntime(char* argBlockStart, const size_t argBlockLength) : AndroidRuntime(argBlockStart, argBlockLength) , mClass(NULL) { }void setClassNameAndArgs(const String8& className, int argc, char * const *argv) { mClassName = className; for (int i = 0; i < argc; + + i) { mArgs.add(String8(argv[i])); } }virtual void onVmCreated(JNIEnv* env) { if (mClassName.isEmpty()) { return; // Zygote. Nothing to do here. }char* slashClassName = toSlashClassName(mClassName.string()); mClass = env-> FindClass(slashClassName); if (mClass = = NULL) { ALOGE(" ERROR: could not find class ' %s' \\n" , mClassName.string()); } free(slashClassName); mClass = reinterpret_cast< jclass> (env-> NewGlobalRef(mClass)); }virtual void onStarted() { sp< ProcessState> proc = ProcessState::self(); ALOGV(" App process: starting thread pool.\\n" ); proc-> startThreadPool(); AndroidRuntime* ar = AndroidRuntime::getRuntime(); ar-> callMain(mClassName, mClass, mArgs); IPCThreadState::self()-> stopProcess(); }virtual void onZygoteInit() { sp< ProcessState> proc = ProcessState::self(); ALOGV(" App process: starting thread pool.\\n" ); proc-> startThreadPool(); }virtual void onExit(int code) { if (mClassName.isEmpty()) { // if zygote IPCThreadState::self()-> stopProcess(); }AndroidRuntime::onExit(code); }String8 mClassName; Vector< String8> mArgs; jclass mClass; };

AppRuntime继承AndroidRuntime,而AndroidRuntime位于
/frameworks/base/core/jni/AndroidRuntime.cpp.
而start()方法便是定义在AndroidRuntime的虚方法:

//这里的className的值就是com.android.intrnal.os.ZygoteInit void AndroidRuntime::start(const char* className, const Vector< String8> & options, bool zygote) { //...省略多行代码static const String8 startSystemServer(" start-system-server" ); for (size_t i = 0; i < options.size(); + + i) { if (options[i] = = startSystemServer) { /* 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))); } }const char* rootDir = getenv(" ANDROID_ROOT" ); if (rootDir = = NULL) { rootDir = " /system" ; if (!hasDir(" /system" )) { LOG_FATAL(" No root directory specified, and /android does not exist." ); return; } setenv(" ANDROID_ROOT" , rootDir, 1); }//1. 启动虚拟机 if (startVm(& mjavaVM, & env, zygote) != 0) { return; } onVmCreated(env); //2. 调用startReg()注册JNI方法 if (startReg(env) < 0) { ALOGE(" Unable to register all android natives\\n" ); return; }jclass stringClass; jobjectArray strArray; jstring classNameStr; stringClass = env-> FindClass(" java/lang/String" ); assert(stringClass != NULL); strArray = env-> NewObjectArray(options.size() + 1, stringClass, NULL); assert(strArray != NULL); classNameStr = env-> NewStringUTF(className); assert(classNameStr != NULL); env-> SetObjectArrayElement(strArray, 0, classNameStr); for (size_t i = 0; i < options.size(); + + i) { jstring optionsStr = env-> NewStringUTF(options.itemAt(i).string()); assert(optionsStr != NULL); env-> SetObjectArrayElement(strArray, i + 1, optionsStr); }char* slashClassName = toSlashClassName(className); jclass startClass = env-> FindClass(slashClassName); if (startClass = = NULL) { ALOGE(" JavaVM unable to locate class ' %s' \\n" , slashClassName); } else { //3. 本质就是调用com.android.intrnal.os.ZygoteInit类的main函数 jmethodID startMeth = env-> GetStaticMethodID(startClass, " main" , " ([Ljava/lang/String; )V" ); if (startMeth = = NULL) { ALOGE(" JavaVM unable to find main() in ' %s' \\n" , className); /* keep going */ } else { env-> CallStaticVoidMethod(startClass, startMeth, strArray); #if 0 if (env-> ExceptionCheck()) threadExitUncaughtException(env); #endif } } free(slashClassName); // 省略多行代码 }

在start()方法中主要做三件事情:
1. 调用startVM()函数启动虚拟机
2. 调用startReg()注册JNI方法
3. 调用com.android.internal.os.ZygoteInit.java类的main函数.
走进ZygoteInit 关于前两者就不细说了,重点来关注我们熟悉的ZygoteInit.java.它在
rameworks/base/core/Java/com/android/internal/os/ZygoteInit.java,我们直接来看他的main方法:

public static void main(String argv[]) { ZygoteServer zygoteServer = new ZygoteServer(); ZygoteHooks.startZygoteNoThreadCreation(); try { Os.setpgid(0, 0); } catch (ErrnoException ex) { throw new RuntimeException(" Failed to setpgid(0,0)" , ex); }try { Trace.traceBegin(Trace.TRACE_TAG_DALVIK, " ZygoteInit" ); RuntimeInit.enableDdms(); // Start profiling the zygote initialization. SamplingProfilerIntegration.start(); boolean startSystemServer = false; String socketName = " zygote" ; String abiList = null; for (int i = 1; i < argv.length; i+ + ) { if (" start-system-server" .equals(argv[i])) { startSystemServer = 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)) { socketName = argv[i].substring(SOCKET_NAME_ARG.length()); } else { throw new RuntimeException(" Unknown command line argument: " + argv[i]); } }if (abiList = = null) { throw new RuntimeException(" No ABI list supplied." ); } //创建名为zygote的socket zygoteServer.registerServerSocket(socketName); Trace.traceBegin(Trace.TRACE_TAG_DALVIK, " ZygotePreload" ); //省略多行参数 SamplingProfilerIntegration.writeZygoteSnapshot(); // Do an initial gc to clean up after startup Trace.traceBegin(Trace.TRACE_TAG_DALVIK, " PostZygoteInitGC" ); gcAndFinalize(); Trace.traceEnd(Trace.TRACE_TAG_DALVIK); Trace.setTracingEnabled(false); Zygote.nativeUnmountStorageOnInit(); ZygoteHooks.stopZygoteNoThreadCreation(); //由于在init.rc中设置了start-system-server参数,因此 //这里将启动SystemServer,可见SystemServer由Zygote创//建的第一个进程 if (startSystemServer) { //启动SystemServer组件 startSystemServer(abiList, socketName, zygoteServer); }Log.i(TAG, " Accepting command socket connections" ); //等待ActivityManagerService请求 zygoteServer.runSelectLoop(abiList); zygoteServer.closeServerSocket(); } catch (Zygote.MethodAndArgsCaller caller) { caller.run(); } catch (Throwable ex) { Log.e(TAG, " System zygote died with exception" , ex); zygoteServer.closeServerSocket(); throw ex; } }

这里的main()方法中主要做了三件事情
1. 通过registerServerSocket()来创建Socket,它将作为服务端用来和作为客户端的ActivityManagerService进行通信
2. 通过startSystemServer()方法来启动SystemServer
3. 最后通过通过runSelectLoop方法使得刚才创建的Socket进入无限循环,以等待来自ActivityManagerService请求
Zygote中Socket创建 首先来看resiterServerSocket()它在:

void registerServerSocket(String socketName) { if (mServerSocket = = null) { int fileDesc; final String fullSocketName = ANDROID_SOCKET_PREFIX + socketName; try { String env = System.getenv(fullSocketName); //从环境变量env中获取文件描述符, fileDesc = Integer.parseInt(env); } catch (RuntimeException ex) { throw new RuntimeException(fullSocketName + " unset or invalid" , ex); }try { //通过文件描述符创建socket,该描述符代表/dev/socket/zygote文件. FileDescriptor fd = new FileDescriptor(); fd.setInt$(fileDesc); mServerSocket = new LocalServerSocket(fd); } catch (IOException ex) { throw new RuntimeException( " Error binding to local socket ' " + fileDesc + " ' " , ex); } } }

方法主要通过文件描述符创建socket,该文件描述代表/dev/socket/zygote文件,现在看看开头init.rc中的配置:socket zygote stream 660 root system
Zygote启动SystemServer 现在来看startSystemServer()方法:

private static boolean startSystemServer(String abiList, String socketName, ZygoteServer zygoteServer) throws Zygote.MethodAndArgsCaller, RuntimeException { long capabilities = posixCapabilitiesAsBits( OsConstants.CAP_IPC_LOCK, OsConstants.CAP_KILL, OsConstants.CAP_NET_ADMIN, OsConstants.CAP_NET_BIND_SERVICE, OsConstants.CAP_NET_BROADCAST, OsConstants.CAP_NET_RAW, OsConstants.CAP_SYS_MODULE, OsConstants.CAP_SYS_NICE, OsConstants.CAP_SYS_RESOURCE, OsConstants.CAP_SYS_TIME, OsConstants.CAP_SYS_TTY_CONFIG, OsConstants.CAP_WAKE_ALARM ); if (!SystemProperties.getBoolean(PROPERTY_RUNNING_IN_CONTAINER, false)) { capabilities |= posixCapabilitiesAsBits(OsConstants.CAP_BLOCK_SUSPEND); } /* Hardcoded command line to start the system server */ String args[] = { " --setuid= 1000" , " --setgid= 1000" , " --setgroups= 1001,1002,1003,1004,1005,1006,1007,1008,1009,1010,1018,1021,1032,3001,3002,3003,3006,3007,3009,3010" , " --capabilities= " + capabilities + " ," + capabilities, " --nice-name= system_server" , " --runtime-args" , " com.android.server.SystemServer" , }; ZygoteConnection.Arguments parsedArgs = null; int pid; try { parsedArgs = new ZygoteConnection.Arguments(args); ZygoteConnection.applyDebuggerSystemProperty(parsedArgs); ZygoteConnection.applyInvokeWithSystemProperty(parsedArgs); //创建子进程 pid = Zygote.forkSystemServer( parsedArgs.uid, parsedArgs.gid, parsedArgs.gids, parsedArgs.debugFlags, null, parsedArgs.permittedCapabilities, parsedArgs.effectiveCapabilities); } catch (IllegalArgumentException ex) { throw new RuntimeException(ex); }//pid= 0表示子进程,此处就是SystemServer进程 if (pid = = 0) { //用于处理系统中有两个Zygote进程的情况,由于通常我们不会配置两个Zygote,因此暂时不关注 if (hasSecondZygote(abiList)) { waitForSecondaryZygote(socketName); } //Zygote创建的子进程(此处就是SystemServer)不需要使用Zygote中创建的Socket文件描述符,因此通过closeServerSocket()关闭它. zygoteServer.closeServerSocket(); handleSystemServerProcess(parsedArgs); }return true; }

这里首先通过Zygote.forkSystemServer()创建一个系统服务进程.与该方法相似还有forkAndSpecialize(),用于创建一个普通应用进程.进程创建成功后返回pid为0.由于此处生成的新进程和Zygote进程一模一样,也就是说这个新进程中同样包含了刚才创建的Socket,但是该Socket在此处无效,因此要将其关闭.接下来调用handleSystemServerProcess()处理刚才新建的进程即SystemServer进程,需要注意此时已经工作在SystemServer进程中了:

private static void handleSystemServerProcess( ZygoteConnection.Arguments parsedArgs) throws Zygote.MethodAndArgsCaller { //省略多行代码,此处invokeWith为null if (parsedArgs.invokeWith != null) { String[] args = parsedArgs.remainingArgs; if (systemServerClasspath != null) { //省略多行代码 } else {ClassLoader cl = null; if (systemServerClasspath != null) { //为SysteServer进程创建PathClassLoader类加载器 cl = createSystemServerClassLoader(systemServerClasspath, parsedArgs.targetSdkVersion); Thread.currentThread().setContextClassLoader(cl); }RuntimeInit.zygoteInit(parsedArgs.targetSdkVersion, parsedArgs.remainingArgs, cl); }}

该函数继续调用RuntimeInit.zygoteInit()进一步执行启动SystemServer组件的操作.继续来看 RuntimeInit.zygoteInit()的具体实现,它在
/frameworks/base/core/java/com/android/internal/os/RuntimeInit.java文件中:

public static final void zygoteInit(int targetSdkVersion, String[] argv, ClassLoader classLoader) throws Zygote.MethodAndArgsCaller { //...省略多行代码commonInit(); nativeZygoteInit(); applicationInit(targetSdkVersion, argv, classLoader); }

在该方法中主要调用了三个方法:

• commonInit():为当前进程的VM设置未捕获异常处理器
• nativeZygoteInit():Binder驱动初始化,该方法完成后,就可以通过该Binder进行进程通信
• applicationInit():主要用调用com.android.server.SystemServer类的main()方法

由于commonInit()方法比较简单,在此就不做分析.
nativeZygoteInit()是一个本地方法,其对应实现在frameworks/base/core/jni/AndroidRuntime.cpp中:

static void com_android_internal_os_RuntimeInit_nativeZygoteInit(JNIEnv* env, jobject clazz) { gCurRuntime-> onZygoteInit(); }

这里的gCurRuntime是AppRuntime的指针,在frameworks/base/core/jni/AndroidRuntime.cpp中定义,并在AndroidRuntime的够赞函数中初始化:

//定义 static AndroidRuntime* gCurRuntime = NULL; ...//在frameworks/base/cmds/app_process/app_main.cpp的main()方法中被调用 AndroidRuntime::AndroidRuntime(char* argBlockStart, const size_t argBlockLength) : mExitWithoutCleanup(false), mArgBlockStart(argBlockStart), mArgBlockLength(argBlockLength) { SkGraphics::Init(); mOptions.setCapacity(20); assert(gCurRuntime = = NULL); gCurRuntime = this; }

继续来看onZygoteInit():

virtual void onZygoteInit() { sp< ProcessState> proc = ProcessState::self(); ALOGV(" App process: starting thread pool.\\n" ); proc-> startThreadPool(); }

这里调用ProcessState::startThreadPool()方法启动线程池,这个线程池就是用来和Binder驱动程序进程交互的.(Binder驱动本质就是一个文件,位于/dev/binder),关于线程池具体创建的过程暂不做说明.
现在来看applicationInit():

private static void applicationInit(int targetSdkVersion, String[] argv, ClassLoader classLoader) throws ZygoteInit.MethodAndArgsCaller { //省略多行代码 invokeStaticMain(args.startClass, args.startArgs, classLoader); }

这里继续调用了invokeStaticMain()进行后续工作:

private static void invokeStaticMain(String className, String[] argv, ClassLoader classLoader) throws ZygoteInit.MethodAndArgsCaller { Class< ?> cl; try { cl = Class.forName(className, true, classLoader); } catch (ClassNotFoundException ex) { throw new RuntimeException( " Missing class when invoking static main " + className, ex); }Method m; try { m = cl.getMethod(" main" , new Class[] { String[].class }); } catch (NoSuchMethodException ex) { //... } catch (SecurityException ex) { //... }// 省略多行代码/* * This throw gets caught in ZygoteInit.main(), which responds * by invoking the exception' s run() method. This arrangement * clears up all the stack frames that were required in setting * up the process. */ throw new ZygoteInit.MethodAndArgsCaller(m, argv); }

此时要执行的是com.android.server.SystemServer的中mian()方法.此外真正执行的过程是在Zygote.MethodAndArgsCaller的run()方法中:

public static class MethodAndArgsCaller extends Exception implements Runnable { /** method to call */ private final Method mMethod; /** argument array */ private final String[] mArgs; public MethodAndArgsCaller(Method method, String[] args) { mMethod = method; mArgs = args; }public void run() { try { mMethod.invoke(null, new Object[] { mArgs }); } catch (IllegalAccessException ex) { throw new RuntimeException(ex); } catch (InvocationTargetException ex) { //省略多行代码 } } }

MethodAndArgsCaller继承Exception并实现Runnable接口,作为一个异常他被ZygoteInit.main()捕获并处理:

public static void main(String argv[]) { // ... try { //...省略多行代码 startSystemServer(abiList, socketName); } catch (MethodAndArgsCaller caller) { caller.run(); } catch (Throwable ex) { //... } }

现在SystemServer的main()已经被调用,我们顺着来看一下实现:

public class SystemServer{public static void main(String[] args) { new SystemServer().run(); }private void run() { try { //...省略一些初始化操作android.os.Process.setThreadPriority( android.os.Process.THREAD_PRIORITY_FOREGROUND); android.os.Process.setCanSelfBackground(false); //初始化主线程Looper Looper.prepareMainLooper(); //创建SystemServiceManager对象 mSystemServiceManager = new SystemServiceManager(mSystemContext); LocalServices.addService(SystemServiceManager.class, mSystemServiceManager); } finally { Trace.traceEnd(Trace.TRACE_TAG_SYSTEM_SERVER); }// 启动关键服务 startBootstrapServices(); //启动核心服务 startCoreServices(); //启动其他服务 startOtherServices(); //...省略多行代码//启动消息循环 Looper.loop(); }}

在main()方法中调用了run()方法继续启动操作.在run方法中这三个方法非常重要:

1. startBootstrapServices():启动引导服务,比如AMS,PMS等
2. startCoreServices():启动核心服务,比如BatteryService等
3. startOtherServices():启动其他服务,比如NetworkStatsService等.

关于SystemService的具体执行过程,在此不做细解.
Socket循环监听 到目前为止,关于ZygoteServer.registerServerSocket()和startSystemServer()的大体流程我们已经弄清除,接下来就是ZygoteServer.runSelectLoop()方法:

void runSelectLoop(String abiList) throws Zygote.MethodAndArgsCaller { ArrayList< FileDescriptor> fds = new ArrayList< FileDescriptor> (); ArrayList< ZygoteConnection> peers = new ArrayList< ZygoteConnection> (); fds.add(mServerSocket.getFileDescriptor()); peers.add(null); while (true) { StructPollfd[] pollFds = new StructPollfd[fds.size()]; for (int i = 0; i < pollFds.length; + + i) { pollFds[i] = new StructPollfd(); pollFds[i].fd = fds.get(i); pollFds[i].events = (short) POLLIN; } try { Os.poll(pollFds, -1); } catch (ErrnoException ex) { throw new RuntimeException(" poll failed" , ex); } for (int i = pollFds.length - 1; i > = 0; --i) { if ((pollFds[i].revents & POLLIN) = = 0) { continue; } if (i = = 0) { //监听Socket链接,如果你做过Socket编程就发现此处充当了服务端Socket ZygoteConnection newPeer = acceptCommandPeer(abiList); peers.add(newPeer); fds.add(newPeer.getFileDesciptor()); } else { //重点关注runOnce()方法 boolean done = peers.get(i).runOnce(this); if (done) { peers.remove(i); fds.remove(i); } } } } }

该方法非常简单:不断的处理来自客户端AMS的请求,然后交给runOnce().此处可见Android 7.0应用启动流程分析
【Android系统启动流程分析】到现在为止,整个SystemServer进程的启动流程已经明确看,用一张顺序图大体的表示上述的整个流程:

文章图片

总结

1. 系统启动时init进程会创建Zygote进程,Zygote进程负责后续Android应用框架层的其他进程的创建和启动.
2. Zygote进程会首先创建一个SystemSever进程,然后由SystemServer负责启动系统关键服务,如ActivityManagerService或者PackageManagerService等.

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