今天主要分析Android启动流程，重点是system_server相关知识。
一、Android系统启动流程
Android正常启动流程如下：
Step1 系统加电，执行bootloader。Bootloader负责初始化软件运行所需要的最小硬件环境，最后加载内核到内存。
Step2 内核加载进内存后，将首先进入内核引导阶段，在内核引导阶段的最后，调用start_kenel进入内核启动阶段。start_kenel最终启动用户空间的init程序。
Step3 init程序负责解析init.rc配置文件，开启系统守护进程。两个最重要的守护进程是zygote进程和ServiceManager，zygote是Android启动的第一个Dalvik虚拟机，ServiceManager是Binder通讯的基础。
Step4 zygote虚拟机启动子进程system_server，在system_server中开启了核心系统服务，并将系统服务添加到ServiceManager中，然后系统进入SystemReady状态。
Step5 在SystemReady状态，ActivityManagerService与zygote中的socket通信，通过zygote启动home应用，进入系统桌面。
从Step3开始，init启动后，上层的实现。
Step1 init启动的核心Daemon服务包括Android的第一个Dalvik虚拟机zygote。
Step2 zygote定义一个socket，用于接受ActivityManagerService启动应用的请求。
Step3 zygote通过fork系统调用创建system_server进程
Step4 在system_server进程中，将会启动系统核心服务以及其他服务。
Step5 系统服务启动后会注册到ServiceManager中，用于Binder通信。
Step6 ActivityManagerService进入systemReady状态。
Step7 在systemReady状态，ActivityManagerService会与zygote的Socket通信，请求启动Home。
Step8 zygote收到AMS的连接请求后，执行runSelectLoopMode处理请求。
Step9 zygote处理请求会通过forkAndSpecialize启动新的应用进程，并最终启动Home。
二、system_server启动过程
在Step3，系统会创建system_server，下面开始分析system_server创建过程。
系统在预加载了共享资源后，编开始启动system_server进程，system_server是理解framework层的基础。Android中所有的系统服务都由它启动，它的异常会导致zygote的自杀重启，这样整个java就崩溃了。
System_server的启动入口是startSystemServer方法，位于frameworks/base/core/java/com/android/internal/os/ZygoteInit.java。
在ZygoteInit的main函数，调用了方法startSystemServer。
public static void main(String argv[]) {
...
String socketName = "zygote";
...
try {
...
if (startSystemServer) {
startSystemServer(abiList, socketName);
}
....分析startSystemServer方法：
/**
* Prepare the arguments and fork for the system server process.
*/
private static boolean startSystemServer(String abiList, String socketName)
throws 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
);
/* Containers run without this capability, so avoid setting it in that case */
if (!SystemProperties.getBoolean(PROPERTY_RUNNING_IN_CONTAINER, false)) {
capabilities |= posixCapabilitiesAsBits(OsConstants.CAP_BLOCK_SUSPEND);
}
/* Hardcoded command line to start the system server */
/* 设置启动system_server的命令行参数 */
String args[] = {
"--setuid=1000", //设置用户的ID为1000，这个UID代表系统权限
"--setgid=1000",
/// M: ANR mechanism for system_server add shell(2000) group to access
/// /sys/kernel/debug/tracing/tracing_on
"--setgroups=1001,1002,1003,1004,1005,1006,1007,1008,1009,1010,1018,1021,1032,2000," +
"3001,3002,3003,3006,3007,3009,3010",
"--capabilities=" + capabilities + "," + capabilities,
"--nice-name=system_server", //设置进程名
"--runtime-args",
"com.android.server.SystemServer", //设置要启动的类名
};
ZygoteConnection.Arguments parsedArgs = null;
int pid;
try {
//将数组分解成Arguments类型的对象，过程很简单
parsedArgs = new ZygoteConnection.Arguments(args);
ZygoteConnection.applyDebuggerSystemProperty(parsedArgs);
ZygoteConnection.applyInvokeWithSystemProperty(parsedArgs);
/* Request to fork the system server process */
//调用系统fork函数创建子进程，这个子进程就是sysytem_server
pid = Zygote.forkSystemServer(
parsedArgs.uid, parsedArgs.gid,
parsedArgs.gids,
parsedArgs.debugFlags,
null,
parsedArgs.permittedCapabilities,
parsedArgs.effectiveCapabilities);
} catch (IllegalArgumentException ex) {
throw new RuntimeException(ex);
}
/* For child process */
//pid为0说明在子进程system_server中
if (pid == 0) {
if (hasSecondZygote(abiList)) {
waitForSecondaryZygote(socketName);
}
//在子进程system_server中调用了handleSystemServerProcess方法
handleSystemServerProcess(parsedArgs);
}
//在父进程中返回
return true;
}到这里，zygote做了第一次分裂，forck出系统服务的总管system_server进程，这个过程分为以下两个重要的步骤：
1）、通过forkSystemServer创建system_server子进程。
2）、在子进程中调用handleSystemServerProcess方法。
2.1 通过forkSystemServer创建system_server子进程
启动system_server的第一步便是调用zygote的system_server方法，在该方法内部将调用Native方法nativeForkSystemServer，来完成启动system_server进程的任务。nativeForkSystemServer的JNI层实现方法位于frameworks/base/core/jni/com_android_internal_os_Zygote.cpp中，方法名为com_android_internal_os_Zygote_nativeForkSystemServer()。
static jint com_android_internal_os_Zygote_nativeForkSystemServer(
JNIEnv* env, jclass, uid_t uid, gid_t gid, jintArray gids,
jint debug_flags, jobjectArray rlimits, jlong permittedCapabilities,
jlong effectiveCapabilities) {
//间接调用了ForkAndSpecializeCommom() 函数
pid_t pid = ForkAndSpecializeCommon(env, uid, gid, gids,
debug_flags, rlimits,
permittedCapabilities, effectiveCapabilities,
MOUNT_EXTERNAL_DEFAULT, NULL, NULL, true, NULL,
NULL, NULL);
if (pid > 0) {//pid>0，则是在system_server的父进程zygote中
// The zygote process checks whether the child process has died or not.
ALOGI("System server process %d has been created", pid);
gSystemServerPid = pid;//在虚拟机中记录system_server的进程ID
// There is a slight window that the system server process has crashed
// but it went unnoticed because we haven't published its pid yet. So
// we recheck here just to make sure that all is well.
int status;
//退出前，重新检查sysem_server进程是否有异常，如果异常退出，
//则zygote直接自杀，因为在zygote的服务项中配置了onrestart这个*option，
//所以zogote自杀后，init进程会重新启动它
if (waitpid(pid, &status, WNOHANG) == pid) {
ALOGE("System server process %d has died. Restarting Zygote!", pid);
RuntimeAbort(env, __LINE__, "System server process has died. Restarting Zygote!");
}
}
return pid;
}在这里，系统会检查system_server进程是否启动成功，如果启动失败，将导致zygote重启。system_server负责构建系统服务，如果启动失败，Android系统也就无法启动，所以必须重启zygote、因此Dalvik_dalvik_system_Zygote_forkSystemServer方法的功能分成两部分：
1）启动system_server进程。
2）监控system_server进程的启动结果。
2.1.1 启动system_server进程
这部分功能由ForkAndSpecializeCommon()函数实现，位于本类中，
static pid_t ForkAndSpecializeCommon(JNIEnv* env, uid_t uid, gid_t gid, jintArray javaGids,
jint debug_flags, jobjectArray javaRlimits,
jlong permittedCapabilities, jlong effectiveCapabilities,
jint mount_external,
jstring java_se_info, jstring java_se_name,
bool is_system_server, jintArray fdsToClose,
jstring instructionSet, jstring dataDir){
......//省略
//设置信号处理函数
SetSigChldHandler();
//调用fork()函数
pid_t pid = fork();
if (pid == 0){
......//子进程
}else if(pid > 0) {
......//父进程处理
}
return pid;
}ForkAndSpecializeCommon函数通过fork()系统函数创建子进程system_server，并在创建system_server前做了重要工作：注册信号处理函数。该信号处理函数的作用就是代表子线程退出的信号。在setSigChldHandler()函数：
// Configures the SIGCHLD handler for the zygote process. This is configured
// very late, because earlier in the runtime we may fork() and exec()
// other processes, and we want to waitpid() for those rather than
// have them be harvested immediately.
//
// This ends up being called repeatedly before each fork(), but there's
// no real harm in that.
static void SetSigChldHandler() {
//这里是非常通用的Linux注册信号处理函数的流程，调用系统函数sigaction()处理
//SIGCHLD信号，具体的处理函数是SigChldHandler。SIGCHLD信号是子进程退出信号
struct sigaction sa;
memset(&sa, 0, sizeof(sa));
sa.sa_handler = SigChldHandler;
int err = sigaction(SIGCHLD, &sa, NULL);
if (err < 0) {
ALOGW("Error setting SIGCHLD handler: %s", strerror(errno));
}
}SetSigChldHandler注册了信号处理函数sigChldHandler，处理的信号便是SIGCHLD，代表子进程退出信号。
2.1.2 监控system_server进程的启动结果
当子进程退出时，由SigChldHandler处理该信号，接下来分析sigchldHandler处理SIGCHLD信号。
// This signal handler is for zygote mode, since the zygote must reap its children
static void SigChldHandler(int /*signal_number*/) {
pid_t pid;
int status;
// It's necessary to save and restore the errno during this function.
// Since errno is stored per thread, changing it here modifies the errno
// on the thread on which this signal handler executes. If a signal occurs
// between a call and an errno check, it's possible to get the errno set
// here.
// See b/23572286 for extra information.
int saved_errno = errno;
//调用系统函数waitpid等待子进程退出
//第一个参数-1表示等待任何子进程
//第二个参数status用于收集子进程退出时的状态
//第三个参数WNOHANG表示如果没有子进程退出，不需要阻塞
while ((pid = waitpid(-1, &status, WNOHANG)) > 0) {
// Log process-death status that we care about. In general it is
// not safe to call LOG(...) from a signal handler because of
// possible reentrancy. However, we know a priori that the
// current implementation of LOG() is safe to call from a SIGCHLD
// handler in the zygote process. If the LOG() implementation
// changes its locking strategy or its use of syscalls within the
// lazy-init critical section, its use here may become unsafe.
if (WIFEXITED(status)) {
if (WEXITSTATUS(status)) {
ALOGI("Process %d exited cleanly (%d)", pid, WEXITSTATUS(status));
}
} else if (WIFSIGNALED(status)) {
if (WTERMSIG(status) != SIGKILL) {
ALOGI("Process %d exited due to signal (%d)", pid, WTERMSIG(status));
}
if (WCOREDUMP(status)) {
ALOGI("Process %d dumped core.", pid);
}
}
// If the just-crashed process is the system_server, bring down zygote
// so that it is restarted by init and system server will be restarted
// from there.
if (pid == gSystemServerPid) {
ALOGE("Exit zygote because system server (%d) has terminated", pid);
//如果退出的是system_server进程，zygote重启，SIGKILL是自杀信号。
//虽然zygote自杀了，但是init进程会负责救活它。
kill(getpid(), SIGKILL);
}
}从forkSystemServer函数的执行过程可以看出：zygote创建system_server进程是十分谨慎的，不但在创建之初判断system_server是否退出，在创建后还专门注册信号处理函数监控system_server的运行状态。如果system_server出现意外，zygote也会自杀，导致整个java世界崩溃。
接下来分析fork出system_server后的处理。
2.2 在子进程中调用handleSystemServerProcess方法
system_server进程启动之后，便开始调用handleSystemServerProcess方法，该方法位于ZygoteInit.java中，代码如下：
/**
* Finish remaining work for the newly forked system server process.
*/
private static void handleSystemServerProcess(
ZygoteConnection.Arguments parsedArgs)
throws ZygoteInit.MethodAndArgsCaller {
// 关闭之前fork操作时，从zygote继承下来的socket
closeServerSocket();
// set umask to 0077 so new files and directories will default to owner-only permissions.
Os.umask(S_IRWXG | S_IRWXO);
if (parsedArgs.niceName != null) {
Process.setArgV0(parsedArgs.niceName);
}
final String systemServerClasspath = Os.getenv("SYSTEMSERVERCLASSPATH");
if (systemServerClasspath != null) {
performSystemServerDexOpt(systemServerClasspath);
}
// 根据invokeWith参数，执行一个shell命令
if (parsedArgs.invokeWith != null) {
String[] args = parsedArgs.remainingArgs;
// If we have a non-null system server class path, we'll have to duplicate the
// existing arguments and append the classpath to it. ART will handle the classpath
// correctly when we exec a new process.
if (systemServerClasspath != null) {
String[] amendedArgs = new String[args.length + 2];
amendedArgs[0] = "-cp";
amendedArgs[1] = systemServerClasspath;
System.arraycopy(parsedArgs.remainingArgs, 0, amendedArgs, 2, parsedArgs.remainingArgs.length);
}
WrapperInit.execApplication(parsedArgs.invokeWith,
parsedArgs.niceName, parsedArgs.targetSdkVersion,
VMRuntime.getCurrentInstructionSet(), null, args);
} else {
ClassLoader cl = null;
if (systemServerClasspath != null) {
cl = createSystemServerClassLoader(systemServerClasspath,
parsedArgs.targetSdkVersion);
Thread.currentThread().setContextClassLoader(cl);
}
/*
* Pass the remaining arguments to SystemServer.
*/
//调用zygoteInit方法，传入参数
RuntimeInit.zygoteInit(parsedArgs.targetSdkVersion, parsedArgs.remainingArgs, cl);
}
/* should never reach here */
}system_server生成之后，迅速进入工作状态，执行handleSystemServerProcess。在该方法中，首先做了一些清理和初始化工作，接着调用RuntimeInit.zygoteInit()方法。
RuntimeInit.java位于frameworks\base\core\java\com\android\internal\os\RuntimeInit.java,
/**
* The main function called when started through the zygote process. This
* could be unified with main(), if the native code in nativeFinishInit()
* were rationalized with Zygote startup.<p>
*
* Current recognized args:
* <ul>
* <li> <code> [--] <start class name> <args>
* </ul>
*
* @param targetSdkVersion target SDK version
* @param argv arg strings
*/
public static final void zygoteInit(int targetSdkVersion, String[] argv, ClassLoader classLoader)
throws ZygoteInit.MethodAndArgsCaller {
if (DEBUG) Slog.d(TAG, "RuntimeInit: Starting application from zygote");
Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "RuntimeInit");
redirectLogStreams();
commonInit();
nativeZygoteInit();
applicationInit(targetSdkVersion, argv, classLoader);
}zygoteInit封装了对四个方法的调用，对应四个步骤试下 不同的初始化操作，接下来分析四步。
2.2.1 redirectLogStrreams() 方法重定向标准I/O操作
redirectLogStreams()方法重定向Java标准I/O操作到Android日志系统。
/**
* Redirect System.out and System.err to the Android log.
*/
public static void redirectLogStreams() {
System.out.close();
System.setOut(new AndroidPrintStream(Log.INFO, "System.out"));
System.err.close();
System.setErr(new AndroidPrintStream(Log.WARN, "System.err"));
}2.2.2 commonInit() 方法初始化通用设置
private static final void commonInit() {
if (DEBUG) Slog.d(TAG, "Entered RuntimeInit!");
/* set default handler; this applies to all threads in the VM */
//设置未不会异常的默认处理函数
Thread.setDefaultUncaughtExceptionHandler(new UncaughtHandler());
/*
* Install a TimezoneGetter subclass for ZoneInfo.db
*/
//获取persist.sys.timezone 属性
TimezoneGetter.setInstance(new TimezoneGetter() {
@Override
public String getId() {
return SystemProperties.get("persist.sys.timezone");
}
});
TimeZone.setDefault(null);
/*
* Sets handler for java.util.logging to use Android log facilities.
* The odd "new instance-and-then-throw-away" is a mirror of how
* the "java.util.logging.config.class" system property works. We
* can't use the system property here since the logger has almost
* certainly already been initialized.
*/
//Android Log配置
LogManager.getLogManager().reset();
new AndroidConfig();
/*
* Sets the default HTTP User-Agent used by HttpURLConnection.
*/
//默认的HTTP User-Agent，用于HTTP连接
String userAgent = getDefaultUserAgent();
System.setProperty("http.agent", userAgent);
/*
* Wire socket tagging to traffic stats.
*/
NetworkManagementSocketTagger.install();//网络相关
/*
* If we're running in an emulator launched with "-trace", put the
* VM into emulator trace profiling mode so that the user can hit
* F9/F10 at any time to capture traces. This has performance
* consequences, so it's not something you want to do always.
*/
//模拟器上面的trace调试
String trace = SystemProperties.get("ro.kernel.android.tracing");
if (trace.equals("1")) {
Slog.i(TAG, "NOTE: emulator trace profiling enabled");
Debug.enableEmulatorTraceOutput();
}2.2.3 onZygoteInit 方法开启Binder通信
nativeZygoteInit是一个Native方法，其JNI实现方法位于frameworks\base\core\jni\AndroidRuntime.cpp，方法名为com_android_internal_os_RuntimeInit_nativeZygoteInit，具体代码：
static void com_android_internal_os_RuntimeInit_nativeZygoteInit(JNIEnv* env, jobject clazz)
{
gCurRuntime->onZygoteInit();
}onZygoteInit开启了sysytem_serve的Binder通道。
2.2.4 invokeStaticMain方法抛出异常
这样，system_server进程已经基本启动完成，之后system_server进程会启动系统核心服务，以及其他服务。
