前言
Android 是Google开发的基于Linux平台的开源手机操作系统。它包括操作系统、用户界面和应用程序,android的系统架构和其操作系统一样，采用了分层的架构。从架构图看，android分为四个层，从高层到低层分别是应用程序层、应用程序框架层、系统运行库层和linux核心层
附上一图：
当引导程序启动Linux内核后，会加载各种驱动和数据结构，当有了驱动以后，开始启动Android系统同时会加载用户级别的第一个进程init
打开（system\core\init.c）查看init.c的main方法
int main(int argc, char **argv)
{
...
if (!strcmp(basename(argv[0]), "ueventd"))
return ueventd_main(argc, argv);
/* clear the umask */
umask(0);
//创建文件，进行挂载
...
mount("tmpfs", "/dev", "tmpfs", 0, "mode=0755");
mkdir("/dev/pts", 0755);
mkdir("/dev/socket", 0755);
mount("devpts", "/dev/pts", "devpts", 0, NULL);
mount("proc", "/proc", "proc", 0, NULL);
mount("sysfs", "/sys", "sysfs", 0, NULL);
open_devnull_stdio();
//打印日志
log_init();
INFO("reading config file\n");
//加载init.rc文件
init_parse_config_file("/init.rc");
...
}
service zygote /system/bin/app_process -Xzygote /system/bin --zygote --start-system-server
socket zygote stream 666
onrestart write /sys/android_power/request_state wake
onrestart write /sys/power/state on
onrestart restart media
onrestart restart netd
int main(int argc, const char* const argv[])
{
...
// Android运行时环境
AppRuntime runtime;
...
// Next arg is startup classname or "--zygote"
if (i < argc) {
arg = argv[i++];
if (0 == strcmp("--zygote", arg)) {
bool startSystemServer = (i < argc) ?
strcmp(argv[i], "--start-system-server") == 0 : false;
setArgv0(argv0, "zygote");
set_process_name("zygote");
// 启动java代码
runtime.start("com.android.internal.os.ZygoteInit",
...
}
public static void main(String argv[]) {
try {
VMRuntime.getRuntime().setMinimumHeapSize(5 * 1024 * 1024);
// 加载Android依赖的类
preloadClasses();
//cacheRegisterMaps();
preloadResources();
...
if (argv[1].equals("true")) {
// 启动系统服务
startSystemServer();
} else if (!argv[1].equals("false")) {
...
}
private static boolean startSystemServer()
...
args = new String[] {
"--setuid=1000",
"--setgid=1000",
"--setgroups=1001,1002,1003,1004,1005,1006,1007,1008,1009,1010,1018,3001,3002,3003,3006",
"--capabilities=130104352,130104352",
"--rlimit=8,",
"--runtime-init",
"--nice-name=system_server",
"com.android.server.SystemServer",
...
/* Request to fork the system server process */
// 母进程开始分叉服务 启动SystemServer
pid = Zygote.forkSystemServer(
parsedArgs.uid, parsedArgs.gid,
parsedArgs.gids, debugFlags, rlimits,
parsedArgs.permittedCapabilities,
parsedArgs.effectiveCapabilities);
..
}
public static void main(String[] args) {
...
// 加载jni库
System.loadLibrary("android_servers");
// 调用native方法
init1(args);
}
native public static void init1(String[] args);在SystemServer.java使用了jni，调用了c++代码
对应的C++代码com_android_server_SystemServer.cpp
// 类似java的抽象方法
extern "C" int system_init();
static void android_server_SystemServer_init1(JNIEnv* env, jobject clazz)
{
// 转调
system_init();
}
/*
* JNI registration.
*/
static JNINativeMethod gMethods[] = {
/* name, signature, funcPtr */
// 函数指针 把init1方法映射到android_server_SystemServer_init1
{ "init1", "([Ljava/lang/String;)V", (void*) android_server_SystemServer_init1 },
};
extern "C" status_t system_init()
{
...
// 启动硬件的服务
if (strcmp(propBuf, "1") == 0) {
// Start the SurfaceFlinger
SurfaceFlinger::instantiate();
}
AndroidRuntime* runtime = AndroidRuntime::getRuntime();
LOGI("System server: starting Android services.\n");
// 启动完硬件服务后，又回到Systemserver的init2方法
runtime->callStatic("com/android/server/SystemServer", "init2");
...
}
public static final void init2() {
Slog.i(TAG, "Entered the Android system server!");
Thread thr = new ServerThread();
thr.setName("android.server.ServerThread");
thr.start();
}
// 开启Android各种服务并且添加到ServiceManager去管理
Slog.i(TAG, "Device Policy");
devicePolicy = new DevicePolicyManagerService(context);
ServiceManager.addService(Context.DEVICE_POLICY_SERVICE, ottle =
...
// We now tell the activity manager it is okay to run third party
// code. It will call back into us once it has gotten to the state
// where third party code can really run (but before it has actually
// started launching the initial applications), for us to complete our
// initialization.
// 各种服务开启后调用ActivityManagerService.systemReady
((ActivityManagerService)ActivityManagerNative.getDefault())
.systemReady(new Runnable() {
public void run() {
Slog.i(TAG, "Making services ready");
public void systemReady(final Runnable goingCallback) {
...
// 打开第一个Activity
mMainStack.resumeTopActivityLocked(null);
}
}
final boolean resumeTopActivityLocked(ActivityRecord prev) {
// Find the first activity that is not finishing.
// 没有已经打开的Activity next为 null
ActivityRecord next = topRunningActivityLocked(null);
// Remember how we'll process this pause/resume situation, and ensure
// that the state is reset however we wind up proceeding.
final boolean userLeaving = mUserLeaving;
mUserLeaving = false;
if (next == null) {
// There are no more activities! Let's just start up the
// Launcher...
if (mMainStack) {
// 启动lucher应用的锁屏界面
return mService.startHomeActivityLocked();
}
}