梳理了Android的启动流程，记录一下，要不然又忘了。
阅读源码要有窍门：1.带着问题，有一个线索。2.忽视与线索无关的代码。3.画图，做笔记。4.反复跟读。5.不能只看别人的分析，要动手操作。
源码版本4.2.2
使用到的文件的路径如下：
system\core\init\init.c
frameworks\base\cmds\app_process\app_main.cpp
frameworks\base\core\jni\AndroidRuntime.cpp
frameworks\base\core\java\java\com\android\internal\os\ZygoteInit.java
frameworks\base\core\java\java\com\android\internal\os\RuntimeInit.java
frameworks\base\services\java\com\android\server\SystemServer.java
frameworks\base\services\jni\com_android_server_SystemServer.cpp
frameworks\base\cmds\system_server\library\system_init.cpp
frameworks\base\services\java\com\android\server\am\ActivityManagerService.java
frameworks\base\core\java\com\android\internal\os\ZygoteConnection.java
1.init进程启动
init进程是linux kernel的第一个用户进程，也是分析的起点。
init进程的可执行文件:init 。adb shell连接到模拟器，根目录下就可以看到这个文件。对应的源码位置：system\core\init\。
入口：init.c中的main()函数。
最重要的一句话：init_parse_config_file("/init.rc")。解析一个叫Init.rc的文件。adb shell连接到模拟器，根目录下就可以看到这个init.rc文件。这个文件有固定的格式和执行顺序。
service zygote /system/bin/app_process -Xzygote /system/bin --zygote --start-system-server
这句话很重要。用于启动zygote进程。zygote进程的可执行文件，对应的源码：app_main.cpp
2.zygote进程
app_main.cpp的main()函数：
AppRuntime runtime;
runtime.start("com.android.internal.os.ZygoteInit",startSystemServer ? "start-system-server" : "");
AppRuntime继承于AndroidRuntime。
AndroidRuntime的start()方法:
JNIEnv* env;
if (startVm(&mJavaVM, &env) != 0) {
return;
}
//jni调用ZygoteInit的main()方法
ZygoteInit的main()方法：
public static void main(String argv[]) {
startSystemServer();
}
private static boolean startSystemServer(){
/* Hardcoded command line to start the system server */
String args[] = {
"--setuid=1000",
"--setgid=1000",
"--setgroups=1001,1002,1003,1004,1005,1006,1007,1008,1009,1010,1018,3001,3002,3003,3006,3007",
"--capabilities=130104352,130104352",
"--runtime-init",
"--nice-name=system_server",
"com.android.server.SystemServer",
};
ZygoteConnection.Arguments parsedArgs = null;
int pid;
try {/* Request to fork the system server process */
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 */
if (pid == 0) {
//子进程的代码
handleSystemServerProcess(parsedArgs);
}
return true;
}
/**
* Finish remaining work for the newly forked system server process.
*/
private static void handleSystemServerProcess(
RuntimeInit.zygoteInit(parsedArgs.targetSdkVersion, parsedArgs.remainingArgs);
}
public static final void zygoteInit(int targetSdkVersion, String[] argv)
throws ZygoteInit.MethodAndArgsCaller {
nativeZygoteInit();
applicationInit(targetSdkVersion, argv);
}
AndroidRuntime.cpp
static void com_android_internal_os_RuntimeInit_nativeZygoteInit(JNIEnv* env, jobject clazz)
{
gCurRuntime->onZygoteInit();
}
app_main.cpp
onZygoteInit()是AppRuntime的方法
private static void applicationInit(int targetSdkVersion, String[] argv)
throws ZygoteInit.MethodAndArgsCaller {
//调用SystemServer的main()方法
invokeStaticMain(args.startClass, args.startArgs);
}
SystemServer的main方法：
init1();//native方法
com_android_server_SystemServer.cpp
static void android_server_SystemServer_init1(JNIEnv* env, jobject clazz)
{
system_init();
}
system_init.cpp
extern "C" status_t system_init(){
ALOGI("Entered system_init()");
spproc(ProcessState::self());
spsm = defaultServiceManager();
ALOGI("ServiceManager: %p\n", sm.get());
spgrim = new GrimReaper();
sm->asBinder()->linkToDeath(grim, grim.get(), 0);
char propBuf[PROPERTY_VALUE_MAX];
property_get("system_init.startsurfaceflinger", propBuf, "1");
if (strcmp(propBuf, "1") == 0) {
// Start the SurfaceFlinger
SurfaceFlinger::instantiate();
}
property_get("system_init.startsensorservice", propBuf, "1");
if (strcmp(propBuf, "1") == 0) {
// Start the sensor service
SensorService::instantiate();
}
// And now start the Android runtime. We have to do this bit
// of nastiness because the Android runtime initialization requires
// some of the core system services to already be started.
// All other servers should just start the Android runtime at
// the beginning of their processes's main(), before calling
// the init function.
ALOGI("System server: starting Android runtime.\n");
AndroidRuntime* runtime = AndroidRuntime::getRuntime();
ALOGI("System server: starting Android services.\n");
JNIEnv* env = runtime->getJNIEnv();
if (env == NULL) {
return UNKNOWN_ERROR;
}
jclass clazz = env->FindClass("com/android/server/SystemServer");
if (clazz == NULL) {
return UNKNOWN_ERROR;
}
jmethodID methodId = env->GetStaticMethodID(clazz, "init2", "()V");
if (methodId == NULL) {
return UNKNOWN_ERROR;
}
env->CallStaticVoidMethod(clazz, methodId);
ALOGI("System server: entering thread pool.\n");
ProcessState::self()->startThreadPool();
IPCThreadState::self()->joinThreadPool();
ALOGI("System server: exiting thread pool.\n");
return NO_ERROR;
}
//a.启动ServiceManager b.调用SystemServier的init2方法
init2{
new ServerThread().run();
}
ServerThread中启动ActivityManagerService，AMS准备好后，启动了launcher.
3.Zygote进程孵化App进程
ZygoteInit的runSelectLoopMode()