Android 7.0系統啟動流程分析
阿新 • • 發佈：2019-01-01
隨著Android版本的升級,aosp專案中的程式碼也有了些變化,本文基於Android 7.0分析Android系統啟動流程.當我們按下電源鍵後,整個Android裝置大體經過了一下過程:
今天我們只想來分析init程序及其後的過程,也就是下圖所示部分:
init程序
init程序會解析init.rc檔案(關於init.rc中的語法,可以參見之前寫的深入分析AIL語言及init.rc檔案),載入相關分割槽,並啟動相關服務.
init程序在/system/core/init/init.cpp
init.rc檔案在/system/core/rootdir下
init.rc檔案由parser.cpp解析,在/system/core/init/init_parser.cpp
在init.rc中,Zygote程序被啟動.Zygote程序是其他所有程序的孵化器.init.rc通過include引入init.zygote.rc,這裡以init.zygote64.rc為例:
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 對個指令碼簡單分析:
service zygote /system/bin/app_process64:service命令告訴init程序要建立一個名字為zygote的程序,這個zygote程序執行的程式是/system/bin/app_process64,後面是傳給app_process64程式的引數.socket zygote stream 660 root system:表示zygote程序需要一個名為”zygote”的socket,該socket用來實現程序間的通訊.當新啟動一個應用時,ActivityManagerService想向該Socket發起請求,請求zygote程序fork出一個新的程序.- 後面的onrestart表示zygote重啟時需要執行的動作.
Zygote程序啟動
上面說到init程序會根據init.rc執行相關的操作,其中有一項就是建立Zygote程序.Zygote程序所對應的程式是/system/bin/app_process,
位於/frameworks/base/cmds/app_process/app_main.cpp,其入口函式是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中接受的引數,表示啟動SystemServer元件
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) {
//此處見到了我們熟悉的ZygoteInit,但該方法的具體實現在//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.");
}
}
上述程式碼總體比較簡單,主要是處理相關引數,並建立AppRuntime,由於在init.rc檔案中,app_process啟動引數被設定為--zygote --start-system-server,因此會執行runtime.start("com.android.internal.os.ZygoteInit", args, zygote),現在我們來看看AppRuntime的具體實現,它同樣在
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繼承AndroidRuntime,而AndroidRuntime位於
/frameworks/base/core/jni/AndroidRuntime.cpp.
而start()方法便是定義在AndroidRuntime的虛方法:
//這裡的className的值就是com.android.intrnal.os.ZygoteInit
void AndroidRuntime::start(const char* className, const Vector<String8>& options, bool zygote)
{
//...省略多行程式碼
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. 啟動虛擬機器
if (startVm(&mJavaVM, &env, zygote) != 0) {
return;
}
onVmCreated(env);
//2. 呼叫startReg()註冊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. 本質就是呼叫com.android.intrnal.os.ZygoteInit類的main函式
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);
// 省略多行程式碼
}
在start()方法中主要做三件事情:
1. 呼叫startVM()函式啟動虛擬機器
2. 呼叫startReg()註冊JNI方法
3. 呼叫com.android.internal.os.ZygoteInit.java類的main函式.
走進ZygoteInit
關於前兩者就不細說了,重點來關注我們熟悉的ZygoteInit.java.它在
rameworks/base/core/Java/com/android/internal/os/ZygoteInit.java,我們直接來看他的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.");
}
//建立名為zygote的socket
zygoteServer.registerServerSocket(socketName);
Trace.traceBegin(Trace.TRACE_TAG_DALVIK, "ZygotePreload");
//省略多行引數
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();
//由於在init.rc中設定了start-system-server引數,因此
//這裡將啟動SystemServer,可見SystemServer由Zygote創 //建的第一個程序
if (startSystemServer) {
//啟動SystemServer元件
startSystemServer(abiList, socketName, zygoteServer);
}
Log.i(TAG, "Accepting command socket connections");
//等待ActivityManagerService請求
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;
}
}
這裡的main()方法中主要做了三件事情
1. 通過registerServerSocket()來建立Socket,它將作為服務端用來和作為客戶端的ActivityManagerService進行通訊
2. 通過startSystemServer()方法來啟動SystemServer
3. 最後通過通過runSelectLoop方法使得剛才建立的Socket進入無限迴圈,以等待來自ActivityManagerService請求
Zygote中Socket建立
首先來看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);
//從環境變數env中獲取檔案描述符,
fileDesc = Integer.parseInt(env);
} catch (RuntimeException ex) {
throw new RuntimeException(fullSocketName + " unset or invalid", ex);
}
try {
//通過檔案描述符建立socket,該描述符代表/dev/socket/zygote檔案.
FileDescriptor fd = new FileDescriptor();
fd.setInt$(fileDesc);
mServerSocket = new LocalServerSocket(fd);
} catch (IOException ex) {
throw new RuntimeException(
"Error binding to local socket '" + fileDesc + "'", ex);
}
}
}方法主要通過檔案描述符建立socket,該檔案描述代表/dev/socket/zygote檔案,現在看看開頭init.rc中的配置:socket zygote stream 660 root system
Zygote啟動SystemServer
現在來看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);
//建立子程序
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表示子程序,此處就是SystemServer程序
if (pid == 0) {
//用於處理系統中有兩個Zygote程序的情況,由於通常我們不會配置兩個Zygote,因此暫時不關注
if (hasSecondZygote(abiList)) {
waitForSecondaryZygote(socketName);
}
//Zygote建立的子程序(此處就是SystemServer)不需要使用Zygote中建立的Socket檔案描述符,因此通過closeServerSocket()關閉它.
zygoteServer.closeServerSocket();
handleSystemServerProcess(parsedArgs);
}
return true;
}
這裡首先通過Zygote.forkSystemServer()建立一個系統服務程序.與該方法相似還有forkAndSpecialize(),用於建立一個普通應用程序.程序建立成功後返回pid為0.由於此處生成的新程序和Zygote程序一模一樣,也就是說這個新程序中同樣包含了剛才建立的Socket,但是該Socket在此處無效,因此要將其關閉.接下來呼叫handleSystemServerProcess()處理剛才新建的程序即SystemServer程序,需要注意此時已經工作在SystemServer程序中了:
private static void handleSystemServerProcess(
ZygoteConnection.Arguments parsedArgs)
throws Zygote.MethodAndArgsCaller {
//省略多行程式碼,此處invokeWith為null
if (parsedArgs.invokeWith != null) {
String[] args = parsedArgs.remainingArgs;
if (systemServerClasspath != null) {
//省略多行程式碼
} else {
ClassLoader cl = null;
if (systemServerClasspath != null) {
//為SysteServer程序建立PathClassLoader類載入器
cl = createSystemServerClassLoader(systemServerClasspath,
parsedArgs.targetSdkVersion);
Thread.currentThread().setContextClassLoader(cl);
}
RuntimeInit.zygoteInit(parsedArgs.targetSdkVersion, parsedArgs.remainingArgs, cl);
}
}
該函式繼續呼叫RuntimeInit.zygoteInit()進一步執行啟動SystemServer元件的操作.繼續來看 RuntimeInit.zygoteInit()的具體實現,它在
/frameworks/base/core/java/com/android/internal/os/RuntimeInit.java檔案中:
public static final void zygoteInit(int targetSdkVersion, String[] argv, ClassLoader classLoader)
throws Zygote.MethodAndArgsCaller {
//...省略多行程式碼
commonInit();
nativeZygoteInit();
applicationInit(targetSdkVersion, argv, classLoader);
}
在該方法中主要呼叫了三個方法:
- commonInit():為當前程序的VM設定未捕獲異常處理器
- nativeZygoteInit():Binder驅動初始化,該方法完成後,就可以通過該Binder進行程序通訊
- applicationInit():主要用呼叫com.android.server.SystemServer類的main()方法
由於commonInit()方法比較簡單,在此就不做分析.
nativeZygoteInit()是一個本地方法,其對應實現在frameworks/base/core/jni/AndroidRuntime.cpp中:
static void com_android_internal_os_RuntimeInit_nativeZygoteInit(JNIEnv* env, jobject clazz)
{
gCurRuntime->onZygoteInit();
}
這裡的gCurRuntime是AppRuntime的指標,在frameworks/base/core/jni/AndroidRuntime.cpp中定義,並在AndroidRuntime的夠贊函式中初始化:
//定義
static AndroidRuntime* gCurRuntime = NULL;
...
//在frameworks/base/cmds/app_process/app_main.cpp的main()方法中被呼叫
AndroidRuntime::AndroidRuntime(char* argBlockStart, const size_t argBlockLength) :
mExitWithoutCleanup(false),
mArgBlockStart(argBlockStart),
mArgBlockLength(argBlockLength)
{
SkGraphics::Init();
mOptions.setCapacity(20);
assert(gCurRuntime == NULL);
gCurRuntime = this;
}
繼續來看onZygoteInit():
virtual void onZygoteInit()
{
sp<ProcessState> proc = ProcessState::self();
ALOGV("App process: starting thread pool.\n");
proc->startThreadPool();
}這裡呼叫ProcessState::startThreadPool()方法啟動執行緒池,這個執行緒池就是用來和Binder驅動程式程序互動的.(Binder驅動本質就是一個檔案,位於/dev/binder),關於執行緒池具體建立的過程暫不做說明.
現在來看applicationInit():
private static void applicationInit(int targetSdkVersion, String[] argv, ClassLoader classLoader)
throws ZygoteInit.MethodAndArgsCaller {
//省略多行程式碼
invokeStaticMain(args.startClass, args.startArgs, classLoader);
}
這裡繼續呼叫了invokeStaticMain()進行後續工作:
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) {
//...
}
// 省略多行程式碼
/*
* 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);
}此時要執行的是com.android.server.SystemServer的中mian()方法.此外真正執行的過程是在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) {
//省略多行程式碼
}
}
}MethodAndArgsCaller繼承Exception並實現Runnable介面,作為一個異常他被ZygoteInit.main()捕獲並處理:
public static void main(String argv[]) {
// ...
try {
//...省略多行程式碼
startSystemServer(abiList, socketName);
} catch (MethodAndArgsCaller caller) {
caller.run();
} catch (Throwable ex) {
//...
}
}現在SystemServer的main()已經被呼叫,我們順著來看一下實現:
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);
//初始化主執行緒Looper
Looper.prepareMainLooper();
//建立SystemServiceManager物件
mSystemServiceManager = new SystemServiceManager(mSystemContext);
LocalServices.addService(SystemServiceManager.class, mSystemServiceManager);
} finally {
Trace.traceEnd(Trace.TRACE_TAG_SYSTEM_SERVER);
}
// 啟動關鍵服務
startBootstrapServices();
//啟動核心服務
startCoreServices();
//啟動其他服務
startOtherServices();
//...省略多行程式碼
//啟動訊息迴圈
Looper.loop();
}
}
在main()方法中呼叫了run()方法繼續啟動操作.在run方法中這三個方法非常重要:
- startBootstrapServices():啟動引導服務,比如AMS,PMS等
- startCoreServices():啟動核心服務,比如BatteryService等
- startOtherServices():啟動其他服務,比如NetworkStatsService等.
關於SystemService的具體執行過程,在此不做細解.
Socket迴圈監聽
到目前為止,關於ZygoteServer.registerServerSocket()和startSystemServer()的大體流程我們已經弄清除,接下來就是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) {
//監聽Socket連結,如果你做過Socket程式設計就發現此處充當了服務端Socket
ZygoteConnection newPeer = acceptCommandPeer(abiList);
peers.add(newPeer);
fds.add(newPeer.getFileDesciptor());
} else {
//重點關注runOnce()方法
boolean done = peers.get(i).runOnce(this);
if (done) {
peers.remove(i);
fds.remove(i);
}
}
}
}
}該方法非常簡單:不斷的處理來自客戶端AMS的請求,然後交給runOnce().此處可見Android 7.0應用啟動流程分析
到現在為止,整個SystemServer程序的啟動流程已經明確看,用一張順序圖大體的表示上述的整個流程:
總結
- 系統啟動時init程序會建立Zygote程序,Zygote程序負責後續Android應用框架層的其他程序的建立和啟動.
- Zygote程序會首先建立一個SystemSever程序,然後由SystemServer負責啟動系統關鍵服務,如ActivityManagerService或者PackageManagerService等.