Zygote 启动过程

1.在Android系统中,所有应用程序进程以及系统服务进程都是由Zygote进程孕育(fork)出来的。

而在Linux系统中,所有的进程都是init进程的子孙进程,Zygote也是由init进程创建的。

在system/core/rootdir/init.rc中Zygote定义的导入

........

import /init.environ.rc

import /init.usb.rc

import /init.${ro.hardware}.rc

import /init.${ro.zygote}.rc

import /init.trace.rc

........

在system/core/rootdir/init.zygote64.rc中Zygote定义

#创建一个名为"zygote"的进程,这个zygote进程要执行的程序是/system/bin/app_process64,后面是要传给app_process的参数

#--zygote 在zygote模式启动, --start-system-server 启动系统服务

service zygote /system/bin/app_process64 -Xzygote /system/bin --zygote --start-system-server

class main #指定分组为main,同一分组的服务,可以统一启动与停止

socket zygote stream 660 root system #创建一个unix域的名为/dev/socket/zygote的套接字，并传递它的文件描述符给已启动的进程.

onrestart write /sys/android_power/request_state wake #zygote进程重启时执行/sys/android_power/request_state wake

onrestart write /sys/power/state on #zygote进程重启时/sys/power/state on

onrestart restart media #zygote进程重启时重启media服务

onrestart restart netd #zygote进程重启时重启netd服务

2.由1可以看出Zygote进程执行的便是/system/bin/app_process64,对应的源码是frameworks/base/cmds/app_process/app_main.cpp,入口函数是main

//frameworks/base/cmds/app_process/app_main.cpp

........

namespace android {

........

static AndroidRuntime* gCurRuntime = NULL; // 当前的AndroidRuntime对象

........

AndroidRuntime::AndroidRuntime(char* argBlockStart, const size_t argBlockLength) :

mExitWithoutCleanup(false),

mArgBlockStart(argBlockStart),

mArgBlockLength(argBlockLength)

{

........

assert(gCurRuntime == NULL); // one per process

gCurRuntime = this;

}

........

AndroidRuntime* AndroidRuntime::getRuntime()

{

return gCurRuntime;

}

........

void AndroidRuntime::start(const char* className, const Vector<String8>& options)

{

........

/* start the virtual machine */

JniInvocation jni_invocation;

jni_invocation.Init(NULL);

JNIEnv* env;

if (startVm(&mJavaVM, &env) != 0) { // 调用函数startVM启动虚拟机

return;

}

onVmCreated(env);

if (startReg(env) < 0) { // 调用函数startReg注册JNI方法

ALOGE("Unable to register all android natives\n");

return;

}

........

// 调用com.android.internal.os.ZygoteInit类的main函数

char* slashClassName = toSlashClassName(className);

jclass startClass = env->FindClass(slashClassName);

if (startClass == NULL) {

ALOGE("JavaVM unable to locate class '%s'\n", slashClassName);

/* keep going */

} else {

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); // 调用main函数

#if 0

if (env->ExceptionCheck())

threadExitUncaughtException(env);

#endif

}

}

free(slashClassName);

........

}

........

}

........

//frameworks/base/cmds/app_process/app_main.cpp

........

namespace android {

........

class AppRuntime : public AndroidRuntime // 继承与/frameworks/base/core/jni/AndroidRuntime.cpp

{

........

}

........

}

........

using namespace android;

........

int main(int argc, char* const argv[])

{

........

AppRuntime runtime(argv[0], computeArgBlockSize(argc, argv)); // 定义一个AppRuntime的对象

........

while (i < argc) {

const char* arg = argv[i++];

if (strcmp(arg, "--zygote") == 0) { // 在init.rc中设置了app_process启动参数--zygote

zygote = true;

niceName = ZYGOTE_NICE_NAME;

} else if (strcmp(arg, "--start-system-server") == 0) { // 在init.rc中设置了app_process启动参数--start-system-server

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;

}

}

........

if (zygote) {

runtime.start("com.android.internal.os.ZygoteInit", args); // 调用父类的start启动ZygoteInit

} else if (className) {

runtime.start("com.android.internal.os.RuntimeInit", args);

} else {

fprintf(stderr, "Error: no class name or --zygote supplied.\n");

app_usage();

LOG_ALWAYS_FATAL("app_process: no class name or --zygote supplied.");

return 10;

}

}

3.ZygoteInit类定义在frameworks/base/core/java/com/android/internal/os/ZygoteInit.java文件中

public class ZygoteInit {

........

　　private static final String ANDROID_SOCKET_ENV = "ANDROID_SOCKET_zygote";

　　........

private static void registerZygoteSocket(String socketName) {

if (sServerSocket == null) {

int fileDesc;

final String fullSocketName = ANDROID_SOCKET_PREFIX + socketName; // ANDROID_SOCKET_zygote

try {

String env = System.getenv(fullSocketName); // 提供环境变量ANDROID_SOCKET_zygote获取文件/dev/socket/zygote的文件描述符

fileDesc = Integer.parseInt(env);

} catch (RuntimeException ex) {

throw new RuntimeException(fullSocketName + " unset or invalid", ex);

}

try {

sServerSocket = new LocalServerSocket(createFileDescriptor(fileDesc));

} catch (IOException ex) {

throw new RuntimeException("Error binding to local socket '" + fileDesc + "'", ex);

}

}

}

........

private static boolean startSystemServer(String abiList, String socketName)

throws MethodAndArgsCaller, RuntimeException {

........

try {

........

pid = Zygote.forkSystemServer( // 提供Zygote的forkSystemServer函数来创建一个新的进程来启动SystemServer组件

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) { // 返回值pid等0的地方就是新的进程要执行的路径，即新创建的进程会执行handleSystemServerProcess函数

if (hasSecondZygote(abiList)) {

waitForSecondaryZygote(socketName);

}

handleSystemServerProcess(parsedArgs);

}

return true;

}

........

private static void handleSystemServerProcess(

ZygoteConnection.Arguments parsedArgs)

throws ZygoteInit.MethodAndArgsCaller {

closeServerSocket(); // 由Zygote进程创建的子进程继承Zygote进程在前面创建的Socket文件描述符，而这里的子进程又不会用到它,调用closeServerSocket函数来关闭

........

final String systemServerClasspath = Os.getenv("SYSTEMSERVERCLASSPATH");

if (systemServerClasspath != null) {

performSystemServerDexOpt(systemServerClasspath);// 对Zygote进程启动的SystemServer进行Dex优化

}

........

if (parsedArgs.invokeWith != null) {

........

} else {

........

RuntimeInit.zygoteInit(parsedArgs.targetSdkVersion, parsedArgs.remainingArgs, cl); // 调用RuntimeInit.zygoteInit函数来进一步执行启动SystemServer组件的操作

}

}

........

public static void main(String argv[]) {

try {

........

boolean startSystemServer = false;

String socketName = "zygote";

String abiList = null;

for (int i = 1; i < argv.length; i++) {

if ("start-system-server".equals(argv[i])) { // 在init.rc中传入了--start-system-server参数

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]);

}

}

........

registerZygoteSocket(socketName); // 调用registerZygoteSocket函数创建一个socket接口,用来和ActivityManagerService通讯

........

if (startSystemServer) {

startSystemServer(abiList, socketName); // 调用startSystemServer函数来启动SystemServer组件

}

// 调用runSelectLoopMode函数进入一个无限循环,在前面创建的socket接口上等待ActivityManagerService请求创建新的应用程序进程

Log.i(TAG, "Accepting command socket connections");

runSelectLoop(abiList);

closeServerSocket();

} catch (MethodAndArgsCaller caller) {

caller.run();

} catch (RuntimeException ex) {

Log.e(TAG, "Zygote died with exception", ex);

closeServerSocket();

throw ex;

}

}

........

private static void runSelectLoop(String abiList) throws MethodAndArgsCaller {

........

int loopCount = GC_LOOP_COUNT;

while (true) { // 进入无限循环

int index;

if (loopCount <= 0) {

gc();

loopCount = GC_LOOP_COUNT;

} else {

loopCount--;

}

try {

fdArray = fds.toArray(fdArray);

index = selectReadable(fdArray); //等待ActivityManagerService来连接这个Socket

} catch (IOException ex) {

throw new RuntimeException("Error in select()", ex);

}

if (index < 0) {

throw new RuntimeException("Error in select()");

} else if (index == 0) {

........

} else {

boolean done;

done = peers.get(index).runOnce(); // 调用ZygoteConnection.runOnce函数来创建新的应用程序

........

}

}

}

........

}

4.RuntimeInit函数

public class RuntimeInit {

........

private static final native void nativeZygoteInit();//实现在frameworks/base/core/jni/AndroidRuntime.cpp文件中

........

public static final void zygoteInit(int targetSdkVersion, String[] argv, ClassLoader classLoader)

throws ZygoteInit.MethodAndArgsCaller {

if (DEBUG) Slog.d(TAG, "RuntimeInit: Starting application from zygote");

redirectLogStreams();

commonInit();

nativeZygoteInit();// 调用zygoteInitNative函数来执行一个Binder进程间通信机制的初始化工作，这个工作完成之后，这个进程中的Binder对象就可以方便地进行进程间通信了

applicationInit(targetSdkVersion, argv, classLoader); //调用上面传进来的com.android.server.SystemServer类的main函数

}

........

}

5.环境变量ANDROID_SOCKET_zygote的值的由来

系统启动脚本文件system/core/rootdir/init.rc是由源代码位于system/core/init/init.c文件的init进程来解释执行的

service_start函数来解释init.rc文件中的service命令的

void service_start(struct service *svc, const char *dynamic_args)

{

struct stat s;

pid_t pid;

........

pid = fork();

if (pid == 0) {

struct socketinfo *si;

struct svcenvinfo *ei;

........

for (ei = svc->envvars; ei; ei = ei->next)

add_environment(ei->name, ei->value);

for (si = svc->sockets; si; si = si->next) {

int socket_type = (

!strcmp(si->type, "stream") ? SOCK_STREAM :

(!strcmp(si->type, "dgram") ? SOCK_DGRAM : SOCK_SEQPACKET));

int s = create_socket(si->name, socket_type,

si->perm, si->uid, si->gid, si->socketcon ?: scon);

if (s >= 0) {

publish_socket(si->name, s);// 通过这个函数来将得到的文件描述符写入到环境变量中去

}

}

}

........

}

........

static void publish_socket(const char *name, int fd)

{

char key[64] = ANDROID_SOCKET_ENV_PREFIX; //在system/core/include/cutils/sockets.h定义

char val[64];

strlcpy(key + sizeof(ANDROID_SOCKET_ENV_PREFIX) - 1, name, sizeof(key) - sizeof(ANDROID_SOCKET_ENV_PREFIX));

snprintf(val, sizeof(val), "%d", fd);

add_environment(key, val); // 写入系统属性

/* make sure we don't close-on-exec */

fcntl(fd, F_SETFD, 0);

}