Android -- 系统进程Zygote的启动分析
我们知道，Android系统是基于Linux内核的。Linux中，所有的进程都是由init进程创建出来的；也就是说，所有的进程都是直接或间接被init进程fork产生的。Android进程的孵化器Zygote进程也是如此，它也是在系统启动过程中，被init进程创建出来的。Android系统启动时，会解析init.rc初始化文件，其中就包含启动Zygote进程的命令：
service zygote /system/bin/app_process -Xzygote /system/bin --zygote --start-system-server
class main
socket zygote stream 660 root system
onrestart write /sys/android_power/request_state wake
onrestart write /sys/power/state on
onrestart restart media
onrestart restart netd
关键字service告诉我们要创建一个名为Zygote的进程，它所要执行的应用程序是/system/bin/app_process；之后的是传入的参数：- -Xzygote：jvm使用的参数
- /system/bin：一个未被使用的父目录
- --zygote、--start--system--server：启动Zygote进程要出使用的参数
socket关键字说明该进程需要创建一个套接字资源用于进程间通信，类型是unix domain socket，权限设置为660。onrestart关键字描述的都是该进程重启时需要执行的命令操作。当init.c文件执行时，解析到这个服务，就会去执行zygote进程对应的应用程序。
它对应的文件是/frameworks/base/cmds/app_process/app_main.cpp；直接看它的main()函数：
int main(int argc, char* const argv[])
{
if (prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0) < 0) {
// Older kernels don't understand PR_SET_NO_NEW_PRIVS and return
// EINVAL. Don't die on such kernels.
if (errno != EINVAL) {
LOG_ALWAYS_FATAL("PR_SET_NO_NEW_PRIVS failed: %s", strerror(errno));
return 12;
}
}
//AppRuntime是AndroidRuntime的子类，这里初始化runtime对象时Androidruntime中的gCurRuntime变量会被初始化为AppRuntime对象：runtime
AppRuntime runtime(argv[0], computeArgBlockSize(argc, argv));
// Process command line arguments
// ignore argv[0]
argc--;
argv++;
// Everything up to '--' or first non '-' arg goes to the vm.
//
// The first argument after the VM args is the "parent dir", which
// is currently unused.
//
// After the parent dir, we expect one or more the following internal
// arguments :
//
// --zygote : Start in zygote mode
// --start-system-server : Start the system server.
// --application : Start in application (stand alone, non zygote) mode.
// --nice-name : The nice name for this process.
//
// For non zygote starts, these arguments will be followed by
// the main class name. All remaining arguments are passed to
// the main method of this class.
//
// For zygote starts, all remaining arguments are passed to the zygote.
// main function.
//
// Note that we must copy argument string values since we will rewrite the
// entire argument block when we apply the nice name to argv0.
int i;
for (i = 0; i < argc; i++) {
if (argv[i][0] != '-') {
break;
}
if (argv[i][1] == '-' && argv[i][2] == 0) {
++i; // Skip --.
break;
}
runtime.addOption(strdup(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) {//忽略第一个参数：-Xzygote
const char* arg = argv[i++];
if (strcmp(arg, "--zygote") == 0) { //由参数列表可知，该项成立
zygote = true;
niceName = ZYGOTE_NICE_NAME;
} else if (strcmp(arg, "--start-system-server") == 0) {//由参数列表可知，该项成立
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;//启动Zygote进程时使用的参数列表
if (!className.isEmpty()) {
// We're not in zygote mode, the only argument we need to pass
// to RuntimeInit is the application argument.
//
// The Remainder of args get passed to startup class main(). Make
// copies of them before we overwrite them with the process name.
args.add(application ? String8("application") : String8("tool"));
runtime.setClassNameAndArgs(className, argc - i, argv + i);
} else {
// We're in zygote mode.
maybeCreateDalvikCache();
if (startSystemServer) {
args.add(String8("start-system-server"));//添加参数
}
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);//添加参数
// In zygote mode, pass all remaining arguments to the zygote
// main() method.
for (; i < argc; ++i) {
args.add(String8(argv[i]));//添加剩余的参数
}
}
if (!niceName.isEmpty()) {
runtime.setArgv0(niceName.string());
set_process_name(niceName.string());
}
if (zygote) {
runtime.start("com.android.internal.os.ZygoteInit", args, zygote);//附带参数列表，在Zygote模式下，同过AndroidRuntime::start()启动Zygote进程
} 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.");
return 10;
}
}
进入AndroidRuntime::start()函数：/*
* Start the Android runtime. This involves starting the virtual machine
* and calling the "static void main(String[] args)" method in the class
* named by "className".
*
* Passes the main function two arguments, the class name and the specified
* options string.
*/
void AndroidRuntime::start(const char* className, const Vector<String8>& options, bool zygote)
{
ALOGD(">>>>>> START %s uid %d <<<<<<\n",
className != NULL ? className : "(unknown)", getuid());
static const String8 startSystemServer("start-system-server");
/*
* 'startSystemServer == true' means runtime is obsolete and not run from
* init.rc anymore, so we print out the boot start event here.
*/
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);
}
//const char* kernelHack = getenv("LD_ASSUME_KERNEL");
//ALOGD("Found LD_ASSUME_KERNEL='%s'\n", kernelHack);
/* start the virtual machine */
JniInvocation jni_invocation;
jni_invocation.Init(NULL);
JNIEnv* env;
if (startVm(&mJavaVM, &env, zygote) != 0) {//1、启动虚拟机
return;
}
onVmCreated(env);
/*
* Register android functions.
*/
if (startReg(env) < 0) { //2、注册所需的JNI函数
ALOGE("Unable to register all android natives\n");
return;
}
/*
* We want to call main() with a String array with arguments in it.
* At present we have two arguments, the class name and an option string.
* Create an array to hold them.
*/
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);
}
/*
* Start VM. This thread becomes the main thread of the VM, and will
* not return until the VM exits.
*/
char* slashClassName = toSlashClassName(className);//com.android.internal.os.ZygoteInit
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");//通过JNI获取com.android.internal.os.ZygoteInit类的main()方法的jmethodID值
if (startMeth == NULL) {
ALOGE("JavaVM unable to find main() in '%s'\n", className);
/* keep going */
} else {
env->CallStaticVoidMethod(startClass, startMeth, strArray);//通过JNI调用com.android.internal.os.ZygoteInit类的main()方法，进入Java层代码
#if 0
if (env->ExceptionCheck())
threadExitUncaughtException(env);
#endif
}
}
free(slashClassName);
ALOGD("Shutting down VM\n");
if (mJavaVM->DetachCurrentThread() != JNI_OK)
ALOGW("Warning: unable to detach main thread\n");
if (mJavaVM->DestroyJavaVM() != 0)
ALOGW("Warning: VM did not shut down cleanly\n");
}start()函数中会进行启动虚拟机、注册JNI方法的预处理，最后会通过JNI的方式在native代码中调用ZygoteInit.java的main()函数，处理流程转而进入Java层。ZygoteInit类是zygote进程的启动类，看它的main()函数：
public static void main(String argv[]) {
try {
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; //该标志为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());//值为zygote
} else {
throw new RuntimeException("Unknown command line argument: " + argv[i]);
}
}
if (abiList == null) {
throw new RuntimeException("No ABI list supplied.");
}
registerZygoteSocket(socketName); // 1、创建socket，用来与ActivityManagerService进行通信
EventLog.writeEvent(LOG_BOOT_PROGRESS_PRELOAD_START,
SystemClock.uptimeMillis());
preload(); // 2、预加载资源文件
EventLog.writeEvent(LOG_BOOT_PROGRESS_PRELOAD_END,
SystemClock.uptimeMillis());
// Finish profiling the zygote initialization.
SamplingProfilerIntegration.writeZygoteSnapshot();
// Do an initial gc to clean up after startup
gcAndFinalize();
// Disable tracing so that forked processes do not inherit stale tracing tags from
// Zygote.
Trace.setTracingEnabled(false);
if (startSystemServer) {
startSystemServer(abiList, socketName); // 3、启动system_server进程
}
Log.i(TAG, "Accepting command socket connections");
runSelectLoop(abiList); // 4、开启一个循环，处理ActivityManagerService创建应用进程的请求
closeServerSocket(); // 程序退出时，清除socket资源
} catch (MethodAndArgsCaller caller) {
caller.run(); // 5、注意
} catch (RuntimeException ex) {
Log.e(TAG, "Zygote died with exception", ex);
closeServerSocket();
throw ex;
}
}
代码中共标记出了5个较为重要的处理过程，下面一一分析。（1）、registerZygoteSocket(socketName)
进入registerZygoteSocket(socketName)函数，查看其代码处理：
/**
* Registers a server socket for zygote command connections
*
* @throws RuntimeException when open fails
*/
private static void registerZygoteSocket(String socketName) {
if (sServerSocket == null) {
int fileDesc;
final String fullSocketName = ANDROID_SOCKET_PREFIX + socketName;// fullSocketName:ANDROID_SOCKET_zygote
try {
String env = System.getenv(fullSocketName);//获取该环境变量的值，即此socket对应的文件描述符
fileDesc = Integer.parseInt(env);
} catch (RuntimeException ex) {
throw new RuntimeException(fullSocketName + " unset or invalid", ex);
}
try {
FileDescriptor fd = new FileDescriptor();
fd.setInt$(fileDesc);
sServerSocket = new LocalServerSocket(fd);//用该文件描述符创建一个LocalServerSocket对象，并开始监听该socket
} catch (IOException ex) {
throw new RuntimeException(
"Error binding to local socket '" + fileDesc + "'", ex);
}
}
}从系统环境变量中获取到“ANDROID_SOCKET_zygote”这个socket对应的文件描述符，创建LocalServerSocket对象并监听该socket；此时名为zygote的socket就可以接收消息了。细心地人可能发现了，在我们的分析过程中并没有看到socket和Zygote进程的创建过程。其实这个过程在init.cpp解析init.rc文件时，已经处理完成了。下面来看这一部分内容。系统启动解析init.rc时，每当碰到一个由service关键字声明的服务，就会给他创建一个进程、并初始化该服务相关的资源；这些资源就包括socket的创建。
在init.cpp中，void service_start(struct service *svc, const char *dynamic_args)函数负责启动每个声明的service服务，我们提出一段重要的处理过程：
（3）、startSystemServer()
pid_t pid = fork();//创建一个进程
if (pid == 0) {
struct socketinfo *si;
struct svcenvinfo *ei;
char tmp[32];
int fd, sz;
umask(077);
if (properties_initialized()) {
get_property_workspace(&fd, &sz);
snprintf(tmp, sizeof(tmp), "%d,%d", dup(fd), sz);
add_environment("ANDROID_PROPERTY_WORKSPACE", tmp);
}
for (ei = svc->envvars; ei; ei = ei->next)
add_environment(ei->name, ei->value);
for (si = svc->sockets; si; si = si->next) { //socket创建
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);//socket发布
}
}
...
}
当系统为每个service通过调用fork()创建进程时，如果发现需要创建socket，它就会通过调用create_socket()创建一个socket：/*
* create_socket - creates a Unix domain socket in ANDROID_SOCKET_DIR
* ("/dev/socket") as dictated in init.rc. This socket is inherited by the
* daemon. We communicate the file descriptor's value via the environment
* variable ANDROID_SOCKET_ENV_PREFIX<name> ("ANDROID_SOCKET_foo").
*/
int create_socket(const char *name, int type, mode_t perm, uid_t uid,
gid_t gid, const char *socketcon)
{
struct sockaddr_un addr;
int fd, ret;
char *filecon;
if (socketcon)
setsockcreatecon(socketcon);
fd = socket(PF_UNIX, type, 0);
if (fd < 0) {
ERROR("Failed to open socket '%s': %s\n", name, strerror(errno));
return -1;
}
if (socketcon)
setsockcreatecon(NULL);
memset(&addr, 0 , sizeof(addr));
addr.sun_family = AF_UNIX;
snprintf(addr.sun_path, sizeof(addr.sun_path), ANDROID_SOCKET_DIR"/%s",
name);//设置此socket的地址
ret = unlink(addr.sun_path);
if (ret != 0 && errno != ENOENT) {
ERROR("Failed to unlink old socket '%s': %s\n", name, strerror(errno));
goto out_close;
}
filecon = NULL;
if (sehandle) {
ret = selabel_lookup(sehandle, &filecon, addr.sun_path, S_IFSOCK);
if (ret == 0)
setfscreatecon(filecon);
}
ret = bind(fd, (struct sockaddr *) &addr, sizeof (addr));//绑定该socket，启动listen在ZygoteInit::registerZygoteSocket()处理
if (ret) {
ERROR("Failed to bind socket '%s': %s\n", name, strerror(errno));
goto out_unlink;
}
setfscreatecon(NULL);
freecon(filecon);
chown(addr.sun_path, uid, gid);
chmod(addr.sun_path, perm);
INFO("Created socket '%s' with mode '%o', user '%d', group '%d'\n",
addr.sun_path, perm, uid, gid);
return fd;//返回该socket的文件描述符
out_unlink:
unlink(addr.sun_path);
out_close:
close(fd);
return -1;
}
socket创建完成后，要以环境变量键值对的形式把它发布到系统中：static void publish_socket(const char *name, int fd)
{
char key[64] = ANDROID_SOCKET_ENV_PREFIX;
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);//ANDROID_SOCKET_zygote -- socket的文件描述符
/* make sure we don't close-on-exec */
fcntl(fd, F_SETFD, 0);
}
到这里，socket的创建、注册处理流程就联系起来了。
ANDROID_SOCKET_ENV_PREFIX、ANDROID_SOCKET_DIR两个宏定义在/system/core/include/cutils/Socket.h中：#define ANDROID_SOCKET_ENV_PREFIX "ANDROID_SOCKET_" #define ANDROID_SOCKET_DIR "/dev/socket"
（2）、preload()
preload()函数的处理：
static void preload() {
Log.d(TAG, "begin preload");
preloadClasses();//加载/system/etc/preloaded-classes中的类资源
preloadResources();
preloadOpenGL();
preloadSharedLibraries();
preloadTextResources();
// Ask the WebViewFactory to do any initialization that must run in the zygote process,
// for memory sharing purposes.
WebViewFactory.prepareWebViewInZygote();
Log.d(TAG, "end preload");
}这里调用了5个函数去加载需要使用的类资源、图片资源、库资源等。这几个函数功能单一，我们可以自己阅读代码；这里就不详述了。但由于这部分内容涉及到很多I/O操作，而且加载的资源较多，会影响Android系统启动的时间。一些开机时间优化就是在这一部分处理的。startSystemServer()函数用于启动system_server进程：
/**
* 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_BLOCK_SUSPEND,
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
);
/* 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",
"--capabilities=" + capabilities + "," + capabilities,
"--nice-name=system_server",
"--runtime-args",
"com.android.server.SystemServer",
};//创建system_server的参数列表。设置了进程的uid、gid和进程名
ZygoteConnection.Arguments parsedArgs = null;
int pid;
try {
parsedArgs = new ZygoteConnection.Arguments(args);
ZygoteConnection.applyDebuggerSystemProperty(parsedArgs);
ZygoteConnection.applyInvokeWithSystemProperty(parsedArgs);
/* Request to fork the system server process */
pid = Zygote.forkSystemServer(
parsedArgs.uid, parsedArgs.gid,
parsedArgs.gids,
parsedArgs.debugFlags,
null,
parsedArgs.permittedCapabilities,
parsedArgs.effectiveCapabilities);//根据参数，为systemserver创建进程
} catch (IllegalArgumentException ex) {
throw new RuntimeException(ex);
}
/* For child process */
if (pid == 0) {
if (hasSecondZygote(abiList)) {
waitForSecondaryZygote(socketName);
}
handleSystemServerProcess(parsedArgs);//进程创建完毕后，调用该函数进一步处理
}
return true;
}
首先根据设置的参数列表创建system_server进程，然后在子进程中调用handleSystemServerProcess()做进一步处理：/**
* Finish remaining work for the newly forked system server process.
*/
private static void handleSystemServerProcess(
ZygoteConnection.Arguments parsedArgs)
throws ZygoteInit.MethodAndArgsCaller {
closeServerSocket();//根据fork()机制，system_server是zygote的子进程，它也拥有zygote这个socket资源；但由于system_server不需要使用socket，这里将它关闭
// 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);//system_server
}
final String systemServerClasspath = Os.getenv("SYSTEMSERVERCLASSPATH");
if (systemServerClasspath != null) {
performSystemServerDexOpt(systemServerClasspath);//com.android.server.SystemServer
}
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 = new PathClassLoader(systemServerClasspath, ClassLoader.getSystemClassLoader());
Thread.currentThread().setContextClassLoader(cl);
}
/*
* Pass the remaining arguments to SystemServer.
*/
RuntimeInit.zygoteInit(parsedArgs.targetSdkVersion, parsedArgs.remainingArgs, cl);//重要
}
/* should never reach here */
}直接查看RuntimeInit.zygoteInit()函数： /**
* 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();//调用AppRuntime.cpp::onZygoteInit()，开启线程池，用于Binder通信
applicationInit(targetSdkVersion, argv, classLoader);//通过反射调用SystemServer.java的main函数
}函数主要做了两个处理：native层开启线程池，用于Binder通信；nativeZygoteInit()最终调用： virtual void AppRuntime::onZygoteInit()
{
sp<ProcessState> proc = ProcessState::self();
ALOGV("App process: starting thread pool.\n");
proc->startThreadPool();
}启动线程池，用于Binder通信。然后进入applicationInit()： private static void applicationInit(int targetSdkVersion, String[] argv, ClassLoader classLoader)
throws ZygoteInit.MethodAndArgsCaller {
// If the application calls System.exit(), terminate the process
// immediately without running any shutdown hooks. It is not possible to
// shutdown an Android application gracefully. Among other things, the
// Android runtime shutdown hooks close the Binder driver, which can cause
// leftover running threads to crash before the process actually exits.
nativeSetExitWithoutCleanup(true);
// We want to be fairly aggressive about heap utilization, to avoid
// holding on to a lot of memory that isn't needed.
VMRuntime.getRuntime().setTargetHeapUtilization(0.75f);
VMRuntime.getRuntime().setTargetSdkVersion(targetSdkVersion);
final Arguments args;
try {
args = new Arguments(argv);
} catch (IllegalArgumentException ex) {
Slog.e(TAG, ex.getMessage());
// let the process exit
return;
}
// The end of of the RuntimeInit event (see #zygoteInit).
Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
// Remaining arguments are passed to the start class's static main
invokeStaticMain(args.startClass, args.startArgs, classLoader);//通过反射调用SystemServer.java的main方法
}再看invokeStaticMain()：/**
* Invokes a static "main(argv[]) method on class "className".
* Converts various failing exceptions into RuntimeExceptions, with
* the assumption that they will then cause the VM instance to exit.
*
* @param className Fully-qualified class name
* @param argv Argument vector for main()
* @param classLoader the classLoader to load {@className} with
*/
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 });//获取SystemServer.java的main()函数的域名，但并没有立即调用main函数；
} catch (NoSuchMethodException ex) {
throw new RuntimeException(
"Missing static main on " + className, ex);
} catch (SecurityException ex) {
throw new RuntimeException(
"Problem getting static main on " + className, ex);
}
int modifiers = m.getModifiers();
if (! (Modifier.isStatic(modifiers) && Modifier.isPublic(modifiers))) {
throw new RuntimeException(
"Main method is not public and static on " + className);
}
/*
* 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);//在ZygoteInit.java的main()中第5步处理时，调用SystemServer.java的main()函数
}通过ZygoteInit.MethodAndArgsCaller异常的处理来调用SystemServer.java的main()函数启动各个系统服务，看MethodAndArgsCaller的定义： /**
* Helper exception class which holds a method and arguments and
* can call them. This is used as part of a trampoline to get rid of
* the initial process setup stack frames.
*/
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) {
Throwable cause = ex.getCause();
if (cause instanceof RuntimeException) {
throw (RuntimeException) cause;
} else if (cause instanceof Error) {
throw (Error) cause;
}
throw new RuntimeException(ex);
}
}
}由代码注释可知：这种调用方式会清理堆栈，可以让SystemServer.java的main函数认为自己是system_server进程的入口，虽然这之前已经做了大量的工作。（4）、 runSelectLoop()
runSelectLoop()函数处理如下：
/**
* Runs the zygote process's select loop. Accepts new connections as
* they happen, and reads commands from connections one spawn-request's
* worth at a time.
*
* @throws MethodAndArgsCaller in a child process when a main() should
* be executed.
*/
private static void runSelectLoop(String abiList) throws MethodAndArgsCaller {
ArrayList<FileDescriptor> fds = new ArrayList<FileDescriptor>();
ArrayList<ZygoteConnection> peers = new ArrayList<ZygoteConnection>();
fds.add(sServerSocket.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) {
ZygoteConnection newPeer = acceptCommandPeer(abiList);
peers.add(newPeer);
fds.add(newPeer.getFileDesciptor());
} else {
boolean done = peers.get(i).runOnce();
if (done) {
peers.remove(i);
fds.remove(i);
}
}
}
}
}ZygoteConnection对象是一个socket连接，它请求要创建一个新应用进程。如果ActivityManagerService发送的请求被收到后，就会进入ZygoteConnection::runOnce()处理。runOnce()的主要工作就是会为这个请求fork一个新的进程，并做一些其他的处理。
（5）、MethodAndArgsCaller异常处理
我们退回到ZygoteInit::main()函数中，看MethodAndArgsCaller异常的捕获处理过程：
catch (MethodAndArgsCaller caller) {
caller.run(); // 5、注意
} catch (RuntimeException ex) {
Log.e(TAG, "Zygote died with exception", ex);
closeServerSocket();
throw ex;
} /**
* Helper exception class which holds a method and arguments and
* can call them. This is used as part of a trampoline to get rid of
* the initial process setup stack frames.
*/
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 });//调用mMethod本身代表的方法
} catch (IllegalAccessException ex) {
throw new RuntimeException(ex);
} catch (InvocationTargetException ex) {
Throwable cause = ex.getCause();
if (cause instanceof RuntimeException) {
throw (RuntimeException) cause;
} else if (cause instanceof Error) {
throw (Error) cause;
}
throw new RuntimeException(ex);
}
}
}run()方法中通过invoke()调用SystemServer.java的main()方法： /**
* The main entry point from zygote.
*/
public static void main(String[] args) {
new SystemServer().run();
}SystemServer.run()方法中做了大量的工作，其中就包括启动各种重要的系统服务，如PackageManagerService、PowerManagerService等等。到此，Zygote进程和SystemServer进程的启动过程就结束了。
作者：csdn_of_coder 发表于2016/10/23 14:41:01 原文链接
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