前面已经讲到，从C/C++层进入了Java层，即调用了ZygoteInit.main()函数，那么接下来会发生什么事情呢？
源码参考Android4.1.1，涉及的文件有ZygoteInit.java,RuntimeInit.java(framework文件夹下)
ZygoteInit.main():
public static void main(String argv[]) {
try {
// Start profiling the zygote initialization.
SamplingProfilerIntegration.start();
//创建服务端的socket
registerZygoteSocket();
EventLog.writeEvent(LOG_BOOT_PROGRESS_PRELOAD_START,
SystemClock.uptimeMillis());
// 最终调用preloadClasses();preloadResources();两个函数，
//加载class文件和res资源
preload();
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
gc();
//这里的argv就是在C层产生的字符串数组，有两个元素
// If requested, start system server directly from Zygote
if (argv.length != 2) {
throw new RuntimeException(argv[0] + USAGE_STRING);
}
//显然，条件成立
if (argv[1].equals("start-system-server")) {
//这是要真刀真枪的开启SS进程了
startSystemServer();
} else if (!argv[1].equals("")) {
throw new RuntimeException(argv[0] + USAGE_STRING);
}
Log.i(TAG, "Accepting command socket connections");
if (ZYGOTE_FORK_MODE) {
runForkMode();
} else {
//这个函数其实也很重要，简而言之，让zygote休眠，但是随时能接受消息
//从loop字眼也可以看出循环的意思，后面的文章会在提到这个
runSelectLoopMode();
}
closeServerSocket();
} catch (MethodAndArgsCaller caller) {
caller.run();
} catch (RuntimeException ex) {
Log.e(TAG, "Zygote died with exception", ex);
closeServerSocket();
throw ex;
}
}
再看看startSystemServer（）函数
private static boolean startSystemServer()
throws MethodAndArgsCaller, RuntimeException {
/* Hardcoded command line to start the system server */
String args[] = {//参数
"--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 {
parsedArgs = new ZygoteConnection.Arguments(args);
ZygoteConnection.applyDebuggerSystemProperty(parsedArgs);
ZygoteConnection.applyInvokeWithSystemProperty(parsedArgs);
/* Request to fork the system server process */
pid = Zygote.forkSystemServer(//fork一个子进程
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，即system_server进程
handleSystemServerProcess(parsedArgs);
}
return true;
}
果然看到了forkSystemServer这样的函数了，接下来在子进程中调用了handleSystemServerProcess（）函数：
/**
* Finish remaining work for the newly forked system server process.
*/
private static void handleSystemServerProcess(
ZygoteConnection.Arguments parsedArgs)
throws ZygoteInit.MethodAndArgsCaller {
closeServerSocket();
// set umask to 0077 so new files and directories will default to owner-only permissions.
FileUtils.setUMask(FileUtils.S_IRWXG | FileUtils.S_IRWXO);
//niceName?
if (parsedArgs.niceName != null) {
Process.setArgV0(parsedArgs.niceName);
}
if (parsedArgs.invokeWith != null) {
WrapperInit.execApplication(parsedArgs.invokeWith,
parsedArgs.niceName, parsedArgs.targetSdkVersion,
null, parsedArgs.remainingArgs);
} else {
/*
* Pass the remaining arguments to SystemServer.
*/
//显然就是这个函数最重要了，
RuntimeInit.zygoteInit(parsedArgs.targetSdkVersion, parsedArgs.remainingArgs);
}
}
需要注意的是zygoteInit()函数是RuntimeInit类直接调用的，说明它是静态函数，在RuntimeInit.java文件中，zygoteInit（）函数如下：
/**
* 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)
throws ZygoteInit.MethodAndArgsCaller {
if (DEBUG) Slog.d(TAG, "RuntimeInit: Starting application from zygote");
redirectLogStreams();
commonInit();
//本地zygoteInit函数，主要职责是C++层的初始化
nativeZygoteInit();
//另外的一些初始化工作
applicationInit(targetSdkVersion, argv);
}
本地的zygoteInit函数我们先放在一边，先看applicationInit函数：
private static void applicationInit(int targetSdkVersion, String[] argv)
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 {
//一些参数设定
args = new Arguments(argv);
} catch (IllegalArgumentException ex) {
Slog.e(TAG, ex.getMessage());
// let the process exit
return;
}
// Remaining arguments are passed to the start class's static main
//英文解释的其实已经比较明白了，
invokeStaticMain(args.startClass, args.startArgs);
}
再来看看invokeStaticMain函数，很明显，调用了JNI.
private static void invokeStaticMain(String className, String[] argv)
throws ZygoteInit.MethodAndArgsCaller {
Class<?> cl;
try {
//获取类对象
cl = Class.forName(className);
} catch (ClassNotFoundException ex) {
throw new RuntimeException(
"Missing class when invoking static main " + className,
ex);
}
Method m;
try {
//获取方法
m = cl.getMethod("main", new Class[] { String[].class });
} 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);
##居然主动抛出异常？！！，what‘s the shit?
}
好吧，我在手心连写十个“忍”，继续还是心平气和的看代码，其实只要往上追溯，由于上层的方法都选择抛出该异常，最终追溯到了ZygoteInit.main()方法中，
在回顾main函数（部分）：
try {
.........
.........
.........
if (argv[1].equals("start-system-server")) {
startSystemServer();
} else if (!argv[1].equals("")) {
throw new RuntimeException(argv[0] + USAGE_STRING);
}
Log.i(TAG, "Accepting command socket connections");
if (ZYGOTE_FORK_MODE) {
runForkMode();
} else {
runSelectLoopMode();
}
closeServerSocket();
} catch (MethodAndArgsCaller caller) {
##shit!!,在这里catch到了这个异常，看来是有意为之了
caller.run();
} catch (RuntimeException ex) {
Log.e(TAG, "Zygote died with exception", ex);
closeServerSocket();
throw ex;
}
好，caller对象是(MethodAndArgsCaller类型的，继续看caller.run()做了什么：
......
......
public void run() {
try {
##kidding me ??!!,在这里调用了SystemServer.main()方法？
mMethod.invoke(null, new Object[] { mArgs });
//有兴趣的同学可以研究一下mArgs里面都是些什么
} 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);
}
}
- 问题到这里可以说结束了，最终（fork的子进程）调用了SystemServer.main()函数，相当于创造了SystemS进程。
那么问题来了，为什么不在主动抛出异常的的地方直接调用main方法？
- 说实话，我不是很理解，但是看了邓平凡大大的《深入理解Android I》(是本好书，值得推荐)，里面给出了一种解释那就是释放堆栈中被浪费的资源。
稍微解释一下，就是函数的调用是需要利用调用堆栈来实现的，每调用一个函数，原来的函数的资源会被压入栈，如果嵌套调用的函数多了的话，会占用系统资源。而关键在于SystemServer进程是不会死的（如果死了的话，系统也挂掉了），所以，也就意味着在这之前占用的那些资源不会被释放，so.......
多说一句，这种方式似乎达成了exec的函数调用的效果，上一篇文章提到，fork往往和exec一起用，可以造就两个不同的进程，但是实际上exec系统调用的工作原理是替换原来的进程的执行内容，只保留进程ID，可以说exec系统调用会擦除之前的进程积压的资源，这种抛异常的方式从某种程度上来讲实现了exec的效果。
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