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本文基于 android 9.0 , 代码仓库地址 ：
文中源码链接：
对 zygote 和 systemserver 启动流程还不熟悉的建议阅读下面两篇文章：
zygote 作为 android 世界的受精卵，在成功繁殖出 system_server 进程之后并没有完全功成身退，仍然承担着受精卵的责任。zygote 通过调用其持有的 zygoteserver 对象的 runselectloop() 方法开始等待客户端的呼唤，有求必应。客户端的请求无非是创建应用进程，以 startactivity() 为例，假如开启的是一个尚未创建进程的应用，那么就会向 zygote 请求创建进程。下面将从 客户端发送请求 和 服务端处理请求 两方面来进行解析。
startactivity() 的具体流程这里就不分析了，系列后续文章会写到。我们直接看到创建进程的 startprocess() 方法，该方法在 activitymanagerservice 中，后面简称 ams。
我要评论> activitymanagerservice.java
private processstartresult startprocess(string hostingtype, string entrypoint,
processrecord app, int uid, int[] gids, int runtimeflags, int mountexternal,
string seinfo, string requiredabi, string instructionset, string invokewith,
long starttime) {
try {
checktime(starttime, "startprocess: asking zygote to start proc");
final processstartresult startresult;
if (hostingtype.equals("webview_service")) {
startresult = startwebview(entrypoint,
app.processname, uid, uid, gids, runtimeflags, mountexternal,
app.info.targetsdkversion, seinfo, requiredabi, instructionset,
app.info.datadir, null,
new string[] {proc_start_seq_ident + app.startseq});
} else {
// 新建进程
startresult = process.start(entrypoint,
app.processname, uid, uid, gids, runtimeflags, mountexternal,
app.info.targetsdkversion, seinfo, requiredabi, instructionset,
app.info.datadir, invokewith,
new string[] {proc_start_seq_ident + app.startseq});
}
checktime(starttime, "startprocess: returned from zygote!");
return startresult;
} finally {
trace.traceend(trace.trace_tag_activity_manager);
}
}
调用 process.start() 方法新建进程，继续追进去:
> process.java
public static final processstartresult start(
// android.app.activitythread，创建进程后会调用其 main() 方法
final string processclass,
final string nicename, // 进程名
int uid, int gid, int[] gids,
int runtimeflags, int mountexternal,
int targetsdkversion,
string seinfo,
string abi,
string instructionset,
string appdatadir,
string invokewith, // 一般新建应用进程时，此参数不为 null
string[] zygoteargs) {
return zygoteprocess.start(processclass, nicename, uid, gid, gids,
runtimeflags, mountexternal, targetsdkversion, seinfo,
abi, instructionset, appdatadir, invokewith, zygoteargs);
}
继续调用 zygoteprocess.start() ：
> zygoteproess.java
public final process.processstartresult start(final string processclass,
final string nicename,
int uid, int gid, int[] gids,
int runtimeflags, int mountexternal,
int targetsdkversion,
string seinfo,
string abi,
string instructionset,
string appdatadir,
string invokewith,
string[] zygoteargs) {
try {
return startviazygote(processclass, nicename, uid, gid, gids,
runtimeflags, mountexternal, targetsdkversion, seinfo,
abi, instructionset, appdatadir, invokewith, false /* startchildzygote */,
zygoteargs);
} catch (zygotestartfailedex ex) {
log.e(log_tag,
"starting vm process through zygote failed");
throw new runtimeexception(
"starting vm process through zygote failed", ex);
}
}
调用 startviazygote() 方法。终于看到 zygote 的身影了。
> zygoteprocess.java
private process.processstartresult startviazygote(final string processclass,
final string nicename,
final int uid, final int gid,
final int[] gids,
int runtimeflags, int mountexternal,
int targetsdkversion,
string seinfo,
string abi,
string instructionset,
string appdatadir,
string invokewith,
boolean startchildzygote, // 是否克隆 zygote 进程的所有状态
string[] extraargs)
throws zygotestartfailedex {
arraylist<string> argsforzygote = new arraylist<string>();
// --runtime-args, --setuid=, --setgid=,
// and --setgroups= must go first
// 处理参数
argsforzygote.add("--runtime-args");
argsforzygote.add("--setuid=" + uid);
argsforzygote.add("--setgid=" + gid);
argsforzygote.add("--runtime-flags=" + runtimeflags);
if (mountexternal == zygote.mount_external_default) {
argsforzygote.add("--mount-external-default");
} else if (mountexternal == zygote.mount_external_read) {
argsforzygote.add("--mount-external-read");
} else if (mountexternal == zygote.mount_external_write) {
argsforzygote.add("--mount-external-write");
}
argsforzygote.add("--target-sdk-version=" + targetsdkversion);
// --setgroups is a comma-separated list
if (gids != null && gids.length > 0) {
stringbuilder sb = new stringbuilder();
sb.append("--setgroups=");
int sz = gids.length;
for (int i = 0; i < sz; i++) {
if (i != 0) {
sb.append(',');
}
sb.append(gids[i]);
}
argsforzygote.add(sb.tostring());
}
if (nicename != null) {
argsforzygote.add("--nice-name=" + nicename);
}
if (seinfo != null) {
argsforzygote.add("--seinfo=" + seinfo);
}
if (instructionset != null) {
argsforzygote.add("--instruction-set=" + instructionset);
}
if (appdatadir != null) {
argsforzygote.add("--app-data-dir=" + appdatadir);
}
if (invokewith != null) {
argsforzygote.add("--invoke-with");
argsforzygote.add(invokewith);
}
if (startchildzygote) {
argsforzygote.add("--start-child-zygote");
}
argsforzygote.add(processclass);
if (extraargs != null) {
for (string arg : extraargs) {
argsforzygote.add(arg);
}
}
synchronized(mlock) {
// 和 zygote 进程进行 socket 通信
return zygotesendargsandgetresult(openzygotesocketifneeded(abi), argsforzygote);
}
}
前面一大串代码都是在处理参数，大致浏览即可。核心在于最后的 openzygotesocketifneeded() 和 zygotesendargsandgetresult() 这两个方法。从方法命名就可以看出来，这里要和 zygote 进行 socket 通信了。还记得 zygoteinit.main() 方法中调用的 registerserversocketfromenv() 方法吗？它在 zygote 进程中创建了服务端 socket。
先来看看 openzygotesocketifneeded() 方法。
> zygoteprocess.java
private zygotestate openzygotesocketifneeded(string abi) throws zygotestartfailedex {
preconditions.checkstate(thread.holdslock(mlock), "zygoteprocess lock not held");
// 未连接或者连接已关闭
if (primaryzygotestate == null || primaryzygotestate.isclosed()) {
try {
// 开启 socket 连接
primaryzygotestate = zygotestate.connect(msocket);
} catch (ioexception ioe) {
throw new zygotestartfailedex("error connecting to primary zygote", ioe);
}
maybesetapiblacklistexemptions(primaryzygotestate, false);
maybesethiddenapiaccesslogsamplerate(primaryzygotestate);
}
if (primaryzygotestate.matches(abi)) {
return primaryzygotestate;
}
// 当主 zygote 没有匹配成功，尝试 connect 第二个 zygote
if (secondaryzygotestate == null || secondaryzygotestate.isclosed()) {
try {
secondaryzygotestate = zygotestate.connect(msecondarysocket);
} catch (ioexception ioe) {
throw new zygotestartfailedex("error connecting to secondary zygote", ioe);
}
maybesetapiblacklistexemptions(secondaryzygotestate, false);
maybesethiddenapiaccesslogsamplerate(secondaryzygotestate);
}
if (secondaryzygotestate.matches(abi)) {
return secondaryzygotestate;
}
throw new zygotestartfailedex("unsupported zygote abi: " + abi);
}
如果与 zygote 进程的 socket 连接未开启，则尝试开启，可能会产生阻塞和重试。连接调用的是 zygotestate.connect() 方法，zygotestate 是 zygoteprocess 的内部类。
> zygoteprocess.java
public static class zygotestate {
final localsocket socket;
final datainputstream inputstream;
final bufferedwriter writer;
final list<string> abilist;
boolean mclosed;
private zygotestate(localsocket socket, datainputstream inputstream,
bufferedwriter writer, list<string> abilist) {
this.socket = socket;
this.inputstream = inputstream;
this.writer = writer;
this.abilist = abilist;
}
public static zygotestate connect(localsocketaddress address) throws ioexception {
datainputstream zygoteinputstream = null;
bufferedwriter zygotewriter = null;
final localsocket zygotesocket = new localsocket();
try {
zygotesocket.connect(address);
zygoteinputstream = new datainputstream(zygotesocket.getinputstream());
zygotewriter = new bufferedwriter(new outputstreamwriter(
zygotesocket.getoutputstream()), 256);
} catch (ioexception ex) {
try {
zygotesocket.close();
} catch (ioexception ignore) {
}
throw ex;
}
string abiliststring = getabilist(zygotewriter, zygoteinputstream);
log.i("zygote", "process: zygote socket " + address.getnamespace() + "/"
+ address.getname() + " opened, supported abis: " + abiliststring);
return new zygotestate(zygotesocket, zygoteinputstream, zygotewriter,
arrays.aslist(abiliststring.split(",")));
}
...
}
通过 socket 连接 zygote 远程服务端。
再回头看之前的 zygotesendargsandgetresult() 方法。
> zygoteprocess.java
private static process.processstartresult zygotesendargsandgetresult(
zygotestate zygotestate, arraylist<string> args)
throws zygotestartfailedex {
try {
...
final bufferedwriter writer = zygotestate.writer;
final datainputstream inputstream = zygotestate.inputstream;
writer.write(integer.tostring(args.size()));
writer.newline();
// 向 zygote 进程发送参数
for (int i = 0; i < sz; i++) {
string arg = args.get(i);
writer.write(arg);
writer.newline();
}
writer.flush();
// 是不是应该有一个超时时间？
process.processstartresult result = new process.processstartresult();
// always read the entire result from the input stream to avoid leaving
// bytes in the stream for future process starts to accidentally stumble
// upon.
// 读取 zygote 进程返回的子进程 pid
result.pid = inputstream.readint();
result.usingwrapper = inputstream.readboolean();
if (result.pid < 0) { // pid 小于 0 ，fork 失败
throw new zygotestartfailedex("fork() failed");
}
return result;
} catch (ioexception ex) {
zygotestate.close();
throw new zygotestartfailedex(ex);
}
}
通过 socket 发送请求参数，然后等待 zygote 进程返回子进程 pid 。客户端的工作到这里就暂时完成了，我们再追踪到服务端，看看服务端是如何处理客户端请求的。
zygote 处理客户端请求的代码在 zygoteserver.runselectloop() 方法中。
> zygoteserver.java
runnable runselectloop(string abilist) {
...
while (true) {
...
try {
// 有事件来时往下执行，没有时就阻塞
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) { // 有新客户端连接
zygoteconnection newpeer = acceptcommandpeer(abilist);
peers.add(newpeer);
fds.add(newpeer.getfiledesciptor());
} else { // 处理客户端请求
try {
zygoteconnection connection = peers.get(i);
// fork 子进程，并返回包含子进程 main() 函数的 runnable 对象
final runnable command = connection.processonecommand(this);
if (misforkchild) {
// 位于子进程
if (command == null) {
throw new illegalstateexception("command == null");
}
return command;
} else {
// 位于父进程
if (command != null) {
throw new illegalstateexception("command != null");
}
if (connection.isclosedbypeer()) {
connection.closesocket();
peers.remove(i);
fds.remove(i);
}
}
} catch (exception e) {
...
} finally {
misforkchild = false;
}
}
}
}
}
acceptcommandpeer() 方法用来响应新客户端的 socket 连接请求。processonecommand() 方法用来处理客户端的一般请求。
> zygoteconnection.java
runnable processonecommand(zygoteserver zygoteserver) {
string args[];
arguments parsedargs = null;
filedescriptor[] descriptors;
try {
// 1. 读取 socket 客户端发送过来的参数列表
args = readargumentlist();
descriptors = msocket.getancillaryfiledescriptors();
} catch (ioexception ex) {
throw new illegalstateexception("ioexception on command socket", ex);
}
...
// 2. fork 子进程
pid = zygote.forkandspecialize(parsedargs.uid, parsedargs.gid, parsedargs.gids,
parsedargs.runtimeflags, rlimits, parsedargs.mountexternal, parsedargs.seinfo,
parsedargs.nicename, fdstoclose, fdstoignore, parsedargs.startchildzygote,
parsedargs.instructionset, parsedargs.appdatadir);
try {
if (pid == 0) {
// 处于进子进程
zygoteserver.setforkchild();
// 关闭服务端 socket
zygoteserver.closeserversocket();
ioutils.closequietly(serverpipefd);
serverpipefd = null;
// 3. 处理子进程事务
return handlechildproc(parsedargs, descriptors, childpipefd,
parsedargs.startchildzygote);
} else {
// 处于 zygote 进程
ioutils.closequietly(childpipefd);
childpipefd = null;
// 4. 处理父进程事务
handleparentproc(pid, descriptors, serverpipefd);
return null;
}
} finally {
ioutils.closequietly(childpipefd);
ioutils.closequietly(serverpipefd);
}
}
processonecommand() 方法大致可以分为五步，下面逐步分析。
> zygoteconnection.java
private string[] readargumentlist()
throws ioexception {
int argc;
try {
// 逐行读取参数
string s = msocketreader.readline();
if (s == null) {
// eof reached.
return null;
}
argc = integer.parseint(s);
} catch (numberformatexception ex) {
throw new ioexception("invalid wire format");
}
// see bug 1092107: large argc can be used for a dos attack
if (argc > max_zygote_argc) {
throw new ioexception("max arg count exceeded");
}
string[] result = new string[argc];
for (int i = 0; i < argc; i++) {
result[i] = msocketreader.readline();
if (result[i] == null) {
// we got an unexpected eof.
throw new ioexception("truncated request");
}
}
return result;
}
读取客户端发送过来的请求参数。
> zygote.java
public static int forkandspecialize(int uid, int gid, int[] gids, int runtimeflags,
int[][] rlimits, int mountexternal, string seinfo, string nicename, int[] fdstoclose,
int[] fdstoignore, boolean startchildzygote, string instructionset, string appdatadir) {
vm_hooks.prefork();
// resets nice priority for zygote process.
resetnicepriority();
int pid = nativeforkandspecialize(
uid, gid, gids, runtimeflags, rlimits, mountexternal, seinfo, nicename, fdstoclose,
fdstoignore, startchildzygote, instructionset, appdatadir);
// enable tracing as soon as possible for the child process.
if (pid == 0) {
trace.settracingenabled(true, runtimeflags);
// note that this event ends at the end of handlechildproc,
trace.tracebegin(trace.trace_tag_activity_manager, "postfork");
}
vm_hooks.postforkcommon();
return pid;
}
nativeforkandspecialize() 是一个 native 方法，在底层 fork 了一个新进程，并返回其 pid。不要忘记了这里的 一次fork，两次返回 。pid > 0 说明还是父进程。pid = 0 说明进入了子进程。子进程中会调用 handlechildproc，而父进程中会调用 handleparentproc()。
> zygoteconnection.java
private runnable handlechildproc(arguments parsedargs, filedescriptor[] descriptors,
filedescriptor pipefd, boolean iszygote) {
closesocket(); // 关闭 socket 连接
...
if (parsedargs.nicename != null) {
// 设置进程名
process.setargv0(parsedargs.nicename);
}
if (parsedargs.invokewith != null) {
wrapperinit.execapplication(parsedargs.invokewith,
parsedargs.nicename, parsedargs.targetsdkversion,
vmruntime.getcurrentinstructionset(),
pipefd, parsedargs.remainingargs);
// should not get here.
throw new illegalstateexception("wrapperinit.execapplication unexpectedly returned");
} else {
if (!iszygote) { // 新建应用进程时 iszygote 参数为 false
return zygoteinit.zygoteinit(parsedargs.targetsdkversion, parsedargs.remainingargs,
null /* classloader */);
} else {
return zygoteinit.childzygoteinit(parsedargs.targetsdkversion,
parsedargs.remainingargs, null /* classloader */);
}
}
}
当看到 zygoteinit.zygoteinit() 时你应该感觉很熟悉了，接下来的流程就是：
zygoteinit.zygoteinit()->runtimeinit.applicationinit()->findstaticmain()
和 systemserver 进程的创建流程一致。这里要找的 main 方法就是 activitythrad.main() 。activitythread 虽然并不是一个线程，但你可以把它理解为应用的主线程。
> zygoteconnection.java
private void handleparentproc(int pid, filedescriptor[] descriptors, filedescriptor pipefd) {
if (pid > 0) {
setchildpgid(pid);
}
if (descriptors != null) {
for (filedescriptor fd: descriptors) {
ioutils.closequietly(fd);
}
}
boolean usingwrapper = false;
if (pipefd != null && pid > 0) {
int innerpid = -1;
try {
// do a busy loop here. we can't guarantee that a failure (and thus an exception
// bail) happens in a timely manner.
final int bytes_required = 4; // bytes in an int.
structpollfd fds[] = new structpollfd[] {
new structpollfd()
};
byte data[] = new byte[bytes_required];
int remainingsleeptime = wrapped_pid_timeout_millis;
int dataindex = 0;
long starttime = system.nanotime();
while (dataindex < data.length && remainingsleeptime > 0) {
fds[0].fd = pipefd;
fds[0].events = (short) pollin;
fds[0].revents = 0;
fds[0].userdata = null;
int res = android.system.os.poll(fds, remainingsleeptime);
long endtime = system.nanotime();
int elapsedtimems = (int)((endtime - starttime) / 1000000l);
remainingsleeptime = wrapped_pid_timeout_millis - elapsedtimems;
if (res > 0) {
if ((fds[0].revents & pollin) != 0) {
// only read one byte, so as not to block.
int readbytes = android.system.os.read(pipefd, data, dataindex, 1);
if (readbytes < 0) {
throw new runtimeexception("some error");
}
dataindex += readbytes;
} else {
// error case. revents should contain one of the error bits.
break;
}
} else if (res == 0) {
log.w(tag, "timed out waiting for child.");
}
}
if (dataindex == data.length) {
datainputstream is = new datainputstream(new bytearrayinputstream(data));
innerpid = is.readint();
}
if (innerpid == -1) {
log.w(tag, "error reading pid from wrapped process, child may have died");
}
} catch (exception ex) {
log.w(tag, "error reading pid from wrapped process, child may have died", ex);
}
// ensure that the pid reported by the wrapped process is either the
// child process that we forked, or a descendant of it.
if (innerpid > 0) {
int parentpid = innerpid;
while (parentpid > 0 && parentpid != pid) {
parentpid = process.getparentpid(parentpid);
}
if (parentpid > 0) {
log.i(tag, "wrapped process has pid " + innerpid);
pid = innerpid;
usingwrapper = true;
} else {
log.w(tag, "wrapped process reported a pid that is not a child of "
+ "the process that we forked: childpid=" + pid
+ " innerpid=" + innerpid);
}
}
}
try {
msocketoutstream.writeint(pid);
msocketoutstream.writeboolean(usingwrapper);
} catch (ioexception ex) {
throw new illegalstateexception("error writing to command socket", ex);
}
}
主要进行一些资源清理的工作。到这里，子进程就创建完成了。
process.start() 创建应用进程zygoteprocess 负责和 zygote 进程建立 socket 连接，并将创建进程需要的参数发送给 zygote 的 socket 服务端zygote 服务端接收到参数之后调用 zygoteconnection.processonecommand() 处理参数，并 fork 进程findstaticmain() 找到 activitythread 类的 main() 方法并执行，子进程就启动了到现在为止已经解析了 zygote 进程 ，systemserver 进程，以及应用进程的创建。下一篇的内容是和应用最密切相关的系统服务 activitymanagerservice , 来看看它在 systemserver 中是如何被创建和启动的，敬请期待！
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