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。
客户端发送请求
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。
Process.startProcess()
> 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。
startViaZygote()
> 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。
openZygoteSocketIfNeeded()
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。
zygoteSendArgsAndGetResult()
> 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
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。
processOneCommand()
> 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。
readArgumentList()
> 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。
forkAndSpecialize()
> 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()。
handleChildProc()
> 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。
handleParentProc()
> 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。
总结
- 调用
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- 最后通过
findStaticMain()找到ActivityThread类的 main() 方法并执行，子进程就启动了
