概述
启动一个应用程序要通过AMS，而AMS在启动应用程序时，会先判断该应用程序所需要的进程是否存在，若不存在，则需先创建进程。创建进程的工作由Zygote来完成。AMS向Zygote在Java层创建的Socket发送创建应用请求，Zygote通过fock自身创建。
时序图.png
过程详解
1.AMS的startProcessLocked
private final void startProcessLocked(ProcessRecord app, String hostingType,
String hostingNameStr, String abiOverride, String entryPoint, String[] entryPointArgs) {
//此处省略N行.......
//获取要创建的进程的用户Id
int uid = app.uid;
int[] gids = null;
int mountExternal = Zygote.MOUNT_EXTERNAL_NONE;
if (!app.isolated) {
int[] permGids = null;
//此处省略N行
if (ArrayUtils.isEmpty(permGids)) {
gids = new int[3];
} else {
gids = new int[permGids.length + 3];
System.arraycopy(permGids, 0, gids, 3, permGids.length);
}
gids[0] = UserHandle.getSharedAppGid(UserHandle.getAppId(uid));
gids[1] = UserHandle.getCacheAppGid(UserHandle.getAppId(uid));
gids[2] = UserHandle.getUserGid(UserHandle.getUserId(uid));
}
//此处省略N行
if (hostingType.equals("webview_service")) {
startResult = startWebView(entryPoint,
app.processName, uid, uid, gids, debugFlags, mountExternal,
app.info.targetSdkVersion, seInfo, requiredAbi, instructionSet,
app.info.dataDir, null, entryPointArgs);
} else {
//启动应用程序进程
startResult = Process.start(entryPoint,
app.processName, uid, uid, gids, debugFlags, mountExternal,
app.info.targetSdkVersion, seInfo, requiredAbi, instructionSet,
app.info.dataDir, invokeWith, entryPointArgs);
}
//此处省略N行.......
}
2.Process的start()
内部持有ZygoteProcess对象，调用了它的start()方法
public static final ProcessStartResult start(final String processClass,
final String niceName,
int uid, int gid, int[] gids,
int debugFlags, int mountExternal,
int targetSdkVersion,
String seInfo,
String abi,
String instructionSet,
String appDataDir,
String invokeWith,
String[] zygoteArgs) {
//processClass是前面AMS传进来的entryPonit，而entryPoint实际上是
// if (entryPoint == null) entryPoint = "android.app.ActivityThread";
return zygoteProcess.start(processClass, niceName, uid, gid, gids,
debugFlags, mountExternal, targetSdkVersion, seInfo,
abi, instructionSet, appDataDir, invokeWith, zygoteArgs);
3.ZygoteProcess的start()
public final Process.ProcessStartResult start(final String processClass,
final String niceName,
int uid, int gid, int[] gids,
int debugFlags, int mountExternal,
int targetSdkVersion,
String seInfo,
String abi,
String instructionSet,
String appDataDir,
String invokeWith,
String[] zygoteArgs) {
try {
return startViaZygote(processClass, niceName, uid, gid, gids,
debugFlags, mountExternal, targetSdkVersion, seInfo,
abi, instructionSet, appDataDir, invokeWith, zygoteArgs);
} catch (ZygoteStartFailedEx ex) {
Log.e(LOG_TAG,
"Starting VM process through Zygote failed");
throw new RuntimeException(
"Starting VM process through Zygote failed", ex);
}
}
调用起内部startViaZygote（）
private Process.ProcessStartResult startViaZygote(final String processClass,
final String niceName, final int uid, final int gid, final int[] gids,
int debugFlags, int mountExternal, int targetSdkVersion, String seInfo,
String abi,String instructionSet,String appDataDir,
String invokeWith,String[] extraArgs)throws ZygoteStartFailedEx {
ArrayList<String> argsForZygote = new ArrayList<String>();
argsForZygote.add("--runtime-args");
argsForZygote.add("--setuid=" + uid);
argsForZygote.add("--setgid=" + gid);
synchronized(mLock) {
return zygoteSendArgsAndGetResult(openZygoteSocketIfNeeded(abi), argsForZygote);
}
}
这里主要保存了一些进程启动的参数，最终调用zygoteSendArgsAndGetResult（），该方法将应用程序启动参数写入ZygoteProcess的静态内部类，表示与Zygote进程通信的状态，
【openZygoteSocketIfNeeded(abi)获取zygoteState对象。】
@GuardedBy("mLock")
private ZygoteState openZygoteSocketIfNeeded(String abi) throws ZygoteStartFailedEx {
Preconditions.checkState(Thread.holdsLock(mLock), "ZygoteProcess lock not held");
if (primaryZygoteState == null || primaryZygoteState.isClosed()) {
try {
//通过ZygoteState与Zygote建立连接
primaryZygoteState = ZygoteState.connect(mSocket);
} catch (IOException ioe) {
throw new ZygoteStartFailedEx("Error connecting to primary zygote", ioe);
}
}
if (primaryZygoteState.matches(abi)) {
return primaryZygoteState;
}
if (secondaryZygoteState == null || secondaryZygoteState.isClosed()) {
try {
secondaryZygoteState = ZygoteState.connect(mSecondarySocket);
} catch (IOException ioe) {
throw new ZygoteStartFailedEx("Error connecting to secondary zygote", ioe);
}
}
if (secondaryZygoteState.matches(abi)) {
return secondaryZygoteState;
}
throw new ZygoteStartFailedEx("Unsupported zygote ABI: " + abi);
}
该方法通过与Zygote进程建立的Server端的Sokcet建立连接，获取ZygoteState类型的primaryZygoteServer对象，判断其与启动应用进程所需的ABI是否匹配，若不匹配，则尝试连接Zygote的辅模式。在判断辅模式返回ZygoteState与启动应用程序进程的ABI是否匹配，匹配则连接，不匹配则抛出异常ZygoteStartFailedEx。
最终若连接成功且匹配ABI成功返回ZygoteState对象
回到 zygoteSendArgsAndGetResult(openZygoteSocketIfNeeded(abi), argsForZygote);
我们继续看下面:拿到ZygoteState对象，下面这个对象做了什么？
private static Process.ProcessStartResult zygoteSendArgsAndGetResult(
ZygoteState zygoteState, ArrayList<String> args)
throws ZygoteStartFailedEx {
try {
int sz = args.size();
for (int i = 0; i < sz; i++) {
if (args.get(i).indexOf('\n') >= 0) {
throw new ZygoteStartFailedEx("embedded newlines not allowed");
}
}
//将传入携带应用程序进程启动参数的args写入zygoteState
final BufferedWriter writer = zygoteState.writer;
final DataInputStream inputStream = zygoteState.inputStream;
writer.write(Integer.toString(args.size()));
writer.newLine();
for (int i = 0; i < sz; i++) {
String arg = args.get(i);
writer.write(arg);
writer.newLine();
}
writer.flush();
Process.ProcessStartResult result = new Process.ProcessStartResult();
result.pid = inputStream.readInt();
result.usingWrapper = inputStream.readBoolean();
if (result.pid < 0) {
throw new ZygoteStartFailedEx("fork() failed");
}
return result;
} catch (IOException ex) {
zygoteState.close();
throw new ZygoteStartFailedEx(ex);
}
}
该方法内将携带应用程序进程启动参数的args写入zygoteState，那么Zygote进程就会接收到创建应用程序进程的请求。
这是为什么呢？---我们回顾下Zygote进程创建的时候干了什么？
public static void main(String argv[]) {
ZygoteServer zygoteServer = new ZygoteServer();
//创建一个Server端的Socket
String socketName = "zygote";
zygoteServer.registerServerSocket(socketName);
if (!enableLazyPreload) {
bootTimingsTraceLog.traceBegin("ZygotePreload");
EventLog.writeEvent(LOG_BOOT_PROGRESS_PRELOAD_START,
SystemClock.uptimeMillis());
preload(bootTimingsTraceLog);
EventLog.writeEvent(LOG_BOOT_PROGRESS_PRELOAD_END,
SystemClock.uptimeMillis());
bootTimingsTraceLog.traceEnd(); // ZygotePreload
} else {
Zygote.resetNicePriority();
}
//启动SystemServer进程
if (startSystemServer) {
startSystemServer(abiList, socketName, zygoteServer);
}
//等待AMS的请求
zygoteServer.runSelectLoop(abiList);
zygoteServer.closeServerSocket();
} catch (Zygote.MethodAndArgsCaller caller) {
caller.run();
} catch (Throwable ex) {
zygoteServer.closeServerSocket();
throw ex;
}
}
1.创建名为“zygote”一个Server端的Socket
2.启动SystemServer进程:startSystemServer
3.开启等待AMS请求：runSelectLoop
4.SystemServer的runSelectLoop
这是一个while（true）循环，初次执行，先完成SystemServer与Socket连接，监听Socket是否有可读数据，若有则执行tunOnce()
void runSelectLoop(String abiList) throws Zygote.MethodAndArgsCaller {
ArrayList<FileDescriptor> fds = new ArrayList<FileDescriptor>();
ArrayList<ZygoteConnection> peers = new ArrayList<ZygoteConnection>();
fds.add(mServerSocket.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 {
//监听 socket 是否数据可读，如果没有那么 会在此处等待
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) {
//SystemServer 启动时，首次与Zygote 连接, 此处将其端的连接对象
//ZygoteConnection存储下来,以便之后读取ams 写入socket的参数
ZygoteConnection newPeer = acceptCommandPeer(abiList);
peers.add(newPeer);
fds.add(newPeer.getFileDesciptor());
} else {
//当有数据传入时，获取ams写入socket的参数
boolean done = peers.get(i).runOnce(this);
if (done) {
peers.remove(i);
fds.remove(i);
}
}
}
}
}
boolean runOnce(ZygoteServer zygoteServer) throws Zygote.MethodAndArgsCaller {
String args[];
Arguments parsedArgs = null;
FileDescriptor[] descriptors;
try {
//获取应用程序启动参数
args = readArgumentList();
descriptors = mSocket.getAncillaryFileDescriptors();
} catch (IOException ex) {
closeSocket();
return true;
}
//.........
try {
parsedArgs = new Arguments(args);
//Zygote通过fock自身创建进程,并返回进程号
pid = Zygote.forkAndSpecialize(parsedArgs.uid, parsedArgs.gid, parsedArgs.gids,
parsedArgs.debugFlags, rlimits, parsedArgs.mountExternal, parsedArgs.seInfo,
parsedArgs.niceName, fdsToClose, fdsToIgnore, parsedArgs.instructionSet,
parsedArgs.appDataDir);
} catch (ErrnoException ex) {
logAndPrintError(newStderr, "Exception creating pipe", ex);
} catch (IllegalArgumentException ex) {
logAndPrintError(newStderr, "Invalid zygote arguments", ex);
} catch (ZygoteSecurityException ex) {
logAndPrintError(newStderr,
"Zygote security policy prevents request: ", ex);
}
try {
if (pid == 0) {
//pid为0，表示当前代码执行逻辑运行在新创建的进程中，调用handleChildProc处理进程
zygoteServer.closeServerSocket();
IoUtils.closeQuietly(serverPipeFd);
serverPipeFd = null;
handleChildProc(parsedArgs, descriptors, childPipeFd, newStderr);
return true;
} else {
IoUtils.closeQuietly(childPipeFd);
childPipeFd = null;
return handleParentProc(pid, descriptors, serverPipeFd, parsedArgs);
}
} finally {
IoUtils.closeQuietly(childPipeFd);
IoUtils.closeQuietly(serverPipeFd);
}
}
handleChildProc()内调用了
ZygoteInit.zygoteInit(parsedArgs.targetSdkVersion,
parsedArgs.remainingArgs, null /* classLoader */);
5.ZygoteInit的zygoteInit()
public static final void zygoteInit(int targetSdkVersion, String[] argv,
ClassLoader classLoader) throws Zygote.MethodAndArgsCaller {
if (RuntimeInit.DEBUG) {
Slog.d(RuntimeInit.TAG, "RuntimeInit: Starting application from zygote");
}
Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "ZygoteInit");
RuntimeInit.redirectLogStreams();
RuntimeInit.commonInit();
ZygoteInit.nativeZygoteInit();
RuntimeInit.applicationInit(targetSdkVersion, argv, classLoader);
}
ZygoteInit.nativeZygoteInit()为应用程序进程创建Binder线程池
RuntimeInit.applicationIni（）创建ActivityThread类，通过反射调用起入口main方法
ActivityThread的main
ActivityThread是应用程序主线程的管理类:
1.创建主线程的Looper
2.创建主线程H类（Handler），处理组件生命周期,如启动Activity
3.开启Looper循环
public static void main(String[] args) {
SamplingProfilerIntegration.start();
EventLogger.setReporter(new EventLoggingReporter());
final File configDir = Environment.getUserConfigDirectory(UserHandle.myUserId());
TrustedCertificateStore.setDefaultUserDirectory(configDir);
Process.setArgV0("<pre-initialized>");
Looper.prepareMainLooper();
ActivityThread thread = new ActivityThread();
thread.attach(false);
if (sMainThreadHandler == null) {
sMainThreadHandler = thread.getHandler();
}
if (false) {
Looper.myLooper().setMessageLogging(new
LogPrinter(Log.DEBUG, "ActivityThread"));
}
Looper.loop();
throw new RuntimeException("Main thread loop unexpectedly exited");
}
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