开门见山
所谓Zygote，就如同它的名字“受精卵”一样，它是Android 系统所有进程的源头，所有的进程都是由zygote进程fork出来的。而zygote进程是由init进程fork出来的。关于init进程这里不做进一步扩展，我们只需要知道Android系统启动的时候，会启动init进程，然后init进程会创建zygote进程。
Zygote 的初始化
其实zygote的初始化流程并不在zygote.java类中，而在ZygoteInit.java中。我们先可以看ZygoteInit的main方法：
public static void main(String argv[]) {
ZygoteServer zygoteServer = new ZygoteServer();
// Mark zygote start. This ensures that thread creation will throw
// an error.
ZygoteHooks.startZygoteNoThreadCreation();
// Zygote goes into its own process group.
try {
Os.setpgid(0, 0);
} catch (ErrnoException ex) {
throw new RuntimeException("Failed to setpgid(0,0)", ex);
}
final Runnable caller;
try {
// Report Zygote start time to tron unless it is a runtime restart
if (!"1".equals(SystemProperties.get("sys.boot_completed"))) {
MetricsLogger.histogram(null, "boot_zygote_init",
(int) SystemClock.elapsedRealtime());
}
String bootTimeTag = Process.is64Bit() ? "Zygote64Timing" : "Zygote32Timing";
TimingsTraceLog bootTimingsTraceLog = new TimingsTraceLog(bootTimeTag,
Trace.TRACE_TAG_DALVIK);
bootTimingsTraceLog.traceBegin("ZygoteInit");
RuntimeInit.enableDdms();
boolean startSystemServer = false;
String socketName = "zygote";
String abiList = null;
boolean enableLazyPreload = false;
for (int i = 1; i < argv.length; i++) {
if ("start-system-server".equals(argv[i])) {
startSystemServer = true;
} else if ("--enable-lazy-preload".equals(argv[i])) {
enableLazyPreload = 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());
} else {
throw new RuntimeException("Unknown command line argument: " + argv[i]);
}
}
if (abiList == null) {
throw new RuntimeException("No ABI list supplied.");
}
zygoteServer.registerServerSocketFromEnv(socketName);
// In some configurations, we avoid preloading resources and classes eagerly.
// In such cases, we will preload things prior to our first fork.
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();
}
// Do an initial gc to clean up after startup
bootTimingsTraceLog.traceBegin("PostZygoteInitGC");
gcAndFinalize();
bootTimingsTraceLog.traceEnd(); // PostZygoteInitGC
bootTimingsTraceLog.traceEnd(); // ZygoteInit
// Disable tracing so that forked processes do not inherit stale tracing tags from
// Zygote.
Trace.setTracingEnabled(false, 0);
Zygote.nativeSecurityInit();
// Zygote process unmounts root storage spaces.
Zygote.nativeUnmountStorageOnInit();
ZygoteHooks.stopZygoteNoThreadCreation();
if (startSystemServer) {
Runnable r = forkSystemServer(abiList, socketName, zygoteServer);
// {@code r == null} in the parent (zygote) process, and {@code r != null} in the
// child (system_server) process.
if (r != null) {
r.run();
return;
}
}
Log.i(TAG, "Accepting command socket connections");
// The select loop returns early in the child process after a fork and
// loops forever in the zygote.
caller = zygoteServer.runSelectLoop(abiList);
} catch (Throwable ex) {
Log.e(TAG, "System zygote died with exception", ex);
throw ex;
} finally {
zygoteServer.closeServerSocket();
}
// We're in the child process and have exited the select loop. Proceed to execute the
// command.
if (caller != null) {
caller.run();
}
}
虽然main方法非常的长，但是我们只关注几个比较重要的流程
1 注册socket
zygoteServer.registerServerSocketFromEnv(socketName);
通过这个我们可以看到，其他进程和zygote的通讯其实就是通过socket进行通讯的，比如我们在上一篇Activity 启动流程分析中可以看到，ActivityManagerService通知zygote fork目标进程就是采用的socket。
2 创建SystemServer
if (startSystemServer) {
Runnable r = forkSystemServer(abiList, socketName, zygoteServer);
// {@code r == null} in the parent (zygote) process, and {@code r != null} in the
// child (system_server) process.
if (r != null) {
r.run();
return;
}
}
在这里我们可以看到，zygote其实懒的很。zygote把systemServer fork 出来后就不管了，就相当于zygote是一个大老板，而systemServer是一位秘书。如果某些进程需要和Zygote进行通讯的时候，其实是通过SystemServer做中转，由SystemServer和Zygote进行通讯。
在这里，如果由同学对SystemServer的启动流程感兴趣的话，可以看我之前的另一篇文章 Android SystemServer 启动流程
3开启循环监听socket
caller = zygoteServer.runSelectLoop(abiList);
runSelectLoop()方法里面由一个while(true)，也即是开启一个循环不停的去监听当前有没有ZygoteConnect。
总结
Zygote分析起来其实比较简单，结合前面的两篇文章我们就可以把完整的流程给分析出来
