Android应用框架之应用启动过程详解

在Android的应用框架中，ActivityManagerService是非常重要的一个组件，尽管名字叫做ActivityManagerService，但通过之前的博客介绍，我们知道，四大组件的创建都是有AMS来完成的，其实不仅是应用程序中的组件，连Android应用程序本身也是AMS负责启动的。AMS本身运行在一个独立的进程中，当系统决定要在一个新的进程中启动一个Activity或者Service时就会先启动这个进程。而AMS启动进程的过程是从startProcessLocked启动的。

1.ActivityManagerService.startProcessLocked

public final class ActivityManagerService extends ActivityManagerNative     implements Watchdog.Monitor, BatteryStatsImpl.BatteryCallback {   ......   private final void startProcessLocked(ProcessRecord app,         String hostingType, String hostingNameStr) {     ......     try {       int uid = app.info.uid;       int[] gids = null;       try {         gids = mContext.getPackageManager().getPackageGids(           app.info.packageName);       } catch (PackageManager.NameNotFoundException e) {         ......       }       ......       int debugFlags = 0;       ......       int pid = Process.start("android.app.ActivityThread",         mSimpleProcessManagement ? app.processName : null, uid, uid,         gids, debugFlags, null);       ......     } catch (RuntimeException e) {       ......     }   }   ...... }

可以看到，函数会调用Process.start函数来创建一个进程，其中第一个参数”android.app.ActivityThread”是需要加载的类，而在完成这个类的加载之后就会运行ActivityThread.main函数。

2.Process.start

public class Process {  ......  public static final int start(final String processClass,    final String niceName,    int uid, int gid, int[] gids,    int debugFlags,    String[] zygoteArgs)  {    if (supportsProcesses()) {      try {        return startViaZygote(processClass, niceName, uid, gid, gids,          debugFlags, zygoteArgs);      } catch (ZygoteStartFailedEx ex) {        ......      }    } else {      ......      return 0;    }  }  ......}

这个函数最后会调用startViaZygote来创建进程，而Zygote正是Android孵化进程的服务，所有的进程都是通过Zygotefork出来的，所以这里创建进程的任务又落到了Zygote头上了。

3.Process.startViaZygote

public class Process {  ......  private static int startViaZygote(final String processClass,      final String niceName,      final int uid, final int gid,      final int[] gids,      int debugFlags,      String[] extraArgs)      throws ZygoteStartFailedEx {    int pid;    synchronized(Process.class) {      ArrayList argsForZygote = new ArrayList();      // --runtime-init, --setuid=, --setgid=,      // and --setgroups= must go first      argsForZygote.add("--runtime-init");      argsForZygote.add("--setuid=" + uid);      argsForZygote.add("--setgid=" + gid);      if ((debugFlags & Zygote.DEBUG_ENABLE_SAFEMODE) != 0) {        argsForZygote.add("--enable-safemode");      }      if ((debugFlags & Zygote.DEBUG_ENABLE_DEBUGGER) != 0) {        argsForZygote.add("--enable-debugger");      }      if ((debugFlags & Zygote.DEBUG_ENABLE_CHECKJNI) != 0) {        argsForZygote.add("--enable-checkjni");      }      if ((debugFlags & Zygote.DEBUG_ENABLE_ASSERT) != 0) {        argsForZygote.add("--enable-assert");      }      //TODO optionally enable debuger      //argsForZygote.add("--enable-debugger");      // --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);      }      argsForZygote.add(processClass);      if (extraArgs != null) {        for (String arg : extraArgs) {          argsForZygote.add(arg);        }      }      pid = zygoteSendArgsAndGetPid(argsForZygote);    }  }  ......}

函数里面最为重要的工作就是组装argsForZygote参数，这些参数将告诉Zygote具体的启动选项，例如”�Cruntime-init”就表示要为新启动的运行程序初始化运行库。然后调用zygoteSendAndGetPid函数进一步操作。

4.Process.zygoteSendAndGetPid

public class Process {  ......  private static int zygoteSendArgsAndGetPid(ArrayList args)      throws ZygoteStartFailedEx {    int pid;    openZygoteSocketIfNeeded();    try {      /**      * See com.android.internal.os.ZygoteInit.readArgumentList()      * Presently the wire format to the zygote process is:      * a) a count of arguments (argc, in essence)      * b) a number of newline-separated argument strings equal to count      *      * After the zygote process reads these it will write the pid of      * the child or -1 on failure.      */      sZygoteWriter.write(Integer.toString(args.size()));      sZygoteWriter.newLine();      int sz = args.size();      for (int i = 0; i < sz; i++) {        String arg = args.get(i);        if (arg.indexOf('\n') >= 0) {          throw new ZygoteStartFailedEx(            "embedded newlines not allowed");        }        sZygoteWriter.write(arg);        sZygoteWriter.newLine();      }      sZygoteWriter.flush();      // Should there be a timeout on this?      pid = sZygoteInputStream.readInt();      if (pid < 0) {        throw new ZygoteStartFailedEx("fork() failed");      }    } catch (IOException ex) {      ......    }    return pid;  }  ......}这里的sZygoteWriter

是一个Socket写入流，是由openZygoteSocketIfNeeded函数打开的。而这个Socket由frameworks/base/core/java/com/android/internal/os/ZygoteInit.java文件中的ZygoteInit类在runSelectLoopMode函数侦听的。这个类会返回一个ZygoteConnection实例，并执行ZygoteConnection的runOnce函数。

5.ZygoteConnection.runOnce

class ZygoteConnection {  ......  boolean runOnce() throws ZygoteInit.MethodAndArgsCaller {    String args[];    Arguments parsedArgs = null;    FileDescriptor[] descriptors;    try {      args = readArgumentList();      descriptors = mSocket.getAncillaryFileDescriptors();    } catch (IOException ex) {      ......      return true;    }    ......    /** the stderr of the most recent request, if avail */    PrintStream newStderr = null;    if (descriptors != null && descriptors.length >= 3) {      newStderr = new PrintStream(        new FileOutputStream(descriptors[2]));    }    int pid;    try {      parsedArgs = new Arguments(args);      applyUidSecurityPolicy(parsedArgs, peer);      applyDebuggerSecurityPolicy(parsedArgs);      applyRlimitSecurityPolicy(parsedArgs, peer);      applyCapabilitiesSecurityPolicy(parsedArgs, peer);      int[][] rlimits = null;      if (parsedArgs.rlimits != null) {        rlimits = parsedArgs.rlimits.toArray(intArray2d);      }      pid = Zygote.forkAndSpecialize(parsedArgs.uid, parsedArgs.gid,        parsedArgs.gids, parsedArgs.debugFlags, rlimits);    } catch (IllegalArgumentException ex) {      ......    } catch (ZygoteSecurityException ex) {      ......    }    if (pid == 0) {      // in child      handleChildProc(parsedArgs, descriptors, newStderr);      // should never happen      return true;    } else { /* pid != 0 */      // in parent...pid of < 0 means failure      return handleParentProc(pid, descriptors, parsedArgs);    }  }  ......}

真正创建进程的代码在Zygote.forkAndSpecialize,通过Zygote来fork出一个新的进程作为应用进程。fork函数会有两个返回，其中一个在父进程，一个在子进程，其中自进程的进程号会为0，所以按照上面的代码，这里会执行handleChildProc。

6.ZygoteConnection.handleChildProc

class ZygoteConnection {  ......  private void handleChildProc(Arguments parsedArgs,      FileDescriptor[] descriptors, PrintStream newStderr)      throws ZygoteInit.MethodAndArgsCaller {    ......    if (parsedArgs.runtimeInit) {      RuntimeInit.zygoteInit(parsedArgs.remainingArgs);    } else {      ......    }  }  ......}

因为在创建的时候传入了“�Cruntime-init”，所以这里会运行RuntimeInit.zygoteInit。

public class RuntimeInit {  ......  public static final void zygoteInit(String[] argv)      throws ZygoteInit.MethodAndArgsCaller {    // TODO: Doing this here works, but it seems kind of arbitrary. Find    // a better place. The goal is to set it up for applications, but not    // tools like am.    System.setOut(new AndroidPrintStream(Log.INFO, "System.out"));    System.setErr(new AndroidPrintStream(Log.WARN, "System.err"));    commonInit();    zygoteInitNative();    int curArg = 0;    for ( /* curArg */ ; curArg < argv.length; curArg++) {      String arg = argv[curArg];      if (arg.equals("--")) {        curArg++;        break;      } else if (!arg.startsWith("--")) {        break;      } else if (arg.startsWith("--nice-name=")) {        String niceName = arg.substring(arg.indexOf('=') + 1);        Process.setArgV0(niceName);      }    }    if (curArg == argv.length) {      Slog.e(TAG, "Missing classname argument to RuntimeInit!");      // let the process exit      return;    }    // Remaining arguments are passed to the start class's static main    String startClass = argv[curArg++];    String[] startArgs = new String[argv.length - curArg];    System.arraycopy(argv, curArg, startArgs, 0, startArgs.length);    invokeStaticMain(startClass, startArgs);  }  ......}

这里有两个关键的函数调用，一个是zygoteInitNative函数调用，一个是invokeStaticMain函数调用，前者就是执行Binder驱动程序初始化的相关工作了，正是由于执行了这个工作，才使得进程中的Binder对象能够顺利地进行Binder进程间通信，而后一个函数调用，就是执行进程的入口函数，这里就是执行startClass类的main函数了，而这个startClass即是我们在Step 1中传进来的”android.app.ActivityThread”值，表示要执行android.app.ActivityThread类的main函数。

7. Zygote.invokeStaticMain

public class ZygoteInit {  ......  static void invokeStaticMain(ClassLoader loader,      String className, String[] argv)      throws ZygoteInit.MethodAndArgsCaller {    Class<?> cl;    try {      cl = loader.loadClass(className);    } catch (ClassNotFoundException ex) {      ......    }    Method m;    try {      m = cl.getMethod("main", new Class[] { String[].class });    } catch (NoSuchMethodException ex) {      ......    } catch (SecurityException ex) {      ......    }    int modifiers = m.getModifiers();    ......    /*    * 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);  }  ......}

从代码中可以看到，通过ClassLoader加载对应的android.app.ActivityThread类，然后再获取到对应的main函数句柄，最后调用该类的main函数。不过这里的调用方式比较有意思，不知直接调用，而是通过抛出一个异常。这样做的方式是为了清空堆栈，让系统认为新进程是从ActivityThread的main函数开始的。

8.ActivityThread.main

public final class ActivityThread {  ......  public static final void main(String[] args) {    SamplingProfilerIntegration.start();    Process.setArgV0("");    Looper.prepareMainLooper();    if (sMainThreadHandler == null) {      sMainThreadHandler = new Handler();    }    ActivityThread thread = new ActivityThread();    thread.attach(false);    if (false) {      Looper.myLooper().setMessageLogging(new        LogPrinter(Log.DEBUG, "ActivityThread"));    }    Looper.loop();    if (Process.supportsProcesses()) {      throw new RuntimeException("Main thread loop unexpectedly exited");    }    thread.detach();    String name = (thread.mInitialApplication != null)      ? thread.mInitialApplication.getPackageName()      : "";    Slog.i(TAG, "Main thread of " + name + " is now exiting");  }  ......}

从这里我们可以看出，这个函数首先会在进程中创建一个ActivityThread对象,然后进入消息循环中,这样，我们以后就可以在这个进程中启动Activity或者Service了。

以上就是本文的全部内容，希望对大家的学习有所帮助，也希望大家多多支持脚本之家。

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