Android應用框架之應用啟動過程詳解

在Android的應用框架中，ActivityManagerService是非常重要的一個組件，盡管名字叫做ActivityManagerService，但通過之前的博客介紹，我們知道，四大組件的創建都是有AMS來完成的，其實不僅是應用程序中的組件，連Android應用程序本身也是AMS負責啟動的。AMS本身運行在一個獨立的進程中，當系統決定要在一個新的進程中啟動一個Activity或者Service時就會先啟動這個進程。而AMS啟動進程的過程是從startProcessLocked啟動的。

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，函數會調用Process.start函數來創建一個進程，其中第一個參數”android.app.ActivityThread”是需要加載的類，而在完成這個類的加載之後就會運行ActivityThread.main函數。

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;
    }
  }
  ......
}

這個函數最後會調用startViaZygote來創建進程，而Zygote正是Android孵化進程的服務，所有的進程都是通過Zygotefork出來的，所以這裡創建進程的任務又落到了Zygote頭上了。

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<String> argsForZygote = new ArrayList<String>();

      // --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);
    }
  }
  ......
}

函數裡面最為重要的工作就是組裝argsForZygote參數，這些參數將告訴Zygote具體的啟動選項，例如”–runtime-init”就表示要為新啟動的運行程序初始化運行庫。然後調用zygoteSendAndGetPid函數進一步操作。

4.Process.zygoteSendAndGetPid

public class Process {
  ......

  private static int zygoteSendArgsAndGetPid(ArrayList<String> 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;
  }
  ......
}

這裡的sZygoteWriter

是一個Socket寫入流，是由openZygoteSocketIfNeeded函數打開的。而這個Socket由frameworks/base/core/java/com/android/internal/os/ZygoteInit.java文件中的ZygoteInit類在runSelectLoopMode函數偵聽的。這個類會返回一個ZygoteConnection實例，並執行ZygoteConnection的runOnce函數。

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);
    }
  }
  ......
}

真正創建進程的代碼在Zygote.forkAndSpecialize,通過Zygote來fork出一個新的進程作為應用進程。fork函數會有兩個返回，其中一個在父進程，一個在子進程，其中自進程的進程號會為0，所以按照上面的代碼，這裡會執行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 {
      ......
    }
  }
  ......
}

因為在創建的時候傳入了“–runtime-init”，所以這裡會運行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);
  }
  ......
}

這裡有兩個關鍵的函數調用，一個是zygoteInitNative函數調用，一個是invokeStaticMain函數調用，前者就是執行Binder驅動程序初始化的相關工作了，正是由於執行了這個工作，才使得進程中的Binder對象能夠順利地進行Binder進程間通信，而後一個函數調用，就是執行進程的入口函數，這裡就是執行startClass類的main函數了，而這個startClass即是我們在Step 1中傳進來的”android.app.ActivityThread”值，表示要執行android.app.ActivityThread類的main函數。

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);
  }
  ......
}

從代碼中可以看到，通過ClassLoader加載對應的android.app.ActivityThread類，然後再獲取到對應的main函數句柄，最後調用該類的main函數。不過這裡的調用方式比較有意思，不知直接調用，而是通過拋出一個異常。這樣做的方式是為了清空堆棧，讓系統認為新進程是從ActivityThread的main函數開始的。

8.ActivityThread.main

public final class ActivityThread {
  ......

  public static final void main(String[] args) {
    SamplingProfilerIntegration.start();

    Process.setArgV0("<pre-initialized>");

    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()
      : "<unknown>";
    Slog.i(TAG, "Main thread of " + name + " is now exiting");
  }

  ......
}

從這裡我們可以看出，這個函數首先會在進程中創建一個ActivityThread對象,然後進入消息循環中,這樣，我們以後就可以在這個進程中啟動Activity或者Service了。

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