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<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”–runtime-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<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;
  }
  ......
}

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“–runtime-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("<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");
  }

  ......
}

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我们。