Android開機流程-重新梳理

最近回顧的一些知識，補充了一下。

源碼標准：API : 29「Android 10.0」

android手機是怎么開機的？

android 的底層是 linux kernel「內核」，由 BootLoader「系統啟動加載器」 負責加載（類似於計算機的BIOS系統）。

/bootable/recovery/bootloader.h

首先啟動 init「父進程，第一個進程」進程，接着運行init.rc腳本，腳本文件有個命令啟動了Zygote進程，初始化時會啟動虛擬機。

/system/core/rootdir/init.zygote.rc

 

 

Zygote進程fork出SystemServer進程，然后會調用SystemServer.main()方法。

/frameworks/base/services/java/com/android/server/SystemService.java

/** The main entry point from zygote.*/
public static void main(String[] args) {
    new SystemServer().run();
}

run方法中，主要是在進程中啟動系統的各項服務，比如ActivityManagerService，PackageManagerService，WindowManagerService服務等。

private void run() {
    //創建主線程Looper、ActivityThread、SystemContext
    android.os.Process.setThreadPriority(android.os.Process.THREAD_PRIORITY_FOREGROUND);
    Looper.prepareMainLooper();

    // Initialize native services.
    System.loadLibrary("android_servers");    
    // Initialize the system context.
    createSystemContext();
    
    // Create the system service manager.
    mSystemServiceManager = new SystemServiceManager(mSystemContext);
    mSystemServiceManager.setStartInfo(mRuntimeRestart,mRuntimeStartElapsedTime, mRuntimeStartUptime);
    LocalServices.addService(SystemServiceManager.class, mSystemServiceManager);
    // 並行線程池
    SystemServerInitThreadPool.get();

    // Start services.
    traceBeginAndSlog("StartServices");
    startBootstrapServices();
    startCoreServices();
    startOtherServices();

    // Loop forever.
    Looper.loop();
}

下面是一些主要的初始化方法。

/**
 * 這些服務具有復雜的相互依賴關系，所以需要放一起全部初始化
 */
private void startBootstrapServices() {
    // Start the watchdog as early as possible so we can crash the system server
    final Watchdog watchdog = Watchdog.getInstance();
    watchdog.start();

    //啟動AMS
    ActivityTaskManagerService atm = mSystemServiceManager.startService(
                ActivityTaskManagerService.Lifecycle.class).getService();
    mActivityManagerService = ActivityManagerService.Lifecycle.startService(
                mSystemServiceManager, atm);

    //電源管理器需要提前啟動，因為其他服務需要它
    mPowerManagerService = mSystemServiceManager.startService(PowerManagerService.class);

    // Start the package manager.
    mPackageManagerService = PackageManagerService.main(mSystemContext, installer,
                    mFactoryTestMode != FactoryTest.FACTORY_TEST_OFF, mOnlyCore);

    //設置Application實例並開始
    mActivityManagerService.setSystemProcess();

    //使用 ActivityManager 實例完成看門狗設置並監聽重啟
    watchdog.init(context, mActivityManagerService);
}

真正啟動是在ActivityManagerService的中systemReady方法，調用resumeTopActivityLocked打開鎖屏界面。

/**
 * Starts a miscellaneous grab bag of stuff that has yet to be refactored and organized.
 */
private void startOtherServices() {
    //啟動WMS
    wm = WindowManagerService.main();
    mActivityManagerService.setWindowManager(wm);
    
    //WMS 顯示默認啟動消息
    ActivityManagerNative.getDefault().showBootMessage();
    //開始啟動初始應用程序
    mActivityManagerService.systemReady(new Runnable(){
        //SystemUI
        startSystemUi(context, windowManagerF);
    });
}

/frameworks/base/services/core/java/com/android/server/am/ActivityManagerService.java

/** 通過StackSupervisor運行所有 ActivityStacks */
final ActivityStackSupervisor mStackSupervisor;

public void systemReady(final Runnable goingCallback, TimingsTraceLog traceLog) {
    mStackSupervisor.resumeFocusedStackTopActivityLocked();
}

到這里，android的開機流程結束。