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一文带你看懂Android Application启动流程是怎样的

作者：冬天的毛毛雨

谈到Android Application的启动流程，很多文章都是各种源码类和方法的一堆调用关系，这样的文章就算看一百遍，也只是云里雾里。源码得看，但是最好不要一下子深陷到源码的细节之中，不可自拔。这里站在前人的基础之上做一个总结

基于Android11-API30

总览

详细流程分析

从 ActivityThread.java 的main方法开始看；

public static void main(String[] args) {
    ...
    ActivityThread thread = new ActivityThread();
    thread.attach(system=false, startSeq);//1
    ...
}

进入attach方法；

if(!system){
    final IActivityManager mgr = ActivityManager.getService();
    try {
        mgr.attachApplication(mAppThread, startSeq);//1
    } catch (RemoteException ex) {
        throw ex.rethrowFromSystemServer();
    }
}

非系统应用流程，根据 getSeervice和捕获的RemoteException可以断定，此处在使用Binder进行远程接口调用。
转身看下mAppThread是什么？

final ApplicationThread mAppThread = new ApplicationThread();

private class ApplicationThread extends IApplicationThread.Stub {
    //批量的schedule*接口，比如scheduleReceiver、scheduleCreateService等
    public final void schedule*
    
    //TODO 关键方法
    public final void bindApplication(some args){}//1
    
    //一堆dump方法，比如dumpMemory、dumpActivity等
    
}

可以看到，ApplicationThread是一个实现了远程接口的Binder客户端，内部封装实现了很多远程接口。不过这个客户端什么时候连接的服务器还未可知，没有找到bindService关键字，反正此时应该已经连接上对应的Service了。应该是在RuntimeInit.java类中进行应用进程启动时启动的。

回来看下前一步服务的实例IActivityManager.attachApplication()内部的实现。

先获取AMS的实例，此处获取AMS实例代码跟Activity启动流程中一致

public static IActivityManager getService() {
    return IActivityManagerSingleton.get();
}
private static final Singleton<IActivityManager> IActivityManagerSingleton =
        new Singleton<IActivityManager>() {
            @Override
            protected IActivityManager create() {
                final IBinder b = ServiceManager.getService(Context.ACTIVITY_SERVICE);
                final IActivityManager am = IActivityManager.Stub.asInterface(b);
                return am;
            }
        };

…获取到AMS的Binder后，继续查看ActivityManagerService.java中的attachApplication方法

public final void attachApplication(IApplicationThread thread, long startSeq) {
    
   	synchronized (this) {
        int callingPid = Binder.getCallingPid();
        final int callingUid = Binder.getCallingUid();
        final long origId = Binder.clearCallingIdentity();
        attachApplicationLocked(thread, callingPid, callingUid, startSeq); //1
        Binder.restoreCallingIdentity(origId);
    }
}

单例获取AMS实例，AMS服务在系统启动就已经注册到ServiceManager了，此处直接去获取Binder实例就行，ServiceManager以Binder池的方式管理注册的Server。

AMS的attachApplication方法中进入到attachApplicationLocked方法，捡能看懂的代码看，跟着thread参数查看代码。

private boolean attachApplicationLocked(@NonNull IApplicationThread thread,
            int pid, int callingUid, long startSeq) {

    try {
        AppDeathRecipient adr = new AppDeathRecipient(
                app, pid, thread);
        thread.asBinder().linkToDeath(adr, 0);//1
        app.deathRecipient = adr;
    } catch (RemoteException e) {
        app.resetPackageList(mProcessStats);
        mProcessList.startProcessLocked(app,
                new HostingRecord("link fail", processName),
                ZYGOTE_POLICY_FLAG_EMPTY);
        return false;
    }

    final ActiveInstrumentation instr2 = app.getActiveInstrumentation();

    if (instr2 != null) {//2
        thread.bindApplication(processName, appInfo, providerList,
                instr2.mClass,
                profilerInfo, instr2.mArguments,
                instr2.mWatcher,
                instr2.mUiAutomationConnection, testMode,
                mBinderTransactionTrackingEnabled, enableTrackAllocation,
                isRestrictedBackupMode || !normalMode, app.isPersistent(),
                new Configuration(app.getWindowProcessController().getConfiguration()),
                app.compat, getCommonServicesLocked(app.isolated),
                mCoreSettingsObserver.getCoreSettingsLocked(),
                buildSerial, autofillOptions, contentCaptureOptions,
                app.mDisabledCompatChanges);
    } else {
        thread.bindApplication(processName, appInfo, providerList, null, profilerInfo,
                null, null, null, testMode,
                mBinderTransactionTrackingEnabled, enableTrackAllocation,
                isRestrictedBackupMode || !normalMode, app.isPersistent(),
                new Configuration(app.getWindowProcessController().getConfiguration()),
                app.compat, getCommonServicesLocked(app.isolated),
                mCoreSettingsObserver.getCoreSettingsLocked(),
                buildSerial, autofillOptions, contentCaptureOptions,
                app.mDisabledCompatChanges);
    }

}

先给ApplicationThread这个Binder上个死亡代理，根据这个死亡代理应该可以找到对应的Service是如何重新启动的，感兴趣可以继续深入，咱们继续往下走。
此处调用到thread.bindApplication接口，前面咱们查看ApplicationThread时有看到，直接切入。

private class ApplicationThread extends IApplicationThread.Stub {
    //批量的schedule*接口，比如scheduleReceiver、scheduleCreateService等
    public final void schedule*
    
    //TODO 关键方法
    public final void bindApplication(some args){
        AppBindData data = new AppBindData();
        ...一堆参数
        sendMessage(H.BIND_APPLICATION, data);//1
    }
    
    //一堆dump方法，比如dumpMemory、dumpActivity等
    
}

到达咱们Android开发工程师比较熟悉的点了，封装了一堆参数后，通过H这个Handler对象发了一条BIND_APPLICATION消息，咱们看看这条消息去哪了，直接跟进BIND_APPLICATION这个消息的捕捉位置。

//消息分发
class H extends Handler{
    public void handleMessage(Message msg){
        swich(msg.what){
            case BIND_APPLICATION: 
                AppBindData data = (AppBindData)msg.obj;
                handleBindApplication(data);//1
                break;
            ...省略
        }
    }
}

进入消息分发处理方法，这个方法比较长，注意阅读能看懂的代码，不求甚解，跟踪data的处理。

private void handleBindApplication(AppBindData data) {
    //各种初始化，比如进程名，应用名，AsyncTask线程池的配置，时区，网络发现
    
    //Context的初始化
    final ContextImpl appContext = ContextImpl.createAppContext(this, data.info);
    
    try {
        final ClassLoader cl = instrContext.getClassLoader();
        mInstrumentation = (Instrumentation)//1
            cl.loadClass(data.instrumentationName.getClassName()).newInstance();
    } catch (Exception e) {
        throw new RuntimeException(
            "Unable to instantiate instrumentation "
            + data.instrumentationName + ": " + e.toString(), e);
    }
    
    final ComponentName component = new ComponentName(ii.packageName, ii.name);
    mInstrumentation.init(this, instrContext, appContext, component,//1
            data.instrumentationWatcher, data.instrumentationUiAutomationConnection);
            
    ...
    Application app;
    app = data.info.makeApplication(data.restrictedBackupMode, null);//2

    mInstrumentation.onCreate(data.instrumentationArgs);
    mInstrumentation.callApplicationOnCreate(app);//3
}

通过反射实例化mInstrumentation对象，该对象为Android系统组件的管家，目前看可以控制Application和Activity的生命周期。

创建Application对象，进去看下创建的代码

//LoadApk.java #makeApplication
public Application makeApplication(boolean forceDefaultAppClass,
    Instrumentation instrumentation){
    ...
    app = mActivityThread.mInstrumentation.newApplication(cl, appClass, appContext);//1
    appContext.setOuterContext(app);
    ...
}

//Instrumentation.java #newApplication
public Application newApplication(ClassLoader cl, String className, Context context)
        throws InstantiationException, IllegalAccessException, 
        ClassNotFoundException {
    Application app = getFactory(context.getPackageName())
            .instantiateApplication(cl, className);//2
    app.attach(context);//首先回调attachBaseContext方法
    return app;
}

//AppComponentFactory #instantiateApplication
public @NonNull Application instantiateApplication(@NonNull ClassLoader cl,
        @NonNull String className)
        throws InstantiationException, IllegalAccessException, ClassNotFoundException {
    return (Application) cl.loadClass(className).newInstance();//3
}

可以看出最后还是通过反射初始化了Application。

最后通过mInstrumentation对象完成Application类的onCreate方法的调用。

mInstrumentation.callApplicationOnCreate(app);//1

//Instrumentation.java #callApplicationOnCreate
public void callApplicationOnCreate(Application app) {
    app.onCreate();
}

到此这篇关于一文带你看懂Android Application启动流程是怎样的的文章就介绍到这了,更多相关Android Application 启动流程内容请搜索脚本之家以前的文章或继续浏览下面的相关文章希望大家以后多多支持脚本之家！

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