Application创建起源分析

AndroidSource

主要的类是ActivityThread,分为系统进程和我们的引用进程

系统进程：
SystemServer

public static void main(String[] args) {
    new SystemServer().run();
}
private void run() {
    //...
    // Initialize the system context.
    createSystemContext();
    // Create the system service manager.
    mSystemServiceManager = new SystemServiceManager(mSystemContext);
    LocalServices.addService(SystemServiceManager.class, mSystemServiceManager);
    // Start services.
    startBootstrapServices();
    startCoreServices();
    startOtherServices();
}

    public static ActivityThread systemMain() {
        // The system process on low-memory devices do not get to use hardware
        // accelerated drawing, since this can add too much overhead to the
        // process.
        if (!ActivityManager.isHighEndGfx()) {
            HardwareRenderer.disable(true);
        } else {
            HardwareRenderer.enableForegroundTrimming();
        }
        ActivityThread thread = new ActivityThread();
        thread.attach(true);
        return thread;
    }

private void attach(boolean system){
// ...
// if system
try {
    mInstrumentation = new Instrumentation();
    ContextImpl context = ContextImpl.createAppContext(this, getSystemContext().mPackageInfo);
    mInitialApplication = context.mPackageInfo.makeApplication(true, null);
    mInitialApplication.onCreate();
} catch (Exception e) {
    throw new RuntimeException(
            "Unable to instantiate Application():" + e.toString(), e);
}
}

简化为

LoadedApk packageInfo = new LoadedApk(this);
ContextImpl context =  ContextImpl.createAppContext(this, packageInfo);
mInitialApplication = packageInfo.makeApplication(true,null);
mInitialApplication.onCreate();

普通Application创建的起源：
在系统进程的控制下进入main函数，
ActivityThread.main() -> ActivityThread.attach(fasle) [handleBindApplication] -> LoadedApk.makeApplication() -> Instrumentation.newApplication()

Application类的调用链:
Application() -> Application.attach() -> Application.attachBaseContext() -> Application.onCreate()

ActivityThread的attach方法区分是否系统的进程

private void attach(boolean system) {
}

LoadedApk类的方法

public Application makeApplication(boolean forceDefaultAppClass,
        Instrumentation instrumentation) {
    if (mApplication != null) {
        return mApplication;
    }
    Application app = null;
    String appClass = mApplicationInfo.className;
    if (forceDefaultAppClass || (appClass == null)) {
        appClass = "android.app.Application";
    }
    try {
        java.lang.ClassLoader cl = getClassLoader();
        if (!mPackageName.equals("android")) {
            initializeJavaContextClassLoader();
        }
        ContextImpl appContext = ContextImpl.createAppContext(mActivityThread, this);
        app = mActivityThread.mInstrumentation.newApplication(
                cl, appClass, appContext);
        appContext.setOuterContext(app);
    } catch (Exception e) {
        if (!mActivityThread.mInstrumentation.onException(app, e)) {
            throw new RuntimeException(
                "Unable to instantiate application " + appClass
                + ": " + e.toString(), e);
        }
    }
    mActivityThread.mAllApplications.add(app);
    mApplication = app;
    if (instrumentation != null) {
        try {
            instrumentation.callApplicationOnCreate(app);
        } catch (Exception e) {
            if (!instrumentation.onException(app, e)) {
                throw new RuntimeException(
                    "Unable to create application " + app.getClass().getName()
                    + ": " + e.toString(), e);
            }
        }
    }
    // Rewrite the R 'constants' for all library apks.
    SparseArray<String> packageIdentifiers = getAssets(mActivityThread)
            .getAssignedPackageIdentifiers();
    final int N = packageIdentifiers.size();
    for (int i = 0; i < N; i++) {
        final int id = packageIdentifiers.keyAt(i);
        if (id == 0x01 || id == 0x7f) {
            continue;
        }
        rewriteRValues(getClassLoader(), packageIdentifiers.valueAt(i), id);
    }
    return app;
}

Instrumentation

static public Application newApplication(Class<?> clazz, Context context)
        throws InstantiationException, IllegalAccessException, 
        ClassNotFoundException {
    Application app = (Application)clazz.newInstance();
    app.attach(context);
    return app;
}

Application类的attach方法,实际上主要是调用到了attachBaseContext方法

/* package */ final void attach(Context context) {
    attachBaseContext(context);
    mLoadedApk = ContextImpl.getImpl(context).mPackageInfo;
}

所以我们的关注点在attachBaseContext方法中,该方法在其父类ContextWrapper中定义的.

protected void attachBaseContext(Context base) {
    if (mBase != null) {
        throw new IllegalStateException("Base context already set");
    }
    mBase = base;
}

子类重写的时候i.e.

 protected void attachBaseContext(Context base) {
    super.attachBaseContext(base);
    MultiDex.install(this);
}

常用作动态部署,热更新机制在这里操作 i.e.

@Override
protected void attachBaseContext(Context context) {
    // ...
    createRealApplication();
    // This is called from ActivityThread#handleBindApplication() -> LoadedApk#makeApplication().
    // Application#mApplication is changed right after this call, so we cannot do the monkey
    // patching here. So just forward this method to the real Application instance.
    super.attachBaseContext(context);
    if (realApplication != null) {
        try {
            Method attachBaseContext =
                    ContextWrapper.class.getDeclaredMethod("attachBaseContext", Context.class);
            attachBaseContext.setAccessible(true);
            attachBaseContext.invoke(realApplication, context);
        } catch (Exception e) {
            throw new IllegalStateException(e);
        }
    }
}