Android Activity組件的啟動過程

1、總圖中的第一步

如圖：第一步 <喎?/kf/ware/vc/" target="_blank" class="keylink">vcD4KPHA+ICAgICAgIH4vQW5kcm9pZC9mcmFtZXdvcmtzL2Jhc2UvY29yZS9qYXZhL2FuZHJvaWQvYXBwPC9wPgo8cD4gICAgICAgLS0tLUFjdGl2aXR5TWFuYWdlck5hdGl2ZS5qYXZhPC9wPgo8cD48cHJlIGNsYXNzPQ=="brush:java;">class ActivityManagerProxy implements IActivityManager { public int startActivity(IApplicationThread caller, Intent intent, String resolvedType, Uri[] grantedUriPermissions, int grantedMode, IBinder resultTo, String resultWho, int requestCode, boolean onlyIfNeeded, boolean debug) throws RemoteException { Parcel data = Parcel.obtain(); Parcel reply = Parcel.obtain(); data.writeInterfaceToken(IActivityManager.descriptor); data.writeStrongBinder(caller != null ? caller.asBinder() : null); intent.writeToParcel(data, 0); ......... data.writeStrongBinder(resultTo); mRemote.transact(START_ACTIVITY_TRANSACTION, data, reply, 0); } 其中caller為：

final ApplicationThread mAppThread = new ApplicationThread();

繼承於ApplicationThreadNative，ApplicationThreadNative繼承於Binder實現了IApplicationThread。

resultTo如下圖所示：

java層的Parcel，writeStrongBinder方法，最後映射到C++層，執行如下：

~/Android/frameworks/base/core/jni

----android_util_Binder.cpp

static void android_os_Parcel_writeStrongBinder(JNIEnv* env, jobject clazz, jobject object)//clazz為Parcel，object指向了在Java層中創建的硬件訪問服務FregService
{
    Parcel* parcel = parcelForJavaObject(env, clazz);//獲取java層Parcel對象data的引用
    if (parcel != NULL) {
        const status_t err = parcel->writeStrongBinder(ibinderForJavaObject(env, object));
        if (err != NO_ERROR) {
            jniThrowException(env, "java/lang/OutOfMemoryError", NULL);
        }
    }
}

ibinderForjavaObject實現如下：

~/Android/frameworks/base/core/jni

----android_util_Binder.cpp

sp ibinderForJavaObject(JNIEnv* env, jobject obj)
{
    if (obj == NULL) return NULL;

    if (env->IsInstanceOf(obj, gBinderOffsets.mClass)) {
        JavaBBinderHolder* jbh = (JavaBBinderHolder*)
            env->GetIntField(obj, gBinderOffsets.mObject);//這裡把obj對象的mObject成員變量強制轉為JavaBBinderHolder對象
        return jbh != NULL ? jbh->get(env) : NULL;
    }

    if (env->IsInstanceOf(obj, gBinderProxyOffsets.mClass)) {
        return (IBinder*)
            env->GetIntField(obj, gBinderProxyOffsets.mObject);
    }

    LOGW("ibinderForJavaObject: %p is not a Binder object", obj);
    return NULL;
}

（1）如果傳入的是caller.asBinder()，那麼首先生成一個JavaBBinder本地對象。

（2）如果傳入的是resultTo，那麼生成一個代理對象。

writeStrongBinder實現如下：

～/Android/frameworks/base/libs/binder

----Parcel.cpp

status_t Parcel::writeStrongBinder(const sp& val)
{
    return flatten_binder(ProcessState::self(), val, this);
}

status_t flatten_binder(const sp& proc,
    const sp& binder, Parcel* out)
{
    flat_binder_object obj;
    
    obj.flags = 0x7f | FLAT_BINDER_FLAG_ACCEPTS_FDS;
    if (binder != NULL) {
        IBinder *local = binder->localBinder();
        if (!local) {
            BpBinder *proxy = binder->remoteBinder();
            if (proxy == NULL) {
                LOGE("null proxy");
            }
            const int32_t handle = proxy ? proxy->handle() : 0;
            obj.type = BINDER_TYPE_HANDLE;
            obj.handle = handle;
            obj.cookie = NULL;
        } else {
            obj.type = BINDER_TYPE_BINDER;
            obj.binder = local->getWeakRefs();
            obj.cookie = local;
        }
    } else {
        obj.type = BINDER_TYPE_BINDER;
        obj.binder = NULL;
        obj.cookie = NULL;
    }
    
    return finish_flatten_binder(binder, obj, out);
}

（1）如果是本地對象，obj.cookie為本地對象IBinder地址。

（2）如果是代理對象，obj.handle為代理對象的句柄值。

如圖：第二步

Binder Driver：調用binder_transaction:

～/Android//kernel/goldfish/drivers/staging/android

----binder.c

                case BINDER_TYPE_BINDER:
		case BINDER_TYPE_WEAK_BINDER: {
			struct binder_ref *ref;
			struct binder_node *node = binder_get_node(proc, fp->binder);
			if (node == NULL) {
				node = binder_new_node(proc, fp->binder, fp->cookie);
				......
			}
			.......
			ref = binder_get_ref_for_node(target_proc, node);
			if (ref == NULL) {
				return_error = BR_FAILED_REPLY;
				goto err_binder_get_ref_for_node_failed;
			}
			if (fp->type == BINDER_TYPE_BINDER)
				fp->type = BINDER_TYPE_HANDLE;
			else
				fp->type = BINDER_TYPE_WEAK_HANDLE;
			fp->handle = ref->desc;
			......
		} break;
		case BINDER_TYPE_HANDLE:
		case BINDER_TYPE_WEAK_HANDLE: {
			struct binder_ref *ref = binder_get_ref(proc, fp->handle);
			......
			if (ref->node->proc == target_proc) {
				if (fp->type == BINDER_TYPE_HANDLE)
					fp->type = BINDER_TYPE_BINDER;
				else
					fp->type = BINDER_TYPE_WEAK_BINDER;
				fp->binder = ref->node->ptr;
				fp->cookie = ref->node->cookie;
				binder_inc_node(ref->node, fp->type == BINDER_TYPE_BINDER, 0, NULL);
				if (binder_debug_mask & BINDER_DEBUG_TRANSACTION)
					printk(KERN_INFO "        ref %d desc %d -> node %d u%p\n",
					       ref->debug_id, ref->desc, ref->node->debug_id, ref->node->ptr);
			} else {
				.......
			}
		} break;

（1）如果是BINDER_TYPE_BINDER，首先創建實體對象，再創建引用對象。handle為引用句柄值。

（2）如果是BINDER_TYPE_HANDLE，首先獲取引用對象，再獲取實體對象，cookie為本地對象IBinder的地址。

如圖：第三步

~/Android/frameworks/base/core/java/android/app

----ActivityManagerNative.java

public abstract class ActivityManagerNative extends Binder implements IActivityManager
{
    ......
    public boolean onTransact(int code, Parcel data, Parcel reply, int flags)
            throws RemoteException {
        switch (code) {
        case START_ACTIVITY_TRANSACTION:
        {
            data.enforceInterface(IActivityManager.descriptor);
            IBinder b = data.readStrongBinder();
            IApplicationThread app = ApplicationThreadNative.asInterface(b);
            Intent intent = Intent.CREATOR.createFromParcel(data);
            String resolvedType = data.readString();
            ......
            IBinder resultTo = data.readStrongBinder();
            ......
            return true;
        }

Parcel類readStrongBinder映射到C++層，執行如下：

～/Android/frameworks/base/core/jni

----android_util_Binder.cpp

static jobject android_os_Parcel_readStrongBinder(JNIEnv* env, jobject clazz)
{
    Parcel* parcel = parcelForJavaObject(env, clazz);//獲得Java層reply的引用
    if (parcel != NULL) {
        return javaObjectForIBinder(env, parcel->readStrongBinder());
    }
    return NULL;
}

~/Android/frameworks/base/libs/binder

----Parcel.cpp

sp Parcel::readStrongBinder() const
{
    sp val;
    unflatten_binder(ProcessState::self(), *this, &val);
    return val;
}

status_t unflatten_binder(const sp& proc,
    const Parcel& in, sp* out)
{
    const flat_binder_object* flat = in.readObject(false);
    
    if (flat) {
        switch (flat->type) {
            case BINDER_TYPE_BINDER:
                *out = static_cast(flat->cookie);
                return finish_unflatten_binder(NULL, *flat, in);
            case BINDER_TYPE_HANDLE:
                *out = proc->getStrongProxyForHandle(flat->handle);
                return finish_unflatten_binder(
                    static_cast(out->get()), *flat, in);
        }        
    }
    return BAD_TYPE;
}

（1）如果是BINDER_TYPE_BINDER，返回本地對象。

（2）如果是BINDER_TYPE_HANDLE，根據句柄值，返回代理對象。

然後執行javaObjectForIBinder。

~/Android/frameworks/base/core/jni

----android_util_Binder.cpp

jobject javaObjectForIBinder(JNIEnv* env, const sp& val)
{
    if (val == NULL) return NULL;

    if (val->checkSubclass(&gBinderOffsets)) {
        // One of our own!
        jobject object = static_cast(val.get())->object();
        ........
        return object;
    }

    // For the rest of the function we will hold this lock, to serialize
    // looking/creation of Java proxies for native Binder proxies.
    AutoMutex _l(mProxyLock);

    // Someone else's...  do we know about it?
    jobject object = (jobject)val->findObject(&gBinderProxyOffsets);//檢查當前進程之前是否已經為它創建過一個BinderProxy對象
    if (object != NULL) {//如果有返回來的就是一個指向該BinderProxy對象的WeakReference對象object,即一個弱引用對象
        jobject res = env->CallObjectMethod(object, gWeakReferenceOffsets.mGet);//由於弱引用對象object所指向的BinderProxy對象可能已經失效，因此，需要檢查它的有效性，方法是調用它的成員函數get來獲得一個強引用對象。
        if (res != NULL) {//如果不為NULL
            ......
            return res;//直接返回
        }
        .....
        android_atomic_dec(&gNumProxyRefs);//如果為NULL
        val->detachObject(&gBinderProxyOffsets);//解除它與一個無效的BinderProxy對象的對應關系
        env->DeleteGlobalRef(object);//刪除弱引用對象的全局引用
    }

    object = env->NewObject(gBinderProxyOffsets.mClass, gBinderProxyOffsets.mConstructor);//創建一個BinderProxy對象
    if (object != NULL) {
        .......
        env->SetIntField(object, gBinderProxyOffsets.mObject, (int)val.get());//BinderProxy.mObject成員變量記錄了這個BpBinder對象的地址
        val->incStrong(object);

        // The native object needs to hold a weak reference back to the
        // proxy, so we can retrieve the same proxy if it is still active.
        jobject refObject = env->NewGlobalRef(
                env->GetObjectField(object, gBinderProxyOffsets.mSelf));//獲取BinderProxy內部的成員變量mSelf(BinderProxy的弱引用對象)，接著再創建一個全局引用對象來引用它
        val->attachObject(&gBinderProxyOffsets, refObject,
                jnienv_to_javavm(env), proxy_cleanup);//把它放到BpBinder裡面去，下次就要使用時，就可以在上一步調用BpBinder::findObj把它找回來了

        // Note that a new object reference has been created.
        android_atomic_inc(&gNumProxyRefs);
        incRefsCreated(env);
    }

    return object;
}

（1）如果是本地對象，首先向下轉型為JavaBBinder，然後取得ActivityRecord對象，它繼承了IApplicationToken.Stub。而IApplicationToken.Stub繼承Binder，實現了IApplicationToken。所以可以向上轉型為IBinder。

IBinder resultTo = data.readStrongBinder();

（2）如果是代理對象，首先生成BinderProxy對象，裡面的mObject指向代理對象，向上轉型為IBinder。

            IBinder b = data.readStrongBinder();
            IApplicationThread app = ApplicationThreadNative.asInterface(b);

然後生成ActivityManagerProxy對象，裡面mRemote指向BinderProxy對象。

如圖：第四步
～/Android/frameworks/base/services/java/com/android/server/am

----ActivityManagerService.java

    public final int startActivity(IApplicationThread caller,
            Intent intent, String resolvedType, Uri[] grantedUriPermissions,
            int grantedMode, IBinder resultTo,
            String resultWho, int requestCode, boolean onlyIfNeeded,
            boolean debug) {
        return mMainStack.startActivityMayWait(caller, intent, resolvedType,
                grantedUriPermissions, grantedMode, resultTo, resultWho,
                requestCode, onlyIfNeeded, debug, null, null);
    }

待續