status_t BpBinder::transact(
    uint32_t code, const Parcel& data, Parcel* reply, uint32_t flags) {
    if (mAlive) {
        status_t status = IPCThreadState::self()->transact(
            mHandle, code, data, reply, flags);
        if (status == DEAD_OBJECT) mAlive = 0;
        return status;
    }
    return DEAD_OBJECT;
}

真正调用的是IPCThreadState的transact方法，查看IPCThreadState。