  We use a sample of radio-loud active galactic nuclei (AGNs) with estimated
central black hole masses to explore their jet formation mechanisms. The jet
power of AGNs is estimated from their extended radio luminosity. It is found
that the jets in several AGNs of this sample are too powerful to be extracted
from the standard thin accretion disks or rapidly spinning black holes
surrounded by standard thin disks. If the advection dominated accretion flows
(ADAFs) are present in these AGNs, their bright optical continuum luminosity
cannot be produced by pure-ADAFs due to their low accretion rates and low
radiation efficiency, unless the ADAFs transit to standard thin disks at some
radii $R_{\rm tr}$. If this is the case, we find that the dimensionless
accretion rates as high as 0.05 and transition from ADAFs to standard thin
disks at rather small radii around 20GM/c^2 are required to explain their
bright optical continuum emission. We propose that the disk-corona structure is
present at least in some AGNs in this sample. The plasmas in the corona are
very hot, and the pressure scale-height of the corona H\sim R. Powerful jets
with Q_jet \sim L_bol (bolometric luminosity) can form by the large-scale
magnetic fields created by dynamo processes in the disk corona of some AGNs.
The maximal jet power extractable from the corona Q_jet^max\le 0.6L_c (L_c is
the corona luminosity) is expected by this jet formation scenario. The
statistic results on the sample of AGNs are consistent with the predictions of
this scenario. Finally, the possibility that the jet is driven from a
super-Keplerian rotating hot layer located between the corona and the cold disk
is discussed. We find that, in principle, this layer can also produce a
powerful jet with Q_jet\sim L_bol.
