具体可以参考这里 http://wenku.baidu.com/link?url=KLkYho-Opm_-QGdfLkQ7nSC3joCNnhnAmpbO2BJsCHEUd6cOBvYc47ejLHPM1KFE2Xljhtgh2bvpCUdvjZwLxXOcml75xijXfi_Y_NnJu_K
