详见该文档，说的很明白。http://wenku.baidu.com/link?url=msbR8rwkhpyzyfnL8b6JuIDjXbObeuZjWJ928lxl5u4crSczmC92sri03ODQtcvBuqZa6CfGfa8xQMS4QmXjbg16M4BNcX94twM_HAQ1-vy
