java sdk：
https://docs.oracle.com/javase/8/docs/api/
servlet api：
http://tomcat.apache.org/tomcat-8.5-doc/servletapi/index.html
jdbc:
https://docs.oracle.com/javase/8/docs/technotes/guides/jdbc/
c3p0：
https://www.mchange.com/projects/c3p0/
freemarker:
http://freemarker.foofun.cn/
https://freemarker.apache.org/
httpclient:
http://hc.apache.org/httpcomponents-client-ga/httpclient/apidocs/