常用在线java API集合

Apache Poi

http://poi.apache.org/apidocs/index.html

Java SE 6

http://docs.oracle.com/javase/6/docs/api/

Javs SE 7

http://docs.oracle.com/javase/7/docs/api/

Java SE 8

http://docs.oracle.com/javase/8/docs/api/

Java 2 EE 1.4

http://docs.oracle.com/javaee/1.4/api/

Java EE 5

http://docs.oracle.com/javaee/5/api/

Java EE 6

http://docs.oracle.com/javaee/6/api/

Apache Struts2

http://struts.apache.org/maven/struts2-core/apidocs/index.html

还在整理中，遇到了还会再添加引用地址。