java project classpath in emacs

[QinGW <dealover@21cn.com>]

hello,everybody!
        how can I get different classpath value when I load different
        java project?

        for example:
        
1)        project A
          it's classpath as following:
          /home/qingwu/source/A/src
          /home/qingwu/source/A/lib
          /home/qingwu/source/A/build/classes 
2)        project B
          it's classpath as following:
          /home/qingwu/source/B/src
          /home/qingwu/source/B/lib
          /home/qingwu/source/B/build/classes 

I have setted "jde-global-classpath" variable using different value in
prj.el and add hook to jde-mode, but I still can't find lib package when I compile one java
source file.

my env:emacs jde ant

Thanks very much, qingwu