当我引用一些jre库时，如何使用ant构建android项目

I used eclipse to develop my android app and used JRE System Library in the project. and i can build the project using Eclipse. but i could not build it using ant. it could not found the classes in the JRE System Library. How can i add the JRE System Library to the ant build path?I used eclipse to develop my android app and us