如何用ant判断一个文件是否存在

 <?xml version="1.0" encoding="UTF-8" ?> 
- <project name="unzip" default="unzip" basedir=".">
- <target name="unzip">
  <available file="archiva.xml" property="archiva.exist" /> 
  <echo>${archiva.exist}</echo>   
  </target>
  </project>

运行：
E:\automatize>ant -buildfile build2.xml
Buildfile: E:\automatize\build2.xml

unzip:
[echo] true

BUILD SUCCESSFUL
Total time: 0 seconds